Archive for February 15th, 2017


From km/s to parsecs per million years

February 15, 2017

I’ve been following a fascinating thread on Cloudy Nights called “Classic Rich Field“, in which Danish observer Allan Dystrup is posting his observations and sketches of stellar associations, especially OB associations of big, hot, young stars. It’s a fascinating observing program, not least because he’s doing it all with a 55mm telescope. I’ll no doubt be talking more about OB associations in the future, as my interest in this area – both intellectual and aesthetic – has been steadily growing over the last few years.

Canadian astronomer Glenn LeDrew has made some very substantial contributions to the thread as well. His two posts on runaway stars alone are worth copying, pasting, and saving (1, 2). Especially for this arresting fact that he related:

1 km/s is almost exactly 1 parsec/million years [1.023 pc/myr, in fact, according to this page – ed.]

What a stunning way to put the scale of a parsec into common terms – there are about as many kilometers in a parsec as there are seconds in a million years. Or, if you prefer the more familiar light years, about as many kilometers in a light year as there are seconds in 313,000 years. Suddenly the universe feels ungraspably, inhumanly big. Which of course it always has been – it’s just easy to forget that.

Who thinks on these scales, besides astronomers? Paleontologists. And the question that popped into my head immediately upon reading that near-equivalence was, “If an object had been traveling at 1 km/s since the Late Jurassic, how far would it have traveled?” By Late Jurassic I was thinking about 145 million years ago, when Apatosaurus, Stegosaurus, and Allosaurus roamed the American West. When I go dig each summer, it’s in sediments laid down during that time, preserving the bones of those animals. And if you travel at 1.023 pc/myr, after 145 million years you will have traveled 1.023 x 145 = 148.3 parsecs, or 485 light years.


A diagram I made for my RTMC talk last year, showing the size of the galaxy and our place in it.

That’s nothing. That’s barely farther than the distance to the Pleiades, one of the closest open star clusters to earth. It’s about a third of the way to the Orion Nebula. (I have these distances loaded in RAM for a reason.) It’s a little less than half of the 1,100-light-year thickness of the ‘thin disk‘ of the Milky Way, which holds about 85% of the stars in the disk of the Milky Way, including our sun.

That’s amazing. If you started out right on the centerline of the plane of the galaxy, halfway between the bottom and the top of the galactic disk, and you started flying directly up or down (galactic north or south) at 1 km/s (fast as a speeding bullet), you’d have to fly for more than 170 million years to reach the edge of thin disk. To get to the edge of the galaxy’s halo at 30,000 parsecs would take 29.3 billion years, or just over twice the age of the universe (and 3.5 times the age of the Milky Way itself).

Space is big, y’all.