Date: December 24, 2010
Title: The Color of Stars
Podcaster: Rob Knop
Organization: Quest University Canada
Links: My home page : http://www.questu.ca/academics/faculty/rob_knop.php
Description: Go outside and look at the stars. It may or may not surprise you to find out that the stars don’t all look white to you; you can pick out some stars that look reddish, even to the unaided eye. In this podcast, I talk about the different colors that stars have, why they have those colors, and why we see the colors of stars that we do in the night sky.
Bio: Rob Knop obtained a PhD in Physics from Caltech in 1997. He then worked with the Supernova Cosmology Project and was part of the discovery that the expansion of the Universe is accelerating. After six years as an assistant professor at Vanderbilt University, he worked in the computer industry for two years. He now teaches physics the new college Quest Unviersity in British Columbia. He gives regular astronomy talks in Second Life in association with the Meta-Institute of Computational Astronomy.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored anonymously.
Additional sponsorship for this episode of “365 Days of Astronomy” has been provided by Mike Smithwick, author of the award winning astronomy software, Distant Suns, now for the iPad and iPhone.
Transcript:
Go outside on a clear, moonless night, and look up. What do you see? Stars, of course! Depending on when it is, you might also see a couple of planets, and depending on where you are, you might also see the haze of the Milky Way, or perhaps the Magellanic clouds, and you might just see a cluster or two, or, if you know where to look, the Andromeda Galaxy. But, mostly, you’re looking at stars.
This is Professor Rob Knop, a tutor of Physical Science at Quest University Canada. I’ve spent a lot of my life working in and teaching astronomy by now. However, it wasn’t until I was in college– or, in University, as they say it here in Canada– that I realized that all the stars didn’t look white to me. What is that, you say? Either you’re surprised that I ever thought that, or you’re surprised to find out that all the stars don’t look white. After all, if you’re making a quick drawing of the night sky, you sprinkle white dots around to indicate the stars. Most drawings and illustrations of the night sky show the stars as white pinpricks, or perhaps as white dots with little rays– what somebody who takes pictures with a telescope might call diffraction spikes– coming out of them. And, yet, they’re not all white.
Around this time of year, if you go out in the few hours after sunset and look to the Southeast, you’ll see the constellation Orion. Exactly where it is depends on what time you look, and also where you are on Earth. Indeed, if you’re in the Southern hemisphere, you’ll have to look to the Northeast! Orion, however, is one of the most distinctive constellations in the sky, second in familiarity only to the Big Dipper. The constellation is characterized by the three stars that form Orion’s belt, the four stars in a sort of rectangle (two shoulders and two legs) around those three stars, and the sword or dagger than hangs down from the belt. (Although, again, in the Southern hemisphere, everything is upside-down, and the sword sticks *up* from the belt.)
Focus in particular on two of the brightest stars in the constellation. The bright star in the upper-left shoulder is Betelgeuse, and the bright star in the lower-right leg is Rigel. Look again at Betelgeuse. Notice that it doesn’t look white! Rather, it looks reddish. You can see this all the better if you point a pair of binoculars at the star.
To the Northeast of Orion in the sky– directly up, or up and to the right, if you’re in the Northern hemisphere after sunset and facing the Southeastern horizon– is the smattering of stars known as the Pleiades. These stars are indeed white. Halfway between them and Betelgeuse is another star, known as Aldebaran. This is the brightest star in the constellation Taurus. Look closely again at Aldebaran; this star, too, has a reddish hue to it, although perhaps it looks more orange, or a bit paler, than the angry red of Betelgeuse.
As you look around the sky, you’ll see that, indeed, the vast majority of stars you see look white to your eye. But you will notice, if you’re thoughtful in your observation, that there are a number of them that appear reddish, that some may appear nearly golden, and, just perhaps, you’ll see some– such as Rigel– that look blue-white to you.
You can do even better; if you have a friend with a small telescope, or if you have a small telescope yourself, point it at the star Beta Cygni, or at the star Gamma Andromedae. Both of these are binary star systems where the two stars have very different colors. The comparison of the two stars next to each other greatly increases the color contrast you perceive as opposed to when you are comparing stars across the sky.
What is it that gives stars their color? The short answer is “their temperature”. When astronomers talk about the spectrum of a star, they’re talking about the brightness of the light as a function of wavelength. Your eye can see wavelengths between 300 and 700 nanometers. At the lower end of that, your eye perceives colors as blue or purple; at the upper end, red. Different types of objects show different spectra– that is, the graph of intensity versus wavelength has a different shape. However, all stars, to a decent approximation, have a shape that we call a blackbody spectrum. This is the spectrum of an object that is emitting thermally– it’s glowing just because it’s hot.
