Orbiting our Milky Way galaxy like two bickering siblings are the Magellanic Clouds, galaxies in their own right, though far smaller than ours. The smaller of the two – named, shockingly, the Small Magellanic Cloud – is also the farther of the two, about 200,000 light years to the Larger cloud’s 180,000 or so. The SMC is loaded with gas and dust, and is actively churning out stars.
The Spitzer Space Telescope, which observes infrared light from astronomical sources, took this incredibly beautiful image of the SMC:
[Click to embiggen, including getting access to a huge 7800 x 7000 40Mb version.]
Remember, this is not a visible light image! In the picture, blue represents light at a wavelength of 3.6 microns, about 5 times longer than what the human eye can see. Green is 8 microns, and red 24. So what you see here as blue is really what we would think of as red stars if we saw them with our eyes. Green shows light from big organic molecules called PAHs, for polycyclic aromatic hydrocarbons. Red is light from stars deeply embedded in dust, and is where stars are busily being born.
All together, this image shows starbirth on a vast scale, thousands of light years across. And this may be a new phenomenon for the SMC: measurements of the elements in the stars there show that they have far fewer heavy elements (like oxygen, iron, and so on) than stars in the Milky Way, as little as 1/5th as abundant. Since these elements are created inside of stars over time, this indicates that stars in the SMC are on the whole younger than in the Milky Way.
Even though the two Clouds are the closest galaxies we can see – and you can spot them easily with the unaided eye in the southern hemisphere – there’s still a lot we don’t know about them. In fact, we’re not even sure if they are orbiting the Milky Way, or just passing by! Even over decades, measuring their actual motion across the sky is very difficult; their mind-numbing distance of quintillions of kilometers away shrinks any real motion into apparently microscopic amounts. It may be quite some time before this question is finally resolved.
Another image from Spitzer also shows a tail of gas streaming away from the SMC, material ripped out of the body of the galaxy itself by the gravity of the Milky Way. It’s possible that interactions with the Milky Way and the other Magellanic Cloud are what triggered the star formation in the SMC, too.
It’s rather convenient to have such a nice laboratory for dwarf galaxies and starbirth so close to us. That makes it easier to study, giving us access to really high resolution images like this one. And the bonus? They’re pretty, too.