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Joe Bauman: The colors of stars

The red supergiant star Antares, photo by Aubrey Brickhouse, taken at the Brickhouse Observatory near Clifton, Texas, June 9, 2016; used with permission.
The red supergiant star Antares, photo by Aubrey Brickhouse, taken at the Brickhouse Observatory near Clifton, Texas, June 9, 2016; used with permission.
Aubrey Brickhouse

Editor's note: Portions were previously published on the author's blog.

Take a mental trip to a dark site in the western Utah desert, accompanied by a friend who's an experienced star guide, this early June on a clear, cool moonless midnight.

"Want to see something 10,000 times as bright as the sun?" the guide asks.


"Look directly south," and you peer past the ragged silhouettes of sagebrush toward the glowing bulge of the Milky Way. It’s always a lovely sight, especially now, when it looks like a tilted flying saucer with one edge continuing up and up past the dome, higher into the darkness. Our galaxy appears as a glorious stream of filmy grays and dark patterns, bright stars and clusters, stretching from the south up to the east.

"Not that high," your friend says. "Closer to the horizon, just to the right of the Milky Way's dome. Here, hold your arm straight out and extend your fingers as far as you can."

From far off comes a quavering howl answered by a series of sharp little barks.

“Did you hear that?" you whisper. "Coyotes talking."

"Yes, wonderful," says the friend. "Now keep your fingers perpendicular, with the tip of your little finger at the horizon. Close to the end of your thumb is a magnificent star."

It's a bright gleam, twinkling in the breeze. Even with your naked eye you see that it is red.

"You found Antares."

Your friend starts to explain.

Antares is called "the heart of the scorpion" because it's lodged within the constellation Scorpius near the head. Scorpius is a rare star formation that really looks like its namesake, a long sprawling scorpion with a curving fishhook tail that is stuck in the Milky Way's bulge at the left, with a pair of claws spread at the other end. The blazing heart, Antares, is the brightest star in the design.

The name comes from the words anti, meaning against, and Ares, the Greek god of war. Ares was called Mars by the Romans and is personified by the planet Mars. The planet is a ruddy color appropriate for war and Antares’ color is the reason it is deemed Ares’ rival.

A red supergiant 10,000 times as bright as the sun, Antares is 15 times as massive and 700 times the diameter of our local star, according to NASA. However, because of its gigantic size, it is fluffy, having expanded drastically and cooled off. The density of Antares is less than that of water.

It is not the brightest star in the night sky but it would be fantastically dazzling if it were close — 500 light-years will reduce the glare of the shiniest. Antares isn’t the intrinsically brightest star in our galaxy, either. Eta Carinae, a double star that may go supernova soon, is as bright as 4.7 million suns; only its distance of 7,500 light-years makes it seem dim.

While Antares is not the biggest or the brightest, as a supergiant whose ruby glitter is easily visible from Earth it makes a good subject to launch a simplified discussion of star colors.

"It’s beautiful," you say.

According to astronomers, differences among stars are caused mainly by age, composition and mass. Mass contributes to gravity and compression, which means that the more dense and massive a star is, the hotter. That causes it to burn through its fuel quickly, so that massive stars have relatively short lifetimes before they cool and expand prior to destruction. Some astronomers refer to this as the "live fast and die young" syndrome.

Low-mass stars are believed capable of surviving much longer than the universe has existed (about 13.8 billion years so far); medium stars, like the sun, may continue on the main sequence of star life for about 9 billion years before they puff off their outer envelopes and become white dwarfs (we’re halfway there); red supergiants are close to their demise. Antares is expected to explode as a supernova anytime between now and a million years in the future (see

Star color is related to the temperature of the photosphere, the surface. Through decades of research, scientists have classified star temperature, and therefore color, into seven basic types that range from 71,500 degrees Fahrenheit to 5,000 degrees. From hottest to coolest, they are O (blue), B (light blue), A (white), F (yellow-white), G (yellow), K (orange) and M (red). As all budding astronomers know, the mnemonic to remember the order is "Oh be a fine girl (or guy), kiss me."

The sun is a beaming type G.

Antares is a red supergiant type M ready to explode. In the past, it was a huge, massive and dense blue star. It burned through much of its hydrogen fuel, converting it to helium. The next phase of its nuclear furnace was to start fusing helium into carbon atoms.

According to NASA, "Gravity continues to pull carbon atoms together as the temperature increases and additional fusion processes proceed, forming oxygen, nitrogen, and eventually iron."

The star can't fuse iron into other elements because iron's atomic bonds are the strongest known. When a star is left with a large proportion of iron that it can't fuse, it runs out of fuel and dies in a supernova.

Your guide notes, "Knowing the color and spectral details of stars helps to plot their distance and their time of life."