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CHAPTER 1
Reaching for the Stars

Birth and Death

Q. Are stars all burning out, or are new ones forming?

A. Stars are being born as well as dying, but the rate varies greatly from galaxy to galaxy.

Stars form from huge clouds of dust and gas. If a cloud begins to contract because of its own gravity, its interior heats up as gravitational energy is converted to heat energy, reaching millions of degrees, and nuclear reactions begin that change one element into another, releasing energy.

The pressure tends to expand the cloud back out, but eventually equilibrium is reached. That is essentially what a star is—a mass of gas at equilibrium between inward pressure from gravity and outward pressure from nuclear reactions.

A star has a finite lifetime because it is burning fuel. For 90 percent of its life, it burns hydrogen into helium. When the hydrogen is used up, the pressure decreases, but gravity never disappears, so the star contracts until the temperature climbs again, this time reaching hundreds of millions of degrees, while reactions convert helium to carbon and oxygen. The star can then remain stable for a briefer time. Eventually the star dies, when the reactions no longer produce energy, but only consume it.

In a Spin

Q. Is the universe rotating?

A. Most astronomers would say no. There is no known mechanism that would give the universe so much angular momentum, or spin, at its beginning, and few mechanisms for adding spin later.

To know for sure if the universe rotates, scientists would need to know the velocities of millions of galaxies, over all regions of the sky and out to very great distances. Analyses of these velocities would be necessary to see if they indicated a common center about which galaxies were rotating, and a sense of direction on average.

Since it may be a while before the velocities of millions of galaxies are known, astronomers are trying to answer a simpler question: Are there regions of space toward which large numbers of galaxies are moving as a group?

Death by Black Hole

Q. What would kill you if you fell into a black hole?

A. You might not die right away, but you would eventually be pulled apart by the force of gravity. As you fell in and even afterward, you might not lose consciousness, but the pull of gravity on your feet would be stronger than on your head, and you would be stretched, then torn apart. The difference in force is called the tidal force and is like that in the ocean, except in more extreme form. The force would be less if it was a big enough black hole; in a small black hole it might kill you before you disappeared beneath the event horizon, the edge of the hole.

But even in a larger black hole, the tidal force always gets you in the end. Once you fall in, you can't avoid falling toward the center, and the force would kill you before you reached the center. How long it would take depends on how big the black hole is. If it was big enough so that the tidal force didn't kill you before you fell in, you might have an hour or several hours before being torn apart. In a small one, such as one that forms when a star collapses, you wouldn't have much time, perhaps a thousandth of a second. That would happen with a run-of-the-mill black hole, like those that might be found in the Milky Way Galaxy.

Averting Your Eyes

Q. When I look at a constellation, I can see the fainter stars better out of the corner of my eye. Why?

A. Because the eye has two kinds of receptors, cones for fine resolution and color and rods for dim light, and the rods tend to be located around the periphery, for viewing the edges of the field of vision.

Cones are extremely good at high definition and for precise positioning of pinpoints of light. Rods don't give nearly as fine resolution and don't distinguish colors, but are much more sensitive. In effect, rods serve as night-vision sensors.

The result is that when you go out at night, you can clearly see the bright stars and planets at the center of your eye, but you can see the fainter ones out of the corner of your eye. Astronomers call this averted vision. For example, if they look with the unaided eye straight at a galaxy they know is there, they may not see it, but if they look off to one side, they can easily see the fuzzy gray patch that is the galaxy.

Cameras, too, tend to give up resolution and sensitivity to color when they achieve better sensitivity to low light levels.

Seeing Stars

Q. Why can't you see the stars in photos or videos taken by astronauts?

A. Such pictures do not ordinarily show stars because the stars are not bright enough in comparison to the nearby Sun and the things it shines on.

Virtually all of the astronaut photos are of objects brightly illuminated by the Sun. To capture them on film without overexposing the image, you need a relatively short exposure, which does not provide enough time for the film to capture images of stars. If there is no other strong light source in the picture, however, a photo can show stars.

A South Star?

