Astronomer Jay M. Pasachoff, author of A Field Guide to the Stars and Planets (Fourth edition, 2001), is one of the world’s leading experts on the universe and the celestial bodies it contains. Here, Pasachoff answers a wide range of intriguing questions, including what could be done if an asteroid threatened to collide with the Earth, what are brown dwarfs and wormholes, is there life on Mars or on extrasolar planets, and can you ever escape Earth’s gravity.
Q: About how large does an asteroid need to be before its gravity makes it round?
A: The largest known asteroid, 1 Ceres, is about 1,000 km (600 mi) in diameter. Only the six largest asteroids are larger than 300 km (200 mi) in diameter, and they are all round. The dividing line below which asteroids do not become round is somewhere between 100 and 300 km (60 and 200 mi), depending on the circumstances of their formation and cooling. The most studied asteroid is 433 Eros. The Near-Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft, created and run by the National Aeronautics and Space Administration (NASA) and the Johns Hopkins University Applied Physics Laboratory, orbited Eros for about a year, starting on February 14, 2000. Eros is oblong, about 33 by 13 by 13 km (21 by 8 by 8 mi), and images of it are posted on the Web.
Several space missions are now discovering asteroids at an astonishing rate. During the year 2000, 10,000 new asteroids were discovered-–as many as had been discovered during the preceding 200 years.
Q: If a giant asteroid were headed toward Earth, could people do anything to stop it?
A: We need more information about asteroids before we can tackle this task. If we were to put a bomb on an asteroid, we don’t know whether the asteroid would swerve slightly, missing Earth, or just break into bits, with each of the bits still coming straight at us. For this reason, astronomers would like to make a survey of the sky to detect all the asteroids that might intersect Earth’s orbit, and to understand the composition of asteroids well enough to know how solid they are.
Our current understanding of these near-Earth objects indicates that we can expect a big collision every few hundred thousand years, and a really devastating one every few million years. Astronomers think that over 1,000 of these near-Earth objects are more than 1 km (0.6 mi) across. More and more scientists agree that a collision with an asteroid or comet 65 million years ago killed off the dinosaurs and many other species. In fact, mammals like us benefited from that event, since mammals survived while the dinosaurs didn’t. Statistically, scientists think there is a 1 percent chance that a much smaller object, around 300 m (1,000 ft) across, will hit Earth sometime in the next century. An asteroid of that size would still make a devastating crater on land or create a tidal wave if it fell in the ocean.
The spacecraft NEAR Shoemaker (Near-Earth Asteroid Rendezvous, also named for planetary geologist Eugene Shoemaker) went into orbit around the asteroid Eros on February 14, 2000, and it has sent back incredibly detailed images. NEAR Shoemaker will get closer and closer to Eros, eventually landing on the asteroid. Eros seems to be solid, while the asteroid Mathilde, which NEAR Shoemaker passed in 1997, seems to be more of a rubble pile, with only half the density of Eros.
Q: I’ve heard that astronomy absorbs tremendous amounts of money, but I cannot find exact figures. Can you tell me how much is spent yearly? And is it too much for just pondering the mysteries of the universe? Wouldn’t it be wiser to spend a fraction of this money for more mundane purposes? I like seeing the pretty pictures from the Hubble telescope, but I wonder if it is worthwhile.
A: Actually, astronomy absorbs relatively little money compared with other government functions. Over the years, it has been demonstrated that investing money in basic research such as astronomy has a return of several times the investment. Astronomy attracts some of the world’s brightest people to scientific problems and leads to discoveries of physical laws that are important to everybody, although the results of such discoveries are not usually immediately apparent.
Astronomic research that could have important impacts in the near future includes studies that may improve our understanding of Earth’s atmosphere and studies of the runaway greenhouse effect on Venus.
In the United States the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA) are the primary sources of funding for astronomy. The NSF astronomy budget is about $100 million per year. The population of the United States is more than 250 million, so that is 40 cents per person per year.
NASA’s budget is harder to categorize since about one-third of it is devoted to crewed space flight—largely for political reasons or for general exploration rather than for scientific research. Indeed, many scientists decry the current emphasis on the International Space Station because of the limited amount of science that will be carried out on it. However, NASA conducts resource mapping and other studies of Earth that have proved very valuable.
Still, even if we say that about $2.5 billion of NASA’s $15 billion per year budget is related to space sciences, with a smaller fraction devoted to astronomy, that is only around $10 per person. The astronomy part is about $500 million, roughly $2 per person.
Note that for $10 per person, you aren’t going to solve major problems on Earth. You aren’t going to solve poverty or make medical advances that will revolutionize the world, or even provide health insurance for individuals. It seems more worthwhile to invest that level of money in basic scientific research that has the promise of making breakthroughs that will bring new health and prosperity to people in the future. Our country spends about 40 times as much on social programs as it does on space.
Incidentally, the Hubble Space Telescope puts out pretty pictures to show the people that it is working, but its major scientific work isn’t in those pretty pictures. It has spectrographs and special filters that allow details to be investigated. Most people like the pictures, but please don’t be misled to think that pictures are Hubble’s main scientific work.
Q: How rare is the aurora borealis? What causes it?
A: The aurora borealis isn’t rare if you live near one of the Earth’s magnetic poles. The north magnetic pole currently is in the Queen Elizabeth Islands of Canada’s Northwest Territories. Views from space show an auroral oval most of the time.
The aurora borealis and its sister in the Southern Hemisphere, the aurora australis, are caused by particles from the Sun hitting the air molecules in the Earth’s atmosphere. These particles give energy to the air molecules and make them glow. Different molecules glow in different colors.
When a coronal mass ejection or a solar flare sends a large number of high-energy particles into the auroral oval, the oval expands. Such events take place more often near the maximum of the solar-activity cycle (which is most commonly regarded as the sunspot cycle). This 11-year cycle is already or will soon be past its peak, which will probably be shown to have occurred in 2000 and 2001.
Solar coronal mass ejections and flares occur frequently in the declining phase of the cycle, so we have hope of seeing auroras at latitudes closer to the equator than is usual. Such an event can cause an aurora that is visible throughout the whole continental United States. If you live in Alaska, you can see an aurora most of the time when the night sky is clear. Phenomena that result from the interaction of the Sun and Earth—like the aurora—are called space weather.
NASA has several satellites in space that image the aurora regularly, including Polar and IMAGE (Imager for Magnetopause-to-Aurora Global Exploration). The IMAX movie SolarMax (2000) shows auroras and other Earth-Sun phenomena on a huge screen.
Q: What was there in the universe before the big bang?
A: Astronomers’ observations indicate that the universe is expanding, with every cluster of galaxies moving away from every other cluster. Projecting their motion backward in time, the clusters of galaxies would be closer and closer together, and the universe’s density would be very, very high. We can project back in time only as far as this original explosion, which we call the big bang. (The name was given derisively in the 1940s, but it caught on and is now used as the formal name.)
Astronomers measure time from the big bang. As far as we know, time started and space originated in the big bang. So there was no “before the big bang.” The universe simply did not exist.
This explanation may become clearer if we examine a similar example: On the Earth’s surface, the North Pole marks latitude 90 degrees. If you go north along any line of longitude toward the North Pole and then keep going, you don’t go above latitude 90 degrees. Once you reach 90 degrees (the North Pole), you start going south. So the question “what is north of the North Pole” doesn’t have a meaningful answer in the same way that “what was there in the universe before the big bang” doesn’t have a meaningful answer.
Q: What is the main advantage of being on-site for an eclipse? Can’t you create an artificial eclipse anywhere by placing a dark black-out disk over the Sun’s face?
A: Well, you can’t place a dark disk over the Sun’s face and see the corona. The corona is just fainter than the blue sky, and it isn’t visible. If you go to a very high mountain, such as Haleakala in Hawaii, you can make such a coronagraph, although it requires intricate optics. And even then you don’t see as much of the corona as well as you do from an eclipse.
Even from space, where there is a coronagraph on the Solar and Heliospheric Observatory (SOHO) spacecraft, the coronagraphs can’t make as good an eclipse as we see from the ground. SOHO hides (occults) not only the Sun but also all the corona for one solar radius above the ordinary solar disk. This is the interval that we see best from a ground-based eclipse site.
