Transcript Lecture 11

Lecture 11
6/25/07
Astro 1001
White Dwarfs
• A White Dwarf is the exposed
core of a star that has died and
shed its outer layers
• Very small radius, but lots of
mass
– Large amounts of degeneracy
pressure needed to counter
gravity
• The most massive White
Dwarfs are the smallest
The Chandrasekhar Limit
• At 1.4x the mass of the Sun,
gravity overpowers electron
degeneracy pressure
• A type Ia Supernova occurs
if too much mass is added
after the White Dwarf forms
• A nova may occur as
material is dumped onto the
White Dwarf
Neutron Stars
• Remains of a massive star
supernova
• Supported by neutron
degeneracy pressure
– About 10 kilometers in
diameter
• The structure of a neutron
star is somewhat uncertain
– Probably contains a crust and
then a sea of neutrons
Pulsars
• Pulsars are neutron stars that
emit a very regular signal
• As the massive star went
supernova, it contracted and
strengthened its magnetic
fields
• The first extrasolar planets
were discovered around a
pulsar
Neutron Star Binaries
• Immense gravitational field means that lots of
potential energy is released by anything falling
onto the neutron star
• X-Ray Binaries occur when matter is regularly
accreted onto the neutron star
Black Holes
• Sometimes no pressure can
stop gravity from
collapsing a star
• The event horizon is the
point of no return
• Black Holes appear to
make information be
irretrievable
– Can only measure the BH’s
mass, charge, and angular
momentum
Group Work
• The Sun is not massive enough to form a
Black Hole. However, lets say that by some
mysterious process it suddenly collapses to
form a Black Hole of exactly 1 solar mass.
What would happen to Earth’s orbit after
the Sun became a Black Hole?
Visiting a Black Hole
• As you approach the black hole,
time slows down and you
experience a gravitational
redshift
• Whether or not you fall into the
black hole depends on who is
telling the story
• Tidal forces are 1 trillion times
as strong as the one that causes
the tides
Do Black Holes Really Exist?
• Theoretically, Black Holes
must form at 2-3 solar
masses
• You can detect Black
Holes by looking for XRay sources
• Strong evidence for
supermassive black holes
at the center of galaxies
Gamma Ray Bursts
• In the 60s, we began to
detect intense bursts of
Gamma Rays
• In the 90s it was discovered
that the sources were
evenly distributed across
the sky
• Since then, the Bursts have
been traced to massive
explosions in distant
galaxies
What Causes GRBs?
• If the energy was emitted in
all directions, the energy
would be millions of times
that of an entire galaxy
– Energy is probably beamed
• At least some GRBs are
associated with supernovae
• Two types of GRBs: short and
long
– Short bursts do NOT appear to
come from supernovae
The Milky Way
• A faint band goes
across the sky
– Galileo showed that
the band was made
of individual stars
• We are inside of the
galaxy, so its hard to
see what the overall
structure of it is
Structure Basics
• We live in a spiral galaxy
• Has spiral arms in a flat
disk
• In the center is a bulge of
stars
• The outskirts of the
galaxy are called the Halo
• Also a series of nearby
galaxies
Orbits of Stars
• Disk stars go around the center of the galaxy
– Also oscillate above and below the disk
• Halo and bulge stars move around randomly
– Can be very far away from the disk
• We can look at the orbits of stars to figure out the mass
of the galaxy
Galactic Recycling
• Stars dump their
processed material into
the ISM as they die
– Also create cosmic rays
• Gravity drags the gas
together and cools it
• Eventually large gas
clouds are formed, from
which stars can form
Where Do Stars Form?
• Stars don’t form uniformly
in the galaxy
• Stars like to form in the
spiral arms of galaxies
– We say that the arms appear
“blue” while other parts
appear “red”
• Spiral density waves are
probably responsible for
this
Galaxy Formation
• The galaxy formed from a Protogalactic
Cloud in a way similar to how stars form
• There may have been multiple clouds
• Or, many Milky Way stars originally
formed in other cannibalized galaxies
The Galactic Center
• The galactic center lies
in the constellation
Sagittarius
• Probably a black hole 3
million times the mass
of the Sun
– Sgr A*
• Not much matter
appears to be accreted
by the black hole
Other Galaxies
• There are lots of other
galaxies out there
– Over 100 billion in the
observable universe
• Galaxies come in many
different shapes and
sizes
• All galaxies appear to
have formed at the same
time
Types of Galaxies
• Spiral Galaxies
– Like our own galaxy
– Relatively rare
– Might be Lenticular (no
spiral arms)
• Elliptical Galaxies
– Red and round
– Often football shaped
• Irregular Galaxies
– Strange shapes
Elliptical Galaxies
• Small ellipticals are the
most common type of
galaxy
• Usually contain very little
gas or dust
• Large ellipticals are
probably the result of
smaller galaxies being
absorbed
– Contain lots of hot gas and
dust
The Hubble Tuning Fork
• Hubble came up with a system to classify
galaxies
Distances to Galaxies
• Standard Candles
– If we know how bright something is and how bright it
appears, we can figure out how far away it is
• Main Sequence fitting
– Done using the Hyades as an example
• Cepheid Variables
– Historically important
– Period-Luminosity relationship
• Type Ia Supernovae
Cepheid Variables
• Pulsating stars that vary in brightness
• How long they take to repeat their pattern
announces how bright they are
• Used by Hubble to determine how far away
galaxies are
Type Ia Supernovae
• The exact same conditions occur for every
Type Ia Supernova
– A star of exactly 1.4 solar masses goes through
the exact same process
• Supernovae are very luminous, so you can
use this method to determine the distance to
very distant galaxies
Group Work
• A typical Type Ia supernova has a
luminosity of about 1 x 1045 watts. Lets say
that we observe a supernova that appears to
be 5 x 10-15 watts. How far away is it?
Express your answer in meters and in
lightyears (example on page 623).
Hubble
• Shapley-Curtis debate was
unable to resolve whether or
not galaxies were island
universes or part of our own
galaxy
• Hubble used a new 100 inch
telescope to resolve
individual stars in
Andromeda
– Noticed Cepheid variables
– Was a bit off, but close enough
Hubble’s Law
• Hubble realized that the
further away a galaxy
was, the more redshifted
it was
– V = H0 x d
• A few caveats:
– Galaxies do not obey the
law exactly since they
might have speeds not
associated with the
expansion of the universe