It turns out that you are emitting light approximately as a blackbody, but nearly all of that light is coming out in the infrared, so other people can’t see it. If you turn on an electric stove burner, and it gets really hot, it starts to glow red. If you were able to get it hotter still, it would glow more orange, then yellow, and then be white-hot. If you were able to get it even hotter– to the temperature of the flame produced by an arc-welder– it might glow even blue-white, at this point putting out a fair fraction of its light as ultraviolet radiation.
If you think about it, this is backwards from what our culture assumes. If you walk up to a kitchen sink, and there are two handles with colored labels, one red and one blue, you know the blue one is cold water, and the red one is hot water. However, that’s backwards from what you get from blackbody radiation. The stars that are glowing red are not as hot as the stars that are white, and the blue-white stars are the hotter still.
So why are some stars hotter than others? There are two answers, for, broadly speaking, there are two types of stars. Most stars are, like our Sun, what we call “main sequence” stars. Stars like this are in the main part of their life, and they form a sequence based on their mass. Stars that are less massive than the Sun tend to be orangish or reddish, with the smallest stars being very red and very cool (for stars). Stars larger than the Sun become white, or, for the most massive ones, blue-white or even blue.
In the last 10% or so of their life, stars become giants; most of these giants are what we call “red giants”. The Sun will be a red giant in another 4 billion years or so. These stars at the end of their life become much more luminous as they swell to large size, but they also tend to be cooler than they were during their main sequence lifetime.
If you see with your naked eye a star in the sky that looks reddish, you can be sure that it’s a red giant. (That is, if it’s not very near the horizon; all stars can look reddish very near the horizon, for the same reason that the Sun looks reddish at Sunset.) How can you be sure of this? Because the low-temperature main-sequence stars are all much too dim to be seen with your naked eye. Indeed, the closest star to Earth, a star known as Proxima Centauri, is more than a hundred times dimmer than the dimmest star you could see on a very dark night. Proxima Centauri is a little red main sequence star, with a mass less than a quarter the mass of the Sun. Even though it’s the closest star, it’s just not luminous enough for us to see. All of the stars we can see with our unaided eyes are much more luminous than Proxima Centauri.
It turns out that the vast majority of the stars in our Galaxy are little red stars like Proxima Centauri. And, yet, without the aid of a telescope we can’t see any of them. The stars we do see are either the more massive, and thus more luminous, main sequence stars, or the bloated red giants reaching the end of their lives. The same is true when you look at a star cluster, or at another galaxy, perhaps with the aid of binoculars or a small telescope. The light you’re seeing is not coming from the most common sort of star. Rather, the bulk of the light comes from the brighter stars, the main sequence stars at least a couple of times the mass of the Sun, or the red giant stars.
Just as the low-mass main sequence red stars are very common, but very dim, the high-mass blue or blue-white stars are very rare, but very bright. Rigel is an example of a star that’s blue white. Although stars like Rigel are rare, they’re so luminous that we can see them to a comparatively great distance, and as such, there’s a decent number of them gracing our sky.
So why is it that most of the stars we see at night appear whitish? There are two main reasons for this. First, you may have noticed that without the aid of a telescope, it’s only the brightest stars in which you can detect color. Our eye is better at perceiving color when things are bright. As things get dimmer, our eye is more able to perceive things as shades of grey, or as dim white spots. So, for the dimmer stars, the color gets washed out. However, it also turns out that a good fraction of the stars that we can see in our sky really are whitish in color. Among main sequence stars, lower-mass stars are more common, but too dim to see out to any great distance. Higher-mass stars can be seen to great distance, but are intrinsically rare. It is the white stars, similar to the stars Vega and Deneb that you can find after sunset in the Norhtwest, that are at a sort of “sweet spot” between being intrinsically common and intrinsically bright– although it turns out that only Vega is a main sequence star, and Deneb is itself an extremely luminous supergiant star!
Next time you go outside at night, look up at the stars. Try to pick out Betelgeuse or Aldebaran, and notice that not all the stars look white to your eyes. Realize that you’re seeing a lot of stars at vastly different distances, some that are modest in luminosity but relatively close, and some that are rare monsters but far away. And, remember that the most common sort of star, the humble low-mass red main-sequence star, is one that you can’t see at all.
End of podcast:
365 Days of Astronomy
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