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Supernovae Type II show prominent lines of
hydrogen whereas hydrogen lines are absent in
spectra of supernovae Type I. This is because
• A. White dwarfs consist mostly of hydrogen
• B. Massive stars contain a lot of hydrogen, while
white dwarfs are mostly carbon and oxygen
• C. Massive stars have burned all of their hydrogen
into heavier elements
• D. None of the above
One of the reasons for pulsars to experience glitches is
A. Nuclear burning in the neutron star
B. Nuclear burning in the white dwarf
C. Cracking of the neutron star crust as it gets more round
D. Cracking of the neutron star crust as it gets more oblate
The first pulsar was discovered by
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A. Sir Edmund Halley, in 1606
B. Galileo Galilei in 1610
C. Albert Einstein in 1905
D. a female graduate, Jocelyn Bell in 1967
The Escape Velocity Limit
• Recall that the velocity necessary to • Also recall that nothing can travel
avoid being gravitationally drawn
faster than the speed of light, c, or
back from an object (the escape
3108 m/s
velocity) is:
Vesc 
2GM
R
2G  M
RS 
c2
Mass Warps Space
• Mass warps space in its
vicinity
• The larger the mass, the bigger
“dent” it makes in space
• Objects gravitationally
attracted to these objects can
be seen as rolling “downhill”
towards them
• If the mass is large enough,
space can be so warped that
objects entering it can never
leave – a black hole is formed.
Black Holes
• It takes for a test
particle infinite time
to fall onto a black
hole. How can black
holes grow in mass?
Viewing a black hole
• You may be asking, “If light
cannot escape a black hole,
how can we see one?”
• If a black hole is in orbit
around a companion star,
the black hole can pull
material away from it.
• This material forms an
accretion disk outside of the
event horizon and heats to
high temperatures
• As the gas spirals into the
black hole, it emits X-rays,
which we can detect!
Light curves from a black hole
binary system
General Relativity
• Einstein predicted that not
only space would be
warped, but time would be
affected as well
• The presence of mass slows
down the passage of time, so
clocks near a black hole will
run noticeably slower than
clocks more distant
• The warping of space has
been demonstrated many
times, including by
observations of the orbit of
Mercury
• The slowing of clocks has
been demonstrated as well!
Gravitational Redshift
• Photons traveling away from a massive object will
experience a gravitational redshift.
– Their frequency will be shifted toward the red end of the
spectrum
If the Sun were replaced by a 1-solar mass black
hole, then the Earth would
• A. be destroyed by the gravitational force of the hole
• B. continue to orbit the black hole in precisely its
present orbit
• C. spiral quickly into the black hole
• D. head off into interstellar space along a straight line
At what location in the space around a black hole
does the escape velocity become equal to the speed
of light?
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A. at the point where escaping x-rays produced
B. at the singularity
C. at the event horizon
D. at the point where clocks are observed to slow
down by a factor of 2
Thermal radiation emitted by isolated Black Holes can be
understood in terms of
A. Nuclear burning induced by the black hole
B. Expansion of the Universe locally near the black hole
C. Effect of the gravitational field on virtual electronpositron pairs arising due to the quantum mechanical
energy uncertainty
D. Stopping of time in the vicinity of the black hole as the black
hole travels through space-time
Star Clusters
• Stars form in large groups out
of a single interstellar cloud of
gas and dust
• These groups are called star
clusters
• Open clusters have a low
density of stars – there is lots
of space between the cluster’s
members
• They can contain up to a few
thousand stars in a volume 14
to 40 light years across
• The Pleiades is a very familiar
open cluster
Globular Clusters
• Some clusters are much
more densely packed than
open clusters.
• These globular clusters
can have as many as
several million stars, in a
volume 80 to 320 light
years across!
