Sat, 11/24/2018 - 9:45am

Kristen Lindquist

[Star map via Wikimedia Commons, courtesy of Torsten Bronger]

Now is the season for lights: strung up on Christmas trees, along our porch eaves, on the holiday wreaths on the town lampposts, in shop windows, on the shrubbery in the front yard. As we approach the Winter Solstice, as our days shrink and nights lengthen, we seem to want to bring as much extra light as we can into our darkening lives.

The heavens too offer an uplifting light show this time of year. The constellations of this season's sky, dominated by Orion the Hunter, often seem to scintillate in the cold air, not unlike those strings of blinking colored lights hanging off your neighbor's front porch.

Stars twinkle in a range of colors as varied as each home's holiday display: white, red, blue, and yellow. A star's color, which may sometimes be better perceived with binoculars, indicates its surface temperature. The hottest stars are those at the white and blue end of the spectrum, while the coolest are those at the red end. This runs contrary to a typical weather map, on which redder means hotter. Think instead of a candle flame, in which the hottest part is the inner blue area, not the orange tip.

Most stars, like our sun, convert hydrogen into helium through thermonuclear fusion in their cores. Put very simply, these stars will eventually—over billions of years—use up the hydrogen in their core and shift into a red giant phase. Our sun, a medium-hot, medium-sized yellow star, will—a few billions years from now—evolve into a red giant and expand to engulf Mercury, Venus, and probably Earth. Millions of years after that, it will eventually shrink in on itself again to become a white dwarf shrouded by a nebula of shedding layers of gases. Really massive stars evolve even more dramatically, ultimately exploding as supernovas.

As a red giant expands up to 1,000 times larger than the Sun, the heat it generates is dissipated over that much greater surface area; it thus cools as it grows. The surface temperature of a red giant may then be "only" 4,000 - 5,500 degrees or so, about half that of the Sun. Because of its size, however, the Sun as a red giant would be far brighter than the Sun as we now know it.

In the late fall–winter sky here in Maine, a couple of red stars are easily visible to the naked eye. The most noticeable of these is Betelgeuse, the alpha or brightest star in the constellation Orion and the 12th brightest star in our sky. This red star, classified as a supergiant, marks Orion's eastern shoulder.

Its unusual name means "hand of the Central One" in Arabic. (Most stars were first named centuries ago by Arabic astronomers.) Other early Arabic sources referred to the star as the right shoulder or arm of the Giant. To the ancient Greeks, this celestial Giant was Orion, a mythological hunter. The Tipai tribe of southwestern California also associated the constellation with hunting, with an origin story that seems to identify the stars of Orion's belt as three prey animals and Betelgeuse as the bloody tip of an arrow. Betelgeuse may also have been the star called Red Feathers by the Yavapai of Arizona. In Japan's Heian Era, the red star Betelgeuse faced off against the white star Rigel (Orion's left foot), with Orion's Belt as a starry line-in-the-sand between them; these two stars were the adopted symbols of two warring samurai clans: Taira (red) and Minamoto (white).

Betelgeuse is one of the largest stars we can see. Because of its tremendous size, its evolution as a star has been fast and furious compared to that of stars like the Sun. Although relatively young in terms of stellar time, Betelgeuse is expected to go supernova in a million years or so. When that happens, the light from that massive burst of energy will be visible from Earth even during the day, though the star is about 650 light years away.

Red-orange Aldebaran, the alpha star in the constellation Taurus the Bull, is also readily visible in the eastern sky this time of year. Its name in Arabic means "the follower," presumably of the Pleiades, a nearby cloudy cluster of many tiny stars. If you draw an imaginary line in the sky from left to right through the three stars of Orion's belt, Aldebaran is the first bright star you hit. It too is among the top twenty brightest stars in our sky, and, at a mere 65 light years away, much closer than Betelgeuse.

As part of a vee of several stars that comprise the Bull's head, Aldebaran is generally regarded as the angry Bull's glaring right eye. The Spokane tribe has a story explaining it as the eyeball of the trickster Coyote, who lost it in the sky while juggling it to impress a woman—now that's a party trick gone wrong! Several California tribes identified the star with Coyote, as well. Ancient Persians recognized Aldebaran as one of the four Royal Stars of Heaven. Hindu astronomy equates it with Rohini, a princess who often took the guise of a red deer. Perhaps a contemporary tale might draw a connection between this star and the red nose of Rudolph, Santa's lead reindeer.

On the night of the Winter Solstice itself, December 21, the eastern sky after nightfall will feature a mini holiday light show: rising Betelgeuse will be there, just above the horizon, with Aldebaran higher above, and the almost-full moon in the sky between them.

Although slightly fainter now compared to the glowing orb that you couldn't miss this summer, Mars remains visible in the southwestern sky, at its highest point right after dusk. Along with the red stars and possibly Rudolph's shiny nose, the red planet offers one more colorful light to look for in our holiday season night sky.

Kristen Lindquist is an amateur naturalist and published poet who lives in her hometown of Camden.

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