Figure 2 3 Planetary nebula NGC 475 f. After about a billion years as a red giant a star like the sun will puff off its outer envelope while the core shrinks to become a white dwarf. A planetary nebula is the beautiful transition from a gaseous star with nuclear fusion to a solid star with no energy source. Courtesy. NASA and the Hubble Heritage Team (STSd/AURA). (Also see color insert)

A single white dwarf emits a little light from its surface, but no longer has a nuclcar furnacc to replace the heat it radiates away. A white dwarf cools and fades away like a memory, slowly slipping below the edge of detection. This is the way the sun will end: it will go not with a bang, but with a whimper. Simple physical principles of heat conduction show how these faint stellar clinkers dim as they age. The coolest, dimmest, most boring white dwarfs provide a cosmic clock to compare with the globular clustcr ages. The dullest white dwarfs took almost 10 billion years to cool—they appear to be just a shade younger than the oldest globular cluster stars.

This is a good result. Globular duster experts can keep their dollars, goldfish, houses, and dogs. When astronomical measures agree, in this ease, the age of the galaxy from white dwarf cooling and the ages of red giants in globular clusters, it makes you think we may be blundering toward the truth. While both arguments are complex and have uncertainties that are hard to evaluate, there are many ways to disagree but only one way to agree. It doesn't prove that both are right, but when independent paths lead to the same conclusion, there's hope that we're not just fooling ourselves.

White dwarfs with binary partners can do more interesting things than go gently into that good night. Sirius and Sirius B are locked into a dancc by their mutual gravity. Closer stellar pairs can interact: white dwarfs in binaries can explode as type la supernovae as the white dwarf nears the Chandrasekhar limit. This won't happen to the sun, because it is an only star, but most stars are born into multiple systems where it's possible for a too-generous sibling to dump gas onto an orbiting white dwarf, precipitating a disaster. The reasons for thinking type la supernovae comc from exploding white dwarfs are powerful, but mostly theoretical. Nolx>dy has yet identified such a system before the explosion, and then seen it erupt. So far, observations don't show any sign of the not-so-innocent bystander being smashed by the explosion. Another possibility that is not ruled out is that type la supernovae come from binary systems where both stars are white dwarfs that radiate their orbital energy in gravitational waves and spiral together to create an explosion.'' Despite our uncertainty about exactly how a white dwarf meets a violent end, we do know that stars explode, and the circumstantial evidence favors white dwarfs as the origin for type la supernovae.

Type la supernovae are found in galaxies of all types, the spiral and irregular galaxies where massive stars are forming today, and the elliptical galaxies where, as in globular clusters, most of the star formation took place 12 billion years ago. SN la are the only type seen in elliptical galaxies today, where the currcnt rate of star formation is very low. This suggests that the path to becoming this type of supernova must be long and slow, as it might well be for a one solar mass white dwarf in a binary. It could easily take several billion years for a star of modest mass to use its fuel, spend some time as a red giant, and settle in as a white dwarf. If the companion is also a low-mass star, there could be a long delay before it begins to gently rain down the extra mass that nudges a white dwarf to thermonuclear destruction.

SN la arc thcrmonuclcar explosions—nuclear bombs with the mass of a star. When the carbon and oxygen in the interior of a white dwarf start to fuse, the reaction releases heat that speeds more fusion, powering an intense nuclcar burning flame that rips through the dense little star. The flame burns much of the star all the way up to iron with such a huge release of energy that for a few weeks a single little star becomes as bright as four billion suns. That's the event we see as a type la supernova.

These explosions are spectacular and distinctive. Even though hydrogen is the most abundant element in the universe, spcctra of type la supernovae don't show any hydrogen. This is a good hint that supernovae come from stars that have undergone significant changcs. Type la supernovae get bright and then dim in a very distinctive way, taking about 20 days to rcach maximum light, then declining by a factor of two in the next two weeks, and then slowly declining by about 1 percent per day for the next year and a half. Computations show that this light curve is powered by the decay of radioactive elements near iron in the periodic tabic that arc produced when an explosion tncincratcs a white dwarf. More precisely, the nuclcar burning in the violent dcstmction of a white dwarf makes radioactive nickel. This decays, with half the remaining nickel decaying every 6.1 days (the "half-life") to cobalt, and cobalt decays with a half-life of 77.1 days to stable iron. Type la light curves provide a nuclear-powered clock.

This is not just an idea. If SN la arc powered by the dccay of nickcl to cobalt to iron, wc should see the abundance of those elements change. Spectrum lines of cobalt should grow weak as those nuclei changc into iron. In 1994, a Harvard undergraduate doing his senior thesis with mc, Marc Kuchner, along with postdocs Phil Pinto and Bruno Leibundgut, used spcctra of SN la to look for these

Figure 2.4. Supernova I994D. This type la superoova (bright spot at lower left) is in a galaxy at a distance of about 50 million fight years in the Virgo duster of galaxies For a month, the light from a single exploding white dwarf is as bright as A billion stars like the sun Courtesy of P ChaJlis. Center for Astraphysics/STScl/NASA, (Also see color insert)

changcs. We measured spectra taken in the weeks after maximum light and we found a decrease in cobalt while the abundance of iron was rising. Just as predicted. Over a time of months, wc could see before our eyes the gradual transformation of one chemical element into another by radioactive dccay/1

Type la supernovae arc responsible for making the iron in the Earth's core, in the Eiffel Tower, and in your own blood. In the explosion of a type la, the star is totally destroyed. Wre expect these supernovae to leave nothing behind but a hot, glowing, iron-rich cloud of shredded star, emitting X-niys Best of all for measuring the universe, the explosions are all more or less similar, possibly because they erupt in stars pressed up against the Chandrasckhar upper mass limit for white dwarfs.

