Our Galaxy
T
he main problem about trying to find out the shape of
the Galaxy is that we live inside it; the situation is
rather like that of a man who is standing in Piccadilly
Circus and trying to work out the shape of London.
Originally – and quite naturally – most people assumed
that the Sun, with its planets, must lie near the centre of
the Galaxy; for example William Herschel found that star
numbers are much the same all along the Milky Way,
though admittedly some parts of it are richer than others.
The first really reliable clue came from radio astron-
omy, during the 1940s. It was known that there is a great
deal of thinly spread matter between the stars, and it was
reasonable to assume that much of this must be hydrogen,
which is by far the most plentiful of all the elements. In
1944, H. C. van de Hulst, in Holland, predicted that clouds
of cold hydrogen spread through the Galaxy should emit
radio waves at one special wavelength: 21.1 centimetres.
He proved to be right. The positions and the velocities of
the hydrogen clouds were measured, and indicated a spiral
structure – which was no surprise, inasmuch as many of
the other galaxies are also spiral in form.
By now we are in a position to draw up what we
believe to be a reliable picture of the shape and structure
of the Galaxy. It is about 100,000 light-years from one
end to the other (some authorities believe this to be some-
thing of an overestimate), with a central bulge about
10,000 light-years across. The Sun lies between 25,000
and 30,000 light-years from the galactic centre, not far
from the main plane and near the edge of a spiral arm.
Beyond the main system there is the galactic halo, which
is more or less spherical, and contains objects which are
very old, such as globular clusters and highly evolved
stars. There are in fact two distinct ‘stellar populations’;
the relatively young Population I, found in the nuclei of
our Galaxy and others, and the older Population II, which
is dominant in globular clusters and other halo objects.ATLAS OF THE UNIVERSE
▲ In our Galaxy, neutral
hydrogen (in blue) is
aggregated mostly along
the four large spiral arms
where also HII regions
(in red) and massive
molecular clouds (in black)
are clustered. The galactic
centre contains numerous
expanding regions of
ionized hydrogen and
giant molecular complexes.
It is surrounded by a
huge ring of radius about
5 kiloparsecs where a great
quantity of atomic and
molecular hydrogen is
concentrated.Galactic centreSpiral armRing at 5kpc
We cannot look directly through to the centre of the
Galaxy, because of obscuring material; the centre lies
beyond the lovely star-clouds in Sagittarius. Infra-red radi-
ations are not blocked in the same way, and the centre was
located as long ago as 1983 by IRAS, the Infra-Red
Astronomical Satellite. It had long been suspected that
there might be a massive central black hole, associated
with a compact X-ray and radio source known as
Sagittarius A* (pronounced Sagittarius A-star). This was
confirmed in 2002 by observations made with the Yepun
mirror of the VLT. Stars were detected in the central
region, and one of these stars, lettered S2, was found to
orbit the central object in a period of 15.2 years, approach-
ing the object to a mere 17 light-hours (three times the
mean distance between our Sun and Pluto). The orbital
speed reached over 5000 kilometres (3100 miles) per sec-
ond. This indicates that the black hole, Sagittarius A*, has
a mass around 2.6 million times that of the Sun. Near it
there are swirling gas-clouds and highly luminous stars.
We know that the Galaxy is rotating round its centre,
and that our Sun takes about 225 million years to complete
one circuit. Yet the general rotation does not follow the
expected pattern. Kepler’s Laws show that in the Solar
System, bodies moving close to the centre (in this case the
Sun) move quicker than bodies which are further out, so
that, for instance, Mercury moves at a greater rate than
the Earth, while the Earth moves faster than Mars. In the
Galaxy, this sort of situation does not arise, and the speeds
are actually greater near the edge of the disk. The only
explanation is that the main mass of the Galaxy is not
concentrated near the centre at all, and there must be a
tremendous amount of material further out. We cannot
see it, and we do not know what it is – all we can say for
certain is that it exists. The ‘missing mass’ problem is one
of the most puzzling in modern astronomy.
Nowadays the term ‘Milky Way’ is restricted to
describing the luminous band in the sky, though it is true
that we still often refer to the Milky Way Galaxy.
Sweeping along it with binoculars or a wide-field tele-
scope is fascinating, and it is not always easy to remember
that each tiny speck of light is a true sun.F Atl of Univ Phil'03stp 3/4/03 5:41 pm Page 196