Bath Astronomers

July Night Sky

Midsummer is our one chance to look at the night sky in the direction of the galactic centre. This is in Sagittarius, which never rises far above the southern horizon. (Our compensation is that we in the northern hemisphere are better placed to look at nearby galaxy clusters in Virgo and Coma Berenices!).

We can’t see all the way to the galactic centre in visible light, as dark clouds of interstellar dust  get in the way, but if we look in that general direction there are many interesting nebulae  near enough to be seen in binoculars, and offering very good views in a good size telescope. Most of these are areas of  active star formation, and therefore  have gases energised by very hot young stars. This means they  radiate strongly at some particular wavelengths and, by using a special filter, the eyepiece of your telescope can let these wavelengths through, while shutting out the rest. It may seem strange to shut out some light from these clusters, but doing so for these objects can turn a dull glow into a fascinatingly  complex pattern of light. The best filter to use is generally O (ie Oxygen) III. Oxygen is reasonably abundant in these clouds; the excitation state that leads to the emission of light at this wavelength is fairly common; and the light itself is a strong green colour readily visible to the eye.

The map below shows the main objects of interest in this region. The best ones are M16, the Eagle Nebula ( also the source of the famous Hubble Pillars of Creation, but don’t expect to see that kind of detail!); just below that, M17, the Omega or Swan Nebula; and then further south M8, the Lagoon Nebula. In the latter two cases at least, the name tells you what kind of shape to expect. All three are best seen with the help of an OIII filter.

Being so far south, they will never appear at their best from the UK, but if you get a clear enough, and dark, night, they can still look very fine. The loss of detail they also suffer from being so low matters less than for nearby Saturn and Mars, which both need high magnifications to be seen at their best.

July 2016 Sagittarius