High overhead in July you’ll find the constellation Hercules. The four stars making up the top of Hercules are known as the Keystone, and if you point a telescope or binoculars to the west side of the Keystone you’ll find one of my favorite objects: M13, a wonderful globular cluster.
West of Hercules, you’ll see Corona Borealis and Bootes. Bootes is a kite-shaped group of stars and easy to pick out because of brilliant Arcturus. You can find Arcturus using the handy sky key that tells us to use the Big Dipper’s handle to “Arc to Arcturus then speed on to Spica”, which leads us directly to those two bright stars.
East of Hercules is Lyra, and then you’ll find Cygnus. The Summer Triangle, which we will explore later, is very apparent in the eastern sky.
Looking south, you’ll see the “Teapot” of Sagittarius and a fishhook, which is the constellation of Scorpius above the southern horizon. They are both in the thick of the Milky Way, which you can follow upward in the sky going by Aquila, Cygnus and stretching through Cassiopeia in the northeast.
There are nine objects this month shining at first magnitude or brighter and in order of brightness are Jupiter, Arcturus, Vega, Saturn, Altair, Antares, Spica, Deneb and Regulus. If you are away from city lights, try to find these naked-eye objects.
Sweep down from under the handle of the big dipper looking for a dark area with a big glint of light on the sky; that is the galactic star cluster known as the Coma Cluster, which contains more than 40 stars and covers over 4 degrees of the sky. Now look between the spout of the Teapot and the tail of Scorpius for the Lagoon Nebula; a cloud of gas and dust where stars are forming. Explore the summer Milky Way with binoculars because you’ll find all sorts of amazing things!
The Summer Triangle is an asterism made up of the three bright stars Vega, Altair and Deneb. This summer landmark is observable to us in the northern hemisphere most of the year, but is highest in our evening sky during July and August. When you see the Summer Triangle rising in the late spring, you know summer is near, and when the Summer Triangle is starting to set in the the west, then winter is upon us.
An asterism is a well-known group of stars, but not an official constellation. Stars making up an asterism don’t have to be in the same constellation but are often well known such as The Big Dipper, which is an asterism made of most of the brightest stars found in Ursa Major (The Great Bear).
You can find the Summer Triangle in less than ideal skies because it is made up of bright stars. The Summer Triangle is huge and covers a large area of the sky but, once you see it, you’ll always be able to pick out this large and bright asterism. To locate the Summer Triangle: look east as the sky darkens this month to find a blue-white star; that will be Vega. Vega is the brightest star in Lyra and is the brightest of the 3 stars making up the Summer Triangle. To the lower right of Vega you’ll find the Summer Triangle’s second brightest star Altair in Aquila, the Eagle. Now go back to Vega and look to the lower left for another bright star and you’ll find Deneb, the brightest star in Cygnus, the Swan.
If you see the Summer Triangle away from any city lights you’ll see it lies in a rich region of the Milky Way. Regions of the sky known as the Milky Way are actually spiral arms of our galaxy and are so rich with stars, gas and dust that they blend together and we see them as a glowing band of light across the sky. Deneb is in the thick of the Milky Way and is a great place to start exploring the sky with binoculars. You can find star clusters, glowing nebula, star clouds and even dark bands where nothing is shining. The Milky Way you see in the summer sky is an edge-on view of our own Milky Way Galaxy. We are looking into our galaxy’s flattened disk. In August you’ll see the Milky Way more prominent above the southern horizon where you are looking towards the center of our galaxy.
The stars that make up the Summer Triangle (as most stars) are varied and interesting.
Vega is a blue-white main sequence star with a magnitude of zero, making it the fifth-brightest star in our sky. From the northern hemisphere, it is very bright and easy to pick out in the night sky. Vega is 40 times more luminous than the sun, and one of the most luminous stars in the vicinity of our solar system. The color of a star is a direct indicator of it’s temperature so since Vega is a blue-white star we know it is much hotter than our yellow-orange Sun. The surface of our Sun is about 10,000 degrees while the hotter star Vega has a surface temperature of about 17,000 degrees. Vega is about two times wider than our Sun with a diameter of about 2 million miles. If the sun was at the same distance as Vega; Vega would be 58 times brighter than the sun.
Polaris is currently our North Pole Star but in about 11,500 years Vega will be the bright star that points out our North Pole. Because the Earth wobbles on its axis (like a spinning top) our axis doesn’t always point to the same point in the sky. Over the course of 26,000 years our axis traces out a circle on the sky making Vega the North Pole Star roughly every 26,000 years. Our cave man ancestors would have watched stars spin every night around Vega.
Also our solar system is headed in the direction of Vega. In fact, our sun and earth are racing at an incredible speed of 12 miles per second towards Vega, but since Vega is 27 light years away, even at this tremendous speed it would take our sun almost 500 million years to reach Vega, if it remained in the same place, which it won’t.
Altair is the brightest star in Aquila and has 2 fainter stars on each side of it. Altair is a 1st magnitude star. As most amateur astronomers knows there are many strange stars in the sky, but Altair is one of the strangest. Altair is only 17 light years away, which is fairly close for a star, and it is the 12th brightest star we can see. If we compare it to our almost million miles wide sun, Altair is 1 1/2 times its size. However, because it is not a relatively cool yellow star like our sun, but a hot white star, it is actually nine times brighter.
The really peculiar thing about Altair has to do with its rotation or the length of its day. A day for any star or planet is defined as the amount of time it takes for a star or planet to make one complete turn on its axis. The earth makes one turn on its axis every 24 hours so an earth day is 24 hours long. Our Sun has a much longer day because it makes one complete turn on its axis every 25 1/2 earth days. So one sun day is 25 1/2 earth days long. You’d think that because our sun is so much larger than our earth and turns so much slower, that an even larger star like Altair would rotate even slower than our sun does. Surprisingly thought that is not the case.
Altair doesn’t turn slower than our sun, instead it is one of the fastest rotating stars known. Altair’s rotational speed at its equator is 160 miles per second which means that Altair rotates once every six and a half hours. So one Altair day is only 1/4 of an earth day long, plus its incredibly fast rotational speed produces one very weird effect. Altair spins so rapidly that it bulges out all around its middle… so much so that Altair is twice as wide from side to side as it is from top to bottom making it more of an ellipsoid shape rather than round. In fact, astronomers find the faster a star or planet turns, the wider it gets. Jupiter rotates once every 10 1/2 hours, it too is wider from side to side than from top to bottom which you can easily see in images of the giant planet.
Deneb is slightly dimmer than the Altair but is still a first magnitude star. Even though Altair and Deneb appear to have roughly the same magnitude keep in mind that Altair is 17 light years away and Deneb is far more distant at 1500 light years away. If Deneb is almost as bright as the much closer star Altair it must be a much bigger and brighter star. Deneb is a whopping 116 times the diameter of our Sun. Deneb is actually one of the most luminous stars in our sky, roughly 60,000 times brighter than our Sun. The only other star that can rival its brightness is Rigel in Orion. In fact Deneb is so bright if it were at the same distance as our brightest star Sirius (8.8 light years away) then Deneb would be as bright as our full moon. If Deneb was located by Alpha Centauri, our nearest star (four light years away) you could easily read by it at night.
Deneb is a blue-white super giant star with a surface temperature close to Vega. Deneb is a massive star with a mass 25 times that of our Sun. Stars this massive do not stay on the main sequence more than a few million years. These massive stars become red super giants eventually ending their lives as supernova.