The Lives of Stars by Andrew Fraknoi
"The Sun is just one among a hundred billion stars in the Milky Way Galaxy, each with its own cosmic tale to tell." — Timothy Ferris, in the film Seeing in the Dark
The stars seem to change little if at all—night after night, century after century—so people used to assume that they lasted forever. The Greek philosopher Aristotle even proposed that the stars were made of a special element, not found on Earth, that never changes. Since Aristotle's time, astronomers have discovered that stars, like people, have finite lifetimes. Stars are born from the great reservoirs of cosmic material (mostly hydrogen and helium, the most common elements in the universe), and they shine with exuberant energy for millions or billions of years. Eventually, however, they exhaust their energy supply and then shudder and die.
Understanding the life stories of the stars has been one of the great tasks of astronomy during the last hundred years, and astronomers have made remarkable progress in tracking the progression of stages through which stars pass. Let's review the broad outlines of the story, and see how some of the astronomical objects discussed in Seeing in the Dark fit into the big picture.
1. The Pre-natal Stage
Stars begin their careers in a kind of incubation. When a dense region happens to form in a huge cloud of gas and dust, the material that will make a star falls inward under the influence of gravity. As the clump of star-stuff gets more and more compressed, it heats up, and starts to glow—first with infrared energy and then with visible light.
2. A Star is Born
As gravity continues to pull the infant star together, the center of the collapsing ball of gas keeps getting hotter, until temperatures in the middle reach an astounding 10 billion degrees Celsius (18 billion degrees Fahrenheit). At that temperature, protons can collide so forcefully that nuclear fusion can occur. Physicists call the reactions "fusion" because protons that originally formed the nucleus of hydrogen atoms fuse together to make the nuclei of heavier atoms. In a star like the Sun, four atoms of hydrogen eventually combine via fusion to make a heavier nucleus of helium.
What makes fusion inside a star so useful is that the transformation of a light element into a slightly heavier one releases energy. The combined energy released by billions and billions of individual fusion reactions flows outward from the star's core. Some of it eventually emerges from the star's surface as light. The pressure of starlight, along with particles of matter blown off the surface of the newborn star, sweep away the surrounding cloud of dust and gas, and the star emerges into view. PhotoView a photo of Rosette Nebula, by Rob Gendler.
Most stars are born in groups, called star clusters. You can see pictures of some of these clusters, like the Pleiades, in the film and in our website gallery. Rich clusters stay together and allow us to track the stories of their stars, while less massive clusters will "dissolve" and eventually mix their stars with the general population of our galaxy.
After a brief period of adolescent turmoil, the star then settles into a long period of stable existence. The outward push from nuclear fusion in its core now nicely balances the inward push of gravity, and the star remains balanced this way for about 90% of its lifetime. This is the stage our Sun is now in—it's been in this stage for almost five billion years and will continue for at least five billion years more. Having a stable star is thought to be crucial to the development and evolution of life on a planet like ours.
How long a star lives depends on how much material (mass) it has. Massive stars have a lot of gravity, which makes their centers extremely hot. Thus they tear through their hot hydrogen at a rapid clip and exhaust their fuel very quickly. The most massive known stars last only a few million years. The core of a lower-mass star like the Sun is less hot and burns its fuel much more slowly so the star lasts billions, rather than millions, of years. The lowest mass "dwarf" stars burn so slowly that they may last a hundred billion years in the adult stage—much longer than the current age of the universe.
Nevertheless, the core of every star eventually runs low on nuclear fuel. At this point, the star experiences a crisis. Gravity is still compressing the star, but the outward push of fusion begins to falter. A complex series of events occurs which leads the star to collapse on the inside while it swells up on the outside. As the outer layers of the star grow larger, they cool off and become reddish in color. A red star is cooler at its surface than a blue-white star for the same reason that an iron bar, heated until it is white-hot, turns red as it cools. Astronomers call such swollen stars red giants; they can become as large as the orbit of Mars or Jupiter.
A particularly dramatic example of a red giant, close enough so that we can study it in some detail, is the star Betelgeuse, 425 light years away in the constellation of Orion. It has grown to be 600 times larger than our Sun; and would, if it replaced the Sun, completely swallow the orbits of Mercury, Venus, Earth, and Mars.
All red giants eventually resolve their moment of crisis, finding a new source of fuel for fusion, but the star is never quite the same again. Some of its outer layers are permanently lost, and its resumed inner stability is only temporary.
5. Old Age
At this point, the life story of stars splits into two branches, depending on the star's mass.
