Galaxies-I
By the 1700’s the old notion that the Earth was the
center of the Universe was overthrown by the success
of Newton’s theory of universal gravitation, a theory
which explained the motion of the planets around the
Sun.
It became respectable to see the stars as other suns, like our
Sun, but scattered through space, with the possibility of other
planets around those suns and even intelligent life on those planets.
And so the question arose: where is our Sun in this universe of stars?
Answering this question required 200 years, roughly from 1750
to 1950. To answer it, astronomers started from careful
observations of the distribution of stars and star clusters in
the night sky.
The Milky Way is a hazy
band of light visible to the
naked eye in the evening sky.
In a telescope it resolves into
clouds of stars.
In 1750 Thomas Wright suggested
that the existence of the Milky Way
proves that the stars are not
randomly distributed, but distributed
in a flattened disk with the Sun off
to one side.
In 1785 William Herschel counted
stars in 683 regions of the sky and
used these counts to estimate the size
of the disk and the direction to the
center.
Milky Way
Scutum star cloud
Sagittarius star clouds
horizon
Herschel’s picture of the system of stars (1785).
Direction of
Sagittarius
star clouds
Herschel’s estimated position
of the Sun.
Illustration by Wright showing
stars confined to a plane (1750).
The system of stars which contains
our Sun is now called the Galaxy
•A composite of 77 photographs showing details of the star clouds in the Milky
Way. By the early 1900’s the dark areas were understood to be obscuring clouds
of dust and gas. This source of interstellar absorption invalidated the star counts
Herschel (and later astronomers) used to estimate the size of the Galaxy.
This is a more recent color mosaic of the Milky Way created by an
amateur astronomer.
•The distribution of
star clusters helps us
understand the structure
of our galaxy.
•Open clusters are a
type of star cluster that
typically contain 100 to
1000 stars.
They are found where
stars are forming out of
the interstellar medium.
•The Pleiades in Taurus
(photo from Bok, 1976)
NorthAmerican nebula
The Milky Way in Cygnus.
Open clusters, yellow circles on
this chart, are concentrated along
the Milky Way. Gaseous nebulae,
in green, are also found
concentrated along the Milky
Way.
“Veil nebula” supernovae remnant
An example of a gaseous nebula is the “North American nebula,”
NGC 7000, near Deneb in Cygnus. Note the evidence of dark clouds by
comparing star counts in the region on the right to the region on the left.
Star clusters condense
out of the dust and gas
between the stars.
HII regions are gaseous
emission nebula glowing
from the ultraviolet
light from hot young
stars, the red color
is distinctive
Globular clusters, in contrast
to open clusters, are concentrated
in one part of the Milky Way.
This picture of Sagittarius star
clouds contains one-third
of all known globulars in an
area less than 3% of the sky.
•A typical globular cluster
has 500,000 stars.
The distribution of
open clusters in the
sky as seen from
Earth, in a coordinate
system aligned with
the Milky Way. The
center (marked by a
cross) is in Sagittarius.
The horizontal line
is the plane of the Milky
Way.
North
Galactic
Pole
South
Galactic
Pole
North
Galactic
Pole
The distribution of
the globular clusters in
the sky. They are
strongly concentrated
in the direction of
Sagittarius.
South
Galactic
Pole
Harlow Shapley
proposed, circa
1918-1921, that
the center of the
distribution of the
globular clusters
was the true center
of the Galaxy.This
was slowly accepted.
The position of the Sun
is marked S in this diagram.
The position of the center
of the Galaxy in Shapley’s
model is marked C. The
gray area around S is about
the size of Herschel’s model
of the Galaxy from star
counts.
(Trumpler, 1930)
Magellanic Clouds
Though Shapley’s
distances were overestimated,
his idea was correct.
By the 1950’s the overall
size and the shape of the
Galaxy was thought to be
well understood.
The Galaxy consists of
a disk population,
Population I, of relatively
young stars and a halo
population, Population II, of
older stars, remnants of the
formation of the Galaxy. The
central bulge also contains
older stars. Two small
satellite galaxies, the Large
and Small Magellanic
Clouds, slowly orbit our
Galaxy.
Disk
Central Bulge
Halo
The Disk:
The open clusters are Population I and found in the disk of the galaxy.
The distribution of HII regions, open clusters, and dark molecular
clouds where stars are forming is not uniform in the disk. These
objects trace out spiral arms. The best measurements of the spiral
arms are made by observing the 21-cm radio emission of neutral
hydrogen gas. Neutral hydrogen is concentrated in the spiral arms.
21-cm radiation is emitted when the spin of the electron in the
hydrogen atom changes from spin-up to spin-down. The hydrogen
emits a photon with an energy equal to the energy difference of the
two states:
Higher energy: spin-up
Lower energy: spin-down
This is a radio
map of the
21-cm radiation
from the disk
of our galaxy.
The dark
areas of strong
21-cm
emission
outline the
spiral arms.
Maps like this
only became
possible after
WWII.
IR view of the Milky Way (2MASS) showing central bulge.
Inside the
bulge is the
center of
the galaxy.
We will study
this in more
detail later.
30 kpc
An artist’s conception of the Milky Way Galaxy seen face-on.
This is based on radio maps and other sources of data. The yellow
dot is the position of the Sun: 10 kpc from the center of the Galaxy.
From the motions of stars in the galaxy, and the 21-cm data, we know
that the galaxy rotates. The Sun takes ~250 million years to complete
one orbit around the galactic center. The inner stars complete their
orbits in shorter times. This differential rotation would destroy the
spiral arms though, winding them up in a few hundred million years.
If the galaxy is billions of years old, why do the spiral arms persist?
Density wave
theory is one solution.
The
density
wave, like
a water
wave,
moves
through
the disk
causing
stars to
form by
compressing the gas and
dust in the disk, but hardly
affecting the stars at all.
Another theory is self-propagating
star formation, where “waves” of
star formation can form bands of
young stars in the galactic disk.
1) Cluster of stars form out of part of
an interstellar cloud.
2) The hottest stars ionize the gas
surrounding gas and an HII
region forms. The HII region
compresses the cloud nearby,
causing new star formation.
3) The most massive stars become
supernovae and the expanding shock
waves also cause star formation.
4) Differential rotation then shears
these regions into arcs we see as
spiral arms.
How a wave of star formation propagates through the
interstellar medium. Successive waves continually recreate
the bright stars and HII regions that trace out the spiral arms.
Both theories have
strong points (and
weak points), perhaps
both processes play a
role in our galaxy.
If we could see our
galaxy face-on, it
would look very
much like this.
Note the color
difference between
disk, nuclear bulge,
and core. HII
regions show as red
patches in the arms.
NGC 7331 is thought to be similar to our galaxy.
NGC 7331 in IR showing dust and star formation regions.
If we could see our Galaxy
edge-on from the outside,
it would look like this.
The Milky Way Galaxy
contains roughly
100 billion stars and our Sun
is one star among these,
located roughly 30 thousand
light years from the center
of the Galaxy in the disk.
What about other
galaxies?