The Interstellar Medium

The space between stars is not empty. It is full of gas and dust which are collectively called the Interstellar Medium or ISM. The ISM accounts for ~10 per cent of the mass of our Galaxy. To see anything beyond the Solar System, we must look through the ISM, and thus all observations are filtered and modified. In our Galaxy the gas forms a disc in the plane of the Milky Way, with diameter ≈30 kpc and thickness ≈0.7 kpc. The density averages about 1 atom/cm3, a far better vacuum than any that could be created here on Earth. This does not sound like much, but it is enough to absorb soft X-rays from most galactic sources. The composition of the gas is close to the usual cosmic abundance: 90 per cent H (by number), 10 per cent He and 0.1 per cent heavier elements. However, it is far from being a uniform medium. The neutral gas exists in a very large range of density, n, and temperature, T. A diffuse cloud might have n ~100 and T ~80 K. The medium between clouds might have n ~1 atoms cm−3 and T ~8000 K. There is also warm (8000 K) and hot (300 000 K) ionised material. Our interest here is in the neutral gas, which does most of the absorbing.

Neutral H in the ISM has, for more than 60 years, been directly observed in the radio band at a wavelength of 21.106 cm.