It is pretty to see the Moon cover the Sun in the sky, but the actual eclipse is pretty small in angle—you can cover it with your thumb at the end of your outstretched arm. The real glory of a total solar eclipse for ordinary watchers (aside from the science that professionals are interested in) is the whole set of atmospheric changes. It is dramatic to be outdoors when the sky grows dark in the middle of the day. Shadows strangely sharpen and the quality of the light changes, making it eerie in a way that is hard to pin down. Then the shadow of the Moon rushes at you at a speed of thousands of miles per hour and within seconds, day is transformed into night. Nobody who is outdoors during these abrupt changes remains unmoved.
Q: In theory if you go into a black hole, when you come out the other side, would you be in a different time? Because if a black hole sucks in light then wouldn’t it suck in time?
A: In principle, you could be in a different time—IF you could go through a black hole and come out somewhere else (the other end would be a location known as a white hole, which is a hypothetical region in space from which stars, light, and other forms of energy explosively emerge). If this were possible, you would be going through a wormhole, a hypothetical passage in space-time connecting widely separated parts of the universe. It is not clear if a wormhole can stay open sufficiently long for anyone to go through it.
With regard to your second question, a black hole doesn’t “suck in light.” If light has the bad luck to go inside, it can’t get out. But the black hole’s effect on space outside itself is the same as that of any equivalent amount of mass.
Q: What are brown dwarfs?
A: Brown dwarfs are failed stars—balls of gas that didn’t become hot enough for fusion of hydrogen to start inside. A star has to have a mass of about 7 percent of the Sun’s mass for its core to get hot enough for fusion to begin. We think that starlike objects with less than that critical mass become brown dwarfs.
Since they aren’t shining brightly, brown dwarfs are very hard to detect. Indeed, until the 1990s, there had been no accepted brown dwarf discoveries. Recently, new large telescopes and satellites sensitive to the infrared radiation that brown dwarfs give off have led to dozens of discoveries.
Some brown dwarfs are as cool as 2000°C (3600°F) at their surfaces. Some gases, such as lithium, which is destroyed at hotter temperatures, survive at the surfaces of brown dwarfs. The presence of such gases convinces astronomers that they are indeed observing this type of star.
For decades, astronomers have divided stars into seven spectral types: O, B, A, F, G, K, and M. O stars are the hottest and M stars are the coolest. Now we have added two cooler types, L and T, to include brown dwarfs. People remember the earlier sequence from the first letters of each word in the sentence “Oh, be a fine girl, kiss me.” Try to think of mnemonics that include not only the original types but also types L and T!
Q: Which star is closest to Earth, other than the Sun?
A: We measure distances to the closest stars using the concept of parallax, which relies on the fact that the background behind an object looks different depending on the angle from which the object is viewed. For example, when the passenger in a car looks at the speedometer needle it appears to indicate a slightly lower speed than the one the driver sees. This is because the driver is looking straight ahead at the speedometer and the passenger is looking at it from an angle, and the needle is projected against the background numbers.
You can check this effect by holding your thumb in front of your face. If you hold your thumb only a few inches from your nose and then look with first one eye and then the other, you can see that your thumb seems to jump a lot across the background. But if you extend your arm and look again with each eye, your thumb seems to jump less.
In astronomy, we project the angle of the nearest stars against a background of much more distant stars. If we look in January, and then look again when Earth has moved halfway around its orbit in June, we find that the nearest stars are measurably displaced compared with farther stars.
Using parallax, astronomers have determined that the nearest star is part of a triple-star system known as Alpha Centauri. Alpha Centauri is so named because it is the brightest system in the constellation Centaurus (alpha is the first letter in the Greek alphabet). Centaurus is too far south in the sky to be visible from most of North America and Europe. One of the three stars in Alpha Centauri, known as Proxima Centauri (from the Greek word for “close”), is the nearest star to us. Still, it is four light-years away. That is, its light travels for four years, at the tremendously great speed of light, before it reaches us. In contrast, light from the Sun reaches us in eight minutes, and light from the Moon reaches us in about one second.
Q: When the Moon is near the horizon, it often takes a red or orange color. Is this caused by the refraction or reflection of light or neither? And why is this “color change” so short-lived?
A: The Moon looks reddish or orange near sunset for the same reason there is a reddish sunset glow on the horizon. And this reason is connected to why the sky is blue.
Sunlight bounces (scientists say “scatters”) off air molecules, and the shorter the wavelength (that is, the bluer the light) the more effectively it bounces. So when the Sun or Moon is overhead, its light comes to us directly but off to the side a bit. Blue light bounces toward us more effectively than red light.
But when the Sun is on the horizon, its light has to pass through a greater length of atmosphere to reach us. The blue light scatters all along the path, making the sky blue for people in that direction. By the time the sunlight reaches us, most of the blue is gone and we are left with relatively more red. The same effect occurs for moonlight, but it is so much fainter than the sunlight that the blue-sky effect is gone. Still, the light that reaches us near the horizon is relatively red because the blue light has been scattered away.
That kind of scattering is called “Rayleigh scattering,” after the great physicist Lord Rayleigh of the 19th century.
Q: Why do the constellations change over time?
A: The visibility of constellations changes over the year, although specific constellations are visible at the same time every year. Constellations also change shape over tens of thousands of years.
First, why do we see different constellations at different times of the year? Earth orbits the Sun once a year. Sunlight that hits Earth’s atmosphere scatters around and makes the sky blue, preventing people on the ground from seeing the stars during the day. So people can only see constellations when they are situated on the side of Earth that faces away from the Sun. At different times of the year, Earth is on different sides of the Sun, so we see different parts of the sky and different constellations. Since our view of the sky changes by 360 degrees in 12 months, the view changes by 30 degrees in 1 month; 30 degrees is about three fists’ width held at the end of your outstretched arm. So each month, stars that you saw last month have moved about 30 degrees.
Why do constellations change over long periods of time? Because stars move through space. We can measure how fast they are coming toward us or going away from us by studying their spectrum, using a phenomenon known as the Doppler effect. We can measure how fast they are moving from side to side only for the closest stars, which have noticeable “proper motions” with respect to other stars. If the proper motion is large enough, then the star will move noticeably if you wait long enough. Stars are so far away, however, that they have to move a long way for us to see any difference. The star with the largest proper motion is known as Barnard’s star; it moves the diameter of the Moon in the sky every 180 years. Unfortunately, Barnard’s star is too faint to see with the naked eye. No star visible to the naked eye moves noticeably in a human lifetime, so no constellation will change noticeably in a human lifetime.
Q: My astronomy professor said that due to the Sun’s cyclical orbit around this arm of the Milky Way, the Earth’s perspective regarding the Sun’s apparent transit through the heavens has changed since 2,000 years ago when the “science” of astrology was formulated. As a result of this quarter-turn shift, the Sun is no longer moving through the same constellations of the zodiac as it was before, and calculations are off by one constellation. Thus, instead of being a Cancer, am I actually a Gemini?
A: Yes, it is true the “signs” you see in the newspaper are not the constellations where the Sun actually is at those times of year, because of the precession of the equinoxes. And the Sun goes through parts of 13 constellations in the course of the year.
Furthermore, the idea that there are 12 “signs of the zodiac” is completely arbitrary—other cultures had different numbers of constellations filling the same space. There is simply nothing to astrology, and I hope that readers of this reply will spend their time on the fascinating topics in the science of astronomy instead of wasting time on the pseudoscience of astrology.
Q: The Discovery Channel recently aired a program called “Planet Storm.” One of the space weather phenomena the show depicted was a coronal mass ejection (CME). While solar flares and sunspots are infrequent events on Earth, nothing of the magnitude of a CME (which could deplete 100 percent of our atmosphere) has ever occurred. In the dramatization, a CME of this level penetrates and vaporizes the ozone layer, creating a brilliant aurora borealis effect for people to enjoy in the short run, only to wake up the next day to be killed off almost instantly by fatal ultraviolet (UV) rays.
Can a CME like this occur and what precautions can be taken? Are we fooling ourselves about this deadly potential as we did with Kiddy Cocoons in the 1950s and 60s?
A: Coronal mass ejections occur daily, and solar physicists spend a lot of time talking about them. Whereas decades ago they thought that giant solar flares were the main Sun-Earth link, they now realize that coronal mass ejections are a more regular link. Whether coronal mass ejections are linked to flares is very much debated; some are, but they may not be cause and effect.
