Cosmic Quiz – Let’s talk about star colors

Seth Jarvis

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.

Stars really do shine in many different colors.

Stars really do shine in many different colors.

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°.

Blue-hot is hotter than yellow-hot.

Blue-hot is hotter than yellow-hot.

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.
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.

Red Dwarf stars such as Proxima Centauri are everywhere.

Red Dwarf stars such as Proxima Centauri are everywhere.

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.

Sure Proxima Centauri is close to Earth, but it's dim!

Sure Proxima Centauri is close to Earth, but it's dim!

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.

The Blue-White Supergiant Star Rigel (upper right) illuminating the "Witch's Head" Nebula.

Supergiant star Rigel illuminating the "Witch Head" Nebula.

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.

Giant stars.  NOW who’s the dwarf?

Giant stars. NOW who’s the dwarf?

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.

The colors really are there - if you take a little time to look for them.

The colors really are there - if you take a little time to look for them.

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14 thoughts on “Cosmic Quiz – Let’s talk about star colors

  1. But are all the photos we see on NASA real color? Dust and nebulae almost look like false color. Also, I remember a Soviet cosmonaut saying that when he was up in orbit everything looked grey!

  2. It really varies image by image. Many images from the Hubble Space Telescope include data from the invisible ranges of the EM spectrum (ultraviolet, infrared, etc.). These data sets are artificially colored, to be sure. The visible spectrum data are natural colors, though enhanced through extremely long exposure times, and software enhancement. Your eyes see only grays because you eye cannot take long exposure-time photographs.

  3. Hmmm, never heard the term used with meters. Seems kind of silly, but here goes. ‘Yotta’ is the prefix meaning ‘septillion,’ or 10^24 (1 followed by 24 zeros). Since one light year is approximately 9.5×10^15 meters, 4 light years = 38×10^15 meters, or 38 quadrillion meters. So, there are 3.8×10^-8 yottameters in 4 light years, or 38 one-billionths of a yottameter in 4 light years. Don’t know why you picked 4 light years, but the distance between Sun and Proxima Centauri, our nearest neighboring star, is 4.2 light years. So, there are 3.99×10^-8 yottameters in one trip to Proxima Centauri. So, it would require 1.25×10^7 round trips to Proxima Centauri in order to have travelled one yottameter, or 12.5 million round trips.

    I’m done. I hope my math is right. I’m doing this at 7 am…

  4. Hello…

    I take long daily morning hikes, usually starting off in the dark, so I often observe the moon and the early morning sun at the same time. It has always bothered me that if I note the position of the moon’s shadow and then draw an imaginary line to where the sun “ought” to be, it’s not there! Never!

    In trying to explain this to myself, I can think of two reasons. First, there must be some light refraction caused by the atmosphere particularly when the sun or the moon is low in the sky. Second, visible light from the sun left its source earlier than simultaneously observed light from the moon; and meanwhile, sun, earth and moon have been in motion relative to each other.

    Are these ideas sufficiently explanatory, or am I missing something?

    Roger Vance

  5. My immediate reaction is that this is a geometry problem. I would guess that your are trying to draw a ‘straight’ line from the Moon’s shadow to the anticipated position of the Sun. The sky needs to be viewed as a spherical surface around the Earth. Therefore, while the path from the Moon’s terminator to the Sun is perpendicular to that terminator, it will appear ‘curved’ from the vantage point of the earthly observer when mapped onto the celestial sphere.

  6. The moon appears whiter than the sun. Why is it not hotter. We have people land on the moon and not burn up. It would be interesting to know how hot the reflective light is on the moon and if some of the “stars” are not stars at all but moons. How can you tell the difference of moon and non-moon by the color?

    You may post this, I would just like to know.

  7. Josh,

    The moon is “white” because it is reflecting sunlight. That’s not the same thing as “white hot.”

    The temperature on the moon’s surface in sunlight is in fact a couple of hundred degrees Fahrenheit, but that’s nowhere near the several thousand degree temperature of the sun’s photosphere (the part of the sun that emits the light we see).

    Is a red rose as hot as a red-hot fireplace poker? Of course not.

    You can know the difference between reflected light from a rose and emitted light from a red-hot poker by studying the spectrum of light from the object in question. The same applies to the surface of the moon and stars.

  8. Are there any stars with a surface color of “green”? What is the surface temperature?

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