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Physics of the cosmos: star death and black holes

What happens when stars die?

In Chapter 20 Nuclear Physics, we learnt that stars are fueled by the nuclear fusion of hydrogen to form helium deep in their interiors. The outflow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight, and the energy by which it shines.

When a star has fused all the hydrogen in its core, nuclear reactions cease. Deprived of the energy production needed to support it, the core begins to collapse into itself and becomes much hotter. Hydrogen is still available outside the core, so hydrogen fusion continues in a shell surrounding the core. The increasingly hot core also pushes the outer layers of the star outward, causing them to expand and cool, transforming the star into a red giant.

If the star is sufficiently massive, the collapsing core may become hot enough to support more exotic nuclear reactions that consume helium and produce a variety of heavier elements up to iron. However, such reactions offer only a temporary reprieve. Gradually, the star’s internal nuclear fires become increasingly unstable – sometimes burning furiously, other times dying down. These variations cause the star to pulsate and throw off its outer layers, enshrouding itself in a cocoon of gas and dust. What happens next depends on the size of the core.

Helix Nebula, glowing expelled gases from a dying star
Helix Nebula, glowing expelled gases from a dying star

Average Stars Become White Dwarfs For average stars like the Sun, the process of ejecting its outer layers continues until the stellar core is exposed. This dead, but still ferociously hot stellar cinder is called a White Dwarf. White dwarfs, which are roughly the size of our Earth despite containing the mass of a star, once puzzled astronomers – why didn’t they collapse further? What force supported the mass of the core? Quantum mechanics provided the explanation. A phenomenon called electron degeneracy prevents further collapse. White dwarfs are intrinsically very faint because they are so small and, lacking a source of energy production, they fade into oblivion as they gradually cool down.

This fate awaits only those stars with a mass up to about 1.4 times the mass of our Sun. Above that mass, electron degeneracy cannot support the core against further collapse. Such stars suffer a different fate as described below.

Supernovae Leave Behind Neutron Stars or Black Holes Stars over eight solar masses are destined to die in a titanic explosion called a supernova. In a supernova, the star’s core collapses and then explodes. In massive stars, a complex series of nuclear reactions leads to the production of iron in the core. Having achieved iron, the star has wrung all the energy it can out of nuclear fusion – fusion reactions that form elements heavier than iron actually consume energy rather than produce it. The star no longer has any way to support its own mass, and the iron core collapses. In just a matter of seconds the core shrinks from roughly 5000 miles across to just a dozen, and the temperature spikes 100 billion degrees or more. The outer layers of the star initially begin to collapse along with the core, but rebound with the enormous release of energy and are thrown violently outward. Supernovae release an almost unimaginable amount of energy. For a period of days to weeks, a supernova may outshine an entire galaxy. Likewise, all the naturally occurring elements and a rich array of subatomic particles are produced in these explosions as stardust. On average, a supernova explosion occurs about once every hundred years in the typical galaxy. About 25 to 50 supernovae are discovered each year in other galaxies, but most are too far away to be seen without a telescope.

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The Crab Nebula is a pulsar wind nebula associated with the 1054 supernova

Neutron Stars. If the collapsing stellar core at the center of a supernova contains between about 1.4 and 3 solar masses, the collapse continues until electrons and protons combine to form neutrons, producing a neutron star. Neutron stars are incredibly dense – similar to the density of an atomic nucleus.

Black Holes If the collapsed stellar core is larger than three solar masses, it collapses completely to form a black hole: an infinitely dense object whose gravity is so strong that nothing can escape its immediate proximity, not even light. Since photons are what our instruments are designed to see, black holes can only be detected indirectly. Indirect observations are possible because the gravitational field of a black hole is so powerful that any nearby material – often the outer layers of a companion star – is caught up and dragged in. As matter spirals into a black hole, it forms a disk that is heated to enormous temperatures, emitting copious quantities of X-rays and Gamma-rays that indicate the presence of the underlying hidden companion.

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Simulated view of a black hole in front of the Large Magellanic Cloud.

If you are interested in black holes, you can watch the video below (The Largest Black Holes in the Universe) for more information! However, you might encounter certain terms being used, so attached are brief explanations of some of the technical terms used.

 

Glossary

Quasar A Quasar, or quasi-stellar radio source, is a very energetic and distant active galactic nucleus. They exhibit high redshift amount of electromagnetic energy, including radio waves and visible light. There is now scientific consensus that a quasar is a compact region in the center of a massive galaxy surrounding its central supermassive black hole.
Supernova A supernova is a stella explosion that results when the production of energy at the core of a star via nuclear fusion is suddenly turned on or off.
Chandra X-ray observatory A space based telescope, the third of NASA’s four great satellite observatories – the first was Hubble Space Telescope; second the Compton Gamma Ray Observatory, and last the Spitzer Splitzer Space Telescope. X-ray telescopes do not work on Earth as the Earth’s atmosphere absorbs the vast majority of X-rays.
Solar Mass The solar mass, kg is the standard unit of mass in astronomy, used to indicate the masses of stars and other galaxies in relation to the mass of the Sun.
Accretion Disc An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of electromagnetic radiation. The frequency range of that radiation depends on the central object. Accretion discs of young stars and protostars radiate in the infrared; those around neutron stars and black holes in the x-ray part of the spectrum.
Event Horizon In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman’s terms it is defined as “the point of no return” i.e. the point at which the gravitational pull becomes so great as to make escape impossible.

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