Popular Astronomy

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Guide to R Ursae Majoris

A Mira type variable star located a few degrees north of the 'pointers' of the Plough.

Like other Mira type variables, R Ursae Majoris is red giant star. The brightness variations are mostly due to pulsations in its outer layers. However, the brightness doesn't rise and fall at a constant rate because as the star's surface cools during the expansion phase, some very simple molecules are able to form and these absorb some of the light being emitted. When the surfaces warms again during the contraction phase, these molecules split apart.

Extreme brightness range 6.7 - 13.4
More typical range 7.2 - 12.7
Period of variation 302 days - approx 10 months
Frequency of observation Worth checking a few times per month
Observe using 50mm binoculars will suffice when it is near maximum. Larger binoculars will be needed when fainter. A telescope is required if you want to follow it down to minimum
Visibility Circumpolar - can be observed all year round.
Upcoming maxima Early July 2017 , Late April 2018

The finder charts which follow show the location of R Ursae Majoris.

The first shows its general location relative to the stars in the 'bowl' of the Plough:

The second chart, which is approx 7 degrees by 4 degrees, shows the area close to R UMa is more detail.

You can follow the changes in R UMa by comparing it with the brightness of the comparison stars. These comparison stars are labelled with their magnitudes (with the decimal point omitted). Thus '76' labels a comparison star of magnitude 7.6.