Lecture 18: Evolution of High Mass Stars

High Mass Stars: Mass > 8 M_Sun

The cores of the high mass stars are so large than when the fuel is burnt out, they cannot be stabilized even by denegerate pressure, and collpase further. High mass stars can have many successive stages of fusion of an element in a core and lighter elements in shells around the core exhaustion of the element core collapse and heating fusion of higher mass elements etc... The general trend is for the star's surface to become cooler and to become a blue giant and later a red supergiant. But there are several oscillations from red (super)giant to blue giant and back phase, correspondent to ignition of the next, heavier, fuel in the core. Star loses signifcant mass during super giant stage

The "Onion-skin" layers of a Supergiant Star

Iron is the End-Point of Nuclear Reactions Under normal circumstances iron doesn't fuse in a star. The strong nuclear force which binds nuclei together is a short range force, so there is a limit to how large a nucleus can be. Iron is the most stable element. Two types of nuclear reactions in nature: Fusion reactions: light elements are fused (or glued) together to form heavier elements. These reactions are exothermic (release energy) as long as the reactants are lighter than iron. In order to fuse iron into a heavier element, energy would have to be supplied. Fission reactions: heavy elements are are split into lighter elements. (Eg. reactions in nuclear reactors which use Uranium) These reactions are exothermic if the reactants are heavier than iron.

The Iron Catastrophe

• When a star develops an iron core, an energy crises occurs, since no more exothermic nuclear reactions are possible.

• However, the core is also degenerate, so no heat source is needed to keep it stable.

• The quantum mechanical degeneracy pressure keeps the core from collapsing.

• The core is essentially an "iron white dwarf".

• But... white dwarf stars aren't always stable.

• If the mass of the white dwarf in the core is larger than 1.4 M_Sun (called the Chandrasekhar limit) the electrons would have to move faster than the speed of light in order to create enough degeneracy pressure to halt the gravitational collapse.

• Electrons can't move faster than light, so a white dwarf with M > 1.4 M_Sun collapses!

• Main sequence stars with mass larger than about 8 M_Sun eventually form white dwarf stars with masses larger than the Chandrasekhar limit and collapse.

• This is the beginning of a Core Collapse Supernova also known as a Type II Supernova

A Supernova in a star with 8 M_Sun < M < 20 M_Sun

• When the supernova begins the iron core collapses rapidly under free-fall and becomes denser.

• When the density is very high, protons and electrons can combine together to form neutrons and neutrinos:
  p + e^- -> n + nu

• This reaction is called inverse beta decay.

• The neutrinos escape easily since they don't interact well with matter and carry off energy.

• The resulting neutron gas collapses until the density is extremely high.

• Neutrons obey the quantum mechanical Pauli exclusion principle. When the density becomes higher than about 10¹⁴ g per cubic cm, neutron degeneracy pressure provides an outward pressure which suddenly halts the gravitational collapse.

• The core of neutrons held stable by neutron degeneracy pressure is called a neutron star.

• The outer layers are still collapsing inwards at this point and these collapsing layers collide with the hard surface of the newly formed neutron star.

• This collision causes a violent rebound and a shock wave bouces outwards, colliding with the outer layers of the star.

• This expanding wave carries an extraordinary amount of energy.

• This energy can provide the fuel which allows the endothermic fusion reactions to create very high mass elements such as Uranium. The supernovae are responsible for all the elements with masses larger than iron found on Earth.

• These supernova explosions are extremely bright. Also large amount of energy is carried away by neutrinos

Supernova 1987a

• Core Collapse Supernovae probably occur about once every 50 years in our galaxy, but most of them are hidden by the dust of the galaxy.
• In 1987 a supernova occurred in the nearby galaxy called the Large Magellenic Cloud.

The B3 I star before the supernova	The supernova explosion

A Supernova in a star with M > 20 M_Sun

• The evolution of very massive stars is similar with the formation of a neutron star at the core in a supernova.

• However, neutron stars (like white dwarfs) have a maximum mass near 3 M_Sun, over which neutron degeneracy pressure can't balance gravity.

• In the very high mass stars, the neutron star goes over the critical mass, and the neutron star collapses.

• No other sources of pressure are available, and the collapsing material forms a black hole.

• Black holes have a "surface" called an event horizon where the force of gravity is so strong that the escape velocity is larger than the speed of light. No light can escape from a black hole, hence its name. (More about black holes next week!)

White Dwarf Stars in Binary Systems can also go Supernova

A close binary system In any binary system, there is a point between the stars called the Inner Lagrangian Point. At the Lagrangian point, the forces acting on a particle exactly cancel out. A Roche Lobe is an imaginary surface surrounding the stars that goes through the Lagrangian point. If a star is large enough to fill up its Roche lobe, matter can flow from the star to the other star. A White Dwarf in a Semi-Detached Binary Suppose that a white dwarf is receiving mass from a companion star. The White dwarf's mass will slowly increase. If it receives enough mass, the White Dwarf's mass will approach the Chandrasekhar limit and collapse. The collapse causes the degenerate Carbon gas in the White dwarf to begin fusing together explosively. This type of supernova is essentially a giant Carbon bomb!

The Two Types of Supernovae

Type Ia: Carbon-bomb caused by a white dwarf accreting material. Source of energy is nuclear fusion reactions. Type II: Core Collapse of a Supergiant Star Source of energy is gravity. There are also Type Ib and Ic , but they are core collapse as Type II are

• Note that our Sun can never go supernova!
• The Sun's mass isn't large enough to become a supergiant star, so it can't undergo a Type II supernova explosion.
• Our Sun will only become a White Dwarf star in the future.
• Since our Sun is not in a binary system, once it becomes a white dwarf, it won't accrete matter and will not undergo a Type Ia supernova explosion.