Blowing a Cosmic Bubble

Contributed by
Dec 5, 2012
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The Sun is hardly an average star; by mass it’s in the upper 80th percentile of all stars in the entire Universe. But much, much beefier stars exist, like SAO 20575 (or, if you like these alphabet soup catalog names, it’s also called BD+60 2522). It tips the cosmic scale with at least 15 times the Sun’s mass: It’s a hot, luminous star, so energetic it’s blasting out a fierce wind of subatomic particles that’s actually carving a gigantic bubble in the surrounding material.

That all sounds a bit clinical, until you see a picture of what this awesome power means:

This structure is called the Bubble Nebula, and it is literally a cavity, an evacuated region, left behind as the stellar wind from the star expands. It’s huge, roughly 10 light years across. That’s 100 trillion kilometers (60 trillion miles), hundreds of thousands of times the size of our entire solar system.

The star sits near the edge of a molecular cloud, a dense clump of gas and dust that may be dozens of light years across. The star itself is near the top of the bubble. It’s not centered because the surrounding material isn’t evenly distributed; it’s denser near the top of the bubble, so the star’s wind has a harder time pushing against it. It’s also possible the star is physically moving in that direction, contributing to it not being centered in the cavity.

But still, it’s pushing hard. Bear in mind our own Sun’s wind barely reaches 20 billion kilometers, and even then it’s stopped by the ethereal wisps of material between stars. The material in the molecular cloud can be thousands of times denser than the space near the Sun, so it’s very thick by galactic standards. But then, the wind from SAO 20575 is nearly a billion times stronger than our own Sun’s solar wind and moving dozens of times faster than a rifle bullet. That’s a lot of force.

All of this attests to what a monster that star is.

In fact it’s not clear just how massive it is. Violent stars make it difficult to measure their mass, but one study found it to be 45 times the Sun’s mass, which is flipping huge. We see powerhouse stars like this scattered around the galaxy, though not very many. It’s hard to make stars a lot bigger than the Sun, and the more massive they are the more rare they are. Stars make energy by fusing elements in their core, similar to the way a nuclear bomb does. The more massive the star, the hotter the core, and the faster the reactions go, making more energy. A star more than about 150 or so times the mass of the Sun generates so much energy that it quite literally and enthusiastically tears itself apart.

Yeah, think on that for a moment.

While SAO 20575 isn’t that massive, it’s still plenty luminous. It gives off so much energy that it floods the surrounding space with ultraviolet light strong enough to make the gas glow, similar to the process that lights up a neon sign. It’s lighting up its own environment so fiercely we can see it from thousands of light years away. It’s incredible.

This picture was taken with a new camera called the One Degree Imager, mounted on the WIYN 3.5-meter telescope. It sees a big chunk of sky; what I put here is only a piece of the much larger full image. And this camera is still being commissioned—only 13 of its 64 light-sensitive digital detectors were used here!—and will be even more capable soon. Even better news is that I see my old friend and astronomer Travis Rector is behind this image, and he is a master of astronomical imaging. I cannot stress enough how beautiful his work is, and if he’s involved with this camera, then we’ll be seeing even more remarkable images from it over time.