Old Stars, New Stars

Some go back to big bang, others live and die many times; offer portrait of stellar evolution

written by Bob Sheldon

A stronomers peering into the heavens see stars as old as the universe itself and stars recycled time and time again from dust and gases in the interstellar medium.

"Stars are born as fiery cauldrons, about three-quarters hydrogen and one-quarter helium," said Kam-Ching Leung, UNL professor of physics and astronomy. "As they age, their outer envelopes expand, consuming energy generated by fires burning in their cores so lustily that their very atoms are transformed from the original hydrogen to atoms of increasingly complex structure and weight.

"Low-mass stars consume themselves at a slower rate than more massive stars. In its entire lifetime, the conversion of a small-mass star's elemental fuel may go no farther than helium. More massive stars bum hotter and faster, and may end their lifetimes with a core fueled by elements as heavy as carbon or iron." "We look at a star and see how its chemistry changes," Leung said. "The best theory we have now is the 'Big Bang' theory. All of the original stars in the universe were created at about the same time. How long they live depends on their conversion rates, how fast nuclear fusion changes their fuel from one chemical to another. The conversion rate is dependent on high orders of interior temperature, and temperature is strongly dependent on mass.

All stars are in galaxies in a universe of unimaginable dimensions. Our planet orbits a sun that is one of billions of stars in the Milky Way, a galaxy in a group of galaxies that occupies only about onemillionth of the observable universe.

All galaxies are moving away from one another. They have been since the universe was created. "When we look at a galaxy we are looking at its past history," Leung said. "When we look beyond galaxies near to us through our telescopes, we are seeing galaxies as they existed long before our time. The light from the most remote galaxies we can see left those galaxies billions of years ago, and is probably light nearly as old as the universe itself.

Astronomers, therefore, see stars not as bright, twinkling objects of the here and now, but as glimpses into a past stretching back 10 billion years or more. And they know that each and every star they see offers a view of a different stage of stellar evolution, like a frame-by-frame movie that began with creation and ends in cataclysmic chaos.

It is an incomplete panorama, however; a film with many blank frames, offering a jerky, primitive motion picture with too may roles in its cast played by unknown actors.

To study a galaxy, Leung said astronomers have to take what they call look-back time into consideration. "When we look at the sun, we don't see the sun as it is now. We see the sun as it was eight minutes ago. When we look at a galaxy, we don't see the galaxy as it is today, because light from the stars in that galaxy may have taken billions of light years to reach earth."

The yardsticks used by astronomers to determine how many light years away is a star or a galaxy are not highly reliable, because when astronomers look through their telescopes they view a universe in two dimensions. They see objects of different brightness and luminosities at vast, but not easily determined, distances from one another. Therefore, it's hard to tell if the dot they see is an asteroid, a comet, a star, or even an entire galaxy.

To compare one galaxy with another, and to peer to the horizon of the universe itself, astronomers need better "measurement yardsticks," according to Leung. Those "yardsticks" are developed as the result of accumulating knowledge about the evolution of the stars.

What is known about the evolution of stars is learned from information about their physical properties--their brightnesses, luminosities, their masses, sizes and physical properties. From those properties, astronomers have constructed what they believe is a reasonably accurate picture of stellar evolution.

As any star ages, part of its mass evaporates into space, while the remaining mass is squeezed tighter and tighter at the core, according to Leung. Eventually, as the outside envelope continues to expand and evaporate, the core of the star will cool, and the star will become a white dwarf.

"If you were to take a star as big as our sun and squeeze it into an object the size of Earth, it would have a density corresponding to our sun if it were a white dwarf," Leung said. "A cubic inch of this white dwarf material would weigh 10 tons.

Our sun, six to seven billion years or so down the road, will become such a white dwarf. A different fate awaits larger stars, however. "Every star's destiny is a product of its mass," Leung said.

"Stars with masses many times larger than our sun's will not only burn hotter and faster as they age, but their cores, whose density increases more and more as its atoms are converted from ever heavier elements, will become squeezed to densities far more compressed than that in a small star such as our sun.

