Medium-size stars

  For stars heavier than 1.4 solar masses but lighter than about 3-4 solar masses (the calculations are still a bit uncertain), the electron pressure is not strong enough to balance gravity. The contraction then goes crushing the electrons together and braking apart the Iron nuclei into their constituents. These constituents, neutrons and protons, also detest being close to each other and, as mentioned above, produce a (degenerate) pressure which opposes gravity. For a star in the present mass range this pressure is sufficient to stop further collapse, but is effective only when the material is extremely dense which occurs only when the star has contracted to an object a few kilometers in diameter.

The contraction of these stars from their initial solar size to the size of a city is one of the most spectacular events in the heavens: a supernova. Imagine an object weighting 5 × 10²⁷ tons (that is five thousand trillion-trillion tons, or about 2.5 solar masses), which contracts from a size of 10⁶ (one million) kilometers to about 10 kilometers, and it all happens in a fraction of a second. During collapse the amount of energy generated is fantastic, part of it goes into creating all elements heavier than Iron, part into creating neutrinos and part is transformed into light.

Radioactive elements are also created during the collapse. These elements rapidly decay, and the resulting radiation is so intense it produces a fantastic flash of light. At this point the supernova will out-shine a full galaxy of normal stars (several billion or up to a trillion of them!).

After the collapse there is a violent overshoot before equilibrium sets in, at this time all the outer layers of the star are ejected at speeds close to that of light. When this material goes trough any planets around the star (if any) it vaporizes them. In the middle of this cloud the core of the original star remains, a rapidly rotating remnant, protected against further collapse by it neutron degenerate pressure.

The overshoot is so violent that the elements created will be strewn all over the region surrounding the star, part of this material will end up in dust clouds which will become stellar systems ( the shock produced by the supernova material colliding with a dust cloud may initiate the formation of a stellar system); this is how the Earth acquired all elements aside from Hydrogen and Helium. Every bit of tungsten used in our light bulbs came from a supernova explosion, as all the uranium, gold and silver. All the iron in your hemoglobin got there through a supernova explosion, otherwise it would have remained locked into the deep interior of some star.

The most famous supernova was observed by Chinese astronomers more than one thousand years ago (see Sect. 2.2.2), its remnants are what we call the Crab nebula (Fig. 2.5). We also met another important supernova (see Sect. 3.3.1) observed by Tycho Brahe in 1572 (Fig. 9.6). In 1987 a star in our galaxy ``went'' supernova, since then we have observed the ejecta from the star and the remnant of the core (Fig 9.7. There are, of course, many known supernova remnants (see, for example, Fig. 9.9). The evolution of a middle-size star is illustrated in Fig. 9.8.

 

Figure 9.6: An X-ray photograph of the remnant of Tycho's supernova.  

 

Figure 9.7: Left: a picture of the supernova 1987A remnant (the most recent supernova in our galaxy. Right: photograph of the core.).  

 

Figure 9.8: Time and life of a star of mass between 1.4 and 3-4 times the solar mass (between about 3 × 10²⁷ and 7 × 10²⁷ tons).  

After gravity is balanced, and after the exterior shells are ejected the star stabilizes forever. But not without some fancy footwork: the remains of the star usually rotates very rapidly (up to 30 times per second!) and it also possesses a very large magnetic field. These two properties cause it to emit X-rays in a directional fashion, sort of an X-ray lighthouse. Whenever the X-ray beam goes through Earth we detect an X-ray pulse which is very regular since the star's rotation is regular. This is called a pulsar. As time goes on the rotation rate decreases and the star dies a boring neutron star. Neutron stars are very compact objects having radii of about 10 km (6 miles) so that their density is enormous, a teaspoon of neutron-star material would weigh about 10¹² (one trillion) tons on the Earth's surface.

 

Figure 9.9: A radio picture of the Cassiopeia A nebula, a supernova remnant.