I have long extolled the contributions that “amateur” astronomers make in the field. I put the word in quotes because it evokes an image of someone hauling out a home-made telescope to their driveway and staring through an eyepiece; while that stereotype isn’t entirely wrong (it described me, more or less, for a lot of my youth) it’s very unfair to a large group of people who have the knowledge and experience to use astronomical equipment in sophisticated ways.

The contributions to astronomy made by amateurs are equally refined. Examples abound, but one that particularly interests me is when they take wide-field but still very deep images of the sky, meaning using very long exposures to see faint objects and structures. Until very recently this was impossible to do with professional-grade observatories, because they tend to look at very tiny sections of the sky. Plus, getting time on big ‘scopes is extremely competitive, so getting truly deep imaging can be tough.

Some years ago I was contacted by amateur astronomer Marcel Drechsler, who works with others like Xavier Strottner and Yann Sainty. They took wide-field images of the sky to look for objects previously unseen. They do this using a very specific kind of filter that looks for light from oxygen atoms.

Different kinds of atoms in a diffuse gas emit light at very specific wavelengths — really, just different colors. That light can be swamped by the light from stars and other objects that emit all across the electromagnetic spectrum, making it hard to see the specific gas colors. It’s like trying to pick out one voice in a huge choir.

Filters are devices that block light from some wavelengths and allow others to pass through. They can be very broad, allowing, say, yellow light through (like blue blocker sunglasses) or extremely narrow, allowing only a very thin slice of wavelengths to pass through while blocking everything else. These are called narrow-band filters, and are extremely useful in astronomy.

Hydrogen is by far the most common element in space, but there are others like oxygen out there as well. While there’s far less oxygen, due to some quantum mechanical effects those atoms can glow fiercely bright. So, even though there are fewer atoms, they still can emit brightly. One kind of atom that does this is oxygen that’s been ionized, stripped of one or more electrons. This usually happens when the atoms are bombarded by a bright source of ultraviolet light, for example, the electrons absorb that energy and get kicked off the atom like shrapnel. Neutral oxygen, an atom that has all its electrons, is called OI (“oh one”). If it loses an electron it’s called OII. Lose two, and it’s OIII. That doubly-ionized oxygen emits very strongly at a wavelength of about 0.5 microns, in the blue-green part of the spectrum. In the early days of astronomy it wasn’t understood how it could emit this light, and so this was called forbidden emission. We still call it that, even though the mechanism is now well understood. That emission is denoted using brackets, so it’s called [OIII].

This light is emitted in gas clouds like star-forming nebulae, dying stars called planetary nebulae, and exploding stars. Using an [OIII] filter and taking deep images of the sky can reveal structure in previously discovered objects as well as uncover new ones. And that’s what Drechsler and his colleagues were doing.

I’ve written about some of their discoveries before; they’ve been finding quite a few planetary nebulae that were completely unknown before. Some of them have weird structures, and it’s not understood how they form (though almost certainly magnetic fields are involved). They’ve also found objects that have astronomers scratching their heads, like huge arcs of oxygen apparently related to the Andromeda Galaxy.

This caught the notice of many professional astronomers, including Robert Fesen, who studies the gas expanding away from exploding stars. Together, they decided to map out the huge supernova remnant called G150.3+4.5, an immense cloud of gas in the constellation Camelopardalis [link to journal paper]. It’s 2.5° x 3° in size, big enough to fit 30 full Moons inside it as seen from Earth! It’s something like 3,000 light-years from Earth and well over 100 light-years across, which is huge.

But then they saw something funny. Right next to the remnant is another structure, an oblong shell of gas glowing in [OIII]. Sitting right at the center of that shell is a well-known star called CI Camelopardalis (or just CI Cam for short). This is known to be a binary system, where one component is a normal star like the Sun, but the other is a compact massive object, either a white dwarf, a neutron star, or a black hole. The system blasts out high-energy X-rays, which are generated when matter falls onto (or into) such an object; the magnetic fields associated with them can be ridiculously powerful, and can accelerate subatomic particles to very near the speed of light, making high-energy light (via the Compton effect).

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