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Discovering Astronomy : Galaxies and Cosmology
Figure 1: Left : the night sky above Bryce Canyon, showing the Milky Way. Photo taken by amateur astronomer Gary Becker. Many other lovely photos are available at the Star Watch website. Right: sketch
explaining how living in a disc of stars gives rise to the appearance of the Milky Way.
Figure 2: Components of the Milky Way. On the left is an edge-on view, and on the right is a face-on view.
These are of course artist’s impressions, as we cannot step outside our own Galaxy!
The Milky Way
Lecture 1 : Cosmic Perspective 19.1,19.2,19.3
The Milky Way is our own galaxy, the one we live inside. The Sun, the planets, and almost every
star we can see belong to the Milky Way galaxy. It is a spiral galaxy and contains very roughly 100
billion stars. How do we know we live in such a great system of stars ? In the night sky, we see a
faint band of light stretching across the sky, passing through various constellations including Cygnus,
Perseus, and Orion. (See Fig. 1.) This band of light was known in the ancient world as the “Milky
Way”. (In Latin - Via Lactea; in Greek - ↵ ↵⇠i↵ ). We see this band because we live, to a first
approximation, in a flattened disc of stars.
Components of the Milky Way. If we could step outside see our galaxy and view it edge-on we
would see (i) a disc of stars; (ii) a central bulge region; (iii) an extended spherical halo; and (iv)
Globular clusters, which are also spread through the halo. Fig. 2 shows these components and their
approximate size. If we could view the Milky Way from the outside but face-on, we would see that
the disc has spiral arms, where new star formation is taking place. The disc also contains interstellar
gas and dust. The halo on the other hand contains very little gas, and mainly old stars. The structure
of the Milky Way has to be laboriously deduced indirectly, and is very hard to see directly on the
night sky. Why is that?
Obscuring dust in the plane of the Milky Way. The problem is that the disc of the Milky Way
has lots of dust clouds. This “dust” is made up of many very small (micron sized) solid particles actually rather more like smoke than household dust. This smoke blocks visible light from distant
Discovering Astronomy : Galaxies and Cosmology
Figure 3: Illustrating how the centre of our Galaxy is blocked from view, while we can still see out to the rest
of the universe.
Figure 4: Left : Distribution of globular clusters on the sky. Data from C.Mihos, CRWU. Right : IR map of
the sky. Data taken by the 2MASS project (2 Micron All Sky Survey).
objects. The dust clouds can be clearly seen in Fig. 1. The dust clouds make it hard to see through the
disc, so that we can’t see the centre of the Galaxy, but we can see easily see out of the disc vertically
up and down, as illustrated in Fig. 3. This is a good thing, otherwise we would never have seen the
rest of the universe!
Using the globular clusters. Historically, the globular clusters were very important because they
showed where the centre of the Galaxy really was. The globular clusters are in the halo component
of the Galaxy, in a roughly spherical distribution around the centre of the Galaxy. Because they are
out of the disc of the Galaxy, we can see them at large distances. If we plot their positions on the sky
(Fig. 4) they are not centred on the Sun, but on a point about 8kpc from the Sun.
Seeing through the dust in the infra-red. In modern astronomy, we observe the sky in light of many
different wavelengths, not just visible light. Infra-red (IR) pictures are particularly valuable, because
ordinary stars emit lots of IR as well as visible light, but IR is blocked much less by the dust, so we
can see stars all the way through the disc. Fig. 4 shows the sky as seen in the IR. Now we can see the
centre of the Galaxy, the disc looks like nice and flat and thin like it ought to, and we can even see
the bulge component very easily.
The Multi-wavelength Milky Way. Over the last few decades, astronomers have explored the sky
at every possible wavelength of light. This only became possible in the late twentieth century, both
because of the technology needed to detect different wavelengths, and because some wavelengths
are absorbed by the atmosphere, and so are only visible from space. The disc of the Milky Way
looks quite different at different wavelengths, as shown in Fig. 5 This is because different physical
Discovering Astronomy : Galaxies and Cosmology
Figure 5: Appearance of the Milky Way at different wavelengths.
components of the Milky Way emit quite different kinds of light.
