High Mass Stellar Evolution Astrophysics Lesson 13

Learning Objectives To know:-  How high mass stars evolve.  The defining properties of supernovae, neutron stars and black holes.

Homework  To complete the practical mock exam paper by next Friday.

A Short Life…  High mass stars (> 8 M solar ) have more fuel but they use it up much more quickly than low mass stars (higher luminosity).  So they only “burn” for millions of years on the main sequence, instead of billions of years.

Fusion up to Iron The core to shell burning process goes beyond helium fusion and for really massive stars up to iron. Fusing iron nuclei does not release energy (as iron is the most stable element.)

A Spectacular Death…  The radiation pressure rapidly decreases so gravity wins and a rapid collapse occurs until the radius of the inner core reaches about 30 km.  Further collapse is stopped by strong force interactions and the degeneracy pressure of neutrons  The infalling matter, rebounds off the core producing a shock wave that propagates outward  SUPERNOVA

Supernova Remnant

What’s left?  If the mass of the core is < 1.4 M solar then a white dwarf is formed.  If the mass of the core is > 1.4 M solar then a neutron star is formed.  If the mass of the core is > 3.0 M solar then a black hole is formed.  Important: Note that these values are for the mass of the core left over – not the initial mass of the star.  Note: The 1.4 M solar limit is known as the Chandrasekhar limit.

White Dwarfs

Neutron Stars  The core gets so dense that it overcomes the electron degeneracy pressure.  Electrons are squashed onto the atomic nuclei and combine with protons to form neutrons  A density is reached when the repulsive force of the neutrons is sufficient to stop the collapse of the stellar core  neutron degeneracy pressure.

Neutron Stars Neutron stars are incredibly dense (about 4×10 17 kgm -3 ). They are small (diameter 20 km) and can rotate up to 600 times a second. They emit radio waves in two beams as they rotate, which can sometimes be observed from the Earth  these are known as PULSARS.

Neutron Star (Puppis A)

Black Holes  There is no known force in nature that can stop the collapse of cores greater than 3 solar masses.  The collapse continues until the core contracts to an infinitely dense point known as a singularity.  Even light cannot escape from the core within a certain radius called the Schwarzschild Radius.

Black Holes

Defining Properties Supernova: Rapid increase of absolute magnitude  Neutron Stars: Composed of neutrons with a density similar to that of atomic nuclei. Black Holes: The escape velocity > speed of light within the Schwarzschild Radius.

Relative Sizes Typically white dwarfs are about the size of the Earth. Neutron stars have a radius of about 10 km. Black holes are even smaller.

Summary Diagram

Black Hole Applet Instructions: open the applet. Click anywhere within the frame and then click anywhere on the bottom left control panel to activate the controls. Along the bottom of the frame is a scale with a blue circle showing how far you are from the center of the black hole. The three line segments extending from the circle show the current field of view. Your left and right keyboard arrows should change the distance, and your up and down arrows should change the viewing angle. If not, the blue circle can be dragged to a new distance, and the middle line segment can be dragged to change your viewing angle. The location "3 M" is the Schwarzschild Radius, inside of which light (and you) cannot escape. Experiment with different distances and viewing angles to see how space appears to be distorted by the bending of light around the black hole.