Spiral galaxies are inherently interesting. Something about their beauty is so enticing… but when you look at them more carefully, the science and physics behind them is terrifically compelling. And when you use different eyes — say, radio telescopes — then you see something different entirely:

This shows two views of the lovely face-on spiral galaxy NGC 6946. On the left is a visible light image, and on the right is the radio view, taken by the Westerbork Synthesis Radio Telescope (taken over the course of 192 hours). Amazingly, these two images are to the same scale!

Spiral galaxies emit light across the entire electromagnetic spectrum, including visible and radio light, but what emits that light is different. Stars and warm gas emit visible light, but cold hydrogen glows at radio wavelengths. At a wavelength of 21 centimeters (about 8.5 inches, much, much longer wavelength than visible light, by a factor of tens of millions!) cold hydrogen can actually be quite bright, making it a perfect target for big radio telescopes.

In this image on the right I superposed both shots so you can see just how much more there is to NGC 6946 than the eye sees. What this image immediately tells us is that cold hydrogen extends well beyond the region where hydrogen is warmer, toward the center of the galaxy. It also shows the gas still takes on a spiral shape well past the visible boundaries of the galaxy.

A more detailed analysis indicates there are over 100 holes in the cold hydrogen gas as well, and these correspond to areas where stars are actively forming. That’s hardly a surprise! Stars use up hydrogen gas when forming, and then heat up what remains around them in the neighborhood. Once warmed up, the gas doesn’t emit as much 21cm radio waves.

The astronomers also found a lot of this gas is moving at high speeds, up to 100 km/sec (60 miles/second, fast enough to go from the Earth to the Moon in a little over an hour!). This is probably gas that’s been blown up and out of the galaxy by stars and supernovae, only to fall back down due to the gravity of the galaxy. That’s not known for sure, but we do see such fountains in other galaxies, including our own.

I’ll be honest: I’m more of a medium-to-high energy guy than radio guy. That’s why I tend to talk more about X-rays and gamma rays from astronomical objects, but every part of the spectrum tells a story. Radio astronomy has been around for almost a century now, and it is still and always will provide insight into the mechanisms behind the Universe.

Image credit: Rense Boomsma/Digitized Sky Survey/WSRT; ASTRON/JIVE Archive

[This is a gallery of gorgeous images, my favorites, from the orbiting infrared observatory called the Spitzer Space Telescope. Click the thumbnail picture to get a bigger picture and more information, click the big pictures to go to my original blog posts about the pictures, and scroll through the gallery using the left and right arrows.]

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When Galileo first turned his telescope to the sky, almost exactly 400 years ago, he could not possibly have known what he was starting.

Today, four centuries later, we’ve come a long, long way. To celebrate the anniversary of Galileo’s telescopic revolution, NASA’s Great Observatories — Hubble, Spitzer, and Chandra — have released a jaw-dropping mosaic of the very heart of the Milky Way galaxy. Behold!

[Oh yes, you want to click to embiggen that-- what I show here is a very compressed version. Or you can go here for a massive copy. You can also get wallpaper versions here.]

This image is nothing less than a heroic effort of astronomical artistry. It’s a chunk of the sky 38 x 14 arcminutes across, or about half the size of the full Moon, and it’s aimed right into the core of our galaxy. See the bright spot just to the right of the center? Buried in there behind light years of dust and gas is the monster of the Milky Way, a black hole with four million times the mass of the Sun. But even that is dwarfed by the 400 billion solar mass heft of the entire galaxy.

There is so much going on in this image it’s hard to know where to start. But first… the Hubble images are in the near-infrared, with a wavelength a little more than twice what the eye can see (1.87 microns for those playing at home). That’s represented in the image as yellow. Spitzer contributed observations in four infrared wavelengths (3.6, 4.5, 5.8, and 8.0 microns), and those are depicted in red. Chandra sees X-rays which are normally written as units of energy, but to remain consistent with the other two images, they were at wavelengths of 0.0005, 0.00025, and 0.00016 microns, and are shown in blue.

What does all this mean? Different objects emit light at different characteristic wavelengths. Warm dust, for example, emits strongly in the infrared. Stars and warm gas emit visible and near-infrared light. Violently heated gas, affected by huge magnetic fields or shocked by colossal collisions glows in X-rays. So this image is a polychromatic view of the crowded downtown region of a bustling city: our galaxy.

You might want to look at an annotated version of this image so you can get your bearings. It’s worth it!

The huge arches of gas on the left are actually the edges of gigantic molecular clouds (dense nebulae where stars are born), lit up by the torrential blast of light from a clutch of massive stars nearby. This clot of stars, called the Arches Cluster due to the arcs it excites, can be seen as a small spot glowing blue just to the left of center in the picture. Don’t be deceived by its diminutive appearance: the Arches cluster has thousands of superstars in it, each dwarfing our Sun, and each capable of sleeting out vast amounts of radiation that lights up the gas surrounding it. Were this cluster much closer than its 25,000+ light year distance, it would blaze in our sky like a beacon. Replace the Sun in our solar system with just one of those stars, and the Earth would be fried beyond the capability of any life to survive. You might as well try living in the flame of an arc-welder.

Below and just to the left of the Arches is a clumpier, more twisted arc of gas called the Sickle. That’s a giant cavity being carved out of dense gas by the Quintuplet cluster, the pinkish glow in its center. It’s another nursery of stars like the Arches cluster, which is also blasting out light and stellar winds which eat away at the gas enveloping it. The Pistol Star resides there, perhaps one of the most massive stars in the Milky Way.

And there’s more! The blue glow on the left is from an X-ray binary called 1E1743.1-2834, what is probably a massive star being orbited by either a neutron star or a black hole. Matter is being stripped from the star and piling up outside the collapsed companion, where it gets heated up to millions of degrees and emits X-rays.

Supernovae remnants dot the image, as do stars, filaments of gas, clouds of dust, and more. This picture is an astronomer’s dream, a map of everything someone might want to visit with a starship — as long as the shields are at full strength. This image is also a map of violence, turbulence, and unrest… a typical scene, so we think, of any normal spiral galaxy like ours. And our Galaxy’s center is considered quiet by astronomers! Some are far worse.

But this is home for us. It’s a place of unimaginable fury but also astonishing beauty… and we see it now as we do because we have dared to examine the world around us, to use tools we invent to peer closer, to magnify the tiny, to extend our eyes into realms we once didn’t even know existed. And every time we do — every single time — we find more questions, more puzzles, more things to examine.

And we find art. Galileo wasn’t the first to turn his telescope to the sky, nor was he the first to record what he saw. But he was the one who made everyone see what he did, and for that, all these years later, he is owed a debt of gratitude.