Daily Archives: September 13th, 2010

Spiral Death Star: Is It Natural, Or A Beacon?

Last week Phil Plait, the Bad Astronomer, posted a photo from the Hubble Telescope of a star that was going into death spirals. Death spirals? Ol’ Phil, being the debunker he is, thought it was fake. But after further investigation, found it was real!

When I first saw this picture, my reactions, in order, were: 1) What the frak is that? followed immediately by 2) This must be a fake! But it’s not fake. It’s real, and it’s the dying gasp of a very, very strange star system. The name of this thing is AFGL 3068. It’s been known as a bright infrared source for some time, but images just showed it as a dot. This Hubble image using the Advanced Camera for Surveys reveals an intricate, delicate and exceedingly faint spiral pattern. It’s so faint no one has ever detected it before! So what’s going on here? First off, this is not a spiral galaxy! It’s a binary star*, two stars that orbit each other, located about 3000 light years away from us. One of the stars is what’s called a carbon star, similar to the Sun but much older. The Sun is still happily fusing hydrogen into helium in its core, but older stars run out of available hydrogen. Eventually, they fuse helium into carbon. When this happens the star swells up and becomes a red giant (note: that’s the brief version; the actual events are a tad more complicated). Red giants tend to blow a lot of their outer layers into space in an expanding spherical wind; think of it as a super-solar wind. The star surrounds itself with a cloud of this material, essentially enclosing it in a cocoon. In general the material isn’t all that thick, but in some of these stars there is an overabundance of carbon in the outer layers which gets carried along in these winds. In these cases the material is very dense and opaque (carbon forms long, complex molecules that are very effective at absorbing visible light), and can completely block the light from the star. All we see is the warm glow from the cocoon as an infrared glow. keck_afgl3068_llpegAFGL 3068 is a carbon star and most likely evolved just like this, but with a difference: it’s a binary. As the two stars swing around each other, the wind from the carbon star doesn’t expand in a sphere. Instead, we see a spiral pattern as the material expands. This is called the sprinkler-head effect. As a sprinkler spins, the jet of water appears to take a spiral shape. Each individual drop is moving directly away from the sprinkler head, but the rotation of the head itself creates a global spiral pattern, with the arms appearing to expand. It’s not precisely an illusion — the spiral pattern is definitely there — but the arms aren’t an actual physical structure. It’s just the way we interpret the way the drops move away from the sprinkler. We see the same thing (on a much smaller scale!)when spinning comets give off gas, too. Going back to our sprinkler analogy, if you’re standing in the yard as the sprinkler spins, you get hit with a blast of water. Wait a few seconds and you get hit again. Obviously, the time between soakings is the time it takes the sprinkler head to spin once, right? That means that we can measure the arms of AFGL 3068’s spiral and calculate the rotation period of the binary! The expansion rate of the spiral material is about 15 km/sec (9 miles/sec). Given that distance, the time it takes between spirals turns out to be a little over 700 years. So if you were hovering in space outside this object and an arm swept past you, you’d have another 700 years before the next one blew your way again. But there’s more! Using the monster Keck 10-meter infrared telescope, the astronomers who observed the object were able to see through the dark material to the binary inside (IR gets through the thick cloud of material more easily than visible light). Making some simple assumptions on the masses of the stars, they find the orbital period is about 800 years: very close to the spiral pattern’s period, given their estimations. That also lets me measure the number of spirals — roughly five — and calculate the size of this object: about a third of a light year across, or more than 3 trillion kilometers! Coooool.

Now lately the science community have been racking their brains out wondering why SETI (Search for Extraterrestrial Intelligence) wasn’t discovered any radio signals from outer space from civilizations like ours (or ones more advanced). Some have suggested that perhaps we should be looking for more physical objects like Bracewell Probes, Dyson Spheres and other space architecture that might be recognizable.

The key here is “recognizable.” Would other civilizations recognize spirals as a beacon? Plait says the phenomenon is a third of a light-year across, that’s a good size object and it’s noticeable. Is it a beacon, or a natural object? Food for thought.

Awesome Death Spiral of a Bizarre Star

Messaging With Cost-Optimized Interstellar Beacons

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