Hipparcos: High Proper Motion Stars

The proper motion of a star is its apparent angular movement per year on the celestial sphere. It is a combination of its actual motion through space and its motion relative to the solar system. Most stars are so distant that the proper motion is almost negligible.

The full space motion of a star is the combination of this proper motion (which is a quantity measured by the Hipparcos satellite) with the "radial velocity" of the star, along the line of sight. The radial velocity of stars or galaxies are usually measured from the Doppler shifts in their spectra: this quantity was not measured by Hipparcos.

Barnard's star (HIP 87937) has the greatest proper motion; the new Hipparcos measurements give:

• proper motion in right ascension = -797.84 mas/yr
• proper motion in declination = 10326.93 mas/yr
• giving a total proper motion =10357.704 mas/yr

Check the Hipparcos Catalogue for more information on the notation. The units of the proper motion for all stars in the Hipparcos and Tycho Catalogues is mas/yr (or milliarcseconds per year). The same number gives the angular distance that the star would move, in seconds of arc, in one thousand years.

The following figure (also available as a pdf file) is taken from the introduction to The Millennium Star Atlas, and was produced by Dennis di Cicco for Sky Publishing Corporation. Note that proper motion of the star stands out clearly as a movement of the star against the background stars with time. Dennis di Cicco's observations were so accurate that the effect of the parallax (the distance) of the stars is also evident - it is seen as the "wavy" motion of the star (the individual observations are shown as black circles with the relevant observation date) about its linear motion (shown as the straight line dissecting the figure); this wavy motion has a period of one year, corresponding to the Earth's orbital motion around the Sun.

Other high proper motion stars measured by the Hipparcos satellite are listed, and the motions of many of these stars could be determined by amateur observers with good equipment and sufficient patience!

A dramatic illustration of the effect of proper motion on the appearance of the night sky over time can be seen using this animation. (Note this spawns a second window which you may need to adjust in order to view both this window and the animation window.) Do note that this is an animation using the real Hipparcos satellite results, not a simulation using fabricated data!

To run the animation, enter the RA and Dec values of the region you would like to view and click on "View". A sky region 8 x 8 degrees is extracted. As for the sky plots presented elesewhere within these pages, click on an object to have the HIP number and magnitude returned (for fast moving objects this may be a challenge!), and click "get info" to have the catalogue details returned. Only Hipparcos Catalogue objects (and not Tycho Catalogue objects) are returned for this sequence.

You can select stars brighter than a magnitude V(lim) to plot. V(lim) is the V magnitude given in the Hipparcos and Tycho Catalogues (Field H5/T5).

When the stars have been drawn, click on animate to begin the animation. You can see the year corresponding to the new sky configuration at the right of the panel. Click "reset" if you want to restart the animation from the current time. You can also change the "step" interval in years (use the smallest step interval for the highest proper motion stars).

The "Tails" button allows you to monitor the motion of the stars by drawing the path of the star as it moves across the sky. For slowly moving stars you may have to wait some time before the star motions become apparent. For stars with high proper motion, the path will appear as a sequence of yellow dots as the path is broken up into discrete frames. This action can be toggled on and off. Some regions will appear better with the tails feature active or inactive.

Watch as high proper motion stars move across the field. A small region beyond the "active field" is used for the calculations, so you may also see some higher proper motion stars appearing in the field as you watch.

To view some particular interesting areas try the following suggestions:

The Hyades (at RA=66.5, dec=15.5): this is a cluster of stars at a distance of about 40 parsecs (about 150 light years). It is the nearest moderately rich star cluster, and at the limit of visibility of the naked eye. Try the animation first with the tails switched off, and see whether you can pick out the highest moving stars. Then switch the tails on, and let the animation run for a few thousand years. Already by the year 3000 AD you will see that half a dozen of the highest proper motion stars have moved significantly from their present positions. By the year 8000 AD or so, notice that the motions of most of the stars in this region are concentrated in the same direction. This is how the sky will really appear to our descendents in thousands of years from now! The brightest star, Aldebaran (HIP 21421) does not participate in this common motion - it is much closer to us than the rest of the Hyades members. By the year 20000 AD or so, you will start to see that the motions are showing "convergence" towards a particular point on the sky. This is a consequence of the geometry of the stars which move with a common motion through space. It is the basis of the methods used to estimate the distance of this important cluster from the ground before the Hipparcos parallaxes became available.

The Pleiades (at RA=56.7, dec=24.2): this is another important cluster, at a distance of about 110 parcsecs. As the animation evolves, the high proper motion stars in this region show up quickly. Let the animation run for a few thousand years, with the tails on, and see again that the central cluster objects move with a common proper motion on the sky.

The following objects are the ten highest proper motion stars contained in the Hipparcos Catalogue (the 61 Cygni binary is seen at this resolution only as a single object). You will see these objects move rapidly across the screen as a discrete sequence of yellow dots:

Look at different parts of the sky, and see if you can observe interesting motions or patterns.

Contact Us Last modified: Fri Sep 10 09:05:57 MET DST 1999