Stellar Evolution

Life Cycle of Stars

Post-Main Sequence Reactions

Red Giant to Horizontal Branch After helium burning begins, a star has two sources of energy, hydrogen fusion in a shell around the core and helium fusion in the core The core of the star becomes rich in carbon and oxygen nuclei, and the star's surface temperature goes up to become a horizontal branch star Stars with masses greater than or equal to the Sun become smaller and hotter at a constant luminosity. They evolve to horizontal branch stars by moving across the HR diagram at constant brightness

AGB Stars Once the helium in the core of the star is exhausted, a helium burning shell will develop beneath the hydrogen burning shell The electrons in the core again become degenerate and the star expands and cools to become an asymptotic giant branch star Most of the energy is coming from the hydrogen burning shell. However, the hydrogen shell is dumping helium ash onto the helium shell. After sometime, enough helium is built up so that the helium shell undergoes an explosive event A wind develops in the star's envelope which blows the outer layers into space. It is in this wind that dust particles are formed.

Planetary Nebula Phase An asymptotic giant branch star becomes more luminous and the rate at which it loses mass increases For stars less than 8 solar masses, a strong stellar wind develops and the outer layers of the star are removed to expose the hot degenerate core As the gas is expelled and the core is visible, the color of the star becomes much bluer and moves to the left in the HR diagram The star begins to emit large quantities of UV radiation, ionizing the hydrogen shell of matter. This shell of ionized hydrogen is a planetary nebula and the core is a white dwarf

White Dwarfs White dwarf stars are much smaller than normal stars, such that a white dwarf of the mass of the Sun is only slightly larger than the Earth.

White Dwarf Evolution Once a white dwarfs contracts to its final size, it no longer has any nuclear fuel available to burn. However, a white dwarf is still very hot from its past as the core of a star The white dwarf cools by radiating its energy outward. As a white dwarf cools, the ions can arrange themselves in a organized lattice structure when their temperature falls below a certain point. The white dwarf will eventually give up all its energy and become a solid, crystal black dwarf.

Summary of Stellar Evolution Low Mass Stars (~Msun) High (>8 Msun)Mass Stars

Life After Death for White Dwarfs The best explanation for novae is surface fusion on a white dwarf. White dwarfs no longer have any hydrogen to burn in a fusion reaction. A white dwarf in a binary system ‘steals’ extra hydrogen from its companion by tidal stripping. Hydrogen gas will build up on the surface of the white dwarf where the surface gravity is extremely high. The hydrogen outer shell will reach the point where fusion can begin and the shell explodes in a burst of energy.

Nova in a Binary System

Nova in Hercules

Post Main Sequence

Supernova 1994D

Supernova 1987A Supernova 1987A--The two large rings are not yet completely understood, though they appear to be associated with the supernova. The rings result from something that the star did before it became a supernova, probably associated with strong stellar winds expected in such stars.

Possibilities for High Mass Stars

Ends for Various Mass Stellar Remnants

Neutron Stars A neutron star is a star that is composed solely of degenerate neutrons. The mass of a star is squeezed into a small enough volume that the protons and electrons are forced to combine to form neutrons. For example, a star of 0.7 solar masses would produce a neutron star that was only 10 km in radius. Their extremely small size implies that they rotate quickly, according to the conservation of angular momentum.

Neutron Stars The interior of a neutron star consists of neutrons packed into such a dense state that it becomes a superfluid sea This dense mixture of neutrons (with zero electric charge) can become a friction-free superfluid at high temperatures The interior of a neutron star will consist of a large core of mostly neutrons with a small number of superconducting protons. These superconducting protons, combined with the high rotation speeds of the neutron star, produce a dynamo effect which gives rise to an enormous magnetic field

Pulsars A powerful magnetic field, combined with the rapid rotation, will produce strong electric currents on the surface of the neutron star. Loose protons and electrons near the surface of the neutron star will be sweep up and stream along the magnetic field lines The magnetic axis of the neutron star does not necessarily have to be aligned with the rotation axis (like the Earth), they can be inclined from each other Only when the Earth lies along the axis of the neutron star is the energy detected as a series of pulses, and the object is called a pulsar.

Black Hole Terms Rs—Schwarzschild radius Event Horizon—Schwarzschild Radius—distance beyond which no event can be seen since light cannot escape Photon Sphere—distance at which light “orbits” a black hole

Frame-Dragging in Black Holes Black holes are gravity wells that can not only draw mater in but can spin it as well. This effect, called frame-dragging, is most prominent near massive, fast spinning objects. Matter in this system gets caught up and spun around the black hole. Such discoveries help scientists better understand gravity itself.

Detection of Neutron Stars and Black Holes

Black Holes at Galactic Centers? Recent results by astronomers using the Hubble Space Telescope now indicate that most - and possibly even all - large galaxies may harbor a black hole. In all the galaxies studied, star speeds continue to increase closer the very center. This indicates a center millions of times more massive than our Sun is needed to contain the stars. This mass when combined with the limiting size make the case for the central black holes.