Greetings stargazers.
Orion’s belt is making its annual return to our early evening sky. From east to west, the belt stars are Alnitak, Alnilam and Mintaka. These three blue giant stars form one of the most recognizable asterisms in the night sky, as they are almost the same visual magnitude and closely spaced along a straight line.
Collectively they have been known, among other things, as the Three Kings, the Three Marys and the String of Pearls. As with many of the stars in the night sky, their modern names are derived from Arabic.
All three have been studied extensively, and Alnitak and Mintaka are parts of multiple star systems. Blue giant stars are the hottest and among the most luminous of all stars. They are typically much more massive than our sun, but because they are burning their fuel thousands of times faster, they will be very short-lived. There will likely be a supernova in the group within the next million years or so. These three stars do not appear as bright as the more prominent stars in the constellation Orion – Rigel, to the south, or Betelgeuse, to the north, but that is only because they are much farther away.
Such luminous stars have tremendous stellar winds, as the intense radiation is very effective at blowing mass away from the stars’ surface. These stars are losing mass through their stellar winds at rates thousands of times faster than our sun.
The belt is conveniently located along the celestial equator, with the star Mintaka being much less than one degree away, so these stars are equally visible in both hemispheres. It has long been used for navigation and has been referenced in literature since ancient times among many cultures. For example, the belt of Orion is mentioned in the book of Job. A more modern reference to the belt was for a cat’s collar in the first “Men in Black” movie.
Useful links
Orion’s Belt
https://en.wikipedia.org/wiki/Orion%27s_Belt
Astronomy picture of the day
http://apod.nasa.gov/apod/
An Astronomer’s forecast for Durango
http://www.cleardarksky.com/c/DrngoCOkey.html?1
Old Fort Lewis Observatory
http://www.fortlewis.edu/observatory
The winter solstice is Dec. 21. Although it has the fewest daylight hours, we have already passed the date of the earliest sunset in Durango, which was at around 4:52 p.m. Monday. The latest sunrise isn’t until Jan. 5. These apparent sunrise and sunset quirks have to do with the Earth’s orbit being elliptical. Perihelion, or the closest approach we make to the sun, is in January and is mostly independent of the tilt of the Earth’s axis. The latest sunrise and earliest sunset times happen every year, but the solstice is the date that gets all the attention.
The Geminid Meteor shower lasts for a couple of weeks each December, and its peak was this weekend. The moon is in a waning crescent phase, so it won’t rise until well after midnight, giving a maximally dark sky in the evening for good meteor shower viewing. These meteors appear to radiate from the constellation Gemini, thus their name. Gemini is the constellation just to the north of Orion. Or, when they are both coming over the horizon in the early evening, Gemini will be just to the left of Orion.
Also prominent in Gemini this month is Jupiter. Saturn is the other prominent planet in the evening sky. In early evening it will be due south and then set in the west around midnight.
If you go outside for some extended meteor shower watching, remember to dress warmly. A hint I have given previously is to take a piece of Styrofoam to put your feet on so the cold from the ground doesn’t make your feet too cold too quickly. And if you are going to sit, you should also sit on something insulating for the same reason.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. He can be reached at hakes_c@fortlewis.edu.
