::Interactive Astronomy::

4. Stellar Death of Low Mass Stars

Planetary Nebula

The ageing star goes through various fluctuations in luminosity and ejects shells of glowing hot gas. Eventually, all that remains of the low mass star is a fiery, exposed core, engulfed in glowing shells of ejected gas. This is the late stage of the life of a star and is called a planetary nebula.

Over time, the gas shells expand as radiation from the star pushes them further out and dissipate far away from the rapidly cooling star to be observed.

White Dwarfs

White Dwarfs are formed by stars that are less than 4 solar masses. A cooling white dwarf does not shrink as the burnt-out core is so dense that its electrons are degenerate and prevents further collapse. This also results in an interesting property- the more massive a white dwarf, the smaller its radius. However, there is a limit to this known as the Chandrasekhar limit, which states that all white dwarfs must have solar masses of less that 1.4 solar masses.

Inside the white dwarf, there are mainly ionized carbon and oxygen atoms amidst a sea of degenerate electrons, which slow down as the dead star cools. Ions arrange themselves in rows and degenerate electrons move around freely in this solid crystallized carbon, thus the white dwarf in fact is an immense spherical diamond!

Over time, the surface temperature and luminosity decreases and cool over a few billion years as the surface temperature heads for absolute zero. The star fades away to become a black body.