The importance of coronal mass ejections has been shown especially by the movies made by the Large Angle Spectrographic Coronagraph (LASCO) instrument on the Solar and Heliospheric Observatory (SOHO) spacecraft, a joint project of NASA and the European Space Agency.
Usually, LASCO shows CMEs shooting off to the side of the Sun, but every once in a while it sees a “halo event,” when material seems to surround the “occulting disk” that hides the bright ordinary solar disk. These halo events no doubt mean that the CME is pointing at the Earth. Particles from the CME arrive hours or days later. CMEs have been known to disrupt terrestrial communications and, probably, to damage spacecraft.
CMEs have varying strengths. I know no reason why the Sun’s magnetic field can’t rearrange itself to shoot off a much more powerful CME than normal. It is statistically unlikely that such a powerful CME, as depicted in your question, would arise. But if it should happen, I see no way that we earthlings could prevent it or deal with it. There are no precautions we can take, short of moving to another planet—which is at least centuries in our future for more than a handful of individuals.
Q: How does Earth’s distance from the Sun affect the seasons?
A: Basically it doesn’t. Earth’s orbit, although elliptical, is so close to round (only 1.7 percentage points from being perfectly round) that Earth’s distance from the Sun does not vary enough to affect the seasons. Indeed, Earth is closest to the Sun each year on January 4, when it is winter and cold in the Northern Hemisphere.
The seasons are caused by Earth’s tilt on its axis. Earth’s tilt is constant with respect to the stars, and as Earth goes around the Sun each year sometimes the Northern Hemisphere is tilted toward the Sun and sometimes it is tilted away. The Southern Hemisphere, of course, is tilted the opposite way. When one hemisphere is tilted toward the Sun, it is summer there and winter in the opposite hemisphere.
Earth’s tilt affects the seasons because a beam of sunlight 1 m (3 ft) in diameter warms a patch of ground 1 m across if the beam shines directly down on Earth’s surface. But when the beam hits a part of Earth’s surface that is tilted, that same beam has to warm a larger patch of ground. Thus the energy in the beam is diffused, and each square centimeter of Earth’s surface receives less energy per second.
Q: If the Moon is receding from the Earth at approximately five centimeters per lunar year, is the Earth going toward the Sun or receding from the Sun? If so, by what distance per Earth year?
A: The Moon is receding from the Earth because of its tidal interaction, which is the result of the gravitational pull of the Moon on the Earth. But the planets have very little mass compared with the Sun, so there is no measurable tidal effect of planets on the Sun.
I am unaware of any yearly change in the Earth’s orbit around the Sun. In the distant future, the Sun will lose enough mass that Earth’s orbit will stabilize at a greater distance than it is now. But that is still billions of years away. If Earth’s orbit were to double in size over 3 billion years, that would mean an increase of 150 million km (93,210,000 mi), or approximately 50 m (164 ft) each year.
Although the Earth is thought to be on a track to recede in the future, some planets spiral in to some stars. In some recently discovered planetary systems, giant planets orbit their parent stars in days (instead of the ten years it takes Jupiter to orbit our Sun). That means these planets orbit very closely. They may have formed farther out and gradually moved close in, according to theories now being developed. There has also been a report of elements detected (spectroscopically) on a star that could well have resulted from a planet crashing into the star.
Q: What is the safest way to observe a solar eclipse?
A: You shouldn’t stare at the Sun, except during the total phase of a total eclipse. During the total phase, the solar surface is completely covered by the Moon. When the Moon covers the Sun only partially during an eclipse, some of the solar surface remains visible, so you cannot safely look at it directly. This part of an eclipse is known as the partial phase; some eclipses are never more than partial when seen from Earth. The Sun is also partially eclipsed before and after the total phase of a total eclipse.
Whenever the solar surface is visible—during a partial eclipse or the partial phase of a total eclipse—you must take special precautions to see the eclipse. Special solar filters are inexpensive; they cut out more than 99.99 percent of the Sun’s light. Another option is to make a pinhole camera. Just punch a hole the size of a pencil in a piece of paper, cardboard, or aluminum foil. Then hold that hole up to the Sun, next to your shoulder, and look away from the Sun at the image formed on the floor or a wall. Since you are looking away from the Sun, there is no hazard.
If you are lucky enough to be in a location where the Sun is totally eclipsed, then the solar surface will be entirely covered. At that time, the solar corona comes into view. The corona is a million times fainter than the solar surface. It is about the same brightness as the full Moon and is equally safe to look at.
So, look at the Sun directly ONLY when the solar surface is completely covered during a total eclipse. Use special filters or a pinhole camera whenever the Sun is partially eclipsed. There are no special rays that come out of the Sun during an eclipse; the hazard is that you are more tempted to stare.
Q: How far away do you need to be to escape Earth’s gravity?
A: As long as you remain in the universe, you can never actually escape Earth’s gravity. Isaac Newton discovered the law of universal gravity, which shows that the strength of gravity declines according to the square of the distance between objects. Thus when you go twice as far away from an object, you feel one-fourth the strength of its gravitational force. But each object in the universe retains some gravitational pull, however minuscule.
If you go nine-tenths of the distance toward the Moon, the Moon’s gravity becomes stronger than Earth’s gravity. If you go one-hundredth of the distance toward the Sun, the Sun’s gravity becomes stronger than Earth’s gravity.
French mathematician Joseph Louis Lagrange (1736-1813) worked out a set of solutions to describe how the gravitational attraction between two large objects and their orbital velocities balance each other such that a small body placed in the orbital plane of the larger bodies will remain balanced there. He found five such points at which smaller objects remain balanced.
For example, there is a point partway to the Moon at which Earth’s gravity and the Moon’s gravity are equal. If you were to pass that point, you would fall toward the Moon instead of toward Earth. This point, known as L1, is closer to the Moon than to Earth since Earth has more mass than the Moon.
Similarly, there is an L1 point partway to the Sun at which Earth’s gravitational pull and the Sun’s gravitational pull on an object are equal. This point is relatively close to Earth. The joint European Space Agency/NASA spacecraft called the Solar and Heliospheric Observatory (SOHO) is located at L1, hovering so the Sun remains in steady view for scientific study, and needs little fuel to remain in place.
Other Lagrangian points in a line with two massive objects include L2, which is on the far side of the smaller of the two objects. L3 is on the far side of the larger object, and is the same distance from it as the smaller body.
Other Lagrangian points, L4 and L5, exist one-third of the way around an orbit. For example, the Trojan asteroids are in Jupiter’s orbit around the Sun, 60 degrees (one-sixth of a circle) ahead of and behind Jupiter. Many people have proposed installing an inhabited space station at the L5 Lagrangian point.
Q: Not knowing your religious beliefs I would like to ask you: As you explore the universe through a telescope do you find the existence of God to be more, or less, believable as you discover the universe’s qualities? Why or why not?
A: My belief in God is independent of what I learn about the universe. The universe seems to follow a set of laws, such as the laws of gravity and orbits and Maxwell’s equations for electromagnetism. Whether you choose to think that God made those laws or that they arose naturally is up to you and is a matter of religious belief. I see no need to mix science and religion.
Q: What are geostationary and geosynchronous orbits, and what is the difference between them?
A: There is no difference. The word used depends on whether you are looking up at a satellite or down from a satellite. A satellite only a few hundred kilometers above Earth’s surface orbits in about 90 minutes. The higher the satellite, the longer it takes to orbit. If the satellite is about 6.5 Earth radii high, or about 40,000 km (about 25,000 mi) above Earth’s center, it orbits in 24 hours.
Earth also spins once every 24 hours, however, so a satellite with a 24-hour orbit is in synch with Earth’s rotation. We call such an orbit geosynchronous. To an observer on the ground looking up, the satellite seems stationary overhead, so the orbit may also be called geostationary. The satellite has to be orbiting above the equator to appear still in the sky, so the band of orbits over the equator has a lot of satellites in it.
Geosynchronous/geostationary satellites are especially useful in relaying television and other communications signals. A radio telescope or satellite dish on Earth can remain fixed and still always point at the satellite.
Q: How hot is the Sun?
