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chapter i events on the scale of atomic nuclei that keep stars shining. For stars, these predictions can be tested, because we see stars of various ages, and we can trace their life cycles from birth through maturity to a death that can be quiet or violent.

The finite speed of light is woven into the language of astronomy—we use the term "light-year" to mean the distance that light travels in a year.'' The time it takes for light to reach us from a star 100 light-years away is fust a century. You can walk out tonight and see stars whose light was emitted before your parents were born. Light from the most distant supernova so far observed carries information about the way the universe has been expanding over the past 10 billion years, two-thirds of the way back to the origin of time at the Big Bang. Measuring the light from these very distant stars is not easy—the sky is bright, the stars are dim, and there are many pitfalls for the unwary—but the rewards for assembling a coherent picture of the universe are great.

Tn 1917, when Einstein began to connect his newly minted grav-ity-as-geometry with the universe, astronomers thought the stars of the Milky Way were the entire contents of the universe. Now we know the Milky Way galaxy is not the whole universe but just a small part of it. Stars form in colossal galaxies and the galaxies, each one 100 billion stars like the sun, are the units we can see that trace the underlying properties of the universe.

The sun is located in one of the outer spiral arms of the Milky Way, about 20,000 light-years from the center. All the stars you can see at night are in the Milky Way galaxy, and many of them are in that faint Rattened band of light that city dwellers never see. The generous size of the galaxy means that many momentous events have already taken place, but we just carry on in ignorance because the news hasn't yet reached us. Andrew Jackson, Old Hickory, won the Battle of New Orleans in 1815, 15 days after the peace treaty with the British was signed in Ghent, Belgium. It took time for the message to reach him so he soldiered on until he heard the news. The Hash from a supernova exploding in the Milky Way travels at the speed of light, but there is a similar lag as information travels across a great distance: there are many supernova explosions in the Milky Way for which we haven't yet seen the light. Supernovae

Figure 1.3. in 1917, Einstein was advised that the Milky Way whi the Universe Mistaking a part for the whole is common with large entities. Copyright © 2001 The New fodder Collection from cartoonbank.com. All Rights Reserved

Figure 1.3. in 1917, Einstein was advised that the Milky Way whi the Universe Mistaking a part for the whole is common with large entities. Copyright © 2001 The New fodder Collection from cartoonbank.com. All Rights Reserved erupt every 100 years or so in a galaxy like ours. Since the light from a supernova might take 20,000 years to travel to us, light from hundreds of supernovae in our own galaxy is on the way to us row, the flash from each one a growing shell traveling outward at the speed of light, like a ripple in a still pond from a fish leaping at twilight. Will one of those little waves lap up on our shores tonight? Will we get to see a supernova in our own galaxy, the way Tycho Brahe, the world's last great observer before the invention of the telescope, did in 1572? We don't know. We can't know, since no information travels faster than light to give advance warning. The last really bright supernova was seen in 1987—not in our galaxy, but in our southern neighbor, the Large Magellanic Cloud. Personally, I am ready for another one.

Individual stars are very small compared to the distances between stars, but galaxies are not so tiny compared to their separations. If you imagine a scale model where a star like the sun has the size of a pea, neighboring stars would be 100 miles away. Since

figure 1.4. The spiral galaxy pair NGC 2207 and IC 11 £3. Distances between galaxies are not always large compared to the sizes of galaxies These two are colliding Note the absorpuon of light from one galaxy by dust lanes in the other. Courtesy of NASA and the Hubble Heritage Team (STSd/AURA). {Also see color insert)

figure 1.4. The spiral galaxy pair NGC 2207 and IC 11 £3. Distances between galaxies are not always large compared to the sizes of galaxies These two are colliding Note the absorpuon of light from one galaxy by dust lanes in the other. Courtesy of NASA and the Hubble Heritage Team (STSd/AURA). {Also see color insert)

stars are so small compared to the distances between them, they rarely collide and our galaxy seems a spacious place with a dark sky. But. the distances between galaxies, although a million times bigger than the distances between stars, are not so big when compared to the galaxies themselves. If you imagine our galaxy as a dinner plate, then our nearest big neighbor galaxy, the Andromeda galaxy (also known as M31, from its place in the Messier catalog of fuzzy objects), would be just ten feet away, at the other end of the Thanksgiving tablecloth down by Uncle Bill As galaxies move under their mutual gravitational pull, it is not rare for them to collide and possibly merge. But galaxies undergo a strange sort of collision, quite different from two plates smashing together near tile gravy boat, because the individual stars that make up each galaxy are still quite unlikely to hit one another. In about 5 billion years, the Milky-Way where we live and M31, now a little over 2 million light-years away but heading our way, will collide. Hie individual stars will miss one another, iikt^ intersecting swarm.s of ht^s.

Galaxies are distributed throughout the observable universe, with typical separations of a few million light-years. They are quite gregarious, forming loose groups and dense clusters where the galaxies crowd together, leaving large voids a few hundred million light-years across where galaxies are rare. The Milky Way is in a

figure f.5. The neartoy spiral M3I. M3f is part of the Local Group of galaxies. In the 1920s, Hubble observed individual cepheid variable stars in this spiral galaxy that showed it was Coo distant to be part of the Milky Wfry and must: be a distant system as about as b|g as The Milky Way. Courtesy of P. Challis. Hansard-Smithsonian Center far Astrophysics from the Digital Sky Survey small group we call the Local Group that includes the Large and Small Magellanic Clouds, M31, and M33 (another nearby spiral galaxy), among others. The nearest moderate-sized cluster of galaxies is in the direction of the constellation Virgo and dubbed the Virgo Cluster Judging distance from the apparent brightness of stars in those galaxies as seen with the Hubble Space Telescope, Virgo Cluster galaxies are located about 50 million light-years away. With a small telescope at a site with a dark sky, it's no problem at all to see these and still more distant, galaxies whose light was emitted when dinosaurs still roamed the Earth.