Low-mass stars (including the Sun) have only limited ability to squeeze their cores and to continue fusion reactions. They experience a period of instability, where their outer layers lift off, but are they are then ready to give in to the inexorable pressure of gravity and to die. The period of instability for these stars produces some of the most beautiful temporary objects in the sky. As the star sheds its outer layers, the gas in it is illuminated by the light of the collapsing star and glows with eerie colors. When William Herschel found the first of these "last gasps" of dying stars, they reminded him of the disks of planets and he named them planetary nebulae. (Although we now understand that there is no connection with planets at all, the name has stuck.) Readily seen with even a small telescope, planetary nebulae are among the most attractive targets for amateur stargazers.
High-mass stars, on the other hand, have sufficient pressure and temperature at the core to do what it takes to continue fusing the newly minted atoms they formed earlier. Helium, carbon, and oxygen each in turn become the fuel for making energy, opposing gravity, and keeping the star (briefly) stable. Eventually, however, the massive star's core becomes composed of the element iron, which sets off a final collapse and catastrophe.
When low-mass stars die, they collapse under their own weight until their centers act in some ways like a solid. A star like the Sun will collapse at the end to be about twice the size of Earth, a tremendous reduction in size. These white-hot dying stars are called white dwarfs by astronomers. A white dwarf is so compressed that— if you could survive standing on its hot surface—your weight would be roughly a million times your Earth weight. Perhaps the best known white dwarf orbits the brightest star in the sky, Sirius. Since Sirius is known as the dog star, the companion white dwarf is sometimes nicknamed "the pup"; it requires a significant telescope to see, but is fun to imagine when you see Sirius on winter evenings.
Massive stars have a very different ending in store. When the core of a massive star collapses, its powerful gravity takes it right through the white dwarf stage to produce one of two extremely bizarre objects—either a neutron star or a black hole (see below). In most cases, the rest of the star then blows up, in a gargantuan explosion called a supernova. These explosions produce so much energy that the star can briefly become brighter than the entire galaxy in which it is located (brighter than 100 billion Suns). Extreme versions of a supernova explosion, sometimes called "hypernovae," produce gamma-ray bursts like the one discovered by Michael Koppelman, as described in our film.
The most famous supernova in history was seen in July 1054. Records of it still survive, from China all the way to North American Indian lore. Seen today, roughly a thousand years later, its remnant, called the Crab Nebula, is one of the most interesting objects in the sky. It can be found with good binoculars or small telescopes in the constellation of Taurus, the Bull.
7. The Graveyard
Every star death leaves a corpse. Star corpses come in three bizarre varieties.
A white dwarf fades away as the billions of years pass. Since no new energy is being produced inside and light is escaping into space, the star corpse cools down until it becomes black as space. Astronomers call these corpses black dwarfs.
A supernova explosion leaves behind a far more compressed corpse than a white dwarf. The violent end of a massive star produces so much pressure that the star's core experiences a remarkable subatomic change. Electrons, negatively charged particles which normally "orbit" the nucleus of the atom, are actually squeezed into that nucleus. There, the electrons "join" with protons and become neutrons, creating a neutron star. This process also removes all the space within the atom, leading to a fantastic compression of the star's remains. To make something as dense as a neutron star on Earth, you would have to take all the people in the world and squeeze them into a raindrop!
If the dying star is especially massive, the corpse becomes a completely collapsed object called a black hole. In these objects gravity is so strong that nothing, not even light, can escape. Space, time, matter, and energy are all trapped within a tiny region. Understanding the behavior of black holes is one of the great frontier areas of astronomy.
8. A Cosmic Connection
From reading the life story of the stars so far, you might think that it is a sequence of events quite removed from the concerns of those of us who watch PBS television shows on Earth. But none of us would be here, were it not for the life and death of previous generations of stars.
All our observations of the chemical makeup of the stars confirm that the universe began with only the two simplest types of atoms—hydrogen and helium. It is not possible to make something as complex as a human being out of only those two elements; you need carbon, oxygen, calcium, iron, and a host of other atoms to make us. Where did these heavier elements come from, if they were not present at the start of the universe?
Heavier atoms had to be produced by nuclear fusion at the centers of stars. Indeed, we see much evidence that stars create new elements in the process of making energy. When stars die, and particularly when stars explode, they recycle their newly-made elements into the cosmos. The next generation of stars then forms from this enriched mixture of elements, as do any planets accompanying them. It took several generations of stars to enrich our neighborhood, so that when the Sun and the Earth formed some 5 billion years ago, they would contain enough of the heavier atoms to make living creatures, ourselves included, possible. As you look at your fingers on the computer keyboard, every atom inside them that is not hydrogen or helium was once inside a star and is now on loan to you from the great library of atoms in the universe. In this sense, we are all made of stardust—and the life story of the stars is our story.