A: The Sun is a ball of gas, heated at its center by nuclear fusion. In its center, hydrogen atoms merge one at a time to make helium, which has almost four times the mass of hydrogen. The mass difference is converted into energy, in an amount that follows Albert Einstein’s famous formula E = mc2, where E is energy, m is the mass converted, and c is the speed of light. A tiny bit of mass converts to a tremendous amount of energy, and as a result the Sun is extremely hot.
The Sun is a little over 5500°C (10,000°F) at its visible surface, but it is much hotter inside. Astronomers can tell the temperature of the Sun’s surface in several ways. One way is that it is brightest in the yellow-green part of the spectrum, a characteristic of gas at 6000°C (11,000°F). Another way is to study the spectral lines caused by atoms in the Sun’s outer atmosphere, which are also characteristic of that temperature.
We can’t see the inside of the Sun directly, but calculations show that its temperature is about 15 million degrees C (about 27 million degrees F). There are two ways to find out about the center of the Sun. One is helioseismology, another name for solar seismology. In helioseismology, astronomers look at vibrations on the Sun’s surface to find out about its inside, just as geologists on Earth find out about the Earth’s inside by studying seismic waves on the planet’s surface. A second way is to study solar neutrinos, elementary particles that come straight out through the Sun once they are formed in fusion.
Q: How many people have walked on the Moon? Can you tell me something about each of them?
A: Six National Aeronautics and Space Administration (NASA) Apollo missions reached the Moon from 1969 to 1972. Many people at that time hoped that the Apollo program would lead to a permanent station on the Moon, and it is very disappointing to realize that nobody has been to the Moon in about 30 years.
Each lunar-landing Apollo mission (which began with Apollo 11) carried three people into orbit around the Moon. One of the astronauts then remained in the command module, while two others used the lunar module to descend to the Moon’s surface.
The first Moon landing was made by Neil Armstrong and Buzz Aldrin in Apollo 11’s lunar module, named Eagle, on July 20, 1969. Armstrong’s first words on the Moon were “That’s one small step for man, one giant leap for mankind.” There has been much discussion over the years as to why he didn’t say “for a man,” and there has been speculation that the word “a” was merely swallowed by a radio glitch, but the consensus seems to be that Armstrong just said “step for man,” perhaps out of nervousness. Armstrong became a professor of engineering and shunned public appearances, although he was arguably the most famous person in the world. Aldrin has recently coauthored a novel, The Return, which is a murder mystery involving the commercialization of space tourism.
Pete Conrad and Alan Bean landed on the Moon in Apollo 12’s lunar module, Intrepid, in November 1969. Conrad quipped, “That may have been a small one for Neil, but it’s a long one for me.” Conrad, always a daredevil, died in a motorcycle accident in 1999. Bean, an artist, has painted space scenes.
Apollo 13 suffered an explosion en route to the Moon. The astronauts were able to return safely to Earth, but they missed their chance to walk on the Moon.
Apollo 14’s lunar module, Antares, shuttled Alan Shepard and Ed Mitchell to the Moon’s surface in February 1971. The long hiatus was the result of the commission of inquiry over the cause of the Apollo 13 explosion. Shepard was chief of the Astronaut Office until he retired from NASA and later went into business. He died in 1998.
Apollo 15’s lunar module, Falcon, carried Dave Scott and Jim Irwin to the lunar surface in July 1971. They were able to go farther on the Moon than earlier astronauts because they had a vehicle, the Lunar Rover. With it, they were able to cover 27.3 km (17 mi). Scott had the idea of demonstrating the physics experiment of dropping a feather and a hammer in the Moon’s airlessness. Confirming Galileo’s experiment (which traditionally involved dropping weights off the Leaning Tower of Pisa), the feather and the hammer landed simultaneously, something shown in video clips to generations of science students since. Irwin retired from NASA to form a religious organization. He has since died.
John Young and Charles Duke went to the lunar surface in April 1972 in Apollo 16’s lunar module, Orion. They landed in the highlands, a rougher and therefore more dangerous region to explore than the smoother areas that NASA had chosen for the earlier flights. Young later became chief of NASA’s Astronaut Office. Duke retired from the astronaut corps to go into business; he also has a religious ministry.
Apollo 17’s lunar module, Challenger, carried Gene Cernan and Jack Schmitt to the Moon’s surface in December 1972. Schmitt was the only Ph.D. scientist and the only trained geologist to walk on the Moon. He later became a United States senator. Cernan went into business and is currently president of Cernan Energy Group, an energy and aerospace consulting company.
Originally NASA planned for longer missions to eventually take place, once the Apollo program got the bugs out. These longer missions were intended to accomplish more scientific investigations. Scientists looked for Apollo 18 to 20 for those scientific opportunities, but these missions were cancelled for financial reasons.
A few spacecraft went back to the Moon in the 1990s, notably the Clementine and Lunar Prospector missions in 1994 and 1998, respectively. A few uncrewed missions are scheduled for the coming decade, but there are presently no plans to send humans back to the Moon.
Q: How many stars are there?
A: In Antoine de Saint-Exupery’s wonderful book The Little Prince, a businessman sits on his tiny asteroid counting the stars, because he thinks he owns them. We can’t own the stars, and there are more than we can count.
Dust in space hides many stars from us, so we couldn’t count all the stars even if we tried. But astronomers can measure the amount of mass in our galaxy. It turns out that there is about a few hundred billion times more mass in our galaxy than our Sun has, and we know that much of that mass is in the form of stars. So we might estimate that there are a few hundred billion stars in our galaxy, assuming that, on average, each star has a mass that is roughly equal to the mass of the Sun.
Actually, the Sun’s mass is higher than that of an average star. This would indicate that there are even more stars in our galaxy. On the other hand, we are increasingly finding that much of the mass in our galaxy is not in the form of stars. Since it isn’t shining, we say this mass is dark matter. There is probably more mass in the form of dark matter in our galaxy than there is in the form of stars; this would tend to lower our estimate of the number of stars in our galaxy. At any rate, a few hundred billion stars is a reasonable estimate for our galaxy.
Our galaxy is only one of billions of galaxies, however, so there are truly a tremendous number of stars in the universe.
Q: Why does the Hubble Space Telescope take sharper pictures than larger telescopes on the ground?
A: The larger a telescope’s mirror, the finer the detail it can distinguish, according to the laws of optics. But ground-based telescopes look up through the atmosphere, and the atmosphere shimmers and shakes (we call it twinkling), blurring the stars.
One good way of getting around this limitation is to put the telescope in space, as NASA did with the Hubble Space Telescope. This telescope can see about ten times more clearly than typical ground-based telescopes, but it is only 94 in (240 cm) across—not very large for a telescope these days. So it can’t collect a lot of light at any given time.
Since the Hubble telescope was launched in 1990, astronomers have made progress in getting better images from ground-based telescopes. Just making sure that there aren’t sources of heat near a telescope or in the telescope dome, and adding fans to the telescope dome to make sure that no hot air is rising up through the dome slit, has helped improve image quality at many telescopes.
In addition, astronomers are rapidly developing a new technique known as active optics. In active optics, a mirror is made thin enough that it can be deformed several times a second by robotically controlled pins pushing on its back. The deformations compensate for the blurring caused by Earth’s atmosphere. Active optics is providing images that in some cases match those from Hubble. Hubble can point anywhere in the sky, however, while active optics can match Hubble’s high resolution only near certain bright objects, where the blurring can be taken out by deforming the mirror. Since we already know how these bright object should appear, we can adjust the mirror to compensate for atmospheric distortion. Other objects nearby in the sky also appear clearer. Another new technique uses laser beams shined up in the sky to make artificial bright objects anywhere we need to point the telescope.
Q: Could there be life in an ocean beneath Europa’s icy surface?
A: Jupiter’s moon Europa has been observed by the Voyager spacecraft and now, in more detail, by the Galileo spacecraft in orbit around Jupiter. Based on the observations of these spacecraft, scientists think that there is an ocean beneath Europa’s surface. Whether life exists in that ocean is a more difficult question.
Europa is similar in size to our Moon. Cracks in its surface look like cracks in eggshells, and some of Galileo’s close-up views show areas that look like ice floes, refrozen in a jumbled manner. The surface may be cracked because tides caused by other moons of Jupiter push and pull, flexing Europa. We don’t know how thick the ice might be—perhaps only 10 km (6 mi) or perhaps as thick as 100 km (60 mi).
