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SYFY WIRE Bad Astronomy

Red Swan

By Phil Plait

One of my favorite kinds of astronomical photos is when a big, professional telescope is used to observe something I’ve seen myself at the eyepiece of my own home ‘scope. Another kind is when amateur astronomers take such observations and process them with care and love, turning them into breathtaking works of natural art.

And here's an example of both: Roberto Colombari and Robert Gendler’s version of M17, also known as the Omega or the Swan Nebula:

And oh yes, you want to click to embiggen that; I had to shrink the original 1800 x 1700 image to fit the blog.

This stunning image is a combination of observations taken by Gendler (in red, green, and blue, providing “natural” colors similar to what the eye sees) together with 40 frames taken using the monster 8.2 meter Subaru telescope. Those images were using a filter that lets through a very narrow color of light centered at about 671 nanometers (red in color), which is where warm hydrogen and nitrogen glow. Gas clouds like this are mostly hydrogen, so this filter accentuates the places where this gas is excited by the stars within.

The nebula is about 5,000–6,000 light years away, close enough to be one of the brightest in the sky, and in the summer months it was a favorite target of mine. It wasn’t much more than a smudge to my eye (I had to look past a bright streetlight when observing to the south; the nebula is located just above the “teapot lid” in the constellation of Sagittarius), but years later I saw it from a dark site with a bigger ‘scope, and I laughed out loud: The swan shape was obvious. It’s not as clear in this image, which is so detailed you lose the overall shape! I’ve included pictures here where I’ve outlined the part that resembles a swan. It’s also called the Omega nebula because the outer parts make it look like the capital Greek letter.

M17 is a region of star formation, with many dozens of stars embedded in it. The glow from a handful of the most massive and luminous of those stars is what lights up this nebula. It’s not reflected light, like when a light bulb illuminates a room. Instead, the ultraviolet light from the stars energizes the gas, causing it to fluoresce like a neon sign. While the red is from hydrogen and nitrogen, I suspect the blue is from oxygen, which is less abundant but (due to complicated quantum atomic physics) tends to glow strongly. Some of the blue may be light from the stars that gets reflected by the dust in the nebula as well, but I think most is from atomic oxygen.

Note where the blue is coming from: a long strip in the central part of the nebula (the body of the swan). This is typical for big, active stellar nurseries. As the stars inside are born, they flood the inside of the gas cloud with light and wind. This carves a cavity in the middle, a bubble with lower density in the middle.

Oxygen atoms don’t glow much if the density is too high; they bump into each other, which robs them of the energy needed to emit light (to be scientific, collisions knock the electrons down to lower energy levels, and the atoms don’t emit photons when that happens; this is called collision de-excitation). If the density is lower, there’s more room to move around, and the atoms have more chance to emit light (science: electrons tend to hang out in the upper energy levels of the atoms for a while, and then spontaneously drop to lower energy levels, emitting a photon). This sensitivity to density is a great boon to astronomers, because by measuring how much light the oxygen gives off, we can then measure the density of the gas, an important parameter. Other atoms have a similar sensitivity to temperature, so we can measure that as well.

Nature is accommodating; objects trillions of kilometers distant (and sometimes much, much more) glow in ways that allow us to take their measure, giving us insight into their behavior. That’s how we know stars are born in clouds like M17, and what happens when they do. We understand how the Universe works because light travels so far, so quickly, and carries so much information. It’s taken a long time, but we’ve learned how to read the story within.

And there’s this fantastic bonus: It’s also incredibly beautiful.

Note: This image is on today’s APOD, too. Colombari sent this image to me a few months ago, but a mutual misunderstanding about wavelengths kept me from writing it up, but we straightened it out, so I figured now was as good a time as any.

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