Michelle Thaller
Loving the Night Sky Part III: Orion
Some of my reactions to the night sky have nothing to do with my profession as an astronomer. Case in point: every year around this time, I look forward to getting my annual first glimpse of the constellation Orion.
I love Orion. It's been my favorite constellation since I was a little girl, when my father used to take me on long winter evening walks, crunching through the snow in suburban Wisconsin. Seeing Orion meant that winter was truly here, and it was a very personal way to feel the turning of the seasons. It was the return of a friend. I've waited for Orion's return now for many years, even when I lived in distressingly different parts of the world. During a time when I lived in Australia, nothing in the sky put my stomach in knots more than seeing Orion upside down!
Unlike the Big Dipper, which is always visible from the northern hemisphere, Orion is a seasonal constellation. It only becomes prominent in the eastern sky around December, and dips below the western horizon again in May. The reason Orion is only visible during certain times of the year is actually quite simple. The direction we look out into space to see Orion is fairly close to the path that the sun takes across our sky.
During the winter months, the night side of Earth (which is the side of Earth pointed away from the sun) looks out toward Orion. During the summer, when we're on the other side of our orbit around the sun, the same area of the sky is now on the day side of Earth, facing the sun. Orion is still there, but you can't see the stars in the bright daytime sky. The seasons are reversed in the Southern Hemisphere, so "Down-Under" Orion signals the return of summer.
Orion is a wonderful constellation for many reasons, some of which became apparent to me only after I began to study the universe in earnest. But starting off with simple things, Orion is probably the easiest constellation to pick out.
There's something pleasing about the pattern of stars. I could take out a pencil right now and draw (with a fair degree of accuracy), what Orion looks like. There are precious few other constellations (the Big Dipper might be another) I can remember that well. And hanging in a dark winter sky, Orion seems full of vibrant color.
Bright red Betelgeuse, the star in the upper left corner of Orion is one of the most obviously colored stars in the sky. Providing a vivid contrast is Rigel (lower right corner) which glows a brilliant blue. The three blue-white belt stars are found easily by even the most casual observer, and they're also a great tool for navigating around the winter sky. Sloping down to the left, the belt stars point to Sirius (in the constellation Canis Major), the brightest star in our sky. In the other direction, the belt stars point to another famously red star, Aldebaran, in Taurus.
But Orion is more than a lovely pattern of stars. More than any other part of the sky, the Orion region has an incredible story to tell. Let's take a more in-depth tour of my favorite constellation.
The belt stars are called (moving from left to right) Alnitak, Alnilam, and Mintaka. The names of these stars are all various Arabic words for "belt." Most of the stars in Orion have beautiful names, remnants from a time when the Middle East led the world in academic learning, and Arab scholars mapped and named the stars. I studied the belt stars of Orion (as well as other similar stars) for my Ph.D. research.
My particular studies involved very massive, hot stars, which are hard to find in the sky. The belt stars are very rare indeed. They are all about the same distance away from us (about 1,500 light years), and probably all formed at the same time, from the same molecular cloud. They are also all about 20 times more massive than our sun, meaning they will burn through their nuclear fuel in only a few million years. After that, one by one, the belt stars will blow up in spectacular supernova explosions.
Rigel, whose name means "Foot," is the bright blue star at the bottom right of Orion. Rigel is a sister of the belt stars, and probably formed right along side them a million years or so ago. Like it's siblings, Rigel is burning very bright and hot, and probably doesn't have much time left to live. Rigel is somewhat larger than our sun (about 20 times the mass and 70 times the size of our star), but it burns its fuel at such a high rate that it shines with the light of 50,000 suns. A quick calculation reveals that it's burning through it's fuel way too fast to last much longer. Like the belt stars, Rigel will go supernova in the near future, at least in astronomical time (probably in the next few hundred thousand years).
But the most incredible Orion star of all would have to be Betelgeuse. I've always loved to say "Beetle-juice," although the name is actually pronounced "BAY-tell-juice" and means "Hand." One of the reasons Betelgeuse looks so big and bright in our sky is that it actually is big -- it's the only star besides our sun that telescopes can resolve into a disk, instead of a tight point of light. At a distance of 500 light years, that means that Betelgeuse is HUGE, about 700 times bigger than the sun in diameter. If Betelgeuse were placed in our solar system where the sun is, it would swallow up all the planets out to Jupiter.
Why the giant size? Astronomers don't really know the details of what's happening, but they do know that all stars, when they run out of hydrogen in their cores, swell up and cool down. With no hydrogen to fuel its internal nuclear reactions, Betelgeuse is fusing helium, the leftover ash from its former nuclear fires, into carbon and oxygen (see my column "The Lives of the Stars" for more info.) Betelgeuse is very close to dying. The unstable reactions in its core have caused the star's outer atmosphere to bloat up to titanic proportions. This expansion of Betelgeuses's atmosphere has cooled the outer parts of the star down, giving rise to the deep red color we see in the sky.
Of all the "live-fast-and-die-young" stars in Orion, Betelgeuse is likely to be the first to go. Many astronomers bet that it will be the next major supernova in our galaxy, and when it goes off, it will be a spectacular show. Five hundred light years is far enough away for us to be safe, but the resulting explosion and after-glow will rival the moon in brightness, and be easily visible during the day.
There's so much drama in Orion, but it all seems to be about death. The two brightest stars are very near to blowing themselves up, and the belt stars will soon follow. Obviously, humans in the far future will have a very different view when they look towards Orion. But is there no hope? Will all the stars just explode and die?
Actually, there's quite a bit of hope, and that's what I think about most when I look at Orion. Something mysterious is going on right behind the familiar figure in the sky. And it's a well-hidden mystery too. Orion is the site of the largest stellar nursery in our part of the Milky Way Galaxy. There's a giant cloud of dust and gas that, if you could see it, would take up most of the region in and around Orion. Compared to the size of the Moon in the sky, the Orion cloud is fifty times larger.
So why can't you see it? Well, because it's dark. Really. The cloud is so thick with dark dust that it blocks any starlight that tries to get through. The only stars we see in Orion are located in front of this vast darkness. From infrared and radio observations, astronomers know that there are hundreds of new stars forming in the Orion cloud right now, and many of them are hot, giant stars that will someday rival the bright stars we see today.
The dark cloud is all but invisible to us, but there is one small gap where we can peer inside. It's called the Orion Nebula, and it's a blister that's been pushed open by four young stars that are just emerging from their birth cocoons. You can find it in the Sword of Orion, three somewhat faint stars that hang down from the Belt.
The central "star" of the Sword is actually not a star at all, but the Orion Nebula. Even with a small telescope or binoculars, you can see the four newborn stars, called the Trapezium, as well as giant glowing tendrils of gas that make up the interior of the nebula. With the Hubble Space Telescope, astronomers were able to see not only new stars forming in this cloud, but entire planetary systems as well. There are even mysterious giant free-floating planets that don't seem to be associated with any star. We can't even begin to explain how they got there, much less how they formed in the first place.
The inside of the cloud is ablaze with shock waves and radiation from the new stars; you can see all this light pouring out of the tiny hole we call the Orion Nebula. In other regions of the cloud, behind veils of dust too thick to look through, more mysteries are playing themselves out.
Even with so much death and birth swirling around him, my old friend will probably rise, just like he is, every winter of my life. But the cycle goes on, which makes me smile and shiver as I look up at Orion on cold winter nights.