Twinkle twinkle little star
How I wonder what you are
Up above the world so high
Like a diamond in the sky
Twinkle, twinkle little star
How I wonder what… color you are
Stars are giant balls of incredibly hot and dense plasma, not much of a mystery there. But what color are they?
Well, the answer is trickier than you think, so let’s find out what colors are the stars.
What Is Color?
Piero Angela, one of the most renowned scientific journalists in Italy, once said (my rough translation):
“[…] the universe is dark, and black. The stars don’t shine and the Sun is not bright. Everything is frighteningly dark in the universe.
Why? “Because the universe lights up only when there are a set of eyes and a brain able to detect the electromagnetic radiation and understand it as light and colors.”
You may already have guessed that what we call color is our human perception of a physical entity: the electromagnetic radiation, or, as we call it, light.
Light is an electromagnetic wave that, unlike sound, can self-propagate in the vacuum of space, and as all waves, it is solely characterized by its frequency (or its wavelength or its energy). Color is not a physical property of light.
High-frequency light, such as gamma rays, x-rays, and UV light, has a short wavelength and carries a lot of energy.
Low energy light, such as infrared light, micro and radio waves, has a long wavelength and low frequency,
In between, we have the visible light: the electromagnetic radiation that our eyes can detect.
In the visible portion of the electromagnetic spectrum, we have lights with wavelengths ranging from about 350nm to 750nm. We perceive the light of different wavelengths as having a different color.
Light with a wavelength of about 350nm appears violet to us, while light with a wavelength of about 700nm looks red.
Fun fact: the visible light accounts for only 0.0035% of the entire electromagnetic spectrum. When we look at the universe, we are basically blind.
Luckily for us, we have a clever brain and we figured out how to build instruments to look at the cosmos with light of all frequencies.
Another fun fact before digging further into the color of stars: did you know there is no pink light in the universe? Pink is an extra-spectral color.
Whether pink as a color exists or not has been a strongly debated argument on the internet some years ago.
Ok, back to the stars now.
Why Are Stars Colored?
Stars emit light at all frequencies, but their emission spectra peaks at a frequency that depends on their surface temperature.
And their emission spectra is a great approximation of a black-body radiation spectrum, a continuous frequency spectrum that depends only on the body’s temperature. The takeaway message is that things at different temperatures emit light of a given range of frequency.
Therefore, “cold” stars glow red, as their emission peaks on the far infrared light, while the hottest ones glow blue, as their emission peaks in the UV region of the electromagnetic spectrum.
Some stars with temperatures a bit higher than our Sun emit red, green, and blue light in about equal measure and so they glow white.
The gif below explains the relation between star temperature and color.
As you can see from the gif above, as the emission spectra of cold stars peaks in the infrared, there is more red visible light than blue. Therefore the star will look red to us.
Or does it? I told you… the answer is more complicated than what one would think.
What Makes A Star Seem A Different Color?
Until now, we discussed the physics of the colors, but what colors the stars look to us is a different thing.
There are two things that affect the color of a star: our human vision and the environment we are in.
We have recently published an article about our own star, the Sun, but one thing we didn’t discuss is what color the Sun is and that is because we all know the Sun is yellow.
And it smiles too 🙂
Or does it?
What Color Is The Sun?
The Sun has a surface temperature of about 5800K. Behaving approximately as a black body, we can calculate the Sun emits most of its energy at 500nm, which is a blue-green light.
So, why don’t we see a big blue-green Sun in the sky?
Part of the answer is that the Sun, as all stars, does not emit only one kind of light. Instead it emits a lot of the other colors too.
The other part of the answer is how we see the universe with our eyes: if you observe it from space, the Sun will look white to you.
Your brain sees colors by combining the signal coming from three different types of cells in your retina: the cones.
These cells are excited by lightwaves that correspond to red, green, and blue light. The brain combines these signals into the final color perception.
As seen from space, there is so much light from the Sun to “saturate” the signal for all three types of cones: our brain interprets that as white.
Ok, so we shouldn’t see a blue-green Sun, but a white one. But then, why is it that we get to see a yellow Sun?
This is because we do not live in space, but we see our star shining through our atmosphere, which is composed of many different types of molecules and filled with dust, pollen, and other fine particles.
All those particles and molecules scatter the “shorter” blue light more efficiently than the “longer” red light.
When we look towards the Sun, as some of the blue color from the sunlight has been scattered from the atmosphere, we receive more red light.
This effect is even more dramatic at dawn or at dusk when the Sun is low over the horizon and sunlight has to go through more atmosphere to reach us.
