Astronomy and Space articles
by Martin George of the Launceston Planetarium
5 November 2016
A Valuable Map
Look up to the western evening sky once it is dark tonight, and you will be facing toward
the centre of our Milky Way Galaxy. It is a spiral-shaped galaxy, with our Sun located
about two-thirds of the way from the Galaxy's centre to its edge.
However, we can't see the lovely spiral shape from Earth, because we live within the
Galaxy. We see that many other galaxies have a spiral shape, and it is reasonable to
think that ours looks rather like that, even without making special measurements.
You may think that simply observing the locations of the stars in the Galaxy would reveal
the spiral shape, but there is a problem. There is a lot of dust in the Galaxy, especially in
its central plane, and this obscures our view of the stars beyond.
You can see the effect of this dust on a clear, dark night away from the city lights, by
watching out for the dark blobs and streaks that look like gaps in the Milky Way. The
most famous of these is called the Coal Sack, which appears to be tucked up against the
Southern Cross.
However, radio astronomy does not suffer from this problem: radio astronomers can 'look
through' the dust to more distant parts beyond. So it was radio astronomers who
established that our Galaxy is a spiral, based on the distribution of hydrogen.
Hydrogen is a well-known element. As everyone knows, two atoms of hydrogen combine
with one of oxygen in the famous formula H2O, which of course is water.
It is also the least massive of all of the elements: in its basic form it consists of just one
proton and one electron. Perhaps slightly less well known is that it is the most abundant
element in the Universe.
Radio astronomers can detect the hydrogen by making use of the fact that neutral
hydrogen — that is, Hydrogen in its most basic form — emits radio waves at a
wavelength of about 21 centimetres.
This emission, caused by the electron of the atom flipping between two slightly different
energy states, was first predicted in 1944, and first observed in 1951.
Now, radio astronomy has been used to produce the best-ever map of the distribution of
the Milky Way's hydrogen. Our famous Parkes Radio Telescope in New South Wales
and a radio telescope at Effelsburg in Germany have been used to chart the hydrogen in
unprecedented detail.
Astronomy and Space articles
by Martin George of the Launceston Planetarium
5 November 2016
The distribution of Hydrogen in our Galaxy. This map charts the whole sky,
with the Galactic plane through the centre. Credit: HI4PI Collaboration
The colour in the map is not related to anything that is visible. Rather, the colours relate
to whether the hydrogen is approaching or receding from us. The green parts of the map
indicate hydrogen that is receding and the blue parts indicate that it is approaching.
Astronomers can deduce this motion by measuring tiny changes in the wavelength of the
radio waves. It's called the Doppler effect, and you can experience this with sound
waves, too, by noting the pitch of the noise of a car change as it first approaches you and
then recedes.
The map also shows two quite nearby galaxies called the Magellanic Clouds (at the lower
right), and two spiral galaxies called M31 and M33 showing up at the lower left. All of
these galaxies are members of our so-called local group of galaxies.
This amazing map will be used extensively by astronomers studying the structure of our
Galaxy. It's a great resource and we can be proud that our famous Parkes 'dish' played
such an important role!
Martin George is the manager of the Planetarium at the Queen Victoria Museum in Launceston.
Article reproduced with the permission of The Mercury newspaper.
Astronomy and Space articles
by Martin George of the Launceston Planetarium
5 November 2016
The Effelsburg and Parkes radio telescopes. Images: Martin George