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Tue, 30 Apr 2013

Spot the difference – Chi Cyg at max

Cygnus, with Chi Cygni (arrowed) on 30 April
Spot the difference over six months.
Photos: Robin Scagell

An extra naked-eye star is currently visible in the neck of Cygnus the Swan. This is the Mira-type variable Chi Cygni, which is putting in a particularly bright maximum this spring writes Tony Markham.

The extreme brightness range of the Chi Cygni is usually quoted as being between magnitudes 3.3 and 14.2. However, not all maxima of Mira-type variables are equally bright and for Chi Cygni a peak magnitude in the region of 4.5 has been more typical. The upper limit quoted for Chi Cygni's brightness range is largely based on a number of bright maxima that occurred around the middle of the 19th century. Few, if any, 20th century maxima of Chi Cygni exceeded magnitude 4.0. The 2006 maximum was, however, somewhat brighter than this and 2013 looks like producing another bright maximum, with some recent reports indicating that it may have edged brighter than magnitude 4.0 again.

The period of Chi Cygni is around 407 days, meaning that, on average, Chi Cygni should reach peak brightness around 6 weeks later each year. However, in the same way as the peak brightness in each cycle can't be predicted in advance, the timing of the next maximum can only be predicted to within a week or two. In 2013, the best that can be said is that maximum will occur in late April or early May. This means that in order to see Chi Cygni at maximum in 2013 you will need to be able to observe after midnight as Chi Cygni is not visible in the evening sky at this time of the year.

The unpredictability in the timing and brightness of maxima, together with their very large brightness ranges are the reasons why variable star observers find Mira-type variables so interesting to monitor. The SPA comparison chart for the star, including both bright and faint comparison stars, is shown below. It covers a binocular field of view, approximately the same area as in the two comparison photos shown above.

These are red giant stars in which the brightness variations are related to pulsations in their very tenuous outer layers. Over time, the pulsations will slowly cause these outer layers to be shed into space to produce a planetary nebula. Indeed, our Sun is destined become a Mira type variable in the final stages of its evolution.

Incidentally, if you are wondering when Mira itself (Omicron Ceti) will reach its 2013 peak, the answer is that this will occur during June. Unfortunately, Mira is not visible in the night sky from the UK in June. Fortunately, in Mira's absence, Chi Cygni is putting on a good show for us.


Added by: Robin Scagell