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Units to cover: 70, 72,73,74, 75, 76, 78
Homework
Unit 70, Problems 17, 19
Unit 72, Problem 18
Unit 73, Problem 18
Unit 75, Problems 12, 17, 18 19
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.
The Formation of the Milky Way
•
•
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
•
•
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
Galactic Cannibalism
• There are a few observations that
are not explained by this model
– Some stars follow unusual orbits
in the galaxy
– Not all Pop II stars are the same
age
– Model predicts that the first stars
might not have been very massive,
and should still be around!
• Galactic cannibalism provides
some answers
– The Milky Way may be absorbing
another galaxy!
– Observations show streams of
stars coming from our galaxy’s
“victim”.
Composition of Interstellar Clouds
•
•
•
Light passing through an
interstellar cloud can hold
clues as to the cloud’s
composition
Atoms in the cloud absorb
specific frequencies of
starlight passing through,
creating absorption lines
Astronomers can analyze
these spectra to determine
what the clouds are made of.
• Spectra show that interstellar gas clouds are made
of mostly hydrogen and helium, just like the Sun
• Dust particles do not absorb light the same way
that gas atoms do, but using similar methods tells
us that the dust is made of silicates
Interstellar Reddening
•
•
As starlight passes through a dust cloud,
the dust particles scatter blue photons,
allowing red photons to pass through
easily
The star appears red (reddening) – it
looks older and dimmer (extinction) than
it really is.
The Galactic Center and Edge
• Despite the appearance of being
closely spaced, stars in the Milky
Way are very far apart
– At the Sun’s distance from the
center, stellar density is around 1
star per 10 cubic parsecs
• Density is much higher at the core
– Exceeds 100,000 stars per cubic
parsec!
• X-ray and gamma ray telescopes
reveal a supermassive black hole at the
Milky Way’s core
– Called Sag A*
– 5 million solar masses!
Sag A*
Star Formation in Spiral Arms
A History of Galactic Discovery
• In the early 20th century, the existence
of other galaxies was unknown
– The Milky way was the Universe!
– Other galaxies were called nebulae
• Light from galaxies always appears
fuzzy and diffuse, due to the vast
separation between the Sun and the
observed galaxy, as well as the
separation between the stars of that
galaxy!
– The paleness of visible light from distant
galaxies is called the surface brightness.
• Galaxies are therefore difficult to
observe, even with good telescopes.
More History…
• In the 1700’s, Charles Messier was
observing comets, and kept finding objects
that while fuzzy, were not comets
– He made a list (or catalog) of these undesired
objects, so he could avoid seeing them
– They became known as Messier Objects, a
number preceded by an M.
– M31 (the Andromeda galaxy) is one such object
• William and Caroline Herschel (1800’s)
developed a catalog of faint objects in the
heavens
– Now known as the New General Catalog
– Objects are known by a number preceded by the
letters NGC
– Objects can appear in both the Messier and
NGC catalogs!
M31
A Sky Full of Galaxies
• Technology has
advanced to the
point where we
have found as many
galaxies as there are
stars in the Milky
Way!
• Note the gap
running along the
zero latitude line
– Called the zone of
avoidance
– Puzzled
astronomers!
The Zone of Avoidance
Dust and the center of our own galaxy merely blocks our view –
there is no zone of avoidance!
Distances to other galaxies
•
•
•
•
We can use Cepheid variable stars
to measure the distance to other
galaxies
A Cepheid’s luminosity is
proportional to its period, so if we
know how rapidly it brightens and
dims, we know much energy it
emits
If we see a Cepheid in another
galaxy, we measure its period,
determine its luminosity, and
calculate its distance!
Distance between galaxies is huge!
– M100 is 17 million parsecs away.
The Sun’s position in the galaxy is
•
•
•
•
A. unknown
B. in the disk of the galaxy
C. in the spherical halo of the galaxy
D. in the galactic nucleus
Distances to other galaxies
•
•
•
•
We can use Cepheid variable stars
to measure the distance to other
galaxies
A Cepheid’s luminosity is
proportional to its period, so if we
know how rapidly it brightens and
dims, we know much energy it
emits
If we see a Cepheid in another
galaxy, we measure its period,
determine its luminosity, and
calculate its distance!
Distance between galaxies is huge!
– M100 is 17 million parsecs away.
Spiral Galaxies
• Spiral arms and a central bulge
• Type S
Elliptical Galaxies
•
•
•
•
No spiral arms
Ellipsoidal shape
Smooth, featureless appearance
Type E
Irregular Galaxies
• Stars and gas clouds scattered
in random patches
• No particular shape
• Type Irr
Galaxy collision and merger
The Mice
• These two interacting galaxies are tidally distorting
each other.
Quasars
• Quasars are small, extremely
luminous, extremely distant galactic
nuclei
– Bright radio sources
– Name comes from Quasi-Stellar
Radio Source, as they appeared to be
stars!
– Can have clouds of gas near them, or
jets racing from their cores
– Spectra are heavily redshifted,
meaning they are very far away
– Energy output is equivalent to one
supernova going off every hour!
• The HST was able to image a
quasar, showing it to be the active
core of a distant galaxy
Energy Source for Active
Galactic Nuclei
•
•
•
•
•
Active galactic nuclei emit a
tremendous amount of radiation
over a broad range of
wavelengths
A black hole can be both very
small, and have an accretion disk
that can emit enough radiation
Likely that at the centers of these
galactic nuclei, there are
supermassive black holes
Intense magnetic fields in the
accretion disk pump superheated
gas out into jets that leave the
nucleus
There are still many questions to
be answered…
The Redshift and Expansion of the Universe
•
•
•
Early 20th century astronomers
noted that the spectra from
most galaxies was shifted
towards red wavelengths
Edwin Hubble (and others)
discovered that galaxies that
were farther away (dimmer)
had even more pronounced
redshifts!
This redshift was interpreted as
a measure of radial velocity,
and it became clear that the
more distant a galaxy is, the
faster it is receding!
The Hubble Law
• In 1920, Edwin Hubble
developed a simple
expression relating the
distance of a galaxy to its
recessional speed.
• V=Hd
– V is the recessional
velocity
– D is the distance to the
galaxy
– H is the Hubble Constant
(70 km/sec per Mpc)
• This was our first clue that
the universe is expanding!
Which two quantities are shown to be related to one
another in Hubble Law?
•
•
•
•
A. distance and brightness
B. distance and recession velocity
C. brightness and recession velocity
D. brightness and dust content
Large Scale Structure in the Universe
• Using modern technology,
astronomers have mapped the
location of galaxies and
clusters of galaxies in three
dimensions
• Redshift is used to determine
distance to these galaxies
• Galaxies tend to form long
chains or shells in space,
surrounded by voids
containing small or dim
galaxies
• This is as far as we can see!
How are galaxies spread through the Universe?
• A. They are grouped into clusters that in turn are
grouped into clusters of clusters (superclusters)
• B. Galaxies are spread more or less evenly throughout
the Universe
• C. They are grouped around our galaxy
• D. none of the above
Seyfert Galaxies
• Seyfert galaxies are
spiral galaxies with
extremely luminous
central bulges
• Light output of the
bulge is equal to the
light output of the
whole Milky Way!
• Radiation from
Seyfert galaxies
fluctuates rapidly in
intensity
Radio Galaxies
• Radio Galaxies emit large
amounts of energy in the
radio part of the spectrum
• Energy is generated in two
regions
– Galactic nucleus
– Radio lobes on either side
of the galaxy
• Energy generated by
energetic electrons
– Synchrotron radiation
– Electrons are part of the gas
shooting out of the core in
narrow jets