Transcript High Mass Stellar Evolution

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 Msolar) 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 Msolar then a white
dwarf is formed.
If the mass of the core is > 1.4 Msolar then a neutron
star is formed.
If the mass of the core is > 3.0 Msolar 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 Msolar 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
4×1017 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.