Gorgeous and Shocking: Bow Wave of a Runaway Star

Bad Astronomy
The entire universe in blog form
Feb. 27 2014 7:45 AM

A Monster Star Plows Through the Galaxy? Shocking.

Kappa Cas
Runaway star Kappa Cas slams into material floating between the stars, creating a shock wave trillions of kilometers across. Click to enchuckyeagerenate.

Photo by NASA/JPL-Caltech

Correction, March 6, 2014: Well, this will take a moment to explain.

In the article below, I wrote that the star Kappa Cas is moving through the galaxy at a stunning 1,100 km/sec. This turns out to be incorrect: The speed of the stellar wind blowing off its surface is that fast, but the star itself is moving far more slowly.

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I based the original number off the NASA press release, which made that assumption about the speed. However, an email by astronomer Manfred Pakull set me straight. The star is actually moving relative to the Sun at about 25 km/sec, which means it’s moving around the galaxy at around the same speed we are.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

So why is there an arc of material compressed next to it instead of a more spherical shell of dust? The speed of the star is very small compared to the wind speed, so I’m guessing the dusty material the star’s wind is ramming is not evenly distributed. Note that the overall shape is more like part of a sphere as opposed to a bow wave, so this seems likely. Funny, too: When I read the press release I looked for journal papers on the star and found none (Pakull did mention one, which has some relevant numbers for the star). That’s surprising if it really is a runaway star, so now that anomaly makes more sense. Everything else I wrote below is correct, but note that the parts about it being a runaway are almost certainly incorrect. And also? It’s still gorgeous.

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When you look up at the sky, you’d be forgiven to think that the stars are motionless, frozen in time, mounted on the velvet vault of the heavens.

But in fact they are in motion, orbiting the center of our galaxy much like the planets orbit the Sun. The Sun itself, for example, is moving along at roughly 200 kilometers per second (450,000 mph). Some orbit a lot faster.

Take Kappa Cassiopeiae, for example. It’s what’s called a runaway star, screaming through space at a terrifying 1,100 kilometers per second … 2.5 million miles per hour! As it happens, it’s also a blue supergiant, a massive, hot star. These kinds of stars tend to blow out a fast wind of subatomic particles, like the solar wind on steroids. As the star plows through space, its wind rams into the material around it, creating a vast shock wave like air off the nose of a supersonic fighter jet. It’s invisible to the eye, but when you point an infrared telescope like Spitzer at it, you get stunning beauty:

How about that? Kappa Cas is the blue star in the center, and you can see the material arcing around it, snowplowed by the fierce interaction of the star and its surroundings. This image is infrared, which means the colors aren’t “real”; blue is a combination of light at 3.6 and 4.5 microns (five and six times the wavelength of the reddest light the human eye can see), green is from 12 microns, and red is 24 microns. What you see as red is dust that floats between the stars, and green is from complex particles very much like soot (created by stars both when they are born and when they die). In Spitzer images, stars tend to look blue because they give off most of their light toward that end of the spectrum.

The Sun is also moving through interstellar material, but the effect is nowhere near as profound as that from Kappa Cas. But then the wind from Kappa Cas is millions of times more powerful than the Sun’s and is blowing outward several times faster. Add that to the already incredible speed of the star, and you get a bow shock that’s a mind-crushing four light years ahead of the star: 40 trillion kilometers. That’s the same distance as the nearest star from the Sun, so you can see the influence of Kappa Cas extends a long, long way.

Zeta Oph
Zeta Oph, another runaway star, seen by NASA's WISE infrared observatory. Click to embiggen.

Photo by NASA/JPL-Caltech/UCLA

We’ve seen other cases of this as well. Zeta Ophiuchi is one; another massive star barreling through the night. Spitzer has observed it before, and it’s so beautiful that it’s one of my favorite all time astronomical photographs. Another infrared observatory, WISE, also took a great shot of it.

This raises the question: Just why is Kappa Cas on the run? There are a few ways stars can get accelerated to such high velocities. One is if they started out life as a binary, two stars locked in a tight orbit. If the other star exploded as a supernova, the two stars lose their grip on each other, and the angular momentum can fling them both away at high speed, just like a slingshot. Another possibility is that Kappa Cas was born in a cluster of a stars, and a close encounter with a pair of stars in the cluster gave it a kick sufficient to fling it out and into interstellar space.

Cassiopeia
The constellation Cassiopeia, located near the north pole of the sky. Kappa Cas can be seen from moderately dark sites.

Rendering by SkySafari

Kappa Cas is actually bright enough to see with the naked eye; it’s a fourth magnitude star in the W of Cassiopeia. Better take a look while you can, though; being a blue supergiant, Kappa Cas doesn’t have long to live. Even though it has something like 40 times the Sun’s mass, it burns through its nuclear fuel at a far faster rate, shortening its lifespan considerably. Someday, perhaps in the next few hundred thousand years or less, it will explode. It’s 4,000 light years away, so we’re safe, but it’ll get really bright when it goes, getting far brighter than Venus in the sky. What a sight that will be!

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