And since the atmosphere attenuates the sunlight, our cones are not saturated and we can appreciate a beautiful orange/red sunset.
And because blue is scattered more, the part of the sky that is not around the Sun, is, well, blue.
Fun fact: there is another bright object that changes colors while traveling across our sky: the Moon.
The Moon is not emitting light; instead it reflects sunlight, and like the Sun, it looks red at the horizon.
The more the Moon rises, the less atmosphere its reflected light has to go through and its color changes from orange/red to yellow to gray, if very high in the sky.
But this is not a universal law: have you ever wondered how the Sun looks from the surface of other planets?
We have robots (better known as rovers) operating on Mars, and Mars does have a thin atmosphere and dust storms that can envelop the entire planet and last months.
Those rovers, such as the Curiosity rover, travel across the martian surface, happily drilling holes, analyzing soil and rock samples, sniffing the air, and snapping photos.
Well, did you know that sunsets are blue-ish on Mars?
Again, this is due to the Mars atmosphere. Here, the dust that is suspended in the martian atmosphere has the right size that blue light is the one that better penetrates the atmosphere.
And since at sunset the Sun is low on the horizon and its light has to travel through more atmosphere, the red component of sunlight is blocked more than the blue, hence the blue sunset.
What Color Is The Hottest And Coolest Star?
As mentioned before, the hotter the star the more blue it looks. The colder it is, the more red it looks.
Red Dwarfs are the coolest type of stars in their main sequence, and they are by far the more common type of stars in old galaxies, such as our own Milky Way.
Compared to our Sun, red dwarfs have a much lower surface temperature (about 2000K) and a much smaller radii (down to 9% the radius of the Sun).
The hottest star we know has a surface temperature of 210 000K, its emission is well into the UV region and most of the visible light it emits is blue.
Stars can be classified by surface temperature, and thus by their color: our Sun is a type G star.
What color is a dying star?
A dying star the size of our Sun has consumed most of its hydrogen and helium and it has turned into a red giant.
As the star expands, its surface cools as the energy is spread on a larger area and the star glows red.
Why Aren’t There Green Stars?
There are, but because their emission peaks exactly in the middle of the visible range, they are perceived as white. Remember: no star emits light of a single wavelength.
Can We See The Different Star Colors With The Naked Eye?
Because of the atmosphere we live in and how we see colors, we can see colored stars, but that may not be their true color, as I explained above for our Sun.
One of the few objects in the sky we can clearly see in its true color is Mars: the red planet looks, indeed, red.
We can also see colored stars, particularly the brightest ones like Sirius and Vega.
But if you look at them long enough, you will see stars do something planets don’t: they twinkle.
Most stars are so far to look as pinpoint sources of light even when seen through a telescope, while planets are obviously larger than that.
So the light they emit is not as strongly affected by atmospheric turbulence and planets don’t twinkle.
The varying in air density and temperature, in fact, mostly affect starlight, more so when the stars are low in the sky.
This causes the stars to shimmer and its light appears to change colors. These effects are more noticeable for Sirius, the dog star, because Sirius is so bright.
How To Photograph The Colors Of Star
Stars are difficult to photograph, even more so if you want to show their color.
If you do auto white balance on an image of the Sun taken using a classic white light filter, you get a white Sun (which is correct). If you prefer to have it yellow/orange, then you need to tweak the white balance.
For stars other than the Sun, you have to be careful not to clip the highlights to pure white, or all the stars will be pure white. Particularly the larger and the brightest ones.
Using a telescope is easy to isolate stars, as shown before with Vega and Sirius. There is a whole branch of astrophotography dedicated to split double stars and multiple stars systems.
With my small Sky-Watcher Skymax 90/1250 and a 2x Barlow lens, I could resolve Albireo as a double star system and photograph Albireo A (yellow) and Albireo B (blue) with my Olympus E-PL6 camera.
After you take an image that is pinpoint sharp, you can take a second one to use for the colors: by slightly defocusing the image, the star light is spread over a larger number of pixels, and it is easy to record the light color.
While they are not part of the same star system, Pollux and Castor, in Gemini, form another duo that is often photographed. Pollux is yellow, and Castor is Blue.
Finally, if you are after starry landscapes, you can get colors in the big stars. In the landscape below with Orion, you can easily get the red giant Betelgeuse, the hotter (and therefore bluish) Bellatrix, and the other blue stars forming the Orion Belt: Alnitak, Alnilam, and Mintaka.
Twinkle twinkle little star
Now I know what color you are!