Chapter 27: The History of a Star

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People had stared into the heavens for thousands of years and wondered about the stars. However, the first credible models of how stars work date to about 1940. Until people understood fusion, they could nut understand stars. We now understand that stars are born, they live, and they eventually die. Where are stars born? It is instructive to look about our own Milky Way Galaxy. Its diameter is about ( 1, 1000, 100000?) light years, it contains about 100 (thousand, million, billion?) stars, it is rotating and has spiral arms. Our sun is in the Orion arm. If we look along our spiral arm toward the constellation Orion, we see clouds of gas and dust (called "nebula") that are large enough that they obscure our vision of stars behind them. If we look over our shoulder, we see the Veil Nebula in the constellation Cygnus which appears to be the remnant of a giant explosion. Looking inward to the Sagittarius arm, we again see nebulae. We believe that these clouds of gas and dust are the birthplaces of stars. Even to the naked eye, stars are not the same. Some (like our sun) are yellowish, but others are clearly bluish and yet others are reddish in color. We now understand that some of these differences represent different stages in the lifetime of a star. A star begins as an unusually dense pocket of gas in a nebula. ( Force?) begins to pull the matter together, but as the matter collapses, potential energy is converted to energy of the particles and dust. Thus, the temperature of the matter (increases, decreases?) and the pressure outward exerted by the gas (increases, decreases?) . At this point the matter collapse has slowed, and the ball of gas (for something like our sun) is about (2, 200, 2000?) times the size of our sun. The gas has become (state of matter?) and the matter begins to glow, but fusion has not yet begun. This stage is called the . Fusion begins in the core when the temperature there reaches about 30 (thousand, million, billion?) degrees Celsius. The fusion takes place in several steps, but the overall immediate outcome is to fuse protons into nuclei. The star is in its "adult stage" which (for our sun) will last about 10 (thousand, million, billion?) years. (Our sun is about half-way there.) The star is now on the "main sequence" of the Hertzsprung-Russell diagram. In time the fuel is exhausted in the core and the nuclear "fire" spreads outward. The core begins to (expand, collapse?) and get (hotter, cooler?) while the outside of the star begins to (collapse, expand?) and get (hotter, cooler?) . The star is now said to be a . Eventually, the outside stops and also collapses and the cycle may repeat. The Death of a Star. Actually there are three different possible deaths and they each depend on the original of the star. For the least massive stars (such as our sun), the star eventually blows off some of its mass as a nebula, leaving a very dense, hot, glowing (red, white or blue?) dwarf behind. Fusion continues for a time, but the white dwarf eventually dies to become a (black or blue?) dwarf, a burned out cinder. For medium mass stars (up to 2-3 times the mass of our sun), the star goes through several stages of expansion and contraction, but eventually the fusion produces (which element) , beyond which no additional net kinetic energy is released. The star dies in a massive explosion called a which produces (by fusion) all of the heavier elements in the Periodic Table and produces gas from which subsequent stars (like our sun) might form, but which kills the parent star. In at least some cases, a super dense remnant, called a star remains behind. In the most massive stars, gravity wins altogether and the star collapses entirely to become a . Although black holes are not directly visible, the enormous gravitational fields outside, but near the black hole, cause enormous amounts of radiation to be emitted which can be seen as evidence for its existence.

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