SkyWatching: Stars Of Early Summer

Early summer is an in-between time in the skies. The realm of the galaxies has moved off to the west, but the summer Milky Way has not yet arrived. This is the best time of year to observe globular clusters and double stars.

The centrepiece of the early summer constellations is Boötes, the herdsman, with the bright star Arcturus at his heart. Arcturus is easy to find by following the arc of the Big Dippers handle away from the ladle: it is the only bright star in this part of the sky. Alternately, if you live in the northern hemisphere, simply look straight overhead around 11 p.m.

Just after dark on a June evening, look overhead to see the constellations of early summer: Boötes, Corona Borealis, and Serpens Caput.  Credit: Starry Night software.

Although Boötes looks like it might be pronounced like booties, the diaeresis (double dot) over the second o gives you a clue: the two os are pronounced separately: Boh-OO-tes. Its stars form a distinctive kite shape, complete with tail.

Arcturus is the third brightest star in the night sky, after Sirius and Canopus. It is relatively close to us, only 37 light years distant. It is an orange giant star, slightly cooler than the sun, but quite a bit larger in diameter.

Boötes contains relatively few deep sky objects, but is rich in double and variable stars. Izar (Epsilon Boötis) is one of the finest double stars in the sky. With a separation of only 2.9 arc seconds, it requires at least 3 inches aperture, steady skies, and high magnification to see its duality; its stars are gold and greenish in colour. Alkalurops (Mu Boötis) is a much wider double at 2 arc minutes separation, but it is a challenge to see that one of its stars is itself a double.

Although not within Boötes itself, most amateur astronomers use the stars of Boötes to starhop to the Messier globular cluster Messier 3 in the dim constellation of Canes Venatici. M3 forms an almost perfect equilateral triangle with Arcturus and Rho Boötis. This is one of the finest globular clusters in the sky.

Just to the left (east) of Boötes is a small circlet of stars forming Corona Borealis, the northern crown. Look within the circle to see if you can see R Corona Borealis, a very unusual variable star. Some have called this an inverse nova. Most of the time it shines steadily with a brightness of about magnitude 7, just below naked eye visibility, but easily seen in binoculars. At long and irregular intervals, instead of brightening like a nova, it dims by about 6 magnitudes. This dimming is caused by occasional expulsion of a dark obscuring cloud of dust. Currently R is entering its dark phase, but keep watching, and it should soon reappear.

These three constellations contain many interesting objects to look at with binoculars or a small telescope. Credit: Starry Night software.

Below Corona Borealis is one of the most unusual constellations, or rather half constellations. Serpens represents a snake cut in half, each half held in one hand of Ophiuchus. This is the front half: Serpens Caput, or the head of the serpent. The other half, located quite a ways to the east, is Serpens Cauda, the tail of the Serpent. A triangle is supposed to represent the head of the spent, but I always see this and the two stars above as a large X.

The brightest star in Serpens bears the ugly name Unukalhai, which is Arabic for the serpents neck. Just above Unukalhai is Delta Serpentis, a fine pair of pale yellow stars in a telescope.

But the real prize in Serpens Caput is the globular cluster Messier 5, every bit as fine as Messier 3 to the northwest. Like all globular clusters, M5 responds well to aperture and magnification. Besides resolving the cluster into myriads of tiny stars, a large telescope will reveal chains of stars and clusters within the cluster.

Double Stars around Boötes

On a May evening many years ago, I made my first exploration of the night sky. The only star pattern I could recognize was the Big Dipper, but with a star chart in a book, I used that to discover the bright star Arcturus in the constellation Boötes.

The curve of the Big Dipper's handle leads to Arcturus, the brightest star in the kite-shaped constellation of Boötes. Surrounding Boötes is an amazing variety of double stars. Credit: Starry Night software.

The trick to learning the constellations is to begin with the stars you know, and use them to identify their neighbors. This same technique, known as "starhopping" is the key to discovering all the wonders hidden amongst the stars.

Start, as I did, with the Big Dipper, high overhead as the sky gets dark at this time of year. The stars that form the Dipper’s handle fall in a gentle arc, and if you project that arc away from the Dipper’s bowl, you come to a bright star. This is Arcturus, the third brightest star in the night sky, and the brightest star in the northern sky. Only Sirius and Canopus, far to the south, are brighter.

Arcturus is bright in our sky for two reasons, first because it is relatively close to us, 38 light years away, and secondly because it is inherently a bright star, much brighter than our Sun. Though larger and brighter, it is a slightly cooler star than our Sun, so appears orange to our eyes.

Although Boötes is supposed to be a ploughman in mythology, its pattern of stars most resembles a kite, with Arcturus marking the bottom of the kite where the tail attaches. Notice the little dots over the second “o” in Boötes: this indicates that the two "o"s are supposed to be pronounced separately, as "bow-oo’-tees," not "boo’-tees."

Once you have identified Boötes, you can use its stars to identify a number of constellations surrounding it. Between it and the Big Dipper are two small constellations, Canes Venatici (the hunting dogs) and Coma Berenices (Bernice's hair). To Boötes left (towards the eastern horizon) is the distinctive keystone of Hercules. Between Hercules and Boötes is Corona Borealis (the northern crown) with Serpens Caput, the head of the serpent, poking up from the south.

Although most stars appear to our unaided eyes as single points of light, anyone with access to binoculars or a telescope soon discovers that nearly half the stars in the sky are either double or multiple stars. Some of these are just accidents of perspective, one star happening to appear in the same line of sight as another, but many are true binary stars: two stars in orbit around each other, similar to the stars which shine on the fictional planet Tatooine in Star Wars.

Every star labeled on this map of Hercules, Boötes, and Ursa Major is a double star, worth exploring with a small telescope. Some, like Mizar in the Dipper’s handle, can be split with the naked eye. A closer look with a telescope shows that this is really a triple star. Others require binoculars or a small telescope. Some of the finest are Cor Caroli in Canes Venatici, Izar (Epsilon) in Boötes, Delta Serpentis, and Rho Herculis.

One of the joys of double star observing is the colour contrasts in some pairs. Others are striking for matching colours and brightness. My favorites are stars of very unequal brightness, which look almost like stars with accompanying planets.

Also marked on this chart are three of the finest deep sky objects: the globular clusters Messier 13 in Hercules and Messier 3 in Canes Venatici, and the Whirlpool Galaxy, Messier 51, tucked just under the end of the Big Dipper’s handle. You will probably need to travel to a dark sky site to spot this galaxy. A six-inch or larger telescope will begin to reveal its spiral arms, including the one that stretches out to its satellite galaxy, NGC 5195.

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