The Rosette Nebula is a bruiser. It’s a vast gas cloud, over a hundred light-years across — the Orion Nebula is only about 25, for comparison — and is busily birthing hundreds if not thousands of new stars. It sits in the constellation of Monoceros (the Unicorn) and is bright enough to see with just binoculars.

So when you point a four-meter telescope at it and take images with the huge Dark Energy Camera, a 570-megapixel behemoth with a three square degree field of view, what you see is utter magnificence:

Holy HII regions!

The naming of this nebula is obvious enough; it resembles a cosmic flower. It’s massive, with enough gas in it to make many thousands of stars, and in fact in its center is the open star cluster NGC 2244, which has thousands of stars in it. Many of them are extremely massive, and two of them, called HD 46223 and HD 46150, are ridiculously luminous O-type beasts, dozens of times more massive the Sun. They blast out about 400,000 times the energy of the Sun each, much of it in the ultraviolet part of the spectrum. This energizes the gas in the nebula, causing it to glow. This intense radiation also eats away at the gas around them, carving out a huge bubble in the middle of the cloud, which is obvious in the image, too [in fact, if you want to understand the structures in the nebula, I’ve gone over that sort of thing many times with other nebulae like the Pillars of Creation, the Cat’s Paw, and NGC 602].

Looking at this image, I was surprised that the central star cluster wasn’t more obvious, but then I realized, ah, this image must be made using only narrowband filter observations.

So I checked the bottom right part of the press release where they list the observation particulars, and yup. I was right. 

You may be wondering, what does this mean? Well, it means I have a lot of experience in observational astronomy and how filters work! But if what you’re actually wondering is “what does a ‘narrowband filter’ mean”, then hey, I got you.

Light is emitted by astronomical objects in a number of different ways. Stars like the Sun emit light at all wavelengths — think of them as colors — so we call them continuum sources. Objects like gas clouds, though, work differently.

Ultraviolet light from a bright star can knock the electrons off an atom (an atom that has all its electrons is labeled with a I, so neutral hydrogen is HI. When it loses an electron it becomes HII, hence my joke earlier; a nebula like this is called an HII region).

When the electron recombines with the atom, it falls down a series of energy levels you can think of like steps (I describe this in detail in my Crash Course Astronomy episode on light). Each time it drops down a level it emits a very specific wavelength of light, and we can use that wavelength to ID what atom it is. For example, hydrogen loves to emit light at a wavelength of 656.46 nanometers, which is in the red part of the visible spectrum (the kind of light we can see). It emits at different wavelengths as well, but that’s the most common. Oxygen emits strongly at 500.7 and 495.9 nm (green), and sulfur at 671.6 and 673.1 nm (also red). These three elements are very common in nebulae and tend to be the brightest colors we see in them.

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If we want to just see where the hydrogen is, we can use a filter that only lets through a very narrow range of wavelengths centered on 656.5 nm, maybe seeing only a few nanometers to either side (this light from hydrogen is called H-alpha). We can do the same if we want to see just oxygen, or sulfur, or most any other element. 

The Rosette Nebula image above uses three filters just like that. The colors we display them as are arbitrary; though astronomers like to assign them in numeric order (so the shortest wavelength is displayed as blue, the next as green, and the longest as red) it’s not always that way. In this case sulfur is shown in blue, oxygen in green, and hydrogen in red.

But then why do we barely see the stars?

That because their light is so spread out in color. The filters only let through a tiny slice of their light, so they look faint. Think of it like hitting a single key on a piano compared to all the keys at once, the single key is much quieter than all the keys). On the other hand, the gas is emitting all its light at that single wavelength, so it can be quite bright. If we didn’t use filters, the gas would barely be visible while the stars would blaze out in the image (see this image of the Rosette that shows this effect). The filters increase the contrast, making the gas visible.

And we’re not limited to visible light; we can do this in infrared, ultraviolet, and pretty much any other wavelength. Different atoms emit light at different parts of the entire electromagnetic spectrum, and they tell us what’s going on in their environment, like what temperature it is, what’s the density of gas around them, and much more. For something like the Rosette, this in turn allows us to better understand the conditions under which stars are born, which I think anyone would admit is kinda important if you want to know why, say, we exist.

This gets complicated quickly, of course, as physics generally does (I spent a decent fraction of my PhD trying to understand how doubly ionized oxygen emits light in gas around a supernova, and it’s brain-melting complex).

But in broad terms this is how narrow filters work. And it’s why so many images you see of galaxies and gas clouds are so gorgeous.

My latest book, Under Alien Skies, has been selected as a finalist in the One Book program for 2026 by the Sarasota (Florida) County libraries! I’m honored. They interviewed me about the book for the program, so give that a look, and check out the other books selected, too!

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