10th Week: Supernovae and Nucleosynthesis

Supernovae

At the end stage of their evolution stars with masses $0.07M_{\odot }<M<8M_{\odot }$ run out of fuel in their core and form a so called white dwarf star. Depending on the masses of the stars, the newly formed white dwarf could be made of helium in the core ($M<M_{\odot }$) and known as a helium white dwarf. And if the mass of main sequence is in the range of $0.5M_{\odot }<M<8M_{\odot }$, then the carbon oxygen white dwarf will be formed. For star masses of $8M_{\odot }<M<10M_{\odot }$ no white dwarf is formed, since the core fuses neon to iron and the star is no more supported by the electron degeneracy pressure due to high mass of the iron core that exceeds the so called Chandrasekhar limit, a limit that gives the stability of white dwarf star against gravitational collapse. These stars further contract and eventually the gravity is balanced out with the neutron degeneracy pressure and the stars formed are called neutron stars. For more massive main sequence stars the gravitational binding energy becomes sufficient to overcome the neutron degeneracy pressure and star is eventually formed into a black hole.

The supernovae explosion occurs at the end of a star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. If star is not very massive the explosion is due to white dwarf binary system and if the star is very massive, then its core will collapse and in so doing will release a huge amount of energy. This will cause a blast wave that ejects the star's envelope into interstellar space.

When a supernova is observed, it can be categorized according to the absorption lines that appear in its spectrum. A supernovae is first categorized as either a Type I or Type II, then sub-categorized based on more specific traits. Supernovae belonging to the general category Type I lack hydrogen lines in their spectra; in contrast to Type II supernovae which do display lines of hydrogen. The Type I category is sub-divided into Type Ia, Type Ib and Type Ic supernovae.

Type Ib/Ic supernovae are distinguished from Type Ia by the lack of an absorption line of singly-ionized silicon. As Type Ib/Ic supernovae age, they also displays lines from elements such as oxygen, calcium and magnesium. In contrast, Type Ia spectra become dominated by lines of iron.

Type Ia Supernovae

One model for the formation of this category of supernova is a close binary star system. The larger of the two stars is the first to evolve off the main sequence, and it expands to form a red giant. The two stars now share a common envelope, causing their mutual orbit to shrink. The giant star then sheds most of its envelope, losing mass until it can no longer continue nuclear fusion. At this point it becomes a white dwarf star, composed primarily of carbon and oxygen. Eventually the secondary star also evolves off the main sequence to form a red giant. Matter from the giant is accreted by the white dwarf, causing the latter to increase in mass. If a carbon-oxygen white dwarf accreted enough matter to reach the Chandrasekhar limit of about 1.38 solar masses, it would no longer be able to support the bulk of its plasma through electron degeneracy pressure and would begin to collapse. Within a few seconds, a substantial fraction of the matter in the white dwarf undergoes nuclear fusion, releasing enough energy (Failed to parse (syntax error): {\displaystyle 1–2 \times 10^{51}} ergs) to unbind the star in a supernova explosion. An outwardly expanding shock wave is generated, with matter reaching velocities on the order of 5,000–20,000 km/s, or roughly 3% of the speed of light. There is also a significant increase in luminosity, reaching an absolute magnitude of -19.3 (or 5 billion times brighter than the Sun), with little variation.

Type Ib/Ic Supernovae

A type 1b supernova occurs when a star has almost depleted it supply of hydrogen except most of its outer layers have been lost to either heavy solar winds or interaction with a sister star.

Type 1b supernova are believed to be the result of the collapse of a Wolf-Rayet star (an incredibly massive star, about twenty or more solar masses in size). It is believed that they could release gamma ray bursts. Little else is known however.

Type 1c supernova occur in a similar way to a type 1b. Type 1c supernova are known as hypernova and occur in stars where the core mass exceeds the Tolman-Oppenheimer-Volkoff limit, which is believed to be about four solar masses.

The core begins to implode letting off streams of gamma rays at its poles.

If the stars mass is over a certain limit (roughly twenty-five to fifty solar masses) the star will implode into a black hole. Otherwise a neutron star will form.

Type Ib has no silicon lines at maximum light and is about 1 - 2 magnitudes fainter than Ia's. helium lines are present and probably due to helium detonation on a carbon-oxygen core.

Type Ic have no helium lines and also no silicon lines at maximum light.