STARS & THEIR EVOLUTION

Star Dust & Elements About 4 and a half billions years ago something interesting happened in an out-of-the-way part of the galaxy. The sun ignited at the center of a cloud of gas and dust. The rest, as they say, is history. We now know that stars (the sun included) are born in dust clouds, redden and swell to many times their original size as they age, then end their lives in incredible explosions that leave the most bizarre objects in the universe behind. They are the major engines for pumping out atoms heavier than hydrogen -- the stuff that makes up the Earth, its rocks, its oceans and its life forms -- even the two-legged variety reading this page.

Gravity pulls the outer layers of the dust and gas cloud toward the center of the cloud. The center of the cloud is not very hot yet. There is nothing at the center strong enough to hold up the weight of the outer layers So the cloud continues to collapse. This is similar to the egg in the left-hand panel. The weight of the foot is like the weight of the outer layers of the star. The pressure in the egg is like the pressure in the core of the star. What do you think will happen?

When the temperature at the center gets very hot, nuclear reactions begin, converting hydrogen to helium with the release of energy. The temperature increases even further and the gas at the center presses outward holding up the outer layers of the star. Like the bowling ball on the left, high internal pressure means that the weight of the outer layers can no longer crush the star. The star enters the main sequence and is stable for a large part of its life.

Gravity Wins The end, however, is inevitable. When the star's fuel supply is used up and nuclear reactions stop, the star will collapse. Gravity always wins. What's left at the end depends on how massive the star was to begin with.

Stellar Evolution Nebula - Stars begin as a nebula of gas and dust. Protostar- gravity pulls it together, temperature rises. Main Sequence- Temperature, luminosity & mass directly proportional. Red Giant or Supergiant- large, luminous, low temperature. Nova or Supernova- outer layers of star blown off. As the star expands, the outer layers blow off at an incredible rate. A star can lose more than half of its mass during this stage. The gas cloud surrounding the star is called a planetary nebula. Ending is based on initial mass and can be either:

What Happens to Stars Much Heavier than the Sun? Main Sequence Stars that are much heavier than the sun start off as hot white, blue-white or blue stars, rather than yellow stars like our sun. They last only a short time compared to the sun. A star 20 times heavier than the sun will use up its fuel in about 8 million years. The sun takes 1000 times longer. Very big stars that were born when dinosaurs lived on Earth have already used up their fuel and exploded. The sun has been around since long before the dinosaurs and its still here today.

Red Supergiant Phase After the star depletes the hydrogen in its core and moves off the main sequence, the core contracts and heats causing the outer layers to expand. The expansion of the outer layers results in a cooling of the surface temperature. The star gets redder. Meanwhile the core is getting hotter reaching 150 million K. At this temperature, helium (He) fuses explosively into carbon (C) and oxygen (O). Gravitational collapse continues to raise the temperature of the core. When the core reaches an amazing 1 billion K, carbon fuses to produce neon (Ne), magnesium (Mg) and oxgyen (O). These elements fuse to produce even heavier elements. At each stage less total energy is released and thus the stages get progressively more short-lived. Finally an iron core is produced. Since no energy can be gained by fusing iron to make higher mass elements, the core collapses at blinding speeds.

Supernova Stage In the final moments of the star's life, nuclear fusion produces an iron core. No energy can be gained by fusing iron into heavier elements. There is now no energy source to sustain the intense internal pressures holding off gravitational collapse. The core collapses catastrophically inward at 1/4 of the speed of light. In less than 1/10 of a second the core of the star is crushed into a sphere only 100 km across. The gravitational energy released in this fraction of a second is 100 times greater than the sun releases in its entire 10 billion-year lifetime.

Neutron Star After the supernova blast blows off the outer layers of the star, all that is left is the central core. The core now contains a mass between 1.4 and 3.0 times the sun's mass but condensed into a volume 10- to 20- km across - roughly the size of a small town on Earth. The matter in a neutron star would be incredible to behold. It is thought to be no longer gaseous. The surface may be crystalline. Densities are a trillion times greater than those in a white dwarf. In fact, it is so dense that one teaspoonfull would outweigh the Empire State Building.

