Stars over Sisters

After bundling up against the cold, one of the best times to stargaze is on frosty December evenings when the sky is clear, and the stars are bright. Cassiopeia is a prominent wintertime constellation that is easy to recognize because five of its primary stars are arranged in the shape of a "W." Using Ursa Major (the Big Dipper) to locate the North Star (Polaris), extend an imaginary line from the dipper through Polaris to an area of about 30 degrees on the other side of the sky. Cassiopeia will lie nearly overhead at approximately 8 p.m. local time during December. Viewed from our latitude here in Central Oregon this is a circumpolar constellation, meaning that it never sets below the horizon.

Shining at a magnitude of 2.2, Schedar is Cassiopeia's brightest star. It is a red giant star that is four times more massive, 90 times larger, and about 734 times brighter than our sun, and lies at a distance of 228 light-years.

Cassiopeia, as well as several neighboring constellations, are featured prominently in one of the most famous accounts in all of sky lore. Cassiopeia was the vain queen of Aethiopia who boasted that she and her daughter Andromeda were more beautiful than the Nereids, or sea nymphs. This angered Poseidon, who sent a sea monster (Cetus) to wreak havoc on the kingdom of Aethiopia. To stop the sea monster from destroying their kingdom, King Cepheus and Cassiopeia were required to chain their daughter, Andromeda, to a rock as a sacrifice to Cetus. Just as Andromeda was about to be devoured by the sea creature, the hero Perseus noticed her predicament as he was flying above on his winged horse Pegasus. Perseus quickly swooped down and turned the monster to stone by showing it the severed head of Medusa (who he had just killed). Soon after the rescue, the two were married.

Because a branch of the winter Milky Way runs through Cassiopeia, many exceptional deep sky objects are found here. Two of these are open star clusters M52 and M103, both bright and favorite targets of amateur astronomers. M52 is about 35 million years old and contains about 193 stellar members, while M103 has 172 stars that are 25 million years old. Another beautiful open cluster is NGC 7789, which contains at least 1,000 stars that are well over a billion years old. It is also known as Caroline's Rose because it was discovered by Caroline Herschel in 1783.

Photo courtesy NASA/ESA/Hubble St.

The Bubble Nebula is an emission nebula that was created by intense radiation from a hot nearby star.

Less than one degree to the southwest of M52 is the visually striking object NGC 7635, more commonly referred to as the Bubble Nebula. Classified as an emission nebula, this celestial bubble was created by the strong stellar wind of a nearby hot, massive star. Moving at supersonic speeds, these highly charged particles are slamming into surrounding interstellar material, thus shaping the bubble.

The Geminid meteor shower peaks between the evening of December 13 and the morning of the 14th, when up to 120 shooting stars per hour may fly through the sky. Unfortunately, light from a nearly full moon will make many of them hard to see.

All five of the visible planets can be seen this month. When it gets dark, brilliant Venus appears low in the southwest, which will set a short time after sunset. As the month begins, Saturn, in Aquarius, will stand 40 degrees above the southern horizon by nightfall, then set before midnight. But it's dazzling Jupiter (in Taurus) that will rule the evening skies, appearing at dusk and remaining visible all night!

Rising at 8:30 p.m. local time, Mars (in Cancer) will brighten rapidly this month before transitioning to an evening object in January. After about December 10, Mercury, the solar system's smallest planet, becomes well positioned to be observed, rising an hour and a half before the sun on the last day of the month.

Dark sky tip of the month: While Christmas lights may help put you in a festive holiday mood, remember that they are still lights. Make sure they are unlit during normal resting hours and during the day. Recommended hour to turn your lights out is around 10 p.m. If that isn't possible, please switch them off when you go to sleep.