Tuesday, May 1, 2012

Sirius B - a White dwarf

Hubble Space Telescope image of Sirius A
along with its faint stellar companion, Sirius B
Hubble gallery ESA

The brightest star in the night sky is the Dog star Sirius near the great hunter Orion. Sirius is one of the closest stars to us at the distance of mere 8.6 ly. Astronomers tell that this fabulous beacon of the night (Σείριος Seirios) is actually a binary star including Sirius A and an almost invisible, tiny but very heavy companion white dwarf Sirius B.

In 1844 German astronomer Friedrich Bessel deduced from changes in the proper motion of Sirius that it had an unseen companion. Nearly two decades later, on January 31, 1862, American telescope-maker and astronomer Alvan Graham Clark first observed the faint companion, which is now called Sirius B, or affectionately "the Pup". This happened during testing of a 18.5-inch (470 mm) aperture great refractor telescope for Dearborn Observatory, which was the largest refracting telescope lens in existence at the time, and the largest telescope in America

The pair started life about 300 million years ago as a heavenly pair of bright blue stars happily burning hydrogen to helium as main sequence stars do. But alas! Old age does not come alone... As the internal fuel of Sirius B got burned out about 120 million years ago it began to swell into a huge Red giant. It then collapsed and shrank into the tiny hot ball of carbon and oxygen. Although it is only the diameter of Earth it  has the amazing mass of almost one Sun packed very densely. The disintegration probably is the source of unusual iron astronomers have noted on the surface of Sirius.

White dwarf
Wikipedia teaches us about white dwarfs (introduction for your convenience here with my emphasis - please, read the entire article from here)

A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter.

They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored thermal energy.  In January 2009, the Research Consortium on Nearby Stars project counted eight white dwarfs among the hundred star systems nearest the Sun. The unusual faintness of white dwarfs was first recognized in 1910 by Henry Norris Russell, Edward Charles Pickering, and Williamina Fleming the name white dwarf was coined by Willem Luyten in 1922.

White dwarfs are thought to be the final evolutionary state of all stars whose mass is not high enough to become a neutron star—over 97% of the stars in our galaxy.

After the hydrogen–fusing lifetime of a main-sequence star of low or medium mass ends, it will
expand to a red giant which fuses helium to carbon and oxygen in its core by the triple-alpha process.

If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, around 1 billion K, an inert mass of carbon and oxygen will build up at its center.

After shedding its outer layers to form a planetary nebula, it will leave behind this core, which forms the remnant white dwarf. Usually, therefore, white dwarfs are composed of carbon and oxygen.

If the mass of the progenitor is above 8 solar masses but below 10.5 solar masses, the core temperature suffices to fuse carbon but not neon, in which case an oxygen-neon–magnesium white dwarf may be formed.

Also, some helium white dwarfs appear to have been formed by mass loss in binary systems.

The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy, nor is it supported by the heat generated by fusion against gravitational collapse. It is supported only by electron degeneracy pressure, causing it to be extremely dense.

The physics of degeneracy yields a maximum mass for a non-rotating white dwarf, the Chandrasekhar limit—approximately 1.4 solar masses—beyond which it cannot be supported by electron degeneracy pressure.

Supernova SN 1006.
A carbon-oxygen white dwarf that approaches this mass limit, typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation. (SN 1006 is thought to be a famous example.)

A white dwarf is very hot when it is formed, but since it has no source of energy, it will gradually radiate away its energy and cool down. This means that its radiation, which initially has a high color temperature, will lessen and redden with time. Over a very long time, a white dwarf will cool to temperatures at which it will no longer emit significant heat or light, and it will become a cold black dwarf.

However, since no white dwarf can be older than the age of the Universe (approximately 13.7 billion years), even the oldest white dwarfs still radiate at temperatures of a few thousand kelvins, and no black dwarfs are thought to exist yet.

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