Chapter_5 - Chapter 5 Stellar Remnants The life history of...

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Unformatted text preview: Chapter 5 Stellar Remnants The life history of a star is determined primarily by its mass ; if you know a stars initial mass, you know (at least in broad outline) how it will evolve. Other factors, like the stars initial chemical composition and initial angular momentum, have a smaller effect. 1 For instance, the fusion history of a star depends on its initial mass (all mass limits in this table are approximate): M < . 08 M fl : No fusion. The center of a low-mass ball of gas never becomes hot enough for hydrogen fusion. This mass range represents not stars, but brown dwarfs of spectral type L and T. . 08 M fl < M < . 5 M fl : Fusion of 1 H to 4 He. The center of the star never becomes hot enough to fuse 4 He to 12 C & 16 O. Stars in this mass range are M stars on the main sequence, and will eventually end up as white dwarfs made of helium. . 5 M fl < M < 5 M fl : Fusion of 1 H to 4 He and 4 He to 12 C & 16 O. The center of the star never becomes hot enough to fuse 12 C & 16 O to heavier elements. Stars in this mass range are A, F, G, and K stars on the main sequence, and end up as white dwarfs made of carbon and oxygen. 5 M fl < M < 7 M fl : Fusion of 1 H to 4 He, 4 He to 12 C & 16 O, and 12 C and 16 O to 20 Ne & 24 Mg. Stars in this mass range are B stars on the main sequence. 1 The statement a stars properties are determined primarily by its mass is known as the Russell-Vogt theorem, after the ubiquitous Henry Norris Russell and the German astronomer Heinrich Vogt, who were the first to realize its truth. 97 98 CHAPTER 5. STELLAR REMNANTS M > 7 M fl : Fusion of 1 H to 4 He, 4 He to 12 C & 16 O, 12 C & 16 O to 20 Ne & 24 Mg, 20 Ne & 24 Mg to 28 Si, and 28 Si to 52 Fe and 56 Ni. Stars in this mass range are O stars on the main sequence. (Note that the elements produced have atomic masses that are multiples of 4; this is because the heavier elements are built up by fusing on additional 4 He nuclei.) The initial mass of a star also determines what its corpse or stellar rem- nant will be. The lowest mass stars become dense white dwarfs, supported by electron degeneracy pressure. Higher mass stars leave behind even denser neutron stars, and the most massive stars of all end up as black holes, rep- resenting the ultimate in density. We will examine in turn the evolutionary roads that lead to white dwarfs, neutron stars, and black holes. 5.1 White Dwarfs A white dwarf is a stellar remnant supported by electron degeneracy pres- sure. The name white dwarf, as it turns out, is something of a misnomer. Nobody objects to the dwarf part; white dwarfs really are small compared to main sequence stars of comparable mass. As we computed in section 1.5, the radius of the white dwarf Sirius B is r WD = 0 . 0084 r fl , and its mass is M WD = 0 . 96 M fl . However, not all white dwarfs are white-hot. Although the first white dwarfs discovered, such as Sirius B, have high surface temperatures (Sirius B has T 25...
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This note was uploaded on 07/17/2008 for the course ASTRO 292 taught by Professor Ryden during the Winter '06 term at Ohio State.

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Chapter_5 - Chapter 5 Stellar Remnants The life history of...

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