What’s Up in the Sky – April, 2019
Seeing Double
Many readers are familiar with double stars, the most well known probably being two stars in the handle of Ursa Major, the Big Dipper. Mizar, the bright star in the bend of the handle, and Alcor, its fainter companion, form a binary pair, known to the ancients and used as a test of warriors’ eyesight. But a closer look in even a modest telescope reveals that Mizar is itself a double star and was in fact the first double star to be discovered, in 1617. Recent observations with high-tech instruments reveal that all three members of this group are themselves double stars, making the system a true sextuple!
Now and in the coming months a number of double stars will come into view. Some can be observed with amateur equipment, so I would like to present a beginner’s guide to a few of them.
Let’s begin with Sirius, the brightest star in our sky and one of the easiest to find. Follow Orion’s belt toward the left until you come to it, shining like a beacon in the southwest. Its companion, known as Sirius B, was discovered in 1862 by Alvan Clark (using a pretty big telescope), and is a white dwarf star whose mass is roughly equal to that of the Sun, but is packed into a volume about the size of the Earth.
Above Sirius, in the constellation Gemini, is the star Castor, the right-hand member of the Castor and Pollux pair. Castor is an accessible binary for smaller telescopes. Its two components form a spectacular and bright blue-white pair, both of which are what is known as spectroscopic binaries. That means they can only be detected by analyzing their spectrum, the patterns of the light each emits. This is one way astronomers detect exoplanets which are much too dim to observe visually.
Returning to Orion we find another type of double star, a visual double. Look at the bottom star in the sword hanging down from the left side of his belt. A modest pair of binoculars shows it to be two stars, but these are not gravitationally bound to each other as all the previous examples are. One is actually several hundred light years farther away, but along nearly the same line of sight, making them appear to be close together.
I may be getting ahead of myself, but while we are on the subject of double stars, I must mention two of my favorites, the double-double in Lyra and Albireo in Cygnus. Although these two constellations dominate the summer sky, late night observers can spot them beginning in early April and on into May.
Around midnight at the end of the month, look northeast and you will see the star Vega, about thirty degrees above the horizon. It is the top right corner of an equilateral triangle of stars. Look closely at the star just to the left of Vega. Known as Epsilon Lyrae, it is a double star, easily visible in a pair of binoculars. If you are able to observe it through even a small telescope, you will see that each component is itself a double, hence the name double-double. It is truly a spectacular sight for small ‘scopes.
Alberio is the star at the head of Cygnus, the Swan, and consists of a yellow giant and a blue dwarf. The colors are distinct, even in binoculars, and are the reason some refer to Alberio as the U of M star.
I hope you will now be able to enjoy some double-star-gazing during your tours of what’s up in the sky.
This month in history:
April 2: First photograph of Sun taken – 1845
April 9: NASA selects original seven Mercury astronauts – 1959
April 12: Yuri Gagarin becomes first human in space – 1961
April 12: Columbia becomes first space shuttle to be launched – 1981
April 17: Apollo 13 returns to Earth – 1970
April 20: Apollo 16 lands on the Moon – 1972
April 24: China becomes the fifth nation to launch its own satellite – 1970
April 28: Eugene Shoemaker is born – 1928
