2011 August 6
Novae in M31
Nick James reports there are at least two ‘bright’ novae in M31 at the moment. Though at around 17th magnitude both are easily within the reach of amateur astronomers with CCD cameras.
These two novae exploded in the Andromeda Galaxy some 2.5 million years ago but their photons have only just arrived at the Earth where they were first detected on August 4. Some of these photons were captured via telescopes and CCD cameras in Chelmsford to create this montage.
These objects are just like the novae in our galaxy but they a lot further away and so they appear much fainter. A nova is a close binary star system where the primary white dwarf star pulls material off its companion. The material spirals down onto the white dwarf and eventually enough accumulates to cause a runaway nuclear fusion reaction. This dramatically increases the brightness of the system by 10 magnitudes or more.
Most novae have an absolute magnitude of between -7 and -9 at the peak of their outburst (The absolute magnitude is the magnitude the nova would have if it were 10 parsecs, or 32.6 lightyears, away). M31 is around 2.5 million light-years away and so its novae have an apparent magnitude at peak of around 16 to 18 as seen from our vantage point. These latest novae fall right in the middle of that range at around magnitude 17. In fact, if the lightcurve of the nova is carefully measured it can be used as a “standard candle” to give us an idea of its distance.