Stellar Physics

10 lectures, exploring the development of cosmology, and some of the key ideas of Big Bang theory

Access PPT slides athttp://www.astro.gla.ac.uk/users/martin/teaching/aberdeen.ppt

Dr Martin Hendry

Dept of Physics and AstronomyUniversity of Glasgow

martin@astro.gla.ac.uk

25000 10000 8000 6000 5000 4000 3000

Surface temperature (K)

O5 B0 A0 F0 G0 K0 M0 M8

Lum

inos

ity (S

un=1

)

Spectral Type

1

102

104

106

10-2

10-4

-10

-5

0

+5

+10

+15

Abs

olut

e M

agni

tude

0.001 RS

0.01 RS

0.1 RS

1 RS

10 RS

Main Sequence

White dwarfs

Supergiants

1000 RS

100 RS

Giants

We can plot the temperature and luminosity of stars on a diagram

Stars don’t appear everywhere: they group together, and most are found on theMain Sequence

25000 10000 8000 6000 5000 4000 3000

Surface temperature (K)

O5 B0 A0 F0 G0 K0 M0 M8

Lum

inos

ity (S

un=1

)

Spectral Type

1

102

104

106

10-2

10-4

-10

-5

0

+5

+10

+15

Abs

olut

e M

agni

tude

. . . . .. . . .

..

...

. ....

.... .. .. .

.....

..............

.........

... ....... ....

........

. .. .

...........

.. ..

..

............ .

..

....Regulus

Vega

Sirius A

Altair Sun

Sirius B

Procyon B

Barnard’sStar

Procyon A

..

.. ... Aldebaran

Mira

Pollux

Arcturus

RigelDeneb

Antares

BetelgeuseStars on theMain Sequence turn hydrogen into helium.

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 Dwarfs2. Neutron Stars3. 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 SupernovaType Ia SupernovaWhite 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 Will the Universe continue to expand forever?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 expansion is accelerating

The geometry of the Universe is FLAT

The Universe will continue to expand indefinitely

Cosmological Constant?Quintessence?

What is driving the cosmic acceleration?…What is driving the cosmic acceleration?…

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 (launched 1999): high-resolution X-ray map of theGalactic Centre

Chandra has revealed many more X-ray binary sources in the Milky Way, globular clusters and external galaxies.

XRB’s: How do we get so much energy out?

Need something approaching E = mc2

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

But if 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 at radius

Radiation force Thomson cross-section229 m1065.6

rMGML

~acc

r

Pm

crLF T

2rad 4

Radiation force reduces the effective gravitational force to

We can write this as

where the critical, or Eddington, luminosity is

Putting in some numbers we find that

which is close to the maximum observed

OO

LMML 4

crit 103~

crL

rGMmF TP

22grav 4

crit2grav 1

LL

rGMmF P

T

PcGMmL

4crit

XL

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 UniverseGravity in Einstein’s Universe

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

Gravity in Einstein’s UniverseGravity in Einstein’s Universe

“Spacetime tells matter how to move, and matter tells spacetime how to curve”

Gravity in Einstein’s UniverseGravity in Einstein’s Universe

vDifferences between Newtonian and Einsteinian

gravity are tiny, but can be detected in the Solar System – and Einstein always wins!

Gravity in Einstein’s UniverseGravity in Einstein’s Universe

v

1. Precession of orbits – observed for Mercury, matching GR prediction

Gravity in Einstein’s UniverseGravity 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 UniverseGravity in Einstein’s Universe

‘Ultimate’ case of light deflection = ‘Black Hole’: warps spacetime so much that light can’t escape

Pres

sure

, P

Density,

N.R. Electron degeneracy pressure

Rel. Electron degeneracy pressure

3/5P

3/4P

N.R. Proton degeneracy pressure3/5P

Rel. Proton degeneracy pressure

3/4P

Lines of central Pressure, constant mass3/4cP

A

B

CDE

Evidence for stellar black holes from binary systems: e.g. Cygnus X-1

Inferred mass far exceeds OV limit