New evidence from Galileo concerning the strength and direction of Europa’s magnetic field indicates even more strongly than does the presence of jumbled surface ice that a salty ocean lies beneath. Some observations in the infrared part of the spectrum indicate that there may be salt deposits on the ice.
But could there be life? We just don’t know whether life can arise by itself there or anywhere else. There are those who believe that life arises whenever the right chemical mix and energy input occurs. Others believe that life has come from afar—the panspermia theory—and that we have been seeded from other solar systems. If so, Jupiter’s moons could be included. Or it could be that life in our solar system began on Earth or even on Mars and spread to the other inner planets as well as to Europa. We will have to wait for on-site exploration of Europa to know. NASA is in the early stages of planning a Europa lander that will have the ability to drill through the ice.
Q: Is there life on Mars?
A: This question is one of the most interesting of all time. We are planning to send more spacecraft to Mars to find out. NASA’s Viking spacecraft reached Mars in 1976, landed on the surface, and carried out experiments to search for life. All the results were negative.
Mars Global Surveyor, another NASA spacecraft, is now in orbit around Mars and is sending back incredibly detailed pictures. The pictures show many signs that water has flowed on Mars. With the older, less detailed images, scientists thought they saw signs of streambeds with tributaries, indicating that rivers flowed on Mars. The latest images verify that the water that created these features is gone, but we don’t know how recently the latest features were formed.
Based on the knowledge of Mars’s composition gained from spacecraft and landers, scientists conclude that some meteorites found on Earth actually originated on Mars. Large asteroid impacts may have blasted these rocks into space, and the rocks then eventually fell to Earth. Detailed examination of one of these Mars meteorites under powerful microscopes shows tiny structures that some scientists think are fossils of primitive life on Mars. Most scientists disagree with those conclusions and think the shapes formed naturally, but debate continues.
NASA plans to launch a pair of automated rovers to Mars in 2003. These rovers will land on the surface by first using a parachute to slow their descent, then bouncing in giant airbags, much as the Mars Pathfinder did in 1997. They will be able to roam much farther across the surface of Mars than Pathfinder’s rover, Sojourner, did. The United Kingdom is also launching a spacecraft to Mars in 2003, carrying the Beagle 2 rover. Beagle 2 will include a laboratory to search for life.
Q: Is automated spaceflight more efficient and economical than manned spaceflight?
A: That is the conclusion that NASA has reached, and as computer controls and robotic systems get even more sophisticated, automated spaceflight is where we seem to be heading. Since the Apollo missions sent 12 people to the Moon’s surface from 1969 to 1972, we haven’t sent anybody out beyond Earth’s orbit. The missions to the planets—such as the Vikings to Mars, the Voyagers to Jupiter, Saturn, Uranus, and Neptune, and more recently Galileo to Jupiter and Cassini to Saturn—have all been robotic. Most scientists think they are more efficient and economical.
So what is the role of people in space? It is very controversial if people are really necessary or if they are merely inspirational. NASA is now spending incredible sums of money on the International Space Station, and most scientists think the benefits to be gained are not worth the expenditure. But if a rising tide raises all boats, then the space station’s role in keeping NASA’s budget up may indirectly benefit science.
Q: Why is there more matter than antimatter in the universe?
A: A Nobel Prize-winning experiment by James Cronin of the University of Chicago and Val Fitch of Princeton University showed that a certain unusual type of subatomic particle decayed slightly more often in one way than another. Scientists think that about 13 billion years ago, before the universe was one second old, this uneven decay caused a slight imbalance between the number of particles of matter and the number of particles of antimatter. Most of the matter and antimatter particles hit each other and annihilated, destroying each other in a burst of energy. They made photons of energy, similar to light. The remaining small excess of matter makes up all of the matter in the universe today. This process explains why there are 100 million times more photons in the universe than there are particles of matter: Due to uneven decay there were originally 100,000,001 particles of matter for each 100,000,000 particles of antimatter.
The type of imbalance found by Cronin and Fitch is even more obvious in a type of interaction involving a subatomic particle known as a B meson. Particle physicists (physicists who use atom smashers to study subatomic particles) are building machines that will create billions of these B mesons to study. The machines are known as B factories, because of the large number of identical B mesons they will create. The results of these experiments should soon tell us more about the imbalance between matter and antimatter and its origins in the earliest moments of the universe.
Q: Why does the Moon appear larger when it is close to the horizon?
A: The effect is psychological and does not relate to the actual size of the Moon. Apparently, as verified in an experiment widely reported in 2000, people misjudge the distance to objects when the objects are near the horizon. The human brain interprets the Moon to be relatively far and therefore large in angle when the Moon is near the horizon. Recently I was driving in a car on a winding road in which foreground trees sometimes appeared close to the bottom of the Moon in the sky and sometimes were not visible. I noticed the size of the Moon apparently changing from large to small and back again as I drove.
The traditional method of disrupting your brain’s perception of the Moon’s size is to stand with your back to the Moon, and then bend over, looking upside down at the Moon through your legs. You will then no longer see the “moon illusion” of a large lunar disk.
Q: Why does the Moon have more impact craters than Earth?
A: The Moon now has more impact craters than Earth for two reasons. First, Earth never had as many, and second, most of the craters Earth once had have been eroded away.
Unlike the Moon, Earth has a dense atmosphere, so most meteoroids coming at Earth burn up in Earth’s atmosphere. Only the larger ones survive.
Most craters on the Moon apparently formed between 3 and 4 billion years ago. The Moon has been geologically inactive for billions of years, however, so any craters that formed there are still present. The craters that formed on Earth by whatever meteorites did survive their passage through our atmosphere have long since been eroded by a variety of terrestrial processes.
Several craters remain on Earth, but they are large ones. The Meteor Crater in northern Arizona is the most famous and was the first to be recognized. Chicxulub in Mexico’s Yucatan Peninsula is hidden under ground and water but is now being revealed. Chicxulub is thought to be the crater caused by the meteorite that some scientists believe resulted in the mass extinction of the dinosaurs and many other species about 65 million years ago.
Q: Why did the Moon appear larger in the sky in the distant past?
A: Several thousand years ago, before human history, the Moon was close enough to Earth that it would have appeared noticeably larger than it does now. As a result of tidal forces and the conservation of angular momentum, the Moon is gradually receding from Earth by about 5 cm (2 in) per year. However, the effect is difficult to measure because the Moon changes in apparent size (as viewed from Earth) throughout the month as a result of its elliptical orbit.
The tidal bulge goes around faster in the Earth-Moon system than it would were the Moon’s gravity alone making it revolve, because the Earth is rotating. That tidal bulge pulls on the Moon, speeding up its revolution around Earth and making it move farther from Earth.
With the Moon farther out, its angular momentum is increased. Because angular momentum is conserved (according to Newton’s Third Law), the Earth’s angular momentum has to decrease to compensate.
Q: Why can we see only one side of the Moon from Earth?
A: The Moon’s rotation is locked to its orbit around Earth by a bulge of material under the Moon’s surface. This material is denser than the rest of the Moon’s surface material, and Earth’s gravity pulls on it especially strongly. The side of the Moon containing the bulge always faces Earth. We call it the near side and we call the other side, which we never see from Earth, the far side. People often mistakenly use the term “dark side” when referring to the far side of the Moon, forgetting that the far side is fully illuminated by the Sun whenever we see a new moon from Earth.
If we were to look from high above the Earth-Moon system, we would see the Moon rotate once per month with respect to the stars. This is its sidereal rotation period and lasts 27 1/3 days. The Moon revolves around Earth in the same amount of time.
The Moon’s elliptical orbit, combined with other factors, gives the Moon a slightly irregular velocity as it revolves around Earth, so we sometimes see a bit around the Moon’s edge to one side or the other. These “librations” allow us to see about five-eighths of the Moon’s surface from Earth.
Q: How were the sages of India able to accurately calculate eclipses and even the presence of nine planets without the help of the sophisticated equipment we have today? I have heard other nonindustrialized societies such as the Dogon of Mali were also excellent astronomers.
A: There are periodicities in the recurrence of eclipses that allowed scientists as far back as 2,000 years ago to predict them, or at least to predict the seasons when they could occur. The best known of these periodicities is the “saros,” named by Halley around 1700.
