Messier 33 is the third largest galaxy in our Local Group, with an estimated mass of 10 billion to 40 billion times the mass of our sun. Also known as the Triangulum galaxy for its location in the sky, M33 is home to some of the largest known star nurseries; its largest, NGC 604, is something like 100 times larger than the Orion Nebula. If it was as close to us as the Orion Nebula, it would be the brightest object in the night sky, save for the moon.
Because it’s so close, M33 is a perennial favorite of amateur astronomers. Italian astronomer Giovanni Battista Hodierna discovered it some time earlier than 1654, describing it as a nebula, before we even had a word for galaxies. But a team of astronomers using the 4.2m, optical-IR Herschel telescope have just turned up something truly remarkable in one of M33’s star nurseries: a cluster of new stars enclosed by a rare triple “superbubble,” concentric supernovae whose shells enclose each other like Russian nesting dolls in space.
All three shells formed the same way from individual stars: The first star went supernova and ejected a cloudy shell of matter into space, which slowed down and cooled as it went. Then the second supernova happened, with its shell of ejecta, and then the third. The outermost bubble is the slowest, coolest, and oldest of the trio at about 20,000 years old; the innermost ring formed maybe 5,000 years ago. And the clouds of gaseous ejecta are no trivial wisps. Each of these shells, the scientists estimate, is “a couple hundred times the mass of the Sun.”
The hard science behind the discovery is pretty friendly. H-alpha (Hα) spectroscopy measures things like redshift and radial velocity. Using that information, the scientists can tell things like how long the shells have been traveling, and in what direction. “We can roughly calculate when the supernova explosions went off,” said John Beckman, coauthor. “We know how fast the shells are going and how big they are, so we can work out the shells’ ages.”
This discovery was part of a project called BUBBLY, which was established to find cosmic bubbles like these, floating in interstellar space. They vary in size from a few light years to a few thousand light years across. No fewer than 11 concentric superbubbles surround the Cat’s Eye nebula, for example, whose dying red giant core shed the mass equivalent of all the planets in our solar system every 1,500 years. Our own sun will spend some time as a red giant near its end of life.
Astronomers are collecting observations of these superbubbles in order to better understand the birth and death of stars, and the texture of interstellar space. As it turns out, interstellar space is lumpy; we know this because as these concentric but patchy superbubbles sweep outward, they pick up some gas from their surroundings. But only some. There’s some left for the next bubble, and some of it gets swept along with the shockwave, just like the first time.
Ultimately, superbubbles can teach us how the early universe evolved by telling us about how the first galaxies became enriched with metals. By peering across the eons into M33, we’re able to glimpse a process that was vital to life on Earth.