Black Hole If the central core, that remains after the supernova blast, is greater than 3 times the sun's mass, the internal pressure cannot halt the gravitational collapse. The core will continue to collapse to form a black hole. In a black hole, there is so much mass compressed into such a small volume that gravity prevents even light from escaping. Since no light can ever reach your eyes from the collapsed core, it appears black, hence the name. The apparent surface of the black hole is the radius at which light just manages to escape. It is called the event horizon. For example, a black hole with 3 times the solar mass would have a circumference of 55.5 kilometers (the size of a town). The Earth as a black hole would only be a couple of centimeters in diameter (about the size of a marble)!

What happens to stars around the size of the sun? Main Sequence A star's life begins when nuclear reactions start deep in the core. Hydrogen nuclei are fused to form a helium nucleus. Each helium nucleus has slightly less mass than the hydrogen nuclei that formed it. The missing mass is converted into energy. This energy released in the core creates high temperatures and pressures. The high internal pressure would blow the star apart if not for the weight of the outer layers pressing down upon the core. The star spends almost all of its life on the main sequence as a very average star.

Red Giant Phase When the star uses up its hydrogen fuel in the core, it can no longer hold up its outer layers. They fall inward. The center gets hotter. The hot temperatures cause the outer layers of the star to expand. As a result, the star gets bigger and the surface cools. The star is now a red giant. While the outer atmosphere is expanding, the core inside collapses to about the size of the Earth. The temperature again increases, reaching about 100 million degrees. Suddenly helium in the core begins to fuse to carbon and oxygen at a very rapid rate. Within a short time the helium is gone, and gravitational collapse continues.

The Outer Layers Blow Off As the star expands,the outer layers blow off at an incredible rate. A star can lose more than half of its mass during this stage. The gas cloud surrounding the star is called a planetary nebula. The exposed inner part of the star remains as the central star in the planetary nebula. These central stars are the hottest stars known. They are not very luminous because they are extremely small -- about the size of the Earth.

White Dwarf When the fuel is used up, there is nothing to hold the star's layers up against gravity and the star begins to collapse. Gravity squeezes the star down into a very small size. As you might imagine, the matter that makes up a white dwarf is like nothing you've ever seen before. It's as if the mass of an elephant were compressed into the space of a marble. A 70 kg (154 lbs) person would weigh about 600,000 kg (600 metric tons) standing on the surface of a white dwarf. As bizarre as they are, white dwarfs are very common objects in the Universe.

Black Dwarf White dwarfs are some of the hottest objects in the known universe with temperatures ranging from 30,000 K to 200,000 K. They continue to radiate their immense internal energy into space as they continue on their journey to become black dwarfs. When they reach 4000 K these bizarre stellar objects must start to crystallize. The time this requires is very long. In fact, no white dwarf generated has yet had enough time to cool to a black dwarf.

End of a Star’s Life Small Mass Large Mass White Dwarf- small, white star with low luminosity and high temperature. Fuel is used up,gravity squeezes the star down to a very small size. Black Dwarf-cool, star that has crystallized Large Mass Neutron Star (Pulsar is one type)- only has neutrons Black Hole- so small and dense that even light cannot escape it

RED GIANT & SUPERGIANTS Hydrogen fuel is being used up. Outward pressure overcomes gravity. Star swells.

Novas and Supernovas Nova- Outer layer of star swells and leaves Supernova- outer layers explode

White Dwarfs White dwarfs are the hot cores of stars that have blown away their outer layers. They are about the same size as the Earth. http://www.windows.ucar.edu/

Neutron Stars Neutron stars are the cores of stars that have been compressed to a very small size by gravity and are cooling off slowly. They are made of neutrons. They are barely as large as a small town.

Black Hole Black holes are the strangest object in space. They are the core of stars that gravity has compressed down so small that nothing can escape not even light. They are even smaller than neutron stars.

H-R Diagram by Hertsprung-Russell led to understanding of stellar evolution.

Enjar Hertzsprung (Danish)& Henry Norris Russell(American)

The H-R Diagram

http://www.windows.ucar.edu/tour/link=/cool_stuff/tours_main.html http://www.enchantedlearning.com/subjects/astronomy/stars/lifecycle/