July: Double Stars and Black Holes

July 2008

Double Stars and Black Holes

By Calvin L. Chrisman

Summer began astronomically speaking at 7:58 EDT on June 20th. This is when the Sun reached its farthest northern elevation during the year and began moving southward in the sky. Although the shorter nights offer less opportunity to look at the stars, the nights are warm and our current lack of clouds makes for good observing.

July is a good time to go outside and look for double stars. There are a surprising number of stars that are actually doubles; either gravitationally bound to one another or in positions that make them appear to be doubles from our position on Earth. By some estimates, almost fifty percent of the stars we see are doubles, many of which can only be resolved by very large professional telescopes, but many others are easily spotted. Go outside after 10:00 with binoculars, a telescope or just your eyes and see if you can spot some doubles. The first two weeks of July or the last ten days will have the least interference from the bright Moon.

Start out looking south. About twenty degrees above the horizon, you will see a very bright red star. This is the eye of Scorpius, the Scorpion. This is a readily identifiable constellation. If you follow its body down toward the horizon, at the base of the Scorpion’s body is a double star known as Zeta Scorpii. It is about fourth magnitude and should be visible before the Moon is up. Two stars, one an orange shade and the other white will be visible to those with sharp eyes and to anyone with binoculars. The color differences are subtle but perceptible.

While you are looking in this direction, look low in the sky to the east (left) of Scorpius for the constellation Sagittarius. It too is recognizable, looking like a teapot tipping over to the right. Look carefully to see if you can see the teaspoon and lemon wedge that accompany it. Take time on a moonless night in a dark place to let your eyes fully adjust to the dark and look at the sky above the teapot. Boiling out of its spout is the Milky Way, our home galaxy. As you are looking in that direction, you are looking directly into the heart of the galaxy, the center of which contains a large black hole (no, you can’t see it – that’s why it’s called a black hole!). Following directly behind the teapot is a very bright object which is the planet Jupiter. Jupiter will be dominating the sky for a while now, and it will be discussed more next month.

There are other double stars you can look for in the summer sky. Look to the north and find the Big Dipper. The middle star in its handle is called Mizar and has a companion called Alcor which should be visible to the naked eye. Alcor is about three light years beyond Mizar, and so is not a true gravitationally-bound double, but appears so from our perspective. If you look at Mizar with a telescope, you will see that it is also a true double star.

The final double to look for is the famous “double double” known as Epsilon Lyrae. Look high in the sky and a bit to the east of the meridian (an imaginary line running from north to south and passing directly overhead). You will see three bright stars that make up the Summer Triangle. Deneb will be the northernmost of the three representing the head of Cygnus the Swan. Altair is the southernmost and Vega, in Lyra, the Lyre, is to the east. Below Vega is the parallelogram of four stars that makes up the Lyre. In the top right of the Lyre is Epsilon Lyrae, which appears to be a single star to the naked eye. Binoculars will reveal two stars (the “double”) and a telescope will reveal four stars (the “double double”). If you have access to a telescope, it is worth a look.


Clear Skies!


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