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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?
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
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.
Type Sb
Type Sc
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.
Elliptical Galaxies:
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.
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.
Type E3
Type E2
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.
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.
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?
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 1920’s.
L
period
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 parsec=1000 Mpc)
 How do astronomers determine distances to very
distant galaxies?
Mapping the location of galaxies shows us that they live in clusters of 10’s to
1000’s 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.
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
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
don’t the individual galaxies
wander off into intergalactic
space?
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.
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!
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 !
When galaxies collide, their individual stars don’t 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.
Galactic Merger
simulation
observed
Sometimes the two galaxies pass through each other, but modify their
shapes, for example making spiral arms where none existed before.
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
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.
km/
s
velocity
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.
 Discovery of the cosmological red
shift is one of the most important
discoveries of the 20th century
Mpc
distance
Hubble Law:
v = H0 d
When v is measured in km/s and d is Mpc , H0 = 70 (km/s)/Mpc
(today’s 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 can’t 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?
Measuring the mass of Galaxies (similar to discussion of Milky Way
mass in lecture and recitation)
Use Kepler’s Third Law for a small mass (a star) orbiting a large
mass M:
P2 = a3/M
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.
Get P from v (Doppler shift): 2pa = vP,
and thus (a/v)2 ~ a3/M
or
v
a
M
so P ~ (a/v)
v2 ~ M/a or 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 / √a
Expect measured velocity to stay
about constant out to edge of
galaxy, then decrease like 1/√a
v
edge of galaxy
a0
a
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 can’t
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.