Wednesday, February 27, 2008

Looking for proper motions

A star that moves visibly in just a few years has a high proper motion

Image Credit: Sebastien Lepine/SUPERBLINK

Yesterday I mentioned that I am here at Kitt Peak outside of Tucson, Arizona, working on a project to image a large fraction of the sky.  So, why are my collaborators and I trying to image 1/4 of the entire sky visible from Kitt Peak?

The part of the sky we were looking at was imaged between 1999 and 2005 as part of the Sloan Digital Sky Survey, which mapped a large fraction of the Northern Hemisphere sky in five colors.  So, at first glance it may sound silly that we are getting new images of part of the sky that was just mapped within the last decade.

While the stars in the sky appear to be fixed, with constellations staying the same from year to year, generation to generation, and even millennium to millennium, all stars are slowly moving around the Milky Way galaxy.  It takes stars like the sun 250 million years to orbit the Milky Way, so in a human lifetime, we don't cover much of that orbit!  

But, with precise measurements, it is possible to see that the stars are slowly moving in the sky.  If you brought an ancient Greek to modern times, he or she would notice that a few bright stars have moved noticeably in 2300 years.  But with more accurate measurements, we can use ground-based telescopes to measure changes in position as small as about one hundred thousandth of a degree, or about the size of a bacteria seen from 30 feet away.  These star motions are called proper motions.  

The closer a star is to us, the larger its proper motion, on average.  Some nearby stars can move quite a bit, as in the picture above, which shows one red dwarf star moving a large distance (still only the tiniest fraction of a degree!) 45 years, while the other stars appear to stay still.  This particular star is only about 35 light-years away from the sun, while the other stars in the image are probably hundreds of light-years away.

We are looking for the faintest white dwarfs, the remains of dead stars that are slowly cooling off and fading away.  The faintest white dwarfs we can find are therefore some of the oldest stars in our Galaxy, and we hope to use these white dwarfs to tell us how old various parts of the galaxy are. (If we know how faint they are and how fast these stars fade, we can estimate how old they are).  Like the red dwarf in the picture above, faint white dwarfs tend to have relatively large proper motions.  So, although only five years have passed since the first images were taken, the stars have moved enough that we can measure the movement. 

If we showed you most of the stars that have measured motions, you probably wouldn't believe it.  The motions are very tiny, smaller than the spot size of the star.  But my collaborators can still measure a slight shift in the center of that spot!

So, that's why we're here -- searching 1/4 of the visible sky for maybe a fifty or a hundred of the oldest stars in our galaxy, out of millions of stars we'll be imaging.  Hard? Yes!  But the science result will be worth the work.


  1. Hey Professor-- does your survey cover Capricornus? Years ago, a project I was on found a red dwarf there that looked to be one of the 250 closest stars in the sky (judging from the spectrum) but ground-based colors were confusing. Proper motion would clinch it. I'll be happy to give you coords; I'd need to dig them up.

    The Bad Astronomer

  2. Alas, we don't go that far south. You may want to check out Sebastien Lepine's SUPERBLINK catalog; if the star is really high proper motion, he should detect it. The USNO-B catalog (if it goes faint enough) may also have the proper motion. I'd offer to grab you an image, but Capricorn isn't high enough at the end of the night for reliable astrometry.