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March Star of the Month: Betelgeuse

At 7:14 p.m. local time on March 2 (and four minutes earlier every day thereafter), the bright red star depicting Orion's shoulder shines due south, reaching its highest point. It's name is Betelgeuse, which roughly means "Armpit of the Central One." At 7:14 Eastern Standard Time on March 2, our feature star stands at zenith over Columbia -- or more specifically: 7.41 degrees north latitude and 75 degrees west longitude. The star's angular distance from your zenith at this time tells you your distance from this South American country.*

If you have a hard time making out Betelgeuse's ruddy character, use binoculars to compare its hue to those of other stars. Capella, which sparkles overhead, looks yellow in comparison. You'll see most of Orion's other bight stars radiating blue-white.

Astronomers assure us that color is very revealing of a star's true nature. Generally, blue and blue-white stars are the hottest, shining thousands of times more brightly than our yellow Sun; whereas red stars are the coolest, shining hundreds or thousands of times dimmer than the Sun. Hot, massive stars are thought to have short lifetimes (in millions of years); whereas middle-of-the-road stars (like our Sun) have estimated lifespans of billions of years; and tiny, cool red dwarf stars probably have lifetimes of up to trillions of years.

Red dwarf stars may constitute 80% the number of stars populating our Milky Way Galaxy and perhaps 50% of the Galaxy's stellar mass; yet, if you look upward, you'll see that red-colored stars are more the exception than the rule. That's because red dwarf stars are so faint and small that they can not be seen with the unaided eye. Any red star that you CAN see with the eyes alone -- like Betelgeuse -- has to be something other than a red dwarf.

A naked-eye red star must be either a red giant or a red supergiant, the red color indicating that these stars - unlike red dwarfs - are in the autumn of their years. Betelgeuse is a red supergiant, an enormous rarity among stars. With a radius of around 600 to 700 solar radii, and a luminosity of perhaps 60,000 Suns, this monstrous star would fill the solar system past the orbit of Mars, if it were to replace the Sun. Betelgeuse probably shone as a hot, supermassive blue star in its youth and middle age, but upon reaching old age, swelled into a cool, red supergiant star. (Astronomers predict the Sun will even turn into a red giant some five billions years down the road.) Unlike red giants, however, red supergiants end their lives in supernova explosions.

It's been nearly 400 years since a supernova has been seen in our Milky Way Galaxy, and none has been witnessed in our Galaxy since the invention of the telescope. Astronomers feel Betelgeuse's grand finale is inevitable, but whether it'll come tomorrow or in a few million years is anyone's guess.

* For example, from Potsdam, NY, the star's zenith distance would be about 37.25 degrees. Multiplying 37.25 degrees x 69.2 miles gives about 2578 miles as the crow flies.
Professor Kaler's Stars Page

March Feature: Asteroid Vesta in Virgo