Gravity and star death

Stellar observation in Sinai

The Crab Nebula in Taurus – the remnants of a supernova explosion recorded in China in 1054

Stars are thermonuclear fireballs so massive that as the energy is being blasted out, the core is collapsing under its own gravitational weight. For millions or billions of years, the two forces are balanced in a fearsome equilibrium.

Sirius, our brightest star, is in such a state of tension between gravitational collapse and thermonuclear wipe-out. It’s called a Main Sequence star because of its position on a scientist’s graph comparing mass with luminosity.

A Main Sequence star will survive until the hydrogen that fuels the blast is depleted and gravitational collapse overwhelms the star.

But total collapse is arrested when the core gets so dense that new fusion reactions are ignited which fuse helium into carbon and oxygen. The outer shells of incandescent gas then balloon out to form a bigger – but cooler – star, known as a red giant.

Betelgeuse in Orion is such a red giant but so turbulent that its brightness is seen to change erratically.

When the fuel is finally exhausted, core collapse sets in again, leading to final extinction of the star.

For a star of mass similar to our sun, the outer shells of gas are blown out into a ring while the core collapses into a degenerate dwarf. A peculiar epithet for a dead star but scientific jargon describing atoms in which electrons have been squeezed into their lowest energy states. For a time, the energy from the dying core causes the gas ring to fluoresce and it’s known as a planetary nebula since it make you think of planets ringing a star.

For a large star like Betelgeuse, the collapse is so catastrophic that the final fireball lights up the sky like a million stars and the energy released is enough to fuse atoms into uranium, a supernova explosion. The core is either impacted into a neutron star in which electrons are forced into the nuclei of their atoms and the gas blown into a tenuous nebula, or else the whole thing is so immense that gravity sucks it all into a Black Hole.

This blog first appeared as an article for the Cairo BCA magazine in February 2011 in an attemp to interest readers in star gazing in Sinai with an 8″ Meade telescope.

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