U Orionis Observing Notes 2007-9 by Steve Woodbridge

The British Astronomical Association made U Orionis, a long-period variable or “Mira” star, its variable star of year for 2008 and I have been following it since that year’s maximum. This star varies over a period of around 368 days, typically from magnitude 6.5 to 12.0. Like all long-period variable stars the period and range are not constant. The maxima for this star vary from about 5.0 to 7.1. Professional astronomers do not yet know why the size of maxima vary, and the data provided by amateurs over many decades will be a key to trying to find this out in the future. 

U Orionis light curve 2007-9 by Syeve Woodbridge – click to enlarge

Although in the constellation of Orion, this star is near the star Chi Orionis on the borders of Gemini, so actually has a declination of +20 and it is best found starting at Eta Geminorum. So it can be observed for quite a large part of the year from the UK (from mid-August to early May).The predicted maximum for 2009 was February 15th, and for 2010 is February 19th , so it is currently well placed for observation.

I have now been observing it for two seasons, starting in September/October and finishing around May. In the autumn, I have not been able to see it in my binoculars as it was too faint (these observations are marked in yellow on the diagram). I have then followed it right through maximum successfully.

The diagram below summarises my observations over the last 18 months.

2008 was a fairly typical maximum with a peak magnitude around 6.3. The rise to maximum is normally faster than the fall to minimum, as is typical with Mira stars, although this is not clear from my data. 2009 was a much fainter maximum, only about 7.4 and many observers (including myself) did not pick it up in binoculars until well into January.

In terms of the date of maximum, in 2008 it was later than predicted; the date of maximum for 2009 is not clear from my data..

I liaised with another member of the BAA Variable Star section and my early observations for 2007-8 were probably incorrect: I may have been getting confused with a close “companion” star – a common problem with certain variables with a large range.

I plan to continue observing this star until it disappears from my binoculars in the next couple of months, and then to pick it up again in November or December 2009, in advance of the 2010 maximum.

In many ways Long Period variable stars are fun to watch as they do change in brightness so much. We located this star and made observations at the Society meeting in February 2009. If society members (or others) want to join me in watching this interesting star, I will be happy to help advise and collate observations.