Guide to R Bootis

An easy to locate Mira type variable that can reach 6th magnitude.

R Boo

R Bootis is a red giant star. As for other Mira type variables, the brightness variations are primarily due to pulsations in the star’s outer layers, with some ‘irregularities’ during the rise and fall that are related to the formation and break up of very simple molecules in the star’s cool(ish) surface layers.¬†¬†Since these changes don’t repeat exactly from one cycle to the next, the brightness at maximum differs between cycles and published predictions for maxima can be uncertain by +/- 2 weeks.

Extreme brightness range 6.2 – 13.1
More typical range 7.2 – 12.3
Period of variation 223 days (approx 7.5 months)
Frequency of observation Worth checking a few times per month
Observe using 50mm binoculars will suffice when in the upper part of its brightness range. Larger binoculars will be required when it gets fainter. A telescope is required to follow it down to minimum.
Upcoming maxima Mid February 2018, Early October 2018

The finder charts which follow have north at the top and show the location of R Bootis.

The first shows the location of R Bootis relative to the main stars of Bootes.


The second shows the area close to R Bootis in more detail.

You can follow the changes in R Bootis by comparing its brightness with that of the comparison stars. These are labelled with their magnitudes (with the decimal points omitted). Thus, for example, ’70’ labels a comparison star of magnitude 7.0.