Except for the cold, mid-winter is a pretty good time to star gaze. For one thing, there aren't any mosquitoes or other pain-in-the-neck bugs out yet. In addition, because the air is usually drier and therefore more clear than it is in spring or summer, the night sky can be breathtaking. Moreover, it's darker longer, and there just happens to be a lot to see, including bright stars and planets and faint nebulae and deep space objects.

By this weekend, the Moon will begin to wane, so the stars and planets can shine in all their glory. On a clear, moonless winter night, the constellation Orion (the Hunter) quickly captures our attention. It's one of the first constellations that people learn to recognize. Although the Big Dipper (Ursa Major) is up all night, it's relatively low in the northern sky now, and so it's not all that easy to pick out in the early evening. Leo the Lion is just rising in the east around 8 p.m., and the Great Square of Pegasus is dipping down in the western sky. The Big Dipper and Leo will be high in the sky in another few months, but Pegasus won't return to prominence in the evening sky until about September.

When we look at the stars, we notice how bright they look rather than if they're big or small. Their brightness depends on their distance from us and their intrinsic brightness or luminosity. Of course, all the stars aren't the same size, but from our perspective, they're so far away that they all looks like points of light.

Currently, 12 objects with a first order magnitude or brighter are visible. The brightest object is Jupiter, which shines at a magnitude of -2.6 during the tail end of January. Remember, the lower the magnitude, the brighter the object. Each order of magnitude is approximately 2.5 times brighter than the order above it. For example, a -2 magnitude star is 2.5 times brighter than a -1 star and 2.5 times dimmer than a -3 star. On New Year's Eve, Jupiter was almost directly overhead as the clock struck midnight. It's residing for the time being in the constellation of Gemini (the Twins), and it'll rule the night sky for several months to come.

The second brightest object in the sky now is the star Sirius (about -1.5 magnitude) in the constellation Canis Major (Big Dog). Sirius is to the lower left of Orion, and Orion's belt, when followed to the southeast, points in the direction of Sirius. For most of the residents in the Northern Hemisphere, Sirius is the brightest star that we can see.

Saturn, having a magnitude of about -0.3 (about 1/10 as bright as Jupiter) is the third brightest object in the night sky. Though not as striking as Jupiter with the unaided eye, with a modest telescope, Saturn's rings may be visible. Saturn is further west than is Jupiter and at mid evening is nearly overhead.

Mars is also one of the 12 brightest objects in this month's night sky, but our ruddy neighbor is now sinking in the west. It's about +0.7 magnitude or an order of magnitude fainter than Saturn. Earlier in the month, Mercury could easily be seen low in the western sky after sunset, but currently it's pretty much lost in the glare of the setting Sun. As for Venus, it's a "morning star" and can now only be seen before dawn.

Getting back to Orion, aside from being easy to identify, it's one of the more interesting constellations. Not only does it contain bright stars (Rigel and Betelgeuse) and stars that trace out straight lines (Orion's Belt and the sword of Orion), but it also has a star making factory (The Great Nebula) visible to the unaided eye. The Great Nebula is the blurry looking "star" in the middle of the sword of Orion, dangling from Orion's Belt. It's more than 1,500 light years away.

Rigel is a hot blue-white star and the 3rd brightest star in our winter sky. However, considering how far it is from us (about 1,000 light years distant), it may very well be the brightest thing we can see in the Heavens, with the unaided eye. It's absolute magnitude is -6.4, which means that it's approximately 1,600 times more luminous than Sirius, and it's about 33,000 times more luminous than the Sun! Rigel's companion, Betelgeuse, is a fiery orange-red and is also very bright (more about this later). It just happens that Betelgeuse is the only major star in Orion that isn't a white star. Rigel and Betelgeuse are diagonally opposite of each other in Orion and appear to be equidistant from Orion's Belt. A line drawn between them would be nearly perpendicular to the "belt." Betelgeuse is at the left shoulder of Orion, while Rigel marks the hunter's right foot.This configuration is what makes Orion so conspicuous to star gazers throughout the world. Because Orion is located in the night sky near the Celestial Equator (the imaginary line in space directly above the Earth's Equator), its stars can be seen by nearly everyone in both the Northern and Southern Hemisphere.

Betelgeuse is the closest to us of Orion's 10 or so stars - it's about 500 light years away. It's also the biggest star in Orion. In fact, Betelgeuse is the single biggest thing that our eyes can detect. Sure, galaxies and star clusters are bigger, but they're a collection of stars rather than a single object. Compared to other stars, our old Sun is pretty much average, however, Betelgeuse makes our Sun look absolutely puny, and it's gargantuan compared to the Earth. According to astronomer Bob Berman, if Betelgeuse were a ball that enclosed a 20 story building, than the Earth would be the period at the end of this sentence. Using another analogy, if Betelgeuse were a spherical jar, and if Earth-sized objects were poured out every second, it would take 30,000 years to empty the jar!!

Another thing about Betelgeuse is that it's not always the same size or brightness. Over a period of about 14 months, it swells up and then deflates a bit. When it's at its biggest, it's even brighter than Rigel. However, when Betelgeuse is quiescent, it no longer ranks in the top 10 of the brightest stars. Betelgeuse is much more oblate (fatter in the middle) than our Sun since its center of gravity is so far from its surface. If our Sun were replaced by Betelgeuse, in its bloated phase, it would extend out past the orbit of Jupiter! The force of gravity on this massive bag of gas causes Betelgeuse to partially collapse. It'll deflate a bit and then rebound again in an endless cycle. All known red giant stars exhibit this kind of behavior, but since Betelgeuse is the biggest of the big, it's bulging and shrinking is more pronounced.

All-in-all, Orion has a lot going for it. In a single constellation resides the biggest star, the most luminous star and the most impressive nebulae our eyes can see. Even on a cold winter's night, Orion is worthy of more than just a casual glance.

For more about this see Abrams Planetarium Home Page (http://www.pa.msu.edu/abrams/diary.html)
and the book Secrets of the Night Sky by Bob Berman