Introduction to Astrophysics Lecture 13: The Milky Way Galaxy.

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.

Because we are within it, it has not proven to be all that easy to understand its structure.

The galaxy seen in the far infra-red.

M83 gives an indication of what our galaxy might look like if we were able to step back from it.

We would be here.

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

Close up

Optical

In the optical part of the spectrum our view of the galaxy is obscured by large amounts of absorbing dust in the galactic disk.

Close up

Near infra-red

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.

Close up

X-ray

In X-rays we see only a few discrete sources, though the current resolution is much worse than that in the other wavebands.

Close up

Radio

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

Messier 10

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.

Artist’s impression: European Southern Observatory

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.

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!!