Same mass as the sun

Ken Kennedy of Dundee Astronomical Society tells us what we can expect to see in the June sky.

“There are two stars which shine out high in the sky in the lighter June evenings,” he says. “These are Arcturus, above Jupiter but just a bit less bright, and Vega, high, but towards the west by late evenings in June.

“As the two most prominent stars in the summer sky it is interesting to make some comparisons. The first thing which may be noticed is the difference in colour of these stars – Arcturus is a red giant and will appear distinctly orange while Vega is a blue-white star. Because the eye is not as sensitive to orange as it is to white, it seems to many people that Vega is brighter than Arcturus but the opposite is the case.

“Arcturus is the fourth brightest star in the entire sky and Vega is the fifth brightest. Have a look and see what you think.

“Arcturus is interesting as it has nearly the same mass as our sun but is three billion years older and has swollen to 25 times the diameter of the sun. This star gives us a glimpse of what is in store for the sun when it reaches an age of around seven billion years. Vega, on the other hand, is only 455 million years old but because of its greater mass it is expected to have a life span of one billion years.

“Looking northwards, the upper part of the constellation Auriga projects above the horizon. Here we find Capella, quite low in the sky but the third brightest star of the summer sky. Capella is a multiple star and its components are more massive than the sun.

“During the night, Capella moves from a north-westerly direction towards north-east at dawn and is an excellent marker for noctilucent clouds which I briefly mentioned last month.

“June is one of the best months to see noctilucent clouds. These very high altitude clouds are generally quite low towards the north and shine with a pearly white or slightly blue colour. Their delicate structures are illuminated by the sun when it is below the horizon but the clouds are so high they reflect its light through the darkest part of the night, fading at dawn.

“After a long and cold winter we seem to have rushed through the first months of 2018 to arrive at the summer solstice on June 21 at 11.08 BST.

“Mercury can be seen during the last two weeks in June low in the west after sunset. The planet sets about an hour and a half after the sun and it can be found by using brighter Venus which is above and to the left of Mercury.

“Venus is bright and towards the west, setting about two hours after the sun. Even in the June evening sky Venus is so bright that it will easily be seen without any optical aid. Mars rises in the southeast at about 1am but is lost in the twilight by 4am. Jupiter is quite low but bright towards the south at 11pm, setting at 2.30am. Saturn is due south by 2am but is at around the lowest point which it gets in its orbit for those at northerly latitudes.

“The moon is at last quarter on the 6th, new on the 13th, first quarter on the 20th and full on June 28.”