If exploding white dwarfs all emitted exactly the same amount of light, then judging the distance to a SN la from its brightness would be a precise way to measure distances in the universe. In fact, there is a range of energy emitted by SN la explosions, and we have been working hard to understand this variety. Over the past decade, these efforts to improve the precision of SN Ta as cosmic rulers have paid off: supernovae are now the best tools for measuring distances to other galaxies. These are the objects Pete Challis was so desperately seeking in the La Serena data room.

In 1983, I was on the astronomy faculty at the University of Michigan. Very early one October morning, I was awakened by an excited predawn telephone call from the Ann Arbor News.

"I lave you heard about the Nobel Prize?"

It didn't seem possible. What had I done to deserve this? I honestly couldn't think of anything. This was terrible. Maybe I had done something wonderful, but now I had early-onset Alzheimer's and I couldn't remember what it was. Why hadn't they called mc sooner, when I could appreciate it? I sat up in bed, sweating uncontrollably. Luckily, I was too groggy to say anything, and the reporter's voice pulled me out of this inward spiral of self-delusion.

"They gave the Physics Prize to Willy Fowler and, how do you say this name? Chan-dah something something," the reporter went on. "What do you think of that?"

"Oh!. . . Oh, that's great. CHUN-druh-shay-khur, Aspirate the hard k and all the a's are schwas." I stalled for time, slowly regaining brain function. It was OK. I hadn't done anything, but at [cast I knew it.

"It's like moonlight," I said.

"Hunh?" the reporter interrogated deftly.

"We all bask in the reflection! You see these are two of the guys who figured out how to apply nuclear physics and quantum mechanics to stars. The Chandrasekhar limit for white dwarfs, for example,

This is a complex story: White dwarfs, binaries, runaway fusion of carbon and oxygen in a degenerate star, radioactive power from nickel to cobalt to iron decay, and the total destruction of a star. Is there a way to test whether this picture is correct? There's no hope of a laboratory test for the whole complex set of events, but if this Story is right, we should see the essential ingredients by observing supernovae.

Most supernovae wc see arc in very distant galaxies: the information wc can gather is limited by the object's faintness. If you look at thousands of galaxies, you can finds dozens of supernovae each year. Rare things do happen. If our galaxy is like other galaxies, events we see in distant stellar systems have corresponding, if infrequent, events nearby. If you limit your attention to the Milky Way, you must wait for centuries, but an exploding star in our own galaxy, just a few thousand light years away can be an astonishing sight.

In 1572, before the invention of the telescope, the not-yet famous 24-year-old Danish astronomer Tycho Brahe, reported the most recently observed SN la in our galaxy.

On [11, November 1572] a little before dinner . . . and during my walk contemplating the sky here and there in order to continue observations after dinner, behold, direclly overhead a certain strange star was suddenly seen, flashing its light with a radiant gleam and it struck my eyes. Amazed, and as if astonished and stupefied, I stotxl still, gazing for a certain length of time with my eyes fixed intently on it and noticing that same star placed close to the stars which antiquity attributed to Cassiopeia. When I had satisfied myself that no star of that kind had ever shone forth before, I was led into such perplexity by the unbelievability of the thing that I began to doubt the faith of my own eyes, and so, turning to the servants who were accompanying me, I asked them whether they too could see a certain extremely bright star when I pointed out the place directly overhead. They immediately replied with one voice that they saw it completely and that it was extremely bright. But despite their affirmation, still being doubtful on account of the novelty of the thing, I enquired of some country people who by chance were traveling past in carriages whether they could see a certain star in the height Indeed these people shouted out that they saw that huge star, which had never been noticed so high up. And at length, having confirmed that my vision was not deceiving me, but in fact that an unusual star existed there, beyond all type, and marveling that the sky had brought forth a certain new phenomenon to be compared with other stars, immediately I got ready my instrument. I began to measure its situation and distance from the neighboring stars of Cassiopeia, and to note extremely diligently those things which were visible to the eye concerning its apparent size, form, color, and other aspects 7

As day faded into night, and day came again, Pete Challis's list of supernova candidates grew. There were no country people traveling past in carriages to check his work. Carefully screening the images on his monitor, Pete was finding something more valuable than flecks of gold. Pete Challis was picking out supernovae from images of distant galaxies, taking a step toward understanding the history of cosmic expansion. When dawn comes and he hands over his list, other members of the team will jump into action. They will gather the light from Pete's distant discoveries, spread each one into a spectrum, and note extremely diligently things that arc invisible to the unaided eye. The spectrum will reveal each supernova's contribution to the stock of heavy elements in a distant galaxy and form the basis for a scientific prophesy of future cosmic expansion.

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