Guide to U Coronae Borealis

An eclipsing variable in which the primary eclipse is a magnitude deep.


U Coronae Borealis is a similar type of eclipsing variable to Algol (Beta Persei).

Eclipses occur every 3.45 days – approx half of a week -so if you see one, there will be a chance to see another one in just under 7 days (the intervening eclipse having occurred during daylight).

The primary star is a blue-white dwarf (i.e. main sequence) star.

The primary eclipse occurs when the secondary star in the U CrB system – a white sub-giant star – passes in front of it , blocking off its light.

Eclipses last for approx 11 hours. This can make it tricky to observe a whole eclipse in one session.During the spring and summer months, the nights are not this long. During the autumn and winter months, U CrB is not above the horizon for this long.

The best time to catch an eclipse is probably during March, when the nights are almost long enough and U CrB is observable nearly all night. At other times of the year, you will need to observe the fade into eclipse and brightening from eclipse on different nights – possibly weeks apart – and then “join” the two together to create your light curve.

Extreme brightness range 7.8 – 8.8
More typical range always the same
Period of variation 3.45 days
Frequency of observation Check every 30 mins during primary eclipses
Observe using 50-80mm binoculars
Visibility Star is visible all year round, but nightly visibility is rather short from November to January

The two finder charts which follow show the location of U Coronae Borealis.

Chart for U CrB

The second chart, which is approx 6 degrees by 4 degrees, shows the area around U CrB in more detail.

You can follow the changes in U Coronae Borealis by comparing its brightness with that of the lettered and numbered comparison stars. Outside of eclipse it will be slightly brighter than comparison 4. In mid eclipse it becomes fainter than comparison D.

(Note that S CrB is a Mira type variable which currently is only visible in binoculars from August to November).

Chart for U CrB