The Death of a Star

Approaching Death The star has fused the hydrogen in the core. The core begins to collapse. More stellar material begins to fall into the core This results in further core heating.

Shell Burning The hydrogen lying just above the core begins to fuse. This shell burning causes the enormous expansion of the outer layers of the star. The star has now become a red giant

Red Giants Typically between 0.4 & 8 Solar Masses A star of around the sun’s mass will expand to a diameter around the size of the Earth orbit. A star of this mass cannot hold on to its outer material and this is lost into space. Other material falls onto the core. This leaves a highly dense core remnant. The remnant is a White Dwarf.

White Dwarfs The white dwarf is super dense. Density 109kgm3 Electron degeneracy pressure is the only force holding the electrons from being squeezed into the nuclei. A white dwarf has the diameter of a terrestrial planet.

Supernovas For very large stars the no outer material is lost and matter falling onto the core increases the pressure until the electron degeneracy pressure is exceeded and electrons are forced into nuclei. This produces a massive rebound shockwave which blows the star apart as a supernova.

Pulsars The core remnant of a supernova is a neutron star or pulsar. These objects are incredibly dense squeezing over 2.5 solar masses into a diameter of around 20km. They have intense magnetic fields which focus electromagnetic radiation into beams at the poles, They spin rapidly.

Neutron stars can look like ordinary faint stars through an optical telescope. Or if they are recent we may see the expanding shell of gas of the supernova explosion they were created in. Gamma rays from the Vela pulsar They are spinning rapidly and produce regular directional pulses in the radio spectrum. Many are strong X ray or gamma sources

The intense magnetic field of the pulsar concentrates the electromagnetic radiation into two focussed beams emitted from the poles (shown in green). We receive regular pulses if we are in the path of the beam as it sweeps.

Black Holes When super massive stars collapse the neutrons collapse. A central infinitely dense SINGULARITY is produced. There is a region surrounding the singularity from which light cannot escape. The EVENT HORIZON is the boundary of the black hole

Detecting Black Holes Black holes are surrounded by Accretion disks which emit X-rays. These accretion disks can produce jets of material at right angles. Black holes bend light which passes close to them. This gravitational lensing can be observed when a distant galaxy lies behind a black hole.

>10 Ms Red Giant Red Giant Red Giant supernova supernova White dwarf Pulsar Black hole

109kgm3 20km 30km About Earth Size Typical diameter of remnant 4 x 1017kgm3 >1.4Ms >10Ms Neutron Star (infinite at singularity) 109kgm3 Density of remnant >2.5 Ms <1.4Ms Mass of core remnant ~ 40Ms >0.8Ms Typical mass of original star Black Hole White Dwarf