Stellar Birth and Life Help

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By — McGraw-Hill Professional
Updated on Sep 17, 2011

Introduction to Stellar Birth and Life

All stars evolve from clouds of gas and dust. If the original material in the Universe were perfectly homogeneous—equally dense at every point in space—stars could, in theory, never form. The slightest irregularity, however, brought about more irregularity, leading to regions where the gas and dust became concentrated. This is known as the butterfly effect and was discussed in Chapter 6 when we took a “mind journey” to the planet Mars. This effect breeds new stars.

A Star Is Born

Where the clouds of matter were the most dense, the gravitational attraction among the atoms was the greatest. This caused the dense regions to become even more dense and the sparse regions to get more sparse. A vicious circle ensued, which was repeated in countless locations. It is evidently still taking place in the spiral arms of the Milky Way and in other galaxies.

As a cloud of gas and dust contracts, it eventually begins to heat up. The atoms, originally free to move without restriction, get cramped for space and start to collide with one another. This produces outward pressure, but the increased concentration of matter causes a dramatic increase in the gravitational attraction among the atoms. This gravitational force keeps pulling the gas-and-dust cloud tighter and tighter, and it gets hotter and hotter. Finally, the temperature gets so high that hydrogen atoms begin to fuse, forming helium atoms along with great quantities of energy. This causes the star to become extremely hot, and the outward pressure finally rises to meet the inward force of gravitational collapse. Several hundred thousand years, or a few million years, go by between the initial contraction of the gas-and-dust cloud and the start of the hydrogen fusion reaction.

Large stars are born more quickly than small ones. Sometimes two or more stars are formed so close together that they orbit one another; these are binary stars and multiple stars . Sometimes huge gas-and-dust clouds give rise to clusters of stars.

We can follow the metamorphosis of a young star in an H-R diagram. A protostar , as it contracts and heats up, is not very luminous until the fusion reaction starts. Protostars are situated off the scale at the bottom of the graph. As the protostar contracts to the point where fusion begins, the star’s position moves upward (Fig. 13-4). Almost every new star comes to rest on the main sequence. The most massive stars end up at the upper left and become blue and white supergiants. The least massive stars reach their stable positions at the lower right and become orange and red dwarfs. Oddly enough, the dim, cool, and least spectacular stars burn longest, and the bright, massive, and hot stars have much shorter lifespans.

Stars and Nebulae How Bright? How
Distant? A Star Is Born

Figure 13-4. As stars are born and mature, they move onto the main sequence.

Once the fusion process begins and the star shines brightly, the remaining gas in the star’s vicinity becomes ionized by ultraviolet (UV) rays and x-rays. When we look at the Pleiades, for example, through a large telescope, this glowing gas can be seen. Much of the superfluous gas and dust near the star is blown away by the stellar wind , high-speed subatomic particles emitted by the star shortly after it begins to shine. This keeps all the gas and dust in the galaxy in a constant state of turmoil, like a room full of smoke in which people are talking and gesturing.

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