What’s Up in the Sky – September, 2010
By Peter Burkey
September often has cool, clear nights that beckon us out for some stargazing. An interesting and often overlooked category of observing targets is binary, or double stars.
Double stars come in many styles, sizes, and colors. Most are pairs of stars that are gravitationally bound to each other just like Earth and the Sun are and therefore actually orbit each other. Others, called optical doubles, are just two stars that appear close in the sky, but one is lots farther away and they are not gravitationally bound.
Some double stars can be seen with the naked eye. The next time you see the Big Dipper, look closely at the middle star in the handle. You should notice a faint “companion” star right above the brighter primary star. The brighter one is Mizar and the companion is Alcor and they form an optical double. What is more interesting, though, is that if you look at Mizar through a telescope you will see that it is actually two stars, a true binary system. In 1650, Mizar became the first star known to be a binary.
Some binary systems are fairly exotic, especially when one of the companions is much more massive than the other. The heavier one evolves faster and sometimes becomes an invisible white dwarf orbiting a giant blue or red star. Sirius is an example. The intense gravity from the denser dwarf star attracts material from the companion causing the dwarf’s mass to increase. Eventually it reaches a limit, collapses, and explodes (called a Type 1 Supernova).
Even more bizarre stuff happens if the binary system is composed of really massive things like two pulsars. A pulsar is a rapidly spinning neutron star that emits a very precise radio signal that can be used to precisely measure its orbit. A pulsar is so massive it actually warps spacetime which affects the radio signal we receive. The study of such systems may reveal some of the secrets of General Relativity.
For more tips on double stars and the rest of what’s up in the sky, go to:
This month in history:
Sept. 3: Last two lunar landings canceled by NASA – 1970
Sept. 8: Voyager 1 launched – 1977
Sept. 17: First powered flight of X-15 rocket plane – 1959
Sept. 21: Second flyby of Mercury by Mariner 10 – 1974
Sept. 26: Israel launches its first satellite – 1988
Sept. 30: End of daily communication with Pioneer 11 – 1995
Here are this month’s viewing highlights:
Planets this month: Venus continues to dominate the western horizon at dusk. Look for the crescent Moon to the planet’s lower right on the 10th and upper left on the 11th. Jupiter rises just after sunset and is visible in the SE before midnight.
Sept. 1: Last-quarter Moon
Sept. 8: New Moon.
Sept. 10-11: Look low in WSW starting at 8:30p.m. for crescent Moon and Venus.
Sept. 15: First quarter Moon
Sept. 22: Autumnal equinox – first day of fall in Northern Hemisphere.
Sept. 30: Last-quarter Moon