Certainly the ancient Indian sages did not know about nine planets. Indeed, even today we are debating whether Pluto is a planet, so any nontelescopic information about the number of planets past Saturn was not based on reality.
The Jantar Mantar in Jaipur, as well as several other observatories constructed in the 18th century by the Maharaja Jai Singh, have giant instruments that measure the positions of objects in the sky and give time on giant sundials. But the emissaries the Maharaja sent to the Western world were misled by the people they consulted, and they did not bring back news of the telescope, even though it had been invented and Jai Singh would surely have had one built, due to his great interest in modern science.
There was a flurry of interest about the astronomical knowledge of the Dogon of Mali some years ago, but the report they had strangely acquired modern astronomical knowledge turned out to be false. The interest originally dealt with a report that they knew Sirius had a companion, but it turns out contact with travelers in the late 19th century could easily have brought knowledge of that widely reported telescopic discovery.
Q: How are astronomers who view massive, distant objects able to discriminate between redshift components that are due to recessional velocities versus those due to gravitational redshift? Errors in such discrimination may make massive objects seem more distant than they really are, or may make distant objects (strongly redshifted) appear more massive than they really are.
A: Redshift is a change, or shift, in the light radiated by an object, such as a star or galaxy, that indicates the object’s motion. Scientists have used redshifts to measure the velocities (speed and direction) of distant galaxies.
The question you ask was important at the time of the discovery of quasars in 1963. Redshifts of over 13 percent were found in the spectra of objects, and it seemed incredible they could be receding at such a pace. If Hubble’s law—which links redshift and distance—were accepted, the quasars would be the farthest things in the universe. They would have had to be radiating at prodigious rates, given their tremendous distances. No obvious way was known that such huge amounts of radiation could be produced in the volume of quasar emission, which was known to be small because of the speed over which it varied.
Gravitational redshift thus seemed to be a viable alternative. But over the years, the distances of quasars have been verified to be at their Hubble-law (“cosmological”) distances, as direct methods of finding distances have reached farther out into the universe. The linear progression of distances with redshift has been verified for galaxies, and quasars are sometimes found in association with distant galaxies.
The “fuzz” around quasars was eventually resolved conclusively, most recently with the Hubble Space Telescope. On the gravitational redshift model, the amount of redshift should diminish with distance from the quasar; but observationally, it does not. So the gravitational redshift model for quasar redshifts is no longer viable.
Indeed, with the theory of energy production from matter raining down into black holes from their accretion disks, the problem of producing so much energy in a small volume is no longer thought to be a limit.
Hubble’s law is now very well established, most recently from the Key Project on the Cosmic Distance Scale carried out by Wendy Freedman of the Carnegie Observatories and her colleagues over the last decade. (The final results were released in May 2001.) The scientists determined the distances to galaxies using Cepheid variable stars, which Hubble can see farther out than ground-based telescopes can. Secondary distance methods then carried the measurements farther out.
Not only has gravitational redshift been ruled out for producing redshifts of galaxies and quasars but the inverse Compton effect has also been ruled out. Joseph Silk and I, both then graduate students at Harvard, found that such a method of producing redshifts would produce blurring that has not been found.
Q: Why is Earth’s rotation slowing down?
A: In an isolated system, the amount of rotation or revolution, known as angular momentum, doesn’t change. The angular momentum of a rotating object depends on its speed of rotation, its mass, and the distance of the mass from the axis. Thus when an ice skater draws his or her arms in, thereby bringing the distribution of mass closer to the center, the skater spins faster. The amount of angular momentum is conserved.
In the Earth-Moon system, the tidal bulge goes around faster than the Moon’s gravity would make it revolve, because the Earth is rotating. That tidal bulge pulls on the Moon, speeding up its revolution around Earth and making it go farther out. This is due to Newton’s third law of motion, which states that an object experiences a force because it is interacting with some other object. Thus, even the weaker object exerts some force over the stronger one.
With the Moon farther out, its angular momentum is increased. Because the Moon’s angular momentum is conserved, Earth’s angular momentum has to decrease to compensate. Thus Earth’s rotation slows. To slow the rotation, the water at the points of high tide are pulled back by the Moon’s gravity and run into the continents that are rotating forward.
The speed of Earth’s rotation is best measured by studying historical records of eclipses. A total solar eclipse is so dramatic that even written descriptions from thousands of years ago can be interpreted to show whether a total eclipse occurred at the writer’s location. A slowing Earth shifts the positions affected by a total solar eclipse on Earth’s surface. As a hypothetical example, calculations based on Earth’s current rate of rotation might predict that people in ancient Rome would have experienced a total eclipse, when in fact historical records show that the Sun was completely eclipsed over Alexandria, Egypt, instead.
The day is getting longer by about two milliseconds per century, although shifting structures on and in the Earth make this rate vary slightly. Hundreds of millions of years ago, Earth was rotating about four times faster than it is now, making the day about six hours long.
Q: Are SETI (Search for Extraterrestrial Intelligence) projects likely to yield important discoveries?
A: SETI has already yielded an important discovery: There aren’t strong, easy-to-detect signals from other civilizations bombarding the Earth. We wouldn’t know that if we hadn’t searched.
Project Ozma began the search for extraterrestrial civilizations in 1960, using a radio telescope at the National Radio Astronomy Observatory. Now a significant number of astronomers are looking hard for signals from such civilizations, mainly using radio telescopes. The SETI Institute in California is a major leader in the search, especially since the U.S. Congress shut off public funding for such investigations. The SETI Institute has raised private money to continue their efforts.
The SETI Institute collects radio signals from afar and studies them with computers to search for intelligent signals. Another group of scientists, at the University of California at Berkeley, invented the idea of SETI@home, where data are sent out to home computers of people all over the world. These home computers have lots of time when they aren’t computing, either when people aren’t using them at all or even between keystrokes of word processing. Seti@home puts that downtime to use, analyzing the radio signals for signs of intelligence. These two projects identify many signals of potentially intelligent origin that must subsequently be analyzed.
To me, the chance of finding something through these radio searches seems small. However, since the discovery of a distant civilization would be so important, it is still worth pursuing.
Q: Why haven’t astronomers detected planets similar to Earth’s size circling other stars?
A: One of the more exciting things going on in astronomy is the discovery of planets circling other stars. Astronomers detect the planets by observing the effect of their gravitational pull on the stars they circle. Imagine a pair of dancers swirling around each other in a waltz. Even if one dancer were invisible, you would notice the other dancer spinning around some point in front of herself in a way that would be impossible if she were not balanced by the weight of her invisible partner. Similarly, if we look at a star and see it moving alternately toward us and away from us, we know that some other object must be moving in the opposite directions to compensate.
Over the last few years, several groups of astronomers have discovered more than 50 new planets outside the solar system. But the planets have to be massive for them to pull on the stars we are looking at enough for us to be able to measure the effect. At first only planets more massive than Jupiter, which is 318 times the mass of Earth, were detectable. Astronomers have refined their methods and have discovered some planets even less massive than Saturn, which is 95 times the mass of Earth. As methods are further refined, we can expect to detect even smaller planets.
One future space mission that will help in the search is NASA’s Space Interferometry Mission, but you will have to wait until at least 2006 for that. Even farther in the future is the Terrestrial Planet Finder, but when that goes up in two or three or four decades, it should be able to detect the effect even of minuscule planets like Earth.
Q: Will the Sun die someday?
A: Stars evolve. They start as concentrations of gas that collapse under the force of their own gravity. As they collapse, they gain energy from the collapse and heat up. When the temperature and density become sufficiently high, nuclear reactions begin and the star itself is born. The nuclear reactions generate energy that creates an outward pressure, and for most of a star’s lifetime the outward pressure balances the inward force of gravity. This state of equilibrium is known as the main sequence. The more massive a star, the more rapidly it uses up its nuclear fuel and the shorter its main sequence lifetime. For the Sun, and for other stars of the same mass, this main sequence lifetime is about 10 billion years. The Sun is about halfway through this lifetime.
In about 5 billion years, the Sun will have used up so much of the hydrogen in its core, transmuting it into helium, that its insides will heat up and its outsides will swell. The Sun will become a red giant star, so large that its outer layers will cover the orbits of the inner planets. Eventually, the outer layers will drift off, making a type of gas cloud, called planetary nebulae, around the star. These outer shells will dissipate in 50,000 years or so.
