Orion’s Belt
Published on Dec 24, 2006 at 9:43 pm.
1 Comment.
Filed under constellations, stars.
I am now back home from my trip. Driving west early in the morning, leaving for my trip, I saw Orion ahead of me, guiding me onward. As I drove east, returning home, Orion again guided me shortly after sunset. This is the time of year that Orion is up all night long. It is even more obviously so for those of us in the Northern Hemisphere.
Orion is easy to spot. It is one of the few constellations that most people know, even if they know little else. What makes Orion so easy to spot is his belt. Three stars of similar brightness make the belt of Orion. They are (from east to west) Alnitak, Alnilam, and Mintaka.
Alnitak, the leftmost star in the belt when seen from the Northern Hemisphere (rightmost from the Southern Hemisphere) is magnitude 1.70. That’s fairly bright for a star, making it one of the top 35 brightest stars. It is spectral type 09.5 Ib, which is a very hot, very bright, supergiant. It shines with nearly 100,000 times the light of our own Sun. That is is only magnitude 1.7 tells you that it is far away — about 800 lightyears distant, in fact. It is a multi-star system, with two companions, with spectral types B0 and perhaps O9.5. The name Alnitak comes from the Arabic meaning “The Girdle.” Alnitak has Bayer designation Zeta Orionis and a Flamsteed number of 50 Ori.
The middle star of Orion’s belt is Alnilam, and it is one of the common navigation stars. At magnitude 1.70, it is about the same brightness as Alnitak, but being the middle star of the belt makes it hard to confuse with any other star. It is spectral type B0 Ia, only a tiny bit cooler and somewhat more luminous than Alnitak, being perhaps almost 350,000 to 400,000 times brighter than our Sun. But, Alnilam is farther away, at between 1000 and 1300 lightyears distant. There is a very faint reflection nebula lit by Alnilam. The star Alnilam is Epsilon Orionis in the Bayer nomenclature, and has Flamsteed number 46 Ori. The name Alnilam comes Arabic meaning “The String of Pearls.” It represents ornaments on Orion’s belt.
The westernmost star of the belt, the one that rises first, is Mintaka. Mintaka, too, is a multiple star system, and the closest two orbit one another in only 5.7 days, eclipsing one another, causing the magnitude of the star to vary between 2.14 and 2.26. This makes Mintaka the dimmest of the three belt stars. It, too, is spectral type O9.5, but it is considered a giant, not a supergiant like the others, so it is O9.5 II, shining with only 70,000 times the luminosity of the Sun. The companion is a B2 V star. Mintaka is 900 lightyears distant. The name Mintaka means “The Belt.” Mintaka has a Bayer designation of Delta Orionis and a Flamsteen number of 34 Orionis.
As I said, Mintaka means Belt, and that name has been applied to the entire set of three stars. But, so has the name Alnilam, as if the line of stars were the string of pearls, and even the name Alnitak has occasionally been used to represent the set of three stars. Eventually, the names came to be applied individually to the indicated stars.
Each of these stars is a very massive star, with 20 to 40 times the mass of the Sun. Such stars live very short lives and die explosively as a supernova. Each of these stars is similarly far away, and have similar motions in space. Undoubtedly, these stars are fairly young (as stars go) and are part of the active star forming activity going on in the direction of Orion.
Orion was one of the first constellations that I ever learned, and it was a welcome friend in my travels.
-Astroprof
Arabic movies fan on June 13, 2008 at 3:25 pm: 1
thanks for the explanation of those interesting name, and your blog is galactically cool