The amazing stars of Orion
Orion the Hunter is high and bright in these shortening nights of late winter. As seen in the diagram, he stands tall in the southern sky by 7 p.m. Saiph (safe) and Rigel (RYE-gel), marking his knees, are about 35 degrees above the horizon. Reaching toward the Summer Solstice as though to remind us that winter will end, he stretches another 30 degrees to Chi 1 (kye, rhymes with eye) and Chi 2 marking the end of his club.
In my last column, I described the star formation processes going on in the vast molecular cloud complex surrounding Orion. This means that most of the stars are very young … mostly less than 10 million years. That is, they were formed at about the same time that the first bipedal apes were emerging from evolutionary paths in Africa. Our sun, in contrast, is about 5 billion years old.
The three bright, bluish stars along Orion’s belt are some of the most easily identified stars in our sky. These are among the hottest of our familiar stars. Alnitak (ALL-nih-talk), the rightmost (easternmost) of the three is the hottest, with a surface temperature of about 31,000 Kelvin (or Celsius … to convert from Kelvin to Celsius, subtract 273, which makes little difference at 31,000). This is over five times hotter than the Sun’s 5,800 K and most of its luminosity is in the ultraviolet part of the spectrum. For life to survive on an Earth-like planet orbiting this star, it would have to be 300 times farther that we are from the Sun. The immense energy it emits comes from the fact that it is over thirty times the mass of the Sun and fusing hydrogen at a much greater rate. Though it’s only about 6 million years old, it’s close to exhausting its hydrogen fuel and will soon begin its expansion to a red giant and ignition of helium fusion. The middle star of the belt, Alnilam (ALL-nih-lahm) carries the Arabic name that use to apply to the entire belt, meaning “string of pearls.” It is the brightest, and the most distant of the belt stars, 1,342 light years away. Younger than Alnitak at only 4 million years, it is more massive (40 solar masses) and more luminous, though its surface is not quite as hot at only 25,000 K. It’s thought to still be fusing hydrogen. Mintaka is the westernmost and northernmost of the three belt stars. It holds an important position in our sky as it lies a mere quarter of a degree south of the Celestial Equator. This shows that Orion straddles the equator and is visible from all equatorial and mid-latitude locations on Earth. Every human culture recognized, named, and told stories about this bright group of stars that emerged from the mists of gas and dust as our most ancient ancestors began to emerge from the mists of time.
The brightest star in Orion is bright, red Betelgeuse (BAY-tull-juice). Closer than the belt stars at 570 light years, its red color comes from the low temperature of its outer layers. Only 11 solar masses, at an age of 8.5 million years, Betelgeuse has exhausted its helium fuel and is likely fusing helium to carbon. This process has swollen the star into a red supergiant. The outer layers are so diffuse and complicated that it’s difficult to identify a “surface” defining its size. Estimates are that if Betelgeuse replaced the Sun, its outer layers would be between the orbits of Jupiter and Saturn. All the terrestrial planets would be inside the star.
As Betelgeuse fuses elements through neon, magnesium, sodium, and silicon, it will ultimately create and iron core and explode as a Type II supernova. Earth observers will see the exploded star as a nebula as bright as a gibbous moon (between quarter and full) easily seen even in daylight.
North of Betelgeuse are two stars that appear very similar but are extremely different: Chi 1 and Chi 2 at the end of Orion’s club. Chi 1 is slightly brighter, but it is the much less impressive star! It’s actually a bit smaller than the Sun with about the same surface temperature. At a distance of only 28 light years, it’s one of our close stellar neighbors. Chi 2, however, is sixty-four times more distant, sixty times larger and forty times more massive than the Sun. At only 5 million years of age, this star will evolve quickly and may be half-way through its life, but is still fusing its large supply of hydrogen. Its size and mass make this star a hypergiant, another of which is Eta Carinae (car-REEN-eh, rhymes with hay) in the southern hemisphere that has been blowing off layers of itself in to an immense nebula. Chi 2 Orionis, like other hypergiants, is blowing off immense amounts of material in a strong stellar wind. Each 100,000 years it blows off the mass of the Sun! This mass loss prevents these stars from becoming red giants like Betelgeuse. As we continue to observe and study Eta Carinae, we hope to learn of the fate of Chi 2 Orionis, as well.
The two hottest stars in Orion, with temperatures of 35,000 K and 32,500 K are among the less familiar stars. Meissa (mesa, like the elevated table lands of my native southwest) marking Orion’s head and Na’ir al Saif (NAR-all-safe) just south of the Orion Nebula. Meissa, 28 times more massive than the Sun is the hottest star in Orion and illuminates the 150 light year wide Meissa Ring shown in the column for 2/13/2018. This very hot star will, like most of the stars in Orion, end in a supernova and, perhaps, a neutron star creating a pulsar like that in the Crab Nebula not far to Meissa’s north.
Na’ir al Saif is actually the brightest member of an open star cluster, that is, a sibling cluster of stars … a stellar “litter”. It has at least two other gravitationally bound companions and the cluster, NGC 1980, contains at least eighteen other stars.
Saiph and Rigel are the southernmost bright stars of Orion and both are also impressive giants. Saiph is actually emitting more energy than Rigel, but at its hotter temperature, 26,000K, most of its emission is in the ultraviolet band that we can’t see. Rigel is actually a system of three stars of similar ages and brightnesses. The brightest one is suspected to have a dead hydrogen core and is in the process of becoming a red giant that may rival or even surpass the ruddy appearance of Betelgeuse.
The volunteer astronomers at the Adirondack Public Observatory are eager to point out all the stars of Orion and offer views of the nebulae through our telescopes. The Roll Off Roof Observatory (RORO) is open to the public on the first and third Fridays of each month approximately one half-hour after sunset. Whether you’re an avid amateur astronomer or have never visited an observatory, come and view through our telescopes and learn about the Wilderness Above. For updates and notices, check out our website at adirondackpublicobservatory.org and our Facebook page. On our public observing days you can also call the RORO at 518-359-6317 to talk with one of our astronomers.