The inside of the Sun will then collapse, until the electrons in it cannot be pressed closer together. The Sun will then be a white dwarf. A white dwarf is a dead star. It will cool gradually for tens of billions of years.
As the Sun ends its lifetime and its outer layers drift off, it will lose some of its mass. The planets will then orbit farther out than they do now. When the Sun becomes a red giant, the planets will be seared, any water will boil off, and all life that remains may well be extinguished (though who knows what we humans can dream up in 5 billion years). The dead planets may well keep orbiting the dead Sun. Indeed, we know of two or possibly three planets orbiting a single neutron star, a star even more compressed than a white dwarf. Since most of the Sun’s mass will remain, the planets can stay in orbit.
Q: Is it true the Sun’s magnetic cycles may be linked to the ice age cycle the Earth seems trapped in?
A: Nobody knows for sure how the Sun’s magnetic cycles are linked to the Earth’s weather or climate. People have found various supposed links over the years, but most of them have proven wrong once more time has passed or more data has come in.
The most suspicious long-term relation was figured out by John Eddy a few decades ago, reviving something noticed by E. Walter Maunder a hundred years ago. Eddy and Maunder noticed there was a period of decades in the 17th century when there did not seem to be any sunspots on the Sun, and that this period coincided with the “Little Ice Age,” when Europe was cooler than average.
Paintings from that period by Pieter Bruegel and Hendrick Avercamp show people ice skating on Dutch canals, which seldom freeze now.
The dominant cause of Earth’s major ice ages is thought to be related to the Milankovitch cycle, which is not related to the Sun’s magnetic cycles. The Milankovitch cycle is the 41,000-year period of the changing tilt of the Earth’s axis with respect to the plane of its orbit around the Sun, and with the 22,000-year precession cycle of the Earth’s axis; as well as variations in how far out of round the Earth’s orbit is. Predictions based on these cycles forecast at least 100,000 years without an ice age in our future.
Q: Why does the Sun appear yellow?
A: When you heat an iron poker in a fire, it first glows red and then may glow blue-white as it gets hotter. Similarly, the coolest stars are red in color, and intermediate-temperature stars give off more of their energy in the yellow part of the spectrum. The hottest stars give off most of their radiation in the blue or even in the ultraviolet parts of the spectrum. The Sun is a star of intermediate temperature so most of its radiation is given off in or near the yellow.
The kind of radiation any gas gives off depends on its temperature. In the 19th century physicist Wilhelm Wien figured out that the temperature of the gas multiplied by the wavelength at which the continuous radiation from a gas is at its highest yields a constant. So the hotter the gas, the higher the temperature, and the shorter the wavelength. Red light has a long wavelength, yellow has an intermediate wavelength, and violet has the shortest wavelength of all visible light.
Even though the Sun’s radiation peaks in the yellow, all the light from the Sun together is called white light. Our eyes compensate a lot for the mix of colors we see. Some of the pictures of the Sun you see make the Sun look yellow because of the filter photographers use to cut off most of the sunlight. They use filters because the Sun is about a million times brighter than the full moon—too bright to look at directly without a special filter.
Q: Why are the other solar systems that astronomers are detecting arranged so differently from our own?
A: This is such a good question that nobody knows the answer. Until recently, astronomers had only studied our solar system, which has small, rocky planets near the Sun and giant, gaseous ones farther away. We thought that the solar wind and the higher temperatures close to the Sun stripped the inner planets of their mantles of hydrogen and helium, while the outer planets were so cold and had so much gravity that they retained these light elements. But in the last few years, astronomers have found dozens of “hot Jupiters,” giant planets around other sunlike stars but with orbits even smaller than that of the planet Mercury.
Current theories hold that these giant planets were indeed formed at distances comparable to that of our own Jupiter but have migrated toward the Sun over long periods of time. Indeed, there had already been theories like that about our own solar system, theories that suggest Jupiter and Saturn had migrated out a bit while Uranus and Neptune had migrated out much farther from the distances at which they were formed. The reason for the migration might be gravitational interaction.
Your question indicates the problems that can arise when we make generalizations based on only one example, as we had been doing with our solar system. Let us hope we aren’t making the same mistake in generalizing about cosmology from observations of the one universe we know.
Q: How did the invention of the reflecting telescope advance astronomical observation?
A: Isaac Newton invented the reflecting telescope because of a mistake he made! He wrongly assumed that the original type of telescope used by Galileo during the early 17th century, the refracting telescope, could only focus light of different colors at different places, blurring the images. So instead of the refracting telescope, which uses a main lens to bring light to a focus, he invented a reflecting telescope, which uses a mirror. Mirrors reflect light of all colors at the same angle. Scientists later discovered ways of using lenses made of different types of glass to bring several different colors to the same focus even in a refracting telescope.
Newton’s new idea was important in allowing scientists to make bigger telescopes than ever before. In 1845 in Ireland the earl of Rosse made a telescope that had a 79-in (200-cm) mirror. His telescope was so sensitive that he discovered, for example, that many nebulae in the sky had spiral shapes. We now know that these are galaxies, giant collections of billions of stars along with gas and dust that are equivalent to our own Milky Way Galaxy. This telescope was the biggest in the world until 1917, when a huge reflecting telescope with a 100-in (250-cm) mirror was opened in an observatory on Mt. Wilson in California. A telescope twice as big across was opened in an observatory on Palomar Mountain in California in 1950. Huge new reflecting telescopes include the twin Keck telescopes on Mauna Kea in Hawaii. Each has a 400-in (1,000-cm) mirror, composed of several smaller mirrors. The largest refracting telescope is the 40-in (100-cm) refractor of the Yerkes Observatory in Wisconsin. So the largest reflectors are ten times bigger across and 100 times bigger in area than the largest refractor. The reflecting telescopes are therefore much more sensitive.
Q: Why are large telescopes often built on mountaintops?
A: At first, telescopes were set up wherever the astronomers were, starting with Galileo in Venice. In the 19th century Charles Piazzi Smyth, the Astronomer Royal for Scotland, made a trip to the Canary Islands and wrote a report on how good the observation conditions were. In the late 19th century, observatories such as the Lick Observatory on Mount Hamilton were set up in California. From these observatories in the high altitudes of mountains, astronomers found clearer, steadier air than at lower levels, as well as more nights of good weather for observing. Steadier air means that stars twinkle less, making them better to observe.
In the 20th century, with the increased outdoor lighting in commercial and residential areas, it became important for astronomers to choose observation sites far from civilization. That’s another reason why astronomers went to mountaintops.
From 1845 to 1917, the largest telescope in the world belonged to the Earl of Rosse, at Birr Castle in central Ireland. This telescope, the Leviathan of Parsontown, had a mirror that was 6 ft (1.83 m) across; it is now on display at the Science Museum in London. But because of the telescope’s location, few nights a year were suitable for observing. When George Ellery Hale opened his observatory north of Los Angeles on Mount Wilson, he found stable air by first conducting solar observation with a 60-in (152.4-cm) reflector in 1904 and then with a 100-in (254-cm) reflector in 1917. Even now, with the sky too bright for observing dark skies because of the growth of Los Angeles, Mount Wilson is still a useful observatory for studying bright objects because of the steady air. The 100-in (254-cm) telescope on Mount Wilson was superseded in 1950 by the 200-in (508-cm) telescope on Palomar Mountain in southern California, also credited to George Ellery Hale.
Starting in the 1970s, a wonderful mountaintop site was developed at Mauna Kea in Hawaii. Its 4,000-m (14,000-ft) height meant there was little water vapor above, making the site good for infrared observations. The twin 400-in (10-m) Keck telescopes, put up in the mid-1990s, are the largest telescopes on Mauna Kea; other telescopes on Mauna Kea include the 320-in (8-m) telescopes of the Japanese National Astronomical Observatory and the Gemini project.
Chile and the Canary Islands also host major contemporary observatory sites. The European Southern Observatory’s Very Large Telescope in Chile consists of four 26-ft (8-m) telescopes and several smaller telescopes. Texas has a large telescope that can point at limited parts of the sky, and a twin model is being constructed in South Africa.
