Astronomy 101

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Astronomy 101
Lecture 23, Apr. 23 2003
Normal Galaxies (Chapter 24 in text)
Our galaxy, the Milky Way, has a mass of 1011
suns. We see many more such galaxies in the
universe, about 40 billion in all. We are
not unique!!
The nearby Andromeda galaxy is similar to ours
and has pronounced spiral arms. The somewhat
more distant NGC2997 shows a similar structure,
seen here face on.
Others, like M49, are elliptical in shape.
Still others are quite irregular in shape.
What are the types of galaxies? How are they
distributed in space? How do they form and
evolve?
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Spiral galaxies
Spiral galaxies have a basic disk shape with the
spiral arms in the flat disk. There is a central
core containing the bulk of the stars, visible
here as bright hubs (99 of the light) from which
the spirals radiate. And like the Milky Way,
there is a sparsely populated halo of
stars. Spirals come with differing structures of
arms ranging from very tightly wound (type Sa) to
rather loosely wound (type Sc). Milky Way is
typical in size for spirals about 30 kpc across
the disk.
Type Sa
Type Sb
Type Sc
The arms appear bluish white due to the young,
bright O and B stars that produce most of the
light (recall that O, B stars have a short Main
Sequence lifetime and very large luminosity. So
the spiral arms are young.
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Spiral galaxies
Seen edge on, the central bulge is apparent. The
Sombrero galaxy spiral arms are seen edge-on as
a dark band, due to the presence of gas and dust,
characteristic of star-forming regions.
The central bulge is largest for Type Sa and
smallest for Type Sc. The Type Sc galaxies have
the largest concentrations of gas and dust.
Barred spiral galaxies also exist, with similar
variations in tightness of spiral arms and size
of central bulge (Types SBa, SBb, SBc). The
spiral arms project from a central bar rather
than a central ellipsoid.
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Elliptical Galaxies
Type E3
Galaxies without spiral arms are seen with large
variations in size ranging from giant
ellipticals with more than 1012 solar masses (10
times more massive than Milky Way) and a few Mpc
(Mega parsecs) across, to dwarf ellipticals with
a few million solar masses and 1 kpc across.
There are about 10 times as many dwarf
ellipticals as giants. Elliptical galaxies are
labelled E1, E2, E7, according to their
eccentricity (departure from spherical) with E1
being nearly spherical.
Type E2
The elliptical galaxies contain very little dust
and gas, so are not actively making new stars.
They resemble the halo of the Milky Way and other
spirals.
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Irregular galaxies
Many galaxies have rather non-uniform structure
and are called Irregulars. The most famous are
the Large and Small Magellenic Clouds that are
the nearest neighbor galaxies to the Milky Way
(about 50 kpc from the center of our galaxy).
Seen from the Southern Hemisphere, they resemble
bright clouds several degrees across, and clearly
visible to the naked eye.
Magellenic clouds as seen by eye
The Large Magellenic Cloud contains the famous
supernova SN1987a seen via neutrinos on earth in
1987.
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Irregular galaxies
Some irregulars show signs of violent activity or
disruption by collisions. There are many young O
and B stars present and much gas and dust,
indicating that star formation is going on. Quite
possibly, these irregulars are the result of
collisions between galaxies, leaving the
fragmented remains behind.
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Edwin Hubble, who first discovered galaxies
external to ours, provided the classification of
their types used today. The tuning fork
diagram was thought perhaps to be evidence for
evolution of galaxies from one type to another
but this seems incorrect. There is no evidence
for evolution of spirals into ellipticals or vice
versa.

• How do the different types of galaxies differ in
  the amount of dust, gas and young stars?
• How do spiral arms, central bulge and halo
  differ?

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Mapping the galaxies and the large scale
structure of the universe requires methods for
measuring distances deep into space.
All the deep space distance measurements rely on
knowing the true brightness or luminosity of an
object by some means, and obtaining distance from
measuring the apparent brightness on
earth Apparent brightness Luminosity /
distance2 or L 4pd2 Iapp
1. Cepheid variable stars have Period of
oscillation that depends on Luminosity. Allows
distance measurement to 25 Mpc (to galaxies in
our neighborhood). It was Cepheid variables that
enabled Hubble to deduce that there were galaxies
outside our own in the 1920s.
L
period
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2. Tully Fisher method relates the rotational
velocity of a galaxy to its absolute luminosity.
Use Doppler shift of the 21 cm (radio) spectral
line from atomic hydrogen. Radio emission from
opposite sides of galaxy are shifted oppositely
(to red or blue), broadening the line. Works to
about 200 Mpc.

3. Type Ia supernovae have a fixed peak
luminosity (they come from 1.4 solar mass carbon
stars) so are standard candles. Can be used for
distance measurements out to 1 Gpc. (1 Giga
parsec1000 Mpc)

• How do astronomers determine distances to very
  distant galaxies?

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Mapping the location of galaxies shows us that
they live in clusters of 10s to 1000s of
galaxies, bound together by their gravitational
attraction. Probably the clusters reflect the
primordial clumping of matter in the very early
universe.
Coma cluster (all but the bright blue spot are
galaxies)
Magnified view shows even more galaxies in a
small region of space.
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Our Milky Way galaxy is part of the Local Group
a cluster of 45 galaxies. Milky Way and
Andromeda are the two largest galaxies in Local
Group the rest are bound by gravity to Milky Way
or Andromeda, and the two portions are
gravitationally bound rather like a binary star
system. Local group is about 2 Mpc across.
Andromeda
Milky Way
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Virgo cluster, about 18 Mpc from us, is much
larger than the Local Group 2500 galaxies in a
space about 3 Mpc across.

