A very important aspect of astronomy often overlooked is how much our eyes don’t tell us. We see a very limited range of the spectrum of light emitted by astronomical objects, and many times it’s what we don’t see that tells us what’s going on.
That’s why the European Southern Observatory has the HAWK-I camera: it sees in the infrared, in wavelengths invisible to our eyes. And when we train it on the skies, well, we see some pretty cool stuff. Like, say, gorgeous spiral galaxies:
[Click to galactinate.]
That’s NGC 1232, an open face-on spiral some 65 million light years away. This to me is a perfect spiral: the arms are distinct and easy to trace, starting near the center and going all the way around the galaxy. Several spurs — short, disconnected, straighter offshoots — can be seen. There’s also a bar: the center of the galaxy isn’t a sphere, it’s elongated in an oval.
All of these features are due to the weird gravitational field of a galaxy. In our solar system, the Sun dominates in the gravity department, and the planets orbit it. In a big galaxy, though, the gravity of every star adds up, making the force of gravity stronger farther from the center than if all the mass were concentrated there. This causes all sorts of odd manifestations like the bar, the spirals, and the spurs. I have a blog post discussing this in more detail, if you’d like to read that.
Incidentally, given the distance of NGC 1232 of 65 million light years, the photons you are seeing from this galaxy in this image left it when dinosaurs still roamed the Earth… but their time was juuuuussssst about up.
HAWK-I was used to study several galaxies, and not just NGC 1232– five more, in fact, for this particular study, but I want to point out one in particular: NGC 1300, a magnificent elongated spiral, shown here on the right. The bar in this one dominates, stretching over a much larger relative distance than the one in NGC 1232, which is dinky in comparison.