A snapshot of stellar evolution
• Because all stars in a
given cluster formed at
the same time out of the
same cloud of material,
we can learn a lot about
stellar evolution by
examining a cluster’s stars
• We can locate each star in
a cluster on an HR
diagram and look for the
“turnoff point”, the point
on the main sequence
above which the stars in
the cluster have run out of
fuel and become red
giants
We can deduce the age of a cluster by
finding this turnoff point.
Finding a Cluster’s Age
Our Galaxy, the Milky Way
• A galaxy is a large
collection of billions of
stars
• The galaxy in which the
Sun is located is called
the Milky Way
• From our vantage point
inside the galaxy, the
Milky Way looks like a
band of stars across the
night sky, with dark dust
lanes obscuring the
center of the band.
An Early View of the Milky Way
• It is difficult to know
exactly what the Milky Way
looks like from outside the
galaxy!
– Similar to trying to figure
out what kind of car you are
in, from the inside!
• William Herschel (who
discovered the planet Uranus)
created a “map” of the Milky
Way, based on observations.
• He incorrectly placed the Sun
close to the center of the galaxy
The Shape of the Milky Way
Kapteyn’s Universe
• Jacobus Kapteyn improved on
Herschel’s view of the galaxy
• Using more modern equipment,
Kapteyn attempted to count the
number of stars in the galaxy, and
estimate their distance from the Sun
• The model was called Kapteyn’s
Universe, as the existence of other
galaxies was unknown!
• He revised the size of the galaxy to
around 18,000 parsecs (18 kiloparsecs,
or kpc), again with the Sun near the
center
• Both Herschel and Kapteyn were
correct in depicting the shape of the
galaxy as a disk, with most of the stars
lying in more or less the same plane
(the galactic plane)
Moving the Center of the Galaxy
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Harlow Shapley used observations of
globular clusters to correctly deduce the
location of the Sun within the Milky Way
He reasoned that if the Sun were at the
center of the galaxy, then globular clusters
would be found in all directions
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He noted that there were more globular
clusters found in the direction of
Sagittarius than elsewhere
Therefore, the center of the galaxy must be
in the vicinity of Sagittarius!
Today’s view of the Milky Way
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Today we know that the
Milky Way is a spiral galaxy
approximately 30 kps across.
The Sun is located around 8
kpc from the center, in one of
the spiral arms.
Most of the stars are
concentrated in the galactic
plane, or in the central bulge
at the center of the galaxy
Inside the bulge is the nucleus
of the galaxy
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Surrounding the disk is a roughly spherical distribution
of stars called the halo.
Globular clusters are distributed throughout this halo,
surrounding the center of the galaxy.
Two Stellar Populations:
Pop I and Pop II
• Astronomer Walter Baade observed an
interesting segregation between stars in
neighboring galaxies
– Younger, blue stars were found mostly in the disks
and spiral arms
• He called these “Population I” stars
• Typically less than a few billion years old
• Follow circular orbits in the galactic plane
– Older, red stars were found mostly in the halo and
central bulge
• He called these “Population II” stars.
• More than 10 billion years old
• Follow random elliptical orbits around the galactic
center – not necessarily in the plane!
• These populations are in the Milky Way, too!
Mapping the Milky Way’s
spiral arms
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Once this difference between
Population I and II stars was
noted, astronomers could map our
galaxy’s arms
Population I stars are mostly
bright, blue stars (hot O and B
stars) found in the disk
By measuring the location of O
and B stars near the Sun, the first
pictures of the Milky Way’s spiral
structure were produced.
Dust and gas obscure the light
from more distant stars, so the
map is incomplete.
The Formation of the Milky Way
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Our galaxy likely began 13 billion years
ago as a huge cloud of pure hydrogen and
helium, slowly rotating and collapsing
The first stars formed within this cloud,
burning out quickly and violently. This
added heavy elements to the cloud
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Population II stars formed next, capturing
some of the heavy elements and settling
into elliptical orbits around the center of
the cloud
As the collapse continued, a disk formed,
and Population I stars formed from the
ashes of dying Pop I stars