The limit of present-day observation is the image of the "Hubble Deep Field," produced by adding up 342 images taken over 10 days at the end of 1995 with the I lubble Space Telescope. These hours of staring at a very small blank spot in the northern sky have produced

Figure i.6. The Hubble Deep Field- Composed from 342 images taken over 10 days at the end of 1995, the Hubble Deep Field represents the limit of present methods for observing faint, distant, and young objects. Almost every dot. and smudge in this picture is a galaxy, with Sjbc from the most distant ones traveling 12 billion light years to reach us. Courtesy of R. WMIiams/NASA/STScl/AURA. (Also see color insert)

Figure i.6. The Hubble Deep Field- Composed from 342 images taken over 10 days at the end of 1995, the Hubble Deep Field represents the limit of present methods for observing faint, distant, and young objects. Almost every dot. and smudge in this picture is a galaxy, with Sjbc from the most distant ones traveling 12 billion light years to reach us. Courtesy of R. WMIiams/NASA/STScl/AURA. (Also see color insert)

our deepest image of the past. HST is in orbit above the Earth's atmosphere, so it can make images that are not blurred by the ever-chang-ing air. But it is a relatively small telescope, only 1/16 the area of the biggest ground-based instruments, so the Space Telescope takes a king time to gather light from faint and distant galaxies. Almost everything in the Hubble Deep Field image is a galaxy. Galaxies in the foreground overlap with galaxies in the background until the Hubble Deep Field begins to show wall-to-wall galaxies. The Hubble Deep Field is the ultimate in imaging with today's technology, taking us back to the deepest accessible strata of cosmic history, within about 2 billion years of the Big Bang.

I still can call up the sharp pang of disappointment T felt at age 12 when 1 was working my way through the big fat volume of The Complete Sherlock Holmes. When Holmes walked down the path at the Reiehenbach Falls for his deadly encounter with Mori-arty, I felt a boyish sadness at the demise of the best and wisest man Dr. Watson (and I) had ever known. Rut worse was the feeling, "Is that all there is?"

And in a funny way, the Hubble Deep Field evokes a little of the same feeling. Is that it? Is that as far as we can sec? Since we have plausible reasons to think the universe is about 14 billion years old, then the most distant thing we could possibly see emitted its light 14 billion years ago. In other words, the finite time since the Rig Rang and the finite speed of light place a natural limit to our direct, knowledge of the universe—the patch we could possibly observe is only 14 billion light-years in radius. Photons from some objects in the Hubble Deep Field were emitted about 12 billion years ago. So, is that it? Have we reached the edge of knowledge (or at least 12/14 of the edge of knowledge)?

In the same way, it is a little deflating to live in such a small and cramped universe. If the typical distance between galaxies is a few million light-years, then if each galaxy were the size of a dinner plate on a holiday table, we would reside in an observable universe only 20 miles in each direction. The observable universe seems more like crowded, jostling Hong Kong than the big sky country of Montana.

Yet The Complete Sherlock Holmes had another three-inch thickness of pages I had not read. This should have been a hint that Conan Doyle would relent and that there was much more Sherlock ro enjoy. In rhe same way, a moment's thought shows rhere is much we have not yet read in the cosmic text. The Hubble Deep Field image was observed in colors of light that span just a slightly broader range than our eyes can see. But as we look deeper to see more distant galaxies and supernovae, still earlier in cosmic time, the light emitted from the first generation of objects in the universe would have been stretched by cosmic expansion right out of the Space Telescope's view and out into infrared wavelengths.

It's as if we have come in late to a movie. I hate that feeling. We've missed the coded messages of the opening titles and all the important early action—in the universe that's the origin of the expansion, the freezing out of helium, then the formation of the very first objects, the explosions of the very first stars, and tile beginning of chemical change that, makes the rich and varied world we live in, including the carbon, oxygen, calcium, and iron of our bodies. Much of this action took place even farther in (he past than we can hope to see with instruments that operate at the visible wavelengths where our eyes work, Earth's atmosphere is transparent, or where HST has done most of its work.

IiST is not looking in the right way to see the very first light from objects in the early universe. If we want to see the opening sequence, we will need to build an equivalent of the 11ST that works at longer wavelengths, in the infrared: the next-generation space telescope. And we are.

If we want to see the glow of the Big Bang itself, we need to look at even longer wavelengths of light, out where radios work but none of our senses do. And, since 1965, we have been doing that, too. But most of the universe is invisible, even with all our technical means. We know it is there because we see its effects, but we cannot measure it directly. The universe we see is controlled by the universe we do not see: dark matter that, is not like the neutrons and protons that make up our bodies, and an enigmatic dark energy that shows itself in the runaway expansion of the universe.

We can build a coherent picture of the universe through astronomical observation and physical theory. Both are hard work, with many false steps, long periods of drudgery, and brief flashes of excitement. Science is not a vast encyclopedia, it is a thin flame of reason burning across ample reservoirs of ignorance. Discovering how the world works is an adventure. We may be brief and we may be short, but we are kicky enough to be here at a moment when technical advances bring new light to old human questions about the past, and future of the universe. Supernovae form our method of inquiry, the dark energy is our quarry. The game's afoot!

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