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Sunday, July 7, 2013

Barnard's Star

I've been reading up on parsecs and I've gotten a little side-tracked. The ESA (European Space Agency) put up a satellite a few years ago to map all the stars in the sky, well, the brightest million or so anyway. Not all the stars are fixed to the firmament, some of them, like Barnard's star are wandering around loose. You would have be pretty persnickety to tell, but the robot satellite Hipparcos and Dennis di Cicco, astronomer extraordinaire, managed to do so. The jaggy line running up the center of the diagram is the star's path according to Hipparcos.  The round black spots are M. di Cicco's observations. Notice that the period of time covered is about two years while the change in position is about 18 arc-seconds. At an arm's length from your eye that is about 3/1000th's of an inch, or about the thickness of a sheet of 20 pound paper. Not very freaking much. It took two years to travel that far and it's traveling somewhere around 100 miles per second. Compare that to the 5 miles per second you need to reach low Earth orbit.
    Barnard's Star is about 6 light years from Earth, which makes it one of the closest stars and also makes it's motion detectable.


Ole Phat Stu said...

So there is a companion body (planet? dark star? black hole?) and they orbit a common centre of gravity?

Charles Pergiel said...

I presume you are referring to the apparent sine-wave-like path of this star. A good question. I think it might be due to parallax. Note that while the vertical scale is marked at intervals of 2 arc-seconds, the intervals on the horizontal scale are marked in ... I was going to say tenths of an arc-second, but looking closer I am not sure what that superscript character is.

Ole Phat Stu said...

The period is consistent with parallax; I didn't think (to check that first before shooting my mouth off :-( )

Pity he didn't add error bars to his observations.

tomwof said...

I wish I'd seen this last year when it was posted. I can use this in the astronomy lab I teach.

The horizontal oscillating motion is indeed due to parallax. Parallax also has a vertical effect; the vertical motion is a combination of proper motion and parallax. This is why the apparent vertical distance traveled in the three months from April to July is longer than that in the three months from July to October. The horizontal units are seconds of right ascension (RA), which is measured in units of time. But he has scaled his plot correctly (in "square degrees") so that his scale (which appears to be 1 cm = 1 arcsecond) applies in any direction, so you don't have to convert the horizontal units from time to angle.

If you want to do so (this is to get the angular distance between horizontal tick marks), you multiply the difference in RA in seconds by 15 and also by the cosine of the declination (which in this case is almost 1 [cosine of 4.67 deg]). That gives 1.5 arcseconds between horizontal tick marks, while the vertical ticks are 2 arcseconds apart. This agrees with the fact that the linear distance in cm between horizontal ticks is 3/4 of the linear distance between vertical ticks.

-- Tom Wofford, physics & astronomy instructor at Central New Mexico Community College (CNM)

tomwof said...

What is the publication source of this plot, anyway? Did this come from Sky and Telescope magazine? (di Cicco is on the editorial staff.)

Charles Pergiel said...

I got the diagram from the ESA (first embedded link in the text). I don't know where they got it.
As for not seeing it last year, well, we can't be everywhere at once, can we?