At 21cm radio wavelength, we see emission from cold hydrogen gas. This is the raw material for making new stars. At somewhat higher radio frequency, we see radiation from many different molecules.
These molecules exist in the the dense inner parts of gas clouds, and indicate where the next stars are
likely to form. In the far-IR we see emission from the dust clouds that block the visible light. The
energy of the absorbed light heats up the dust to around 20-50 K, which then glows in the IR. In the
near-IR we see normal stars, from throughout the Milky Way, as this light passes through the dust
quite easily. In visible light we see quite a complicated picture. We see nearby stars, but the light from
more distant stars is obscured by the dust, so we see a kind of patchy structure. We also see some
light from gas clouds - those that are ionised by nearby hot stars, and then radiate atomic emission
lines. In X-rays we see hot gas - around a million degrees - which occurs in bubbles throughout the
disc of the Galaxy. Finally in gamma-rays we see the effects caused by highly energetic particles cosmic rays - which collide with the material in the molecular gas clouds.
Stellar motions in the Milky Way.
This is an interesting subject in its own right, but
also important because it allows us to measure
the mass of the Milky Way, as we discuss below.
The orbits followed by stars is rather different in
the different components, as illustrated in Fig.
6. Stars, gas, and dust in the disc all orbit in the
same direction and in the same plane, with typically just a little up-and-down motion as well.
The stars are almost all in more or less circular
orbits, rotating around the Galactic Centre. The
orbits of stars in the bulge and in the halo are
oriented at random. Furthermore they are not
necessarily in circular orbits. In general they are
in elliptical orbits with a variety of ellipticities.
The least elliptical orbits are more or less circu-
Figure 6: Different types of orbit in the Milky Way.
Discovering Astronomy : Galaxies and Cosmology
Figure 7: Left : schematic showing the rotation curve of the Milky Way. Right : artists impression of the Milky
Way sitting inside an extended dark matter halo.
lar and the most elliptical orbits are more or less radial. The orbits of stars in the bulge on average
stay closer to the centre of the Galaxy. Also, the bulge has a slight net rotation. Inside globular clusters, the stars are buzzing about at random with respect to the centre of the cluster, in much the same
way that individual halo stars move with respect to the Galaxy. Meanwhile, the globular cluster as a
whole moves together. The orbits of the globular clusters are just like those of the halo stars - moving
in random orbits with respect to the Galactic Centre.
Galactic rotation. We can measure the rotation speed of the disc as a function of radial distance
from the centre of the Galaxy - see Fig. 7. The very central parts rotate slowly. The speed picks up
as you move outwards, and then flattens off, with rotation speed apparently constant as far out as we
are able to measure. This is not what we would have expected, which is that rotation should increase
at first and then fall off again towards the outer parts of the Galaxy, as we move past the visible edge.
The Galaxy doesn’t really have an edge - it fades out gradually - but past a certain point there isn’t
much more light and so so there can’t be many more stars. This failure to fall off suggests that the
Galaxy has more mass than we can actually see. This “extra mass” is known as dark matter.
Determining the mass of the Galaxy. The rotation speed at a given radius allows to work out the
mass of the Galaxy inside that radius. If the mass inside radius R is M (R) then from Newton’s laws
we know that the rotation speed should be given by v 2 = GM (R)/R. (The matter outside radius
R pulls in all directions and so the force cancels out). Turning that formula round, if we measure
rotation speed v we can calculate M :
M (R) = Rv 2 /G
For example, at the Sun’s orbit the rotation speed is 220 km s 1 , and the radial distance is 8 kpc.
This gives a mass of M = 1011 M - a hundred blllion times the mass of the Sun. At this radius
this estimate agrees quite well with what you might expect from the amount of starlight we see.
However if go further out, the total mass of the Milky Way out as as far as we can measure seems to
be M = 1012 M - a trillion solar masses - whereas there isn’t much more in the way of starlight.
Dark Matter Halo. So, roughly speaking, it seems that 90% of the mass of the Milky Way must
be in “dark matter”. However, the amount of dark matter in the inner parts must be not such a large
fraction, compared to the stellar mass. Putting that the other way round, most of the mass must be in
an extended dark matter halo. The visible Milky Way of stars and gas sits inside this large halo of
dark matter.