These telescopes are for optical and infrared observations. Conditions for observing different parts of the spectrum vary. Most radio telescopes do not need high-altitude sites and can be built in areas where there is no radio interference.
Telescopes to study X rays in space must be placed on rockets or satellites. X rays do not come through the Earth’s atmosphere, so scientists who want to study X rays given off by objects in space must place telescopes on rockets or satellites. The Chandra X-ray Observatory is the largest X-ray telescope in space.
Q: How does a total eclipse affect the solar winds and radiation hitting the Earth and could this affect the magnetic field of the Earth in any ways that could be adverse?
A: A total eclipse of the Sun is a relatively minor event at the solar-system scale and differs only slightly from a normal monthly new moon. The solar wind goes by the Moon fairly easily and can continue on to the Earth even when a solar eclipse is occurring.
The change in radiation hitting the Earth has more of an effect, at least locally. The region in totality on Earth cools by about 11 Celsius degrees (20 Fahrenheit degrees)—as measured at the June 21, 2001, eclipse in Africa—and this cooling (over a period of 90 minutes or so during the partial phases, with the most extreme cooling occurring abruptly at the onset of totality) can cause a local change in weather, especially in cloud formation.
Fortunately, the eclipse cooling did not affect the completely clear day that blessed most of southern Africa for the June 21 total eclipse.
A total eclipse, like a new moon, has no effect on the Earth’s magnetic field.
Q: If the universe were to contract, would time reverse?
A: We are all familiar with the forward-pointing arrow of time: We can stir milk into coffee but can’t unstir the light-colored coffee to separate it into black coffee and milk. If you watch a movie of a natural process, such as running water, backward you can always eventually tell that it is moving backward rather than forward. What makes time seem always to go forward?
Scientists don’t know what time really is. They think that the second law of thermodynamics, which specifies that the universe tends to become more and more disordered, is fundamental. But that law cannot be proved to always be valid.
Until recently, cosmologists argued over whether the universe will always expand or whether it will eventually contract, due to the gravity of the mass in the universe pulling all the matter back together. Current theories, though, indicate that the universe will expand forever. There are even strong indications that the expansion of the universe is accelerating rather than slowing down. If these observations are verified, then we will know that the universe will never contract.
Theoreticians disagree as to whether the arrow of time would be reversed in a contracting universe. At present, it seems as though we will never have the observational opportunity to find out.
Q: What made the universe expand? What force could overcome all that gravity to set things apart so drastically that the universe is still expanding and there’s no chance of it contracting?
A: Once things start moving, they keep moving unless some force acts on them to stop the motion. So once the big bang started things moving outward, the question became: How much gravity is there to pull things back? As it turns out, the universe doesn’t have too much gravity, because there isn’t that much matter in it! Whatever matter exists is so spread out that it hardly counts. You might think that Earth is massive, but by the time its mass is spread over the empty space in the solar system, and all the other planets and stars and galaxies are also spread out, the cosmic density is only about 10-30 g/cu cm, compared with 1 g/cu cm for water.
One way of finding out the density of our universe is to measure how much deuterium there is. Since all the deuterium was formed in the first three minutes after the big bang, while the universe was expanding rapidly, measuring the amount of deuterium tells us how much gravity was pulling back on the expansion at that crucial stage. It turns out that only about 3 percent of the amount of matter necessary to pull the universe back in on itself is present.
Scientists’ latest measurements indicate that another 30 percent or so of the mass in the universe is in the form of “dark matter,” which we detect only by its gravity. That still leaves another 67 percent—the mass needed to pull the universe back on itself.
Discoveries since 1997 have indicated that the remaining 67 percent is in the form of a “dark energy” that makes the universe expand faster, not slower. Scientists are increasingly accepting the idea that the universe’s expansion is accelerating, not slowing down.
I should also introduce the idea of “inflation,” which is a rapid expansion of the universe in its first tiny fraction of a second after the big bang. As the universe inflated to perhaps 1050 times its original size in the first 10-32 second, the amount of “vacuum energy” in each volume of space remained constant. Thus the inflation produced most of the mass and energy in the universe. After about 10-32 second, the inflationary era ended, and the universe resumed its expansion at a more reasonable rate.
We now measure its expansion rate with Hubble’s law, which says that the constant relationship between distance from Earth and the speed of expansion means the universe is expanding uniformly. The Hubble Space Telescope was designed in large part to measure Hubble’s law, and scientists using it gave their final value in the spring of 2001. For each 3.26 million light-years of distance to us, galaxies and clusters of galaxies recede by an additional 72 km/second.
Q: When does something enter orbit? When is something in orbit?
A: Isaac Newton, in his System of the World (published in 1726), drew a series of paths for objects projected forward from a high mountain. He showed that the objects fell to Earth in parabolic arcs, as long as their paths went no more than halfway around the Earth. But once a projectile passed halfway around, it continued forward toward its starting point rather than falling to Earth on its other hemisphere. The object is said to be in orbit.
Something is in orbit around another body when it is held in a fixed path by gravity. Gravity keeps pulling the orbiting object toward the center of the main object. Let us say that the orbiting object is a spacecraft and the main object is the Earth. If the curvature of the Earth is such that the Earth curves away at the rate the spacecraft falls, then the spacecraft always remains at the same distance from the center of the Earth. It is thus in free fall and stays in its orbit. To be in orbit, it must have exactly the proper speed to maintain its distance from Earth. That orbital velocity is about 28,967 km/h (18,000 mph) for an object orbiting within a few hundred miles of Earth’s surface.
Johannes Kepler found three laws governing orbits, which he published in 1609 and 1618. The first law shows the orbit is an ellipse with the central object at one of the ellipse’s foci. The second law describes how the speed of the object varies in its orbit, depending on its distance from the central object. The third law links the period of the orbit (the time it takes to orbit the object once) with its size.
From Kepler’s third law, one can see how an object orbits with a longer period the farther it is from Earth. At about six Earth radii, an object orbits in 24 hours instead of the 90 minutes of an object within a few hundred miles of Earth’s surface. It thus appears to hover overhead since a point on the equator below it is rotating at the same speed that the spacecraft is orbiting. The geosynchronous satellites that relay television signals are in such high orbits.
Q: Do wormholes—shortcuts through time and space—exist?
A: Isaac Newton in 1687 advanced the law of gravity, describing the strength of the force that pulls any two masses together. Albert Einstein in 1916, in his general theory of relativity, explained gravity as a warping of space. An analogy would be to put a heavy weight in the middle of your bed, causing the mattress to sag. If you then rolled a ball across the bed, its path would curve when it entered the sagging part of the mattress. You might think that the weight was pulling the ball to it by gravity. This example shows a two-dimensional surface curving into an extra dimension. Similarly, scientists describe the universe as a four-dimensional space-time, and Einstein’s theory shows how a massive object causes it to curve.
If the weight were extremely heavy, the dent in the mattress would be very deep. A black hole is a curvature of space so extreme that nothing, not even light, can escape. Some scientists have proposed that the opposite also exists: a white hole, out of which matter comes. The question then is whether the black hole and the white hole can be connected, so that matter that goes into the black hole comes out of the white hole. We call this link a wormhole. Once you enter the black hole you are pulled inward and are eventually crushed. Wormholes can, in principle, connect two regions of space without the extreme warp of a black hole.
In the 1980s Carl Sagan, in writing his novel Contact (later made into a major motion picture of the same name), consulted the Caltech theoretician Kip Thorne to see if a wormhole could transport his heroine from one place in the universe to another. Physicists had previously calculated that the throat connecting the two ends of a wormhole would pinch off too quickly for anything to pass through. But following Sagan’s question, Thorne and a student figured out that a theoretical kind of matter, which they call “exotic matter,” might be able to keep the throat open. This type of matter cannot exist in our current understanding of the constituents of the universe, but new laws of physics may be discovered that permit it. When matter is very compressed, for example, we would need a quantum theory to explain gravity, and no such theory yet exists.
So no one knows yet whether wormholes can exist. They can’t in our current understanding of the universe, but we still have a lot to learn. In 2000 Thorne stated, “There is growing evidence, but nothing at all firm, that the laws of quantum field theory in curved space-time may prevent the existence of the kind of stress-energy required to hold open a macroscopic wormhole,” in which case, they can’t exist.
03 Mei 2009
Questions and Answers About Astronomy
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