Corona Borealis

The Northern Crown is a prominent half circle of stars of fourth magnitude, in which the 2.2m bright star Gemma (α Gem) is embedded. Gemma is only 42 light-years away. The constellation contains a galaxy cluster with 400 members, but it is a billion light-years away. Its brightest galaxies reach only 16m and are thus beyond the reach of ordinary amateur telescopes.

R Coronae Borealis is the only object in the Northern Crown which is worth a closer look at through binoculars – if it is visible at all. The star is the namesake of a whole class of variable stars and usually appears as a 6m star. Again and again its brightness decreases over a number of weeks down to about 14m. It takes several months until it reaches its original brightness of about the sixth magnitude again. The reason for the brightness variations are probably clouds of soot in the atmosphere that absorb the light from the star and can be ejected through prominences into the universe. The atmosphere of the star is two-thirds carbon. 3,000 to 7,000 light-years separate us from this luminous old supergiant.

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