Transcript Introduction to Astrophysics, Lecture 13

Introduction to Astrophysics
Lecture 13: The Milky Way Galaxy
The Milky Way: basic properties
The Milky Way is a
giant system of stars. It
contains around a
hundred thousand
million (i.e. 1011) stars,
including our own.
The galaxy seen in the far infra-red.
Because we are within it, it has not proven to be all that
easy to understand its structure.
We would
be here.
M83 gives an indication of what our galaxy might
look like if we were able to step back from it.
Five views of the Milky Way. From top to bottom, this
is how the Milky Way looks in
Radio, infra-red, visible, X-ray and Gamma-ray
Optical
Close up
In the optical part of the spectrum our view of
the galaxy is obscured by large amounts of
absorbing dust in the galactic disk.
Near infra-red
Close up
In the infra-red dust absorption is much less of a
problem and we get a more or less uninterrupted
view of the galaxy, with the bulge clearly visible.
X-ray
Close up
In X-rays we see only a few discrete sources,
though the current resolution is much worse than
that in the other wavebands.
Radio
Close up
A radio map shows fairly continuous emission
across the galactic disk.
Components of the galaxy
 The galactic disk.
 The galactic bulge.
 The globular clusters.
 The dark matter halo.
The Galactic Disk
The majority of the stars in the galaxy are in the disk, mainly
distributed throughout the spiral arms.
The Sun is about 8 kiloparsecs away from the galactic centre,
and orbits the galaxy with a velocity of around 220 km s-1.
Since the galaxy formed, the Sun has undergone perhaps 25
complete orbits.
The spiral structure is not that well understood. Stars nearer
the centre orbit much more quickly, so the arms should `wind
up’. It is thought that instead the spiral structure is due to a
density wave propagating around the galaxy.
The Galactic Disk
The galactic disk contains a lot more than just stars.
We have already seen that views using optical radiation are
highly obscured, by absorbing dust.
The galactic disk is also full of clouds of gas, some hot and
some cold, some actively involved in star formation, some
resulting from supernova explosions when stars die.
There are many energetic particles known as cosmic rays, and
it has a magnetic field.
All this put together is called the interstellar medium.
The Galactic Bulge
The bulge is a central concentration of stars, with a somewhat
flattened spherical shape.
Recently evidence has begun to accumulate that the Milky Way may
be a barred spiral galaxy, rather than a pure spiral. It is hard to tell as
we are looking almost directly down the bar.
Near the centre the stars are orbiting extremely quickly, and there is a
very bright radio source (Sagittarius A*). Because of this, it is
believed that there is a giant black hole at the centre of the galaxy.
Its mass is thought to be equal to a few million Suns, and as time goes
by it is accumulating more and more mass by eating central stars.
The Globular Clusters
The galactic bulge is surrounded by
dense knots of stars known as
globular clusters.
The largest contain around a million
stars, and there are about 100 known.
It is thought that they are remnants
from the original formation of the
Milky Way. Some contain the oldest
stars known.
Messier 10
The globular
clusters don’t just
hang there; they
are in orbit
around the
galactic bulge
and occasionally
lose stars in
passing through
the disk.
Artist’s impression: European Southern Observatory
The Dark Matter Halo
Just like the planets in the solar system, stars in the
galaxy obey Kepler’s laws, which allow us to work out
how fast they orbit. Except …
Stars towards the outer edge of our galaxy (and others)
are orbiting much faster than they ought to, perhaps three
times as fast.
As the velocity v is proportional to the square root of the
mass M of the galaxy, this implies that the galaxy is
really about ten times more massive than it seems to be.
The Dark Matter Halo
The Dark Matter Halo
It is believed that the visible material in the form of stars
makes up only about one tenth of the material in the galaxy.
While there is no doubt more material in the form of cold gas
waiting to be made into stars, it is not believed that that will
explain the difference.
Instead, it is believed that most of the material in the galaxy is
a new form of matter, known as dark matter. So far, we
have little idea what form this dark matter might take, though
there are active attempts to search for it.
It is expected to form a large spherical halo, within which lies
the disk of our galaxy.
Very, very schematic!!