Lois & Clark Forums Forums General Discussion Off Topic Lovely colored trio

I can't resist showing you this lovely picture:



What you can see here is how planet Mars is visiting the constellation of Gemini. Mars is temporarily lined up right next to Gemini's two brightest stars, Pollux (the white star) and Castor (the blue one). Mars itself is strikingly reddish.

(I should say right away that the color balance in this picture is too blue, and Pollux should really be a pale yellow color.)

Why are the colors of Mars, Pollux and Castor so different, and why are their brightnesses so similar? The brightness thing is coincidental, because the brightness of Mars varies. Mars and the Earth follow individual orbits around the Sun, and sometimes Mars is on the same side of the Sun as the Earth and therefore bright, and sometimes it is on the other side of the Sun and therefore relatively faint. In this picture Mars is so bright that it must definitely be on the same side of the Sun as the Earth is. In fact, Mars must be almost as close to the Earth as it ever gets, considering how bright it is. Pollux and Castor are both first-magnitude stars - that is, they are among the brightest and most obvious stars in the heavens - and Mars is every bit as bright as Pollux in this picture, and it is brighter than Castor.

Let's make a little comparison here! Pollux is a so called red giant, but a smallish one for its class. It is "only" about ten times bigger than the Sun - well, its diameter is ten times the diameter of the Sun, which means its volume is a thousand times the diameter of the Sun. That is still small for a red giant. Because Pollux is small as red giants go, it is also relatively warm. Its spectral class is K0 and its surface temperature is 4,770 degrees Kelvin (yes, I just checked it up) which makes it almost exactly a thousand degrees cooler than the surface of the Sun, whose temperature is 5,800 degrees Kelvin. Pollux used to be even smaller and hotter, though, before it became a red giant. It used to be a blue star, hotter but not much bigger than the Sun, and it used to fuse hydrogen to helium in its core, just like the Sun is doing now. But then Pollux used up the hydrogen in its core and swelled up, and now it is fusing helium to carbon in its core. Pollux is 34 light years away.

Castor looks fainter than Pollux does, and it is farther away: 51 light years compared with 34. Castor is also hotter than Pollux: its brightest component is about 9,500 degrees Kelvin, making it hot class A. Fascinatingly, Castor is a sextuple star, made up of three sets of double stars orbiting one another! Wowzers! Together, the six components of Castor emit as much visual light as 49 stars like our Sun. And just like the Sun, the six components of Castor are all "main sequence" stars, fusing hydrogen to helium in their cores.

So Pollux and Castor are 34 and 51 light years away, while Mars in this picture is probably no more than five or six light minutes away. And while Pollux and Castor are suns producing their own light, Mars is a planet, a dark body, which only reflects the light from the Sun that is shining on it.

Why is Mars so red? It is because there is so much rust on its surface! Yes, well, really, there is a lot of oxidized iron there, which is rust, which is very reddish in color, as you know! Mars has been known since antiquity for its red color. The Greeks named it after its god of war, Ares, because they thought that Mars was blood red. The Romans took over most of the Greek mythology and gave most of the Greek gods Roman names, and Ares was re-named Mars.



Ares, the Greek god of war...



...became Mars when the Romans re-named him. Planet Mars is in the background.

So Ares disappeared from the western vocabulary. The name Ares, however, is still there in the heavens, as a name of one of the most prominent stars in the sky, Antares. The name means "Rival of Ares", and Antares got its name because of its striking reddish color. Here is a picture of the constellation Scorpius, and Antares is the brightest individual star, showing off a prominent yellow-orange hue.



Antares is so red becasue it is so big. Being so big, it is also very cool, at least on the surface! And I can't keep this picture from you. Here the size of the Sun is one pixel. You can see Pollux here, too, a comparatively small star, and Antares, a very big one!



Ann

Antares, by the way, is about 600 light years away, and it is about 10,000 times as bright as the Sun in visual light. Its diameter is about 300 times the diameter of the Sun. If it was to replace the Sun in our solar system, it would swallow not only the Earth but Mars, too.