The famous 21-centimetre line (or simply the hydrogen line) is produced as a result of the change in energy level when the spins of the proton and electron in a hydrogen atom relax back to a low-energy configuration in which they point in opposite directions. The 7-metre is fitted with a receiver designed to pick up radio emissions from hydrogen atoms in interstellar space which occur at a wavelength of 21 centimetres. The Lovell Telescope and 42-foot telescope can be seen in the background. This timelapse video shows a day in the life of the 7-metre telescope. Data collection is overseen by Christine Jordan at Jodrell Bank. This telescope, like our 42-foot pulsar monitoring telescope, is an ex-missile tracking dish from Woomera in Australia. On the Stargazing Live show we featured live observations from our 7-metre telescope (actually a misnomer since its actual diameter is 21 foot or 6.4 metres). In particular we can’t easily see its spiral structure. We are seeing it from the inside so it’s not possible to see it as a whole as it is for other galaxies. However, although individual stars and nebulae may be easier to study in the Milky Way than in other galaxies, the overall structure of the Milky Way is very hard to discern. We can see more detail as they appear larger and we can detect fainter features in the light we collect from them with our telescopes. Usually the closer objects are to us, the easier they are to study. My photograph of the centre of the Milky Way taken from the South African Astronomical Observatory in Sutherland From the Earth we see this disc side-on so it appears in the night sky as a band of hazy light stretching from horizon to horizon and threaded by dark dust lanes. The Sun lies in the disc about two thirds of the way from the middle. The stars, gas and dust are found mostly in a disc – like two Frisbees 100,000 light years across stuck face to face.
The Milky Way galaxy is made up of several hundred billion stars plus gas, dust and dark matter, all held together by their mutual gravitational attraction. First up: the spiral structure of the Milky Way galaxy which we discussed in episode 3. I thought I’d write something about a few of the topics we discussed last week during Stargazing Live.