Download ppt

Class 18: Stellar evolution,
Part II

Evolution of a 50 M star…



Black holes.
Hypernovae.
Gamma-Ray Bursts (GRBs)…


Observational characteristics of GRBs.
Possible explanations.
I: Evolution of a 50 M star

Starts off as before…






Forms from gas cloud.
Steady H-burning (for 2-3 million years).
When hydrogen runs out, get whole
succession of nuclear reactions.
Get “onion” structure (like 10 M case).
Core collapses and drives supernova.
Core tries to form neutron star…



… but too much mass.
Neutron star crushes under its own weight.
Undergo complete collapse – get black hole.
Anatomy of a Black Hole
• Singularity – central
“point” containing all of the
mass. Known laws of
physics break down.
RSch = 3 km for the Sun.
• Event horizon – point of
“no return”. Everything
within this radius is
dragged to the singularity
by enormous gravity.
18th century ideas

Idea of an object with gravity so strong that
light cannot escape first suggested by Rev.
John Mitchell in 1783.

Laplace (1798) – “A luminous star, of the
same density as the Earth, and whose
diameter should be two hundred and fifty
times larger than that of the Sun, would not,
in consequence of its attraction, allow any of
its rays to arrive at us; it is therefore possible
that the largest luminous bodies in the
universe may, through this cause, be
invisible.”

Both space and time
are strongly distorted
in the vicinity of a
black hole.
Light seems to bend,
time slows down.

Stellar evolution: Summary

Roughly speaking,
If the star’s initial
mass is…
The star’s fate is
to become a…
< 10 M
White dwarf
 10 M & < 20 M
Neutron star
> 20 M
Black hole
Hypernovae

Possible that very massive stars may
undergo a “hypernova.”



100 more powerful explosion than a
supernova.
Can be seen across vast distances.
May be responsible for “gamma-ray bursts.”
II : Gamma-Ray Bursts

Flashes of gamma-rays discovered by
military in 1960s.




Occur about once per day.
Evenly distributed on the sky.
Last 0.01-1000 seconds.
Mysterious – until recently we…


Didn’t know how far away they are,
nor how powerful they are.

Problem




Gamma-ray
telescopes give blurry
images!
Difficult to say where
GRB was located.
Very many possible
objects located in
“error box.”
Need to pinpoint
“afterglow.”

Have recently located afterglows of
long-duration GRBs…


They are in distant galaxies!



Measurements made at X-ray, visible, and
radio wavelengths.
Must be very powerful…
100-1000 observed energy of a supernova.
Still haven’t located short-duration
GRBs…
What could cause such
powerful explosions?

Possibility I:



Hypernova of a very massive star…
… forms black hole.
Possibility II:


Collision of 2 neutron stars…
… also forms black hole.