<text><span class="style10">tars and Galaxies (4 of 4)Stellar evolution and black holes</span><span class="style7">The manner in which a star evolves depends upon its mass. Protostars with mass less then 0.06 of the Sun will never become hot enough for nuclear reactions to start. Those with mass between 0.06 and 1.4 solar masses quickly move on to the main sequence and can remain there for at least 10 000 million years. When the available hydrogen is used up, the core contracts, which increases its temperature to 100 million deg C (180 million deg F). This produces conditions in which helium can begin a fusion reaction and the star expands to become a </span><span class="style26">red giant</span><span class="style7">. Finally, the outer layers of the star are expelled, forming a </span><span class="style26">planetary nebula</span><span class="style7">. The core then shrinks to become a small </span><span class="style26">white dwarf</span><span class="style7"> star.Stars of between 1.4 and 4.2 solar masses evolve more quickly and die younger. They remain on the main sequence for about one million years before the red giant phase begins. The temperature continues to increase as even heavier elements are synthesized until iron is produced at the temperature of 700 mil lion deg C (1260 million deg F). The star is then disrupted in a huge </span><span class="style26">supernova</span><span class="style7"> explosion producing a vast expanding cloud of dust and gas. At the center of the cloud a small </span><span class="style26">neutron star</span><span class="style7"> will remain. This rotates very rapidly and is incredibly dense: 1 cm3 (0.061 cu in) of neutron-star material has a mass of about 250 million tons.The evolution of more massive stars is stranger still. They may end their lives by producing a </span><span class="style26">black hole</span><span class="style7"> - an object so dense that not even light can escape. The only means of detecting a black hole is by observing its gravitational effects on other objects. The X-ray source Cygnus X-1 may comprise a giant star and a black hole. Material would be pulled away from the star by the black hole and heated - giving off X-rays as it is pulled in.GE</span><span class="style10"></span></text>
</content>
<content>
<layer>background</layer>
<id>26</id>
<text><span class="style10">he relative sizes of different types of stars</span><span class="style7">. Typical red giants are 100 times the size of the Sun, which in turn is 100 times the size of a white dwarf. White dwarfs are 1000 times larger than neutron stars, which typically have a diameter of 10-20 km (6-12 mi). Red supergiants may be 5 times larger than a typical red giant.</span></text>
</content>
<content>
<layer>background</layer>
<id>23</id>
<text>ΓÇó THE UNIVERSE AND COSMOLOGYΓÇó THE SUN AND THE SOLAR SYSTEMΓÇó THE HISTORY OF ASTRONOMYΓÇó ENERGY: COAL, OIL AND NUCLEAR</text>
