Friday, February 20, 2015

Fly by nights

Mamajek et al say a small binary passed close to the Sun about 70,000 years ago (85,000 to 60,000 years). Stars are relatively small compared to the rest of space (you can see through the galaxy between its stars--where there isn't obscuring dust, that is), but from time to time they come "close" to each other. If their extrapolations are correct, this pair got to something like 52,000 AU (75,000AU to 33,000AU) away, which would put it in the Sun's outer Oort cloud. 1 AU is the distance from the Earth to the Sun.

Even if the binary were as luminous as the Sun, it would have been 6000 times dimmer than the full Moon, so our nights would have looked about the same.

They say the intruders were fast and lightweight, and so wouldn't be expected to perturb the cloud much--so we shouldn't expect a rain of comets in the next few centuries from the encounter. And that near-misses at this range are expected to be every 10,000,000 years or so. Which means either this is a rare sighting, or else the estimate is off.

By symmetry, I guess the Sun went through their "Oort" cloud--and so did we. Not very dense, was it? Saturn's rings aren't anything like The Empire Strikes Back and the outer clouds are even sparser.

UPDATE: Per Texan99's comments, I figure the thing would have swept across 90 degrees of the sky in about 6 years. 83km/sec is fast up close, but at a distance it is less dramatic.


Texan99 said...

How fast do they think something like this happens? Would something as bright as the Moon have appeared and stayed in Earth's sky for months, years, decades? I suppose this would have been before people were watching and remembering the sky.

I don't know if you've ever read "The Mote in God's Eye," but there's an astronomical event in it that messes with a lot of people's heads.

james said...

Yes, I've read that one (and the sequel). I should have checked the relative speed. I'll see what I can learn.

If people were watching they'd have seen a "wanderer" appear, I suppose.