(inset) The evolution of a star: A cloud of gases, mostly hydrogen, condenses to form a star. The circles, clockwise from upper left, indicate the star's evolution: Hydrogen atoms fuse into helium atoms, and the core of the star starts to shrink as helium accumulates at the center. Helium fusion slows and heavier elements are formed one after another. The temperature at the core increases. When iron is formed and collects at the core of the star, an implosion occurs (indicated by inward pointing arrows), in which the star collapses rapidly and violently. The implosion is followed by a tremendous explosion (indicated by arrows pointed outward), in which the star's matter rebounds to produce a supernova .

Heavier elements beyond iron are created in the high temperatures resulting from implosion and explosion of a star. The original core of the star, highly compressed and small in size, becomes a neutron star, or, eventually, a black hole.

"The death of a massive star is far less peaceful than that of a small star, Leung said. "The collapse of the lighter elements into a small star's core becomes a very fast collapse in a star whose core is carbon or iron. What happens is that as the star's outer envelope ex-pands rapidly while the core collapses rapidly, a vacuum is developed at the interface of the core and the envelope. The result is comparable to what happens when a building is demolished. An implosion occurs; matter begins hurtling itself inward to the intensely hot core of the star.

"An imploding star is a very dangerous place to be," Leung said. "It's like pouring gasoline on a fire. The material rushing to the core fuels an explosion; a high energy action coupled with an equal and opposite reaction that serves to compress the core of the star even more. The stellar envelope is ejected into space during the explosion. There are binary stars whose periods are even less than that, and these end stage of a star whose core mass has core are so tightly squeezed that they stars, with periods measured in seconds, contracted to something almost dimenbreak apart into neutrons."

Thus, a neutron star is formed. A neutron star, Leung said, is not a star at all. It is a core squeezed into so small a size that a mass like our sun could be squeezed into about half the distance between Lincoln and Omaha.

Leung said that there exist in the universe binary star systems that have undergone all of thoses different varieties of evolution, but revolove around a common center of mass, just as our planet and the sun revolve about a common center of mass. (This is the more accurate description of the relationship between our planet and the sun--Earth does not revolve about the sun, but Earth and the sun revolve about a common center mass.)

"In a binary system, the length of the binary orbital period depends on the separation, or distance, between them, Leung said. "If one star in the system is a white dwarf, the separation between the two can be very small, or the two stars can come in contact. When this happens, the rotation of one around the other can be less than a quarter of a day. If both are white dwarfs, the period could be measured in minutes.

There are binary stars whose periods are even less than that, and these stars with periods measured in seconds, are likely to be neutron stars, according to Leung.

There are other things that can happen to stars in their evolution. From the implosion-explosion of a neutron star comes a masive ejection of matter that produces a super nova. Super novas probably account for the manufacture of all the other elements in the universe beyond iron, Leung said. "Gold, silver, radium, uranium and all the other elements are cureated in the debris during a super nova explosion. Since the explosion only lasted for a very short time, the abundance of elements with atomic weights greater than iron is less and these elements are relatively rare."

Incredibly, in view of the intense concentration of mass in a neutron star, even more intense concentration can occur. There are some stars so large, with cores so tightly concentrated, that the horrendous pressures at their core are such that what those cores contain aren't even neutrons. The masses of these stars are squeezed so tight that their total mass can be concentrated into the size of a point on the tip of a ball point pen, Leung said. These are black holes, which Leung said represents the end stage of a star whose core mass ha contracted to something almost dimensionless; whose density is incalculable.

To an astronomer, all of ehse natural occurrences in the universe offer opportunities to study its evolution. "Some of the original low mass stars created in the'big bang' co-exist today with recent generations of stars that have become increasingly contaminated with heavy chemicals as they have died and been reborn, sometimes time and time again," Leung said. It is this co-existence of generations of stars of differnet ages that makes it possible to study the universe."-RES

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