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.

Type II Supernovae

As massive red supergiants age, they produce "onion layers" of heavier and heavier elements in their interiors. However, stars will not fuse elements heavier than iron. Fusing iron doesn't release energy. It uses up energy. Thus a core of iron builds up in the centers of massive supergiants.

Eventually, the iron core reaches something called the Chandrasekhar Mass, which is about 1.4 times the mass of the Sun. When something is this massive, not even electron degeneracy pressure can hold it up.

The core collapses. Two important things happen:

Protons and electrons are pushed together to form neutrons and neutrinos. Even though neutrinos don't interact easily with matter, at densities as high as they are here, they exert a tremendous outward pressure.

The outer layers fall inward when the iron core collapses. When the core stops collapsing (this happens when the neutrons start getting packed too tightly -- neutron degeneracy), the outer layers crash into the core and rebound, sending shock waves outward. These two effects -- neutrino outburst and rebound shock wave -- cause the entire star outside the core to be blow apart in a huge explosion: a type II supernova. Supernovae are really bright -- about 10 billion times as luminous as the Sun. Supernovae rival entire galaxies in brightness for weeks. They tend to fade over months or years.

During the supernova, a tremendous amount of energy is released. Some of that energy is used to fuse elements even heavier than iron! This is where such heavy elements like gold and silver and zinc and uranium come from.

The collapsed core is also left behind by a type II supernova explosion. If the mass of the core is less than 2 or 3 solar masses, it becomes a neutron star. If more than 2 or 3 solar masses remains, not even neutron degeneracy pressure can hold the object up, and it collapses into a black hole.

Nucleosynthesis

Nucleosynthesis is the process of creating new atomic nuclei from preexisting nucleons. It is thought that the primordial nucleons themselves were formed from the quark-gluon plasma from the Big Bang as it cooled below two trillion degrees. A few minutes afterward, starting with only protons and neutrons, nuclei up to lithium and beryllium were formed but only in relatively small amounts. This first process of primordial nucleosynthesis may also be called nucleogenesis. The subsequent nucleosynthesis of the elements occurs primarily in stars, either by nuclear fusion or nuclear fission.

There are some astrophysical processes which are responsible for nucleosynthesis in the universe. The majority of these occur within the hot matter inside stars. The successive nuclear fusion processes which occur inside stars are known as hydrogen burning , helium burning, carbon burning, neon burning, oxygen burning and silicon burning, which we studied last week. These processes are able to create elements up to iron and nickel, the region of the isotopes having the highest binding energy per nucleon. Heavier elements can be created within stars by a neutron capture process known as the s process or in explosive environments, such as supernovae. Another important important process is the r process, which involves rapid neutron captures, the rp process, which involves rapid proton captures, and the p process, which involves photodisintegration of existing nuclei.

The transmission through th Coulomb barrier decreases drastically with increasing nuclear charges. So charged particle cross sections are too small at moderate stellar temperatures to explain the observed solar system abundances of nuclides with masses beyond $A=60$.

The gross properties of the solar system abundances in the $A>60$ range can be accounted for in terms of two extreme pictures, that is, by relatively low neutron densities achieved in the s-process and by the high neutron exposures characteristic of the r-process. Intermediate exposures between these two extremes seem to play only a minor role for the solar system abundance distribution.