Lives of Stars Notes

The Lives of Stars A star is not organic, therefore it IS NOT alive. Astronomers typically talk about the life cycle of a star as if it were actually a living thing. A star forms, exists, then ceases to exist - equate this to a hurricane, which forms, wreaks havoc, then dies.

The Lives of Stars All stars begin as a nebula. A nebula is a large cloud of gas and dust. A nebula spreads out over huge areas of space. Disturbances cause some of the lighter gases (hydrogen and helium) to begin collapsing under the force of gravity.

Each of these nebulae could give birth to dozens, hundreds, or even thousands of new stars.

Lives of Stars When the gases collapse together with enough mass that a star might form, this is known as a protostar. Eventually, the protostar will begin fusing hydrogen gas into helium, producing nuclear fusion. At this point, it is an actual star, it is said that the star has been born. Animations/ClusterXT1810Z_H264B.mov Animations/ClusterXT1810Z_H264B.mov

Lives of Stars Stars with less mass live longer than stars with more mass. Stars with less mass than Sol can live up to 200 billion years. They burn through their energy very slowly, so they are also cooler. Stars with mass equal to Sol live for about 10 billion years. Stars with masses 15 times the mass of Sol may only love about 10 million years. They burn through their fuel very rapidly and are much hotter.

Death of Stars Eventually, all stars run out of the fuel needed to perform nuclear fusion. When this occurs, the star begins to die. Keep in mind that all stars are in a battle within themselves. There is the outward explosion force of nuclear fusion, and the inward force of its own gravity. For the main life of a star, these two forces are balanced. When a star runs out of fuel, the two forces become unbalanced and the star will begin to change size.

Death of Stars For low-mass and medium-mass stars, when hydrogen runs out, the outward force of nuclear fusion will cause the star to expand outward and cool. This next step forms a RED GIANT (a cooler star about 250 times larger). Eventually, the red giant will run out of fuel as well and blow out its outer layers as a planetary nebula, leaving behind its core, a WHITE DWARF star.

Death of Stars The white dwarf that results is usually about the size of Earth. But it still contains most of the original mass of the sun, meaning that it is about one million times denser than the original star. One teaspoon of white dwarf material would weigh as much as an 18-wheeler. Even though a white dwarf has no fuel, it continues to glow using leftover energy for billions of years. Eventually this core stops glowing and the leftover is called a black dwarf. Our universe (at billion years old) is still too young for any black dwarfs to exist.

Death of Stars A star with a huge mass has a different life cycle. When a supergiant runs out of fuel, it explodes. The explosion of a supergiant star is called a supernova. A single supernova can be so bright that it can outshine the entire galaxy (300,000,000,000 stars) it was in. Supernovae can be seen from Earth. There are historic records of some stars that were so bright that they could be seen during the day for weeks at a time.

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Death of Stars Sometimes, the ejected material from a supernova turns into a nebula that contains not only hydrogen and helium, but also heavier elements such as iron, calcium, carbon, gold, silver, etc. This nebula can begin to collapse and form new stars and planets. Our solar system formed from this type of scenario. All of the material for everything in this solar system came from a supernova that exploded in this area of the Milky Way.

Death of Stars Some of the core of a star that has gone supernova can stay behind and form a neutron star. A neutron star can contain up to three times the mass of our sun, but all packed into the space of only 25 kilometers. This density is approximately equivalent to the mass of the entire human population compressed to the size of a sugar cube. an.ogv

Death of Stars

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Deaths of Stars The stars with the greatest mass become black holes when they die. A black hole is an object with very strong gravity that does not give off any light. A black hole has gravity so strong that nothing escapes its pull - not even light.

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Death of Stars NASA has recently concluded that at the center of most elliptical galaxies (like our own Milky Way) there exists a supermassive black hole, and in some cases, multiple black holes. These black holes can contain millions to billions more mass than our own sun.