Usually, when you boggle slack-jawed at some amazing astronomical image, you’re seeing a combination of three or so individual colors merged together. This mimics how our eyes see color, since our retinae have three different kinds of cells, called cones, that are sensitive to red, green, and blue light (more or less). When light hits the cones, they send a signal to the brain, and those signals get mixed weighted by their strength to represent color. 

Astronomers typically do this by using filters; for example, taking an image with a red filter, then a green one, then a blue one, and finally mixing them together to produce a “natural color” image. The images you see in press releases usually use three but sometimes as many as seven or so filters to create the nifty color pix you see.

But now we can do a wee bit better: The MUSE camera can take images with 1,500 colors!

MUSE stands for Multi Unit Spectroscopic Explorer, a camera that’s used at the Very Large Telescope (or VLT) in Chile (so named because its comprised of four different telescopes each with an 8.2-meter mirror). I’ve explained this before, but in essence what it can do is take a spectrum of an object at each pixel of an image. Think of it this way: when you see an image, there are say 1,000 pixels in the x direction (the rows) and 1,000 in the y direction (columns). Each pixel shows you the brightness at that position, and, in a typical color photo, three different colors combined to show different hues. However, what you see is the combined colors, with the individual color information lost in the merger.

What MUSE can do is not only get the brightness at each pixel, but also the colors individually, about 1,500 of them! So really what it makes is a data cube: 1,500 images, each with a specific color at each position. To make a natural color image out of that you just have to pick your colors, extract the image at those wavelengths, and combine them. Voilá!

Astronomers used MUSE to look at NGC 253, also called the Sculptor Galaxy [link to journal paper]. It’s a spiral galaxy (in the constellation Sculptor, hence the name) about 11 million light-years away, making it one of the closest large spirals to us. That means we can see it in some detail, especially when using the VLT. Couple that with MUSE’s color info, and they had plenty of choices how to make an image. Here’s one example:

Cooooool. There’s a MUCH larger 6,000 x 1,700 pixel version you can grab if you want to see details, and you so do.

This image shows a relatively broad pick of colors to make a red/green/blue image, and added in is a narrow range of colors (around 656 nanometers, in the red part of the spectrum) where hydrogen emits light. Star-forming gas clouds strongly emit this particular wavelength (colloquially called H-alpha), and in the image are colored pink, so you can easily see where stars are being born in NGC 253. That’s pretty nifty. 

But they can pick any color. So here’s another view, this time emphasizing hydrogen, nitrogen, sulfur, and oxygen. Sulfur is prominent in the gas blasted out when a star goes supernova, while oxygen is common in dying stars like the Sun.

That last bit is important. When a star dies and blows off its outer layers, it creates a planetary nebula (like the Ring Nebula and the Helix Nebula; click those links for a lot more info on these kinds of objects). Different planetary nebulae have different luminosities, different amounts of light they give off. If you can look at lots of these nebulae in a galaxy, you can fit their brightnesses to a physical model, called the planetary nebula luminosity function. This in turn tells you how much energy the nebulae are emitting in that galaxy. If you compare that to how bright they appear to be in the image, that gives you the distance to the galaxy (since objects fade with distance at a known rate).

So, by isolating the light from oxygen in these images, astronomers can figure out the distance to the galaxy. Only about 100 planetary nebulae were known in NGC 253 before, but this image revealed about 500 of them, greatly improving the data! The distance to NGC 253 is already pretty well known from other, better methods, but this enlarged and improved set of planetary nebulae helps astronomers adjust the model to better calibrate the method, which can then be used to get distances to other galaxies. Pretty handy.

There are also other structures you can see better if you pick the right wavelengths. Here’s the same image, but I’ve zoomed in on the center of the galaxy using the higher-resolution image:

See that curved cone of light coming out of the center? That’s gas being blasted out of the galaxy’s core by the intense radiation emitted by a flurry of newborn stars there. NGC 253 is a starburst galaxy, meaning it’s making a lot of stars; in the core are several immense clusters with tens of millions of young stars in them. Their combined light is so powerful it can push with significant force on the gas around them, causing it to flow away from the galaxy center. That gas isn’t easy to see in the three-color images, but when you use colors that pick and display the specific light emitted by the gas the structure pops right out.

Clearly, MUSE is a powerful instrument that can do amazing science. And it’s not just limited to galaxies! It can be used to look at any kind of object, providing a detailed map of structure and composition. By looking at Doppler shifts it can even detect motion of material in the object, too (as I showed here).

I love clever things like this. Back when I was working on spectra of different nebula the data were good for the time, but things are hugely improved now. It makes me excited and hopeful for the future of astronomy. We keep getting better at this, which means we keep getting more understanding of the Universe too. That’s what this is all about. 

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