Download Document

Stellar Physics
Dr Martin Hendry
10 lectures, exploring the
Dept of Physicsof
andcosmology,
Astronomy
development
University of Glasgow
and some of the key ideas
of
Big Bang theory
[email protected]
Access PPT slides at
http://www.astro.gla.ac.uk/users/martin/teaching/aberdeen.ppt
Surface temperature (K)
25000
10000
8000 6000
5000 4000 3000
106
-10
We can plot the
temperature and
luminosity of stars
on a diagram
Supergiants
-5
102
0
Giants
1
+5
10-2
+10
10-4
+15
O5 B0
A0
F0
G0
Spectral Type
K0
M0
M8
Absolute Magnitude
Luminosity (Sun=1)
104
Stars don’t appear
everywhere: they
group together,
and most are
found on the
Main Sequence
Surface temperature (K)
25000
106
10000
8000 6000
.
5000 4000 3000
. .
.
.
.. .
. .
. ..
.
. ..
..
.
...
.. . ....
..
. .. ..... ... .
. ..
.
... . . ... ........ .
.. .... .
.. .
. .....
......
......
.. ..
..... .
......
. .
...
. .
. ... ...
..
Deneb
-10
Stars on the
Main Sequence
turn hydrogen into
helium.
Rigel
Betelgeuse
Antares
Luminosity (Sun=1)
Arcturus
102
Aldebaran
Regulus
Vega
Procyon A
Altair
10-2
Pollux
Sun
Procyon B
O5 B0
+5
+10
Barnard’s
Star
Sirius B
10-4
0
Mira
Sirius A
1
-5
A0
F0
G0
Spectral Type
K0
M0
M8
+15
Absolute Magnitude
104
Blue stars are much
hotter than the
Sun, and use up
their hydrogen in a
few million years
Observational Evidence for
Compact Objects
1. White Dwarfs
2. Neutron Stars
3. Black Holes
White Dwarfs
Small but very luminous
(because of high T)
Can be directly observed
Important Type of White
Dwarf for Cosmology:
Type Ia Supernovae
Excellent for measuring
cosmological distances –
good “Standard Candle”
Type Ia Supernova
White dwarf star with a massive binary companion. Accretion
pushes white dwarf over the Chandrasekhar limit, causing
thermonuclear disruption
Good standard candle because:Narrow range of luminosities at peak brightness;
Observable to very large distances
Will the Universe continue
to expand forever?
To find out we need to compare the expansion
rate now with the expansion rate in the distant
past…
Is the Universe speeding
up or slowing down?
Answer depends on the geometry of the
Universe
Closed
Open
Flat
We can measure this using
Type Ia Supernovae
Results:
The geometry of the Universe is FLAT
The Universe will continue to expand
indefinitely
The expansion is accelerating
What is driving the cosmic acceleration?…
Cosmological Constant?
Quintessence?
Neutron Stars
Very much smaller: (almost)
invisible at optical, but can
be seen in X-Rays if their
surfaces are very hot
Crab Nebula: supernova of 1054
There exist large numbers of compact objects in binary
systems. These are powerful emitters of X-rays, many
sources are concentrated near the Galactic plane.
X-Ray Binaries:
compact source orbiting a massive star
Chandra has revealed many more X-ray binary sources in the Milky Way,
globular clusters and external galaxies.
Chandra (launched 1999): high-resolution X-ray map of the
Galactic Centre
XRB’s: How do we get so much energy out?
2
Need something approaching
E = mc
Gravitational energy from accretion
For how long might we expect such an X-ray binary source to shine?...
Suppose we could completely annihilate a source of, say,
So if we want a source lifetime of, say,
we would need to
extract around 10% of the source’s rest mass energy (same efficiency
would give longer lifetime for a less luminous source)
Is this realistic?
Energy source believed to be gravitational infall (accretion) of matter
onto a neutron star from a binary companion.
Energy yield / unit mass
Matter falls in via an accretion disk.
Some orbital angular momentum is lost by viscous friction.
XRB luminosity comes from disk as well as the central source.
Accretion Luminosity and the Eddington Limit
If matter accretes at rate
then we expect, at radius
Lacc
But if
r
GMM
~
r
is large, the accretion process becomes self-limiting, because
the emitted luminosity exerts a significant radiation pressure force on the
infalling material.
Consider a proton of mass
mP
at radius
Radiation force
Frad
LT

4  r 2c
Thomson cross-section
 6.65 1029 m2
Radiation force reduces the effective gravitational force to
Fgrav
We can write this as
Fgrav

GMmP
LT

2
r
4  r 2c
GMmP

r2

L 
1 

L
crit 

where the critical, or Eddington, luminosity is
Putting in some numbers we find that
Lcrit
Lcrit 
4  GMmP c
T
 M 
  3 104 LO
~ 
 MO 
which is close to the maximum observed
LX
Pulsars
Discovered by Jocelyn
Bell, in 1965.
Pulsars
Discovered by Jocelyn
Bell, in 1965.
Extremely accurate
‘clocks’
Rapidly rotating NS,
with beams of radiation
Pulsars
Synchrotron radiation
Pulsars
Observe:
High spin rate
High B field
Electron acceleration
Binary neutron stars
Very strong gravity
provides a test of GR.
Advance of periastron,
Production of GWs
Source of GRB’s?
Gravity in Einstein’s Universe
Matter causes space
to curve or warp
Gravity and acceleration are completely equivalent:
both cause spacetime to become curved or ‘warped’
Gravity is not a force propagating through space and time, but
the result of mass (and energy) warping spacetime itself
Einstein’s Relativity
Gravity in Einstein’s Universe
“Spacetime
matter
how
Matter tells
causes
space
to move,
and
matter
tells
to curve or warp
spacetime how to curve”
Gravity in Einstein’s Universe
v
Differences between Newtonian and Einsteinian
gravity are tiny, but can be detected in the Solar
System – and Einstein always wins!
Gravity in Einstein’s Universe
v
1. Precession of orbits –
observed for Mercury,
matching GR prediction
Gravity in Einstein’s Universe
v
1. Precession of orbits –
observed for Mercury,
matching GR prediction
2. Bending of light close
to the Sun – visible
during total eclipse,
measured in 1919
Gravity in Einstein’s Universe
‘Ultimate’ case of light deflection = ‘Black Hole’:
warps spacetime so much that light can’t escape
Lines of central Pressure, constant mass
Pc   4 / 3
Rel. Proton
degeneracy pressure
4/3
P
N.R. Proton degeneracy pressure
5/3
P
Rel. Electron degeneracy pressure
4/3
E
D
C
B
P
N.R. Electron degeneracy pressure
P   5/3
A
Density,

Evidence for stellar black holes from
binary systems: e.g. Cygnus X-1
Inferred mass far exceeds OV limit