Like families gathering for the holidays, the winter night sky includes stars at varying ages in their lifetimes. Astronomers have not been around long enough to have witnessed the life cycle of any single star. Rather, we have to figure out how stars evolve by looking at these different-age stars. It is as if all the information we have to figure out how people age has to be acquired by looking at a single group picture from a family reunion.
We can see the stellar birthing room this month in the Orion nebula – a huge cloud of gas and dust from which stars are forming. It is found in the sword of Orion, a constellation that stands high in the south about 10 p.m. You can see the nebula even in a small telescope or pair of binoculars. (See a chart at www.upintheair.info.)
Newly formed stars reach their stable state fairly quickly, in astronomical terms, and spend most of their lives with a temperature that depends on their mass. More massive stars are larger in diameter and hotter, glowing blue-white. The smallest stars are cooler and glow orange-red. Between the extremes, the rest are seen as yellow-white.
The brightest star in the sky, Sirius, is found to the lower left of Orion, in Canis Major. It is hotter than our sun but still seen as just white. Rigel, at the bottom-right of Orion and below the sword, is hotter and may be seen to have a bit of a bluish cast. Bellatrix, at the top-right of Orion, is twice as hot as Rigel and shows bluer.
Like some people, as stars age they grow larger in diameter, expanding into a giant phase of their lives. Their surface temperature drops and they glow red. Betelgeuse, at the top left of Orion, is such a star: You should clearly see the difference. Betelgeuse is destined to run out of fuel and explode as a supernova, an event not witnessed in our galaxy since 1604. The remnant of a supernova that went off in 1054 is found in the constellation Taurus, above Orion in the winter sky. A small telescope will reveal the Crab nebula, a faint patch of light that is the expanding gas from the explosion.
Our low-mass sun will end its life with a whimper instead of a bang, puffing off its outer layers in a less dramatic way. You will have to wait until summer to see a bright sample of such a remnant, a so-called planetary nebula. The Ring nebula is seen in small scopes as a smoke ring left over from its star’s demise.
Until our sun dies, enjoy the sky! Don’t worry – we have a few billion years to go.
Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University. Email: firstname.lastname@example.org. More on this month's column: www.upintheair.info.