If our sun was a different colour?

If our sun was a different type of star eg if it was a red dwarf or a blue/violet, what colour would it appear to us, would the sky still look blue, and would it be too cold/hot for humans to survive on earth?


When the Sun does Die out (Progresses on in its life on the "Main Sequence) it will turn into a White Dwarf
((No Red Dwarfs are possible, as far as I know, So sack the Smeghead who told you that…:wink: ))
But before that happens, it will blow up, lose like 90% of it size, and Sizzle all on Earth…

Temperature of a Star (Sun included) Directly relates to its color…
Blue to Red (And virtualy all colors inbetween)

Later in life of most stars the star cools down, and if memory serves me, it gets redder, Blue/Hot Red/Cool

There is a menomic to remember Temperature of Stars and Color… Classified in to different classes, each with a letter… (Starting letter of each word):

Oh Be A Fine (Girl/Guy) Kiss Me (I might be missing some here, but its been some time since my last astronomy class, and this is bordering on way to much additional information)


If it were green with red polka-dots, it’d wreak havoc around Christmas time.

Thanks Meeko. I was more looking for our how sky would look if we had a different sun; or if you prefer - would alien worlds have blue skies?

Sky color is more having to do with Atomasphere IIRC

Ah yes, it scatters the blue light or some such. So to have a different sky colour, we would have to be methane-breathers or something.
Heh, thanks CK. Christmas has quite enough havoc, methinks.

If the sun were red wouldn’t Superman lose all his powers?

I seem to recall reading “somewhere” that if you lived on an Earth-like planet orbiting different colored star (in an orbit close or far away enough to keep an Earth-like temperature, of course.) you’d at least notice that shadows would seem to look a bit different than on Earth.

Shadows cast by objects lit by a blue star would be “sharper,” and with a slightly blue tinge.

Shadows cast by objects lit by a red star would be “blurrier,” and with a slightly red tinge.

Thanks Ranchoth, that’s the kind of thing I was wondering about.

Yep. He got his amazing powers from our yellow sun, so I guess he’ll be SOL when our star exits the main sequence.

One more reason to prefer Batman.


Oh, and as to the blue sky: It is due to the atmosphere. The air in the sky scatters all light coming in (and filters some out, as well), and blue light scatters best. Therefore, blue is the color of our sky. If our atmosphere were thinner, the sky would be darker, the sun would be whiter and more intense, and we wouldn’t have as many problems looking at other stars once the sun had set on a region. We’d have trouble breathing and filtering radiation, but the night sky would be spectacular. :wink:

Would the sky be another color if the sun weren’t yellow? My gut tells me no, even if the sun were reddish. I think the star would still emit enough broad-spectrum radiation to give the air plenty of blue light to scatter around. A red sun in a blue sky would be absolutely striking, especially if it were the bloated, baleful orb a red giant would be.

Main-sequence stars (class V) are also called “dwarf” stars to distinguish them from supergiants (class I) and giants (class III). So there are “red dwarf” stars, in that they’re red main-sequence stars, but you’re right that they’re not the same animal as white dwarfs (class VII). This classification system I’m using is not as common as spectral types (OBAFGKM), but it is used from time to time.

You could in theory have life around any main-sequence star. Stars have blackbody colors, blue, white, yellow, orange, and red. There’s no such thing as a blue-violet star, or a green star. But I guess if you had a green star, it could have red sunspots.

Blue and white stars (spectral type O, B, and A) are massive, hot, short-lived, and violent. Life around these stars would have to be hardy to withstand solar activity, and it would have to evolve within a few 10s or 100s of million years. I think that if the sun were O-type or B-type, Earth would be way too close to be hospitable to life. Maybe Saturn or something.

Orange and red stars (spectral type K and M) are small, cool, and long-lived. Earth would be very cold around an M-type star.

I’ve heard it spectulated that the reason we can see the wavelengths we do is because that’s what our sun gives off. This is true - the sun spews out a lot of light in the optical range. O and B type stars give off more in the ultraviolet, and M and K type stars give off more in the infrared. So if the sun had been B-type instead of G-type, we would probably see everything differently, not just the sun. However, if we did happen to evolve with the same wavelength sensitivity that we have now, stars would appear by and large their actual color.

:rolleyes: That’s so bad. I get it. :slight_smile:

Ok, so Im wrong, but did anyone get the Smeghead reference?

I remember reading that a lot of the stars that are described as orange or red have chromospheres that are app. the same temperature as the filament in a light bulb, so the light would not really look red to us, just not as white as that of our sun.

Shadows on earth have a blue tinge, because it’s only illuminated by the blue sky and not the yellow sun. (Photographers are very aware of this effect.) I think it would be the same regardless of the color of the sun - the sky would always be blue, so the shadow will always have a blue tinge. (Unless you have an atmosphere full of red dust, like Mars, in which case the sky would be red.)

I agree about the sharpness of the shadow. The sharpness depends on the apparent size of the star/sun in the sky. If the sun is big, the shadow would be fuzzy. Since blue stars are hotter (higher temperature and higher power output), a planet must be further away for it to be habitable. Even though blue stars are slightly larger than yellow stars, the difference in distance is much greater so the blue star looks smaller from a habitable planet.

I did, Meeko.

How’m I lookin? I’m lookin goooood.

You can tell that smeghead that most stars in the universe are red dwarfs, unless you count the unknown number of brown dwarfs that may exist;
in order to have Earth-like temperatures on a planet orbiting a red dwarf, you would need to be tenor twenty times closer to the star than Earth is to our Sun.

This would mean that the rotation of such a planet would be tidally locked, or at least in a harmonic relation to it’s orbital period; there would be very long days, or the sun would be eternally in one place in the sky.

So conditions on such a planet would not be very earth like, even if the average temperature was the same as our planet.

Oh, and the sun would be a large, cool ball in the sky, as the planet would need to be very close to such a cool star.

SF worldbuilding at

Thanks Eburacum45. That close to a sun, would radiation prevent life as we know it, or could a thicker ozone help, or am I just way off?

Huh? So the colour of the sun changes the pitch you sing in? :confused: