Chasing the Rainbow: All About Multi-wavelength Astronomy

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Readers of the Spacing Out newsletter will know that on the morning of Thursday September 7^th Japan time (which was the evening of September 6^th Eastern time), the X-Ray Imaging and Spectroscopy Mission, aka XRISM (apparently pronounced “crism”) successfully launched from the Tanegashima Space Center in Japan. In honor of the launch of this newest X-ray telescope, I thought it would be fun to take a look at just why we look at space in different wavelengths and the kinds of telescopes we need to do it.

That’s right kiddos, we’re chasing the rainbow today as we take a look at multi-wavelength astronomy and why it’s necessary if you want to actually know what’s going on in the universe (a trait notorious amongst astronomers).

All the Colors

What do I mean when I say multi-wavelength astronomy? I mean the whole electromagnetic spectrum, everything from radio waves to gamma rays. We humans can see a very narrow part of the entire spectrum with our eyes. Appropriately enough we call this the “visible” part of the spectrum. It just so happens to correspond to the part of the spectrum that the Sun’s energy output peaks in. Clever evolution, making sure our eyes can see best in the kind of light we get the most of from our local star!

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That’s not the only kind of light, it’s just the only kind our eyes can see. There’s infrared light, which you can see if you use night-vision goggles. There’s ultraviolet light, which you can’t see but you can sure feel the effects of if you don’t wear sunscreen on a bright summer day (our Sun’s output may peak in the visible part of the spectrum, but it’s giving off many kinds of light, including plenty of UV light). There’s the microwaves you use to heat food and the X-rays doctors use to look at our bones. There’s radio waves that we use to transmit information and the really high-energy gamma rays that turn you into the Hulk (okay fine, not really).

Which type of light an object emits depends on how energetic it is. Cooler, lower energy things will primarily give off lower energy light like infrared. Higher energy things will primarily emit higher energy light like X-rays. So it turns out that if you only look at the universe through one kind of light, you wind up missing a whole lot of stuff.

What Emits Where

So what can you see if you look in visible light? Well, quite a lot of beautiful stuff. You can, of course, see all the things you see when you look up at the night sky: the planets and moons of our solar system, the stars of the sky, the band of the Milky Way. Plenty of things emit visible light. But, like our Sun, a lot of them are also emitting in other types of light, and some things emit mostly or only other types.

A whole lot of our universe is clouds of gas and dust and things far from the heat of stars. These things are cold and very dark in visible light. Take, for instance, one of my favorite formations in the sky, the Horsehead Nebula. This is what we call a “dark nebula”. It appears as a dark cloud of dust silhouetted against a brightly lit background cloud, and it happens to be in the shape of a horse’s head. The dust of the Horsehead isn’t emitting strongly in the visible spectrum, so it mostly just looks dark. 

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In infrared light it’s a whole different ballgame. Suddenly that brightly lit background is dark—that cloud isn’t bright in infrared. The Horsehead itself is emitting in infrared, and lights up to show the intricate structure within the cloud, along with several of the cooler, dimmer stars that are drowned out in the visible light image by bigger, brighter, hotter ones. So if you want to study clouds like the Horsehead, you really want an infrared telescope.

But let’s say you’re not interested in clouds, you’re interested in things like stars and supernovae and the regions around black holes (all very cool and very proper things to be interested in). Well, the infrared telescope is only going to do so much for you, as these are all high energy targets. This is where you want to be able to see UV, X-rays, and gamma rays, which these objects will all be emitting at high volume.

It’s when you put it all together, though, that the real magic happens.

When Telescopes Team Up

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Not far from us astronomically speaking, a mere 12ish million light years away, is a strange-looking galaxy named Centaurus A. It is suspected that its odd shape is because it’s really two galaxies in the end stage of a collision. As one of our neighbors, it has been heavily studied in many wavelengths, and astronomers see entirely different things depending on which wavelengths they choose.

Looking in X-rays reveals Centaurus A’s energetic center, the region around its highly active central supermassive black hole, which is causing all the gas and dust falling into it to heat up and emit very high energy photons. Looking in optical you can see the light from the stars in the galaxy, though that light is heavily obscured by the thick, heavy dust lanes running around the galaxy’s edge. 

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In infrared, those same dark dust lanes shine bright, and the light of the stars is barely visible. The radio part of the spectrum doesn’t show the galaxy itself at all. What it does show are the two jets of material being spat out from the galaxy’s center due to all the activity around the central black hole. These jets are a million light years long and are fascinating structures in their own rights.

It’s not until you combine the data from all of these different wavelengths of light that you can get a full picture of Centaurus A and what’s going on inside and around it. Astronomers who look in only one type of light will only have a piece of the puzzle. 

Not always, but oftentimes when we see beautiful images from space they are composites made up of light from several different telescopes, or sometimes in different wavelengths from the same telescope, with the different types of light given colors that we can see with our eyes to make it possible for us to fully appreciate our incredible cosmos. So if you actually went and saw many of the things we have incredible images of from our telescopes in person, they might look…well…a lot less impressive.

Observing the Rainbow

As I already hinted, if you want to see different kinds of light you will often need to use different kinds of telescopes, although some can see at least a bit beyond just one type of light. For instance, the mighty Hubble Telescope is our visible light juggernaut in space, but it can also see just a little into the infrared and ultraviolet. You wouldn’t really call it an infrared telescope though—it’s just not designed to maximize its ability to see infrared light. 

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You know what is? The James Webb Space Telescope. If you’ve ever seen a diagram of the telescope you know it has a gigantic pink sunshield and a huge gold mirror. That sunshield is helping to keep the telescope cold, which reduces the amount of infrared light the observatory itself radiates. The gold covering the mirror optimizes it to reflect infrared light into the telescope’s detectors. It also sits out a million miles from Earth and the Moon, which themselves glow brightly in the infrared. All of this makes Webb much better equipped than Hubble for looking in the infrared.

The design of the telescope isn’t the only thing that needs to be considered. Hubble and Webb are in space, but you don’t necessarily have to be in space to observe in visible or certain kinds of infrared light. Many wavelengths of light make it through the atmosphere just fine and can be seen by ground observatories (it’s just better to view from space). There are other kinds of light, however, that you have to be in space to see.

XRISM had to go to space—the X-rays it observes don’t make it through Earth’s atmosphere. That is also true for gamma rays and some kinds of UV light. This is great for us down on the surface who don’t want to get bathed in high-energy radiation from space, but makes life hard for those who want to study high-energy astronomy. If you want to see the energetic universe (which includes black holes, supernovae, and other really cool things, so we definitely do), you have to go up.

On the other end of the spectrum sits radio waves. These are low energy light with very long wavelengths. What that translates to is that you need a big detector (which generally looks more like a giant satellite dish or antenna than what we traditionally think of as a telescope) to observe faint things in radio. Too big, in other words, to practically launch into space. 

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Fortunately radio gets through the atmosphere without much difficulty, which means we can build giant ground-based facilities for detecting in the radio spectrum. Sometimes these take the form of huge arrays of antennas such as the Very Large Array or the Square Kilometer Array, where many dishes can be aligned to be used like they were a single huge dish. Single dishes also exist, and can range from reasonable sizes to stupidly gigantic sizes, like the Five-hundred-meter Aperture Spherical Telescope or the dearly departed Arecibo Observatory (the epic and horrifying destruction of which I still rewatch when I want to feel sad).

Pretty Pictures

With the launch of XRISM our ability to keep exploring the universe in X-rays is secured for the foreseeable future. Across the spectrum, scattered across Earth and flying through space, we have an array of observatories dedicated to making certain we are able to get the most complete view of the universe we can. And when we combine them together we get the ultimate reminder that astronomy comes with a gigantic fringe benefit—it’s the most wondrously beautiful of all the sciences.

(I can feel the volcanologists and marine biologists coming at me for that statement, but I will die on that hill.)