• What holds the clusters of galaxies together
  why dont the individual galaxies wander off into
  intergalactic space?

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Clusters are found in superclusters
associations of thousands to a million individual
galaxies. Our local supercluster, containing the
Local Group, Virgo cluster and many others, is 50
Mpc across.
View of the universe from our vantage point
And even these superclusters have structure
bands of rich clusters such as the Great
Wall Gravity binds these structures
together. On the largest scale, the universe
is far from uniform.
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As far as we can see into deepest space (here
with Hubble telescope in satellite), we see
clusters of galaxies !
We started this course with Ptolemaic idea that
earth is at the center of the universe. Then with
Copernicus thought the universe centered on the
sun.
Then with Shapley, we learned the sun is near the
periphery of the Milky Way. Now Hubble tells us
that our Milky Way is not at the center, and is
just one of billions of galaxies. We are pretty
insignificant!
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Galactic collisions and mergers
Galaxies are not as isolated as stars
Andromeda and Milky Way are separated by about
1000 kpc and are about 30 kpc across the ratio
of separation to diameter is about 30.
Magellenic clouds are much closer to Milky
Way. Like two basketballs on opposite sides of a
court (remember there are 45 basketballs and
baseballs in the Local Group!) Sun and Alpha
Centauri (nearest star to us) are separated by
about 4.3 light years diameter of sun is 1.4
million km. Ratio of separation to diameter of
stars in our neighborhood is nearly 10
million. Like two basketballs, one in New York
and the other in California So, relatively
speaking there is a lot more empty space between
stars than between galaxies.
Thus, collisions between stars are very rare, but
collisions of galaxies happen much more often !
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When galaxies collide, their individual stars
dont bang into each other, but the gravitational
forces tend to disrupt the initial galaxies.
Galaxy collisions have been studied in
supercomputer simulations to see what results.
Sometimes the colliding galaxies can stick
together to form a larger one. Sometimes
the two galaxies pass through each other, but
modify their shapes, for example making spiral
arms where none existed before.
Galactic Merger simulation observed
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Cosmological red shift
18 Mpc
When looking at the very distant galaxies (out to
1000 Mpc) away and measure their velocities
relative to us using the Doppler shift of
spectral lines Hubble made an amazing discovery
The more distant the galaxy (known from standard
candles), the larger the red shift. This implies
that galaxies are moving away from us with speeds
that increase with distance from us.
230
330
600
940
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The distant galaxies are all receding from us
with a velocity that increases in proportion to
their distance (Hubble Law) v H0 d H0 is
called the Hubble constant.
A plot of velocity and distance for some
galaxies. The increase in v with d is clear, but
there is some scatter due to proper motion of a
galaxy relative to its neighbors.
km/s
velocity

• Discovery of the cosmological red shift is one
  of the most important discoveries of the 20th
  century

Mpc
distance
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Hubble Law v H0 d When v is
measured in km/s and d is Mpc , H0 70
(km/s)/Mpc (todays best measurement of Hubble
constant book has H0 65 (km/s)/Mpc) Does
this mean that our Milky Way has bad breath and
all galaxies are rushing away from us? No, we now
understand that the universe as a whole is
expanding so that every galaxy is receding from
every other galaxy! For the most distant objects
in the universe, we can use the Hubble expansion
to estimate distance measure the red shift to
get the velocity of recession and calculate
distance d v/H0

• For the furthest objects in the universe, we
  cant use supernovae, Cepheid variable, Tully
  Fisher to get distance. Explain how the
  cosmological red shift can be used to estimate
  distance? How is H0 determined?

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Measuring the mass of Galaxies (similar to
discussion of Milky Way mass in lecture and
recitation)
Use Keplers Third Law for a small mass (a star)
orbiting a large mass M P2 a3/M
v
a
P is period, a is semimajor axis (radius of
orbit) and M is the total mass inside the orbit
(we neglect the small mass of the orbiting star
compared to the rest of the galaxy.
M
Get P from v (Doppler shift) 2pa vP, so P
(a/v) and thus (a/v)2 a3/M or v2
M/a or v v(M/a) Ifinside the galaxy disk,
mass increases as we go further out in radius
with M a, expect v constant. If outside
the galaxy, expect mass is all inside the orbit
radius a M is a constant as orbit size
increases, expect v 1 / va
Expect measured velocity to stay about constant
out to edge of galaxy, then decrease like 1/va
v
edge of galaxy
a0
a
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We see no decrease in velocity v beyond the
visible edge of the galaxy ! It seems that
there is matter outside the visible stuff, that
is exerting a gravitational force.
There is Dark Matter around all galaxies! The
total amount of dark matter is about 50 times
what we see in visible stars, and about 5 times
the total mass of gas, dust and stars. Its not
burnt out stars it is mostly not neutrinos we
cant understand it as a bunch of black holes.
approximate visible edge of galaxies
Its something new! It may well be new forms of
matter that can be found in labs on earth soon.