Q. What is used as a pole star in the Southern Hemisphere, where navigators can't see the North Star?

A. The closest thing to a south star for navigators south of the equator is a pair of stars in the Southern Cross, Crux Australis (or just Crux to astronomers). Alpha Crucis (its brightest star) and Gamma Crucis (the third brightest) point almost straight to the south celestial pole.

The striking Southern Cross, which has four stars brighter than the second magnitude in a cross or kite shape, takes up only 68 square degrees of the sky. The area defined by the Southern Cross has other interesting features, notably the Coalsack Nebula, which looks like a round blank spot in the sky but is actually a cloud of gas-laden dust that blocks the background starlight, and the Jewel Box cluster, designated NGC 4755, an open cluster of more than 100 stars surrounding Kappa Crucis, which is bright red in a modest telescope.

Hello Out There!

Q. In the search for extraterrestrial intelligence, or SETI, what kinds of signals would be considered evidence of transmission by intelligent life? How would scientists respond?

A. Such a transmission might take many forms, but would probably encode mathematical formulas. The reply would depend on the content; it would not be made by scientists, but would come after extensive international consultation.

As for verification, the main feature distinguishing signals produced by a transmitter from those produced by natural processes is their spectral width—that is, how much room on the radio dial they take up. As far as scientists know, any signal less than about 300 hertz wide is artificially produced.

Other telltale characteristics might be coded information, like the message beamed from the Arecibo telescope in 1974, which included data like the senders' location (third rock from the Sun, in the case of Earth). Another important test would be a confirming observation of the same signal at another radio telescope.

Once confirmed, the discovery would be announced based on a plan set up by six international space agencies. First, the scientific community would be notified through the International Astronomical Union and the United Nations. Then international authorities would draft a reply.

Sitcoms in Space

Q. How far out in space could Earth's television and radio signals be detected?

A. The speed of light can seem fairly slow when you're talking about communicating across galaxies. FM broadcasts and the earliest television programs from, say, half a century of broadcasting have reached a distance of 50 light-years, or about 294 trillion miles, from Earth.

The nearest star is about 4 light-years away, and there are on the order of several thousand stars within the 50-light-year range. So the earliest episodes of I Love Lucy are washing over a new star system at the rate of about one system a day.

Any civilization on the receiving end would need a very large antenna to pick up the broadcasts, about the size of Manhattan, but scientists suggest it could be done. But the chances that one star in a few thousand had some sort of civilization might be a big overestimate.

Shine On, Harvest Moon

Q. Is the harvest moon brighter than other full moons?

A. Ordinarily, the mid-September harvest moon is no brighter than any other full moon, but it does provide more hours of moonlight.

In autumn, the orbital path of the Moon and its visibility in the sky combine so that it stays above the horizon for an unusually long time at the full moon and a day before and after.

The brightness of the Moon is determined by how much sunlight falls on it, which is pretty much constant; how much appears to be lit up from where we see it, from crescent to full; and how high it is in the sky. The higher the Moon is, the more light we see, because it has a shorter path through the dust and smog of the atmosphere.

In 1997, the harvest moon of September 15 was the brightest of the year, by a small margin, because it came within a few hours of perigee, the Moon's closest approach to Earth. The difference in brightness between the Moon's closest approach and farthest point amounts to 12 to 13 percent. It was also the largest full moon in angular size; its apparent diameter, normally about 30 arc-minutes, was 32 or 33 arc-minutes.

The varying Earth-Moon distance is why solar eclipses are sometimes total, when the Moon completely blocks the Sun, and sometimes annular, with a ring of light visible around the Moon.

On the Thin Side

Q. Did the Moon ever have an atmosphere?

A. It has one now, though it is a very thin and highly dispersed collection of molecules, not suitable for breathing by Earthlings.

The existence of a lunar atmosphere was reported in 1933, based on observation of the Moon using a mask that filtered out moonlight in order to study the spectrum of light emitted by sodium. Although sodium is believed to be just a trace in the Moon's atmosphere, it is studied because it is relatively easy to detect and is used as a marker for other components, such as potassium, neon, argon, and helium.

A 1993 study of lunar sodium by Boston University scientists using improved instruments determined that the atmosphere extended at least 5,000 miles above the Moon's surface. The molecules, however, are few and far between, only an estimated 10 million per cubic centimeter near the Moon's surface; Earth's atmosphere is about a billion times as dense.

The sources of the atmosphere are believed to be the release of gases from within the Moon by moonquakes (a phenomenon called outgassing) and the loosening of molecules from the surface by the impact of molecules from the solar wind or by meteorites.

A few moons of other planets have much more impressive atmospheres, like that of Jupiter's Titan, a thick haze of nitrogen and methane, and that of Europa, a thin wisp of oxygen.

Mars Coordinates

Q. How is zero degrees longitude, the equivalent of the Greenwich meridian, determined for Mars?

A. A small, well-defined crater named Airy-0, near the planet's equator, was designated as the starting point for the 360 degrees of Martian longitude.

The satellite Mariner 9 began photographing Mars on November 13, 1971, sending back thousands of detailed pictures of the planet's surface on which to base a map. Using the information captured by this mapping project, the scientists at the U.S. Geological Survey's Center of Astrogeology at Flagstaff, Arizona, were able to refine their calculations so that the line for the prime meridian goes through the exact center of the crater, which is about three-tenths of a mile in diameter.

The zero-degree crater was named Airy-0 in honor of George Biddell Airy, a British astronomer who lived from 1801 to 1892. He became Astronomer Royal in 1835, and among his many accomplishments was the building of the Transit Circle telescope in the Greenwich Observatory's Meridian Building in 1850. The crosshairs in the eyepiece of the telescope define zero degrees longitude for Earth.

Rocks from the Red Planet

Q. How do scientists determine that a rock on Earth came from Mars rather than someplace else?

A. The best idea scientists have of the geochemistry of Mars comes from the two Viking robots that landed on Mars in 1976. Findings from the robots' weeks of readings of things like the Martian atmosphere are compared with the chemical signatures of meteorites found on Earth.

The first such object confidently identified as Martian was reported in 1983. The object, a rock eight inches

in diameter that weighed 17.5 pounds, had been picked up in 1979 at the Elephant Moraine near McMurdo Sound in Antarctica.

Trapped in bits of glass in the object, designated EETA 79001, were some so-called noble gases—neon, argon, krypton, and xenon—strikingly similar in abundance to those of the Martian atmosphere, as determined by the Viking missions.

There are now about two dozen meteorites thought to be from Mars.

Pale in Comparison

Q. Why is Venus so much brighter in the sky than Mercury, which is closer to the Sun?

A. Venus is brighter because it is much larger, because it has a reflective atmosphere, and because it makes close approaches to Earth.

First, Mercury is about 3,100 miles in diameter, compared with about 7,700 miles for Venus.

Second, there is no atmosphere surrounding Mercury, while Venus has a thick atmosphere, composed chiefly of clouds of carbon dioxide. These clouds have a very high reflectivity, causing Venus to appear to shine with great brilliance.

Third, Venus can approach the Earth much more closely than Mercury. In fact, Venus periodically comes closer to Earth than any other known celestial body, except for the Moon, as close as 25 million miles. All of these things allow Venus to shine more brightly than not just Mercury, but than any of the other planets in our solar system.

Because Mercury and Venus are closer to the Sun than Earth is, they can show phases like those of the Moon. When Mercury shines at its brightest, it appears through a telescope as a gibbous or nearly full phase. When Venus appears to shine at its most brilliant, it appears in telescopes not as a nearly full phase, but as a crescent.

On the Track of Venus

Q. I have been told that the Maya made precise measurements of Venus. What did they learn?

A. It would be more accurate to say that the ancient Maya of Central America, who had no telescopes, did not measure Venus itself but kept careful records of its apparent track in the sky.

Because of the relative orbits of Earth and Venus, Venus is first invisible, then appears in the morning sky, then disappears, then reappears in the evening sky, then fades from view again.

An important part of the complex Mayan calendrical system is based on the 584 days it takes for Venus to make that complete cycle. The Venus cycle was meshed with a sacred calendar of 260 days and the 365-day solar calendar to come up with a cycle of about 2,920 days, or 8 solar years, in which the calendars came out approximately even.