This week’s Cosmic Quiz Question was asked by Erik Lindblom.
“Why do stars shine in different colors?”
That’s a great question, Eric!
Stars come in a variety of colors, temperatures, ages, brightness and sizes.
A star’s color is an indicator of the temperature of the outer layers of the star.
We’re used to thinking of something that’s “red hot” as being extremely hot, but for stars “red hot” is actually quite cool.
Here on Earth, we’re already familiar with the relationship between color and temperature. Most folks are aware that something that’s “white hot” is hotter than something that’s “red hot,” and anyone who’s been around oxygen-acetylene welding torches also knows that a blue flame is hotter than a yellow or white flame.
For example, candle flames are a lovely yellow-white color, and indicate a flame temperature of about 1,900° (F), while a propane flame is distinctly blue and indicates a temperature of roughly 4,000°.
There’s no chemical combustion taking place in stars, but the relationship between temperature and color still applies – red stars have relatively cool surface temperatures, white stars are hotter, and blue stars have the hottest surface temperatures.
Astronomers have created a classification system for sorting star colors in this sequence (from hottest to coolest): O, B, A, F, G, K, M.
It looks like this:
- Stars can be classified by their color and temperature.
To help them remember the correct color-temperature sequence of O, B, A, F, G, K, M, astronomers have also created a cute little mnemonic (a kind of memory aid): ”Oh Be A Fine Girl, Kiss Me!” (Who says science can’t be romantic?)
Interesting side note: This classification system for stars was created in 1901 when the field of astronomy was pretty much exclusively a guy-thing. There were very few female professional astronomers a hundred years ago, and fewer still who were permitted into graduate schools to earn their PhDs in astronomy. Nonetheless, a brilliant female astronomer, Annie Jump Cannon, while working at the Harvard Observatory (for one-fourth the salary paid to male astronomers), simplified and organized the earlier complex and unsuccessful attempts to classify hundreds of thousands of stars and developed the OBAFGKM temperature-color classification for stars that’s now in use. It was Annie Cannon herself who created the now-famous “Oh Be A Fine Girl, Kiss Me” mnemonic.
Our Sun is a “G” class star and is fairly commonplace in the universe. It’s middle-aged for a G-type star (4.6 billion years), of average temperature (10,000 °F) and of middling size (860,000 miles).
Our Sun is uncommon, however because although it is smallish and only yellow-white in temperature, it is larger and hotter than the small, cool and relatively dim “Red Dwarf” M-type stars which make up roughly three-fourths of the stars in the universe.
So if three-fourths of the stars in the heavens are Red Dwarfs, then why don’t we see a night sky filled with little red specks of light?
The answer is two-fold:
First, Red Dwarf stars, though numerous, are very dim. The Red Dwarf star Proxima Centauri, a mere four light years from us, is so dim it requires a telescope to see it.
Even a medium-brightness G-type star like our Sun would be too dim to be seen without a telescope from a distance of 100 light years.
Second, of the 6,000 stars visible to the unaided human eye most are stellar freaks – they’re the, “Hey everyone, look at me!” show-offs of the galaxy.
These are typically huge stars many times more massive than our Sun, burning through their nuclear fuel at a terrific rate. They live hard and die young.
As stars go, these giant and supergiant stars are very rare, but they’re tens of thousands of times brighter than our Sun and can be easily seen from enormous distances – anywhere from several hundred light years to well over a thousand light years.
Even though these show-off stars are rare, they’re the ones that get seen.
By carefully studying the light that stars give off, including its color, it is possible to learn a great deal about a star’s size, age, mass, temperature and chemical makeup.
We’re coming into the time of year when going outside and staring up into the starry night is a pleasant way to spend an hour or so before crawling into bed. Naked eye or with binoculars, try paying attention to the relative brightness and color of the stars – you won’t regret it.