13.8

When it is applied to describe anything less than an entire universe,c such as the nature of the orbit of Mercury about the Sun, this is really an approximation, although the approximation can be made as accurate as you like

The difficulty in making these observations is highlighted by the fact that the apparent brightness of these stars, as seen from Earth, is less than one-billionth of the apparent brightness of the faintest stars visible to the naked eye.

This gives an age for the disc of the Milky Way of 9.3 billion years, give or take a billion years or so.

that the oldest white dwarves in the disc of the Milky Way have been cooling off for 9 billion years.

eight times that of the Sun explodes as a supernova and leaves behind a neutron star, in which rather more mass than there is in the Sun is squeezed into ball only a few kilometres across (about the size of Mount Everest)

so the ages of globular clusters should be a bit less than the age of the Universe, defined as the time since the Big Bang. The globular clusters in our Galaxy, the Milky Way, are spread out in a spherical halo that surrounds the Milky Way itself, which is a flattened, disc-shaped congregation of stars.

All of this contributes to our understanding of the origin of the elements in our own bodies.

nuclear burning described in the previous chapter, ejecting more material (stellar ash) into space either through relatively gentle processes or, in the case of stars with more than about eight solar masses, ending their lives in violent explosions, as supernovae, scattering heavy elements through the Galaxy and leaving behind a tiny, dense star, known as a neutron star.

For a star with up to about four solar masses, this is the end of the line. When helium burning comes to an end, the star shrinks into a white dwarf, initially very hot but gradually cooling down into a dense cinder.

temperature of 4,000 K, but can stay on the main sequence for 200 billion years. A star with three times the mass of the Sun will be five times as bright as the Sun, with a surface temperature of 7,000 K, but will only stay on the main sequence for 3 billion years. And a star with 25 times the mass of the Sun will be 80,000 times brighter than the Sun, have a surface temperature of 35,000 K, and use up all the hydrogen fuel in its core in just 3 million years.