Monday February 15th 1999
Announcements:

Lecture notes:

Class 16
At 8 pm tonight find Polaris so facing N then turning 180 degrees so facing S.  The brightest star you'll see is Sirius which is in Canis Major.  Sirius is the brightest star in the sky (other than the sun) 8.6 ly distant.

Actually though it's not 1 star but rather a binary.
Sirius A = A star 26 L_(.)
Sirius B = white dwarf 4% L_(.) 1.1 M_(.) 5570km Radius.
Density of Sirius B is about a million times larger than things familiar to us in the solar system.

Now if you go up and to the right you'll come to 2 other bright stars....
Betelgeuse and Rigel in Orion - "the hunter"  Orion contains a great gas cloud complex in which stars are
forming - the Orion Nebula.  Betelgeuse is a fascinating star.

Red Supergiant - radius is 580 million km nearly 4 times the Earth-Sun distance.  3600 L_(.)  400 ly away 20 M_(.)
in a fairly advanced state of nuclear burning but we can't tell how far along.  Probably has onion skin configuration of nuclear burning as we have discussed.  Will probably go supernova in the next 10 000 years or so.  When it does it will be as bright as the moon.

In the last class we discussed deaths of massive stars.  Fuse elements up to iron but then core collapse and explode as type 2
supernovae (bright as entire galaxy)

outer layers blasted into space at about 10 000 km/s - > supernova remnant.  Easy to find even if we didn't see the
supernova - glow in X-rays and radio.

Inner core of star collapses -> neutron star.  1.4 M_(.) 10km radius

•  dead star - no fusion.
•  gravitationally bound ball of neutrons
•  magnetized and rapidly rotation - pulsar phenomenon
• radio pulses - also X and gamma rays
•  about 800 neutron stars found this way.

Offset magnetic axis leads to "lighthouse" beams of radiation.  Spin with periods ranging from a few s to a few ms.
e.g. Crab pulsar - spins at 30 times/sec

Can also detect neutron stars another way - X-ray binaries.
Some are in interacting binaries undergoing mass transfer.  Mass falls off a normal star when it swells due to stellar evolution.
Also called accretion.

Called X-ray binary since such systems produce prodigious X-ray emission.

The neutron star has a disk of matter near it that is falling on to it - accretion disk.

Inner disk temperature is up to millions of degrees - so hot it makes mostly X-rays (also matter crashes into surface at about
1/3 c)

These X-ray binaries have huge phenomenology.

• Some burst
• Some fire out jets
• Some pulse
• Some eclipse

Now on to black holes
So far we have talked about 2 kinds of dead stars - white dwarfs and neutron stars.  3rd type is even more extreme - can summarize in table.
 

Mass Remaining after star's life	Final Result	Support against gravity
<1.4 M(.)	White Dwarf	Electrons
1.4 - 3 M(.)	Neutron Star	Neutrons
>3 M(.)	Black Hole	nothing!

Nothing can hold the star up.  Shrinks to size of a speck of dust and even smaller.

All the time surface gravity is getting stronger and stronger since the star still weighs several M_(.)

Not even light can escape from this collapsing object (hence the name black hole) once its size gets small enough (about 3
km/M_(.) - due to the enormous surface gravity.

As far as we know collapse continues until we can't use the current laws of physics to describe it any more - singularity.

While the star itself collapses to a point we take the effective surface of the black hole to be the radius from which light
cannot escape -- called the Schwarzschild radius - about 3 km/M_(.)

Events going on inside this radius cannot affect our Universe since they can't escape.  A 10 M_(.) black hole has a Schwarzschild radius 10 M_(.)[3 km/M_(.)] = 30 km.

Not cosmic vacuum cleaners - don't have a magic ability to suck in matter only gravity.  Far away the gravitational force is essentially the same as that for a star of the same mass.

It's only when you get very close to a black hole within 10 Schwarzschild radii do you notice the extreme gravity.

How do we find black holes?
Since not even light can escape it would seem that they would be hard to find.
- We don't detect them directly but rather by their gravitational influence on other objects.
- Some black holes will be in binary systems and will be X-ray binaries.  We detect them by the X-rays they produce.

An example is Cygnus X-1 - found in Uhuru X-ray Sattelite survey of 1970s
Black hole accretes via normal mass transfer and probably also via stellar wind.
X-rays not made by black hole but rather by gas near to it.

Scientists used Newton's law of gravity and other arguments to show that they x-ray emitting object had too large a mass to be a neutron star.

Today we have found solid evidence for about 8 stellar mass black holes in our galaxy and expect hundreds to thousands exist.