Nearby Stars
Nearby Stars
| Group Name | nearstars |
| Reference | Lépine Shara Proper Motion Catalog Sébastien Lépine & Michael Shara (AMNH) |
| Prepared by | Sébastien Lépine (AMNH) |
| Labels | No |
| Files | LSPMstars_100ly.speck |
| Dependencies | halo.sgi, colorbv.cmap |
| Census | 7,031 stars |
Critical toward our understanding of the Galaxy and the Universe are the stars that surround the Sun. If our knowledge of the local stellar population is inaccurate, then astronomer's theories on stellar evolution are unsound. Furthermore, the locations of all stars, even the size and scale of objects in the Universe, rests upon the accuracy of the local stellar neighborhood.
The Lépine Shara Proper Motion (LSPM) Catalog contains the most complete sample of stars within 100 light-years, with over 3,000 newly discovered stars. These stars have high proper motions, that is, they move across the night sky, as seen from Earth, relatively quickly. Of course, we do not see stars streaming across the sky: the fastest star in the sky, Barnard's Star, moves only one-quarter of one degree (about half the width of the full Moon) per century. However, if we observe the same spot over a span of decades, we will see the stars move in the sky.
All stars are in motion within the Galaxy; the Sun orbits the center of the Milky Way galaxy at approximately 500,000 miles per hour. Some stars move alongside the Sun and appear to have little motion in the sky; other stars are moving at angles perpendicular the Sun, giving them higher proper motions. Barnard's Star will be the closest star to the Sun around the year 11,700 AD, when it will be only 3.8 light-years distant.
The new stars in this catalog were discovered by comparing images of the sky from two epochs. When astronomers analyze two images from the same patch of sky taken 50 years apart, the light from each pixel should match between the two images. If there is a bright spot in one image that is not in the other, then it is scrutinized to see if it's a fast-moving star.
Revealing the Unseen
Most of the stars in this catalog are below the brightness limit of our eye, so how do we see them in the Digital Universe? We must represent them conceptually rather than photo-realistically. To see the dimmer stars, which represent the majority, we must increase their luminosity scale in the Digital Universe. By default, the stars in this data set are 200,000 times their normal brightness and the halo on each star has an upper size limit of 20 pixels, so that the few bright stars in the 100-light-year-radius volume, like Sirius, do not overpower the rest of the stars with their brightness. To return these stars to their true brightness, use the commands slum /200000 and polymin 1 1e8 (in this order), then they will be on equal footing with the normal stars.
Local Stellar Density
The LSPM catalog contains over 7,000 stars. Viewing these stars 200,000 times brighter than their true luminosity reveals the sheer density of the local stellar neighborhood. For example, in the same 100-light-year-radius sphere, Hipparcos only observed 2,414 stars (Hipparