AAS #6: Hubble spies interstellar interlopers

Contributed by
Jan 7, 2009

Sure, when you're young, you're reckless. Give a teenager the keys to a sportscar, and how long do you think it'll be before they smash into something?

Turns out stars are the same way. And Hubble just gave us proof.

Hubble spots young stars ramming through gas clouds. Credit: NASA, ESA, and R. Sahai (NASA's Jet Propulsion Laboratory).

Astronomers were using Hubble to look at bright gas clouds, hoping to spot dying stars that were ejecting shells of gas as they shed this mortal coil. What they found by accident were young stars slamming into the gas clouds, creating these gorgeous shock waves.

Picture a boat moving through water. The bow of the boat creates that familiar V-shape as it pushes the water aside. Any object moving through some medium like a liquid or gas will do that. If the object is moving faster than the local speed of sound, then you get a shock wave; a much stronger wave that forms.

That's what these stars are doing. Moving at interstellar speeds (50+ km/sec!), they're ramming the gas and compressing it. What's interesting are the variety of shapes they see. The one on the upper right of the Hubble image above is the only one that has that standard shape. The one on the upper left looks for all the world like a microscopic photo of a sperm swimming! That implies some sort of oscillating motion, a back-and-forth movement to whip up the gas that way. I'm not sure how it manages that. The orbital motion of two stars in a binary might do it, so maybe the stars that have that pattern are actually doubles, traveling through space together. I suspect the one on the bottom right, with the curved tail, is form the star and gas moving in slightly different directions, and the moving gas blows the tail into a curve, like wind distorting a jet's contrail.

We can learn quite a bit just from these images. A star all by itself is way, way too small to make patterns like this in the gas; the bow shock would be too tiny to see. However, if the star is emitting a stellar wind -- like the Sun's solar wind, a stream of subatomic particles emitted from the star's surface -- then this all makes sense. The wind expands, displacing far larger amounts of gas than the star itself could.

But that in turn implies these stars are of intermediate mass (a bit bigger than the Sun) and young! Why? Because very massive stars blow winds that are huge and ionized (that is, glow like a neon sign). That's not seen here, so these stars must be lower mass than that. They have to be more massive than the Sun, because the Sun's wind isn't strong enough to make these patterns. We know they're young because star like these don't blow a wind like this all their lives; it's only when they are very young or very old that their stellar winds are active. The phase when they are old but blowing a wind is pretty short, while the young wind-blowing phase is long (millions of years) so it's far more likely these are youngsters in the scheme of stellar lifetimes.

Why are they moving so rapidly? Most stars plowing through gas like this are plodding along, and wouldn't make such intricate shapes in the gas. The pedal-to-the-metal velocity of these stars indicates they had some sort of encounter earlier in life that kicked them up to high speed. Maybe they formed in a cluster of stars, and a series of gravitational slingshots boosted their speed. Maybe they each used to orbit a high mass star that blew up, giving them a kick that accelerated them. Hard to say.

We know of other high-speed stars like these; but those are high mass stars that will one day explode. Those are rare, but bright and easy to spot which is why we knew of them before. These new guys are lower mass stars, so they're fainter and more difficult to see. The fact that the astronomers found 14 of them without even knowing they existed means that these runaway beasties are more common than their beefier brethren, which is an interesting result all by itself.

But furthermore, this makes you wonder what happens to the gas cloud through which these guys are plowing. The gas is getting stirred up, slammed by the stellar winds. See the parts of the wakes where the gas is brighter? That almost always means the density of the gas is higher (higher density = more gas = more light emitted). Do interlopers like these help compress gas so that more stars can be born? If that's true, and there are lots of these events going on, then there may actually be a process that triggers the births of stars that we were entirely unaware of previously.

That is so cool! There is a lot we still don't know about the Universe, and we learn so much just by keeping our eyes open! That's one of my favorite lessons of all about science.

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