Light stars

For stars lighter than 1.4 solar masses the electron degenerate pressure will balance gravity. The star has by now contracted from its red-giant size to the size of a small planet (like Earth). The material of this star is so dense a teaspoon of it would weight 1 ton on Earth.

When this final contraction occurs there is a certain amount of overshoot and bouncing back and forth before stability is achieved; in this process all the outer layers of the star are ejected. The end result is a beautiful ring of stellar material which spreads out, at the center of which a small star, called a white dwarf, remains (see Fig. 9.4). White dwarfs are is stable and their racy days of nuclear reactions are forever gone; they slowly radiate their remaining heat little by little and eventually become dark cinders. This is the end of a star whose mass is smaller than 1.4 times the mass of the Sun; this process is summarized in Fig. 9.5

 

Figure 9.4: Photograph of a ring nebula. The central white dwarf has, in its last throes, expelled its outer layers appearing here as a ring surrounding the small remnant.  

 

Figure 9.5: Time and life of a star of mass below 1.4 times the solar mass (less than about 3 × 10²⁷tons).  

It is interesting to note that the theory predicts that these objects will always be lighter than 1.4 solar masses. Observations have confirmed this. This theory is a combination of quantum mechanics and gravitation and, in fact, it provided the first application of quantum physics to stellar objects.

For heavier stars the pull of gravity overcomes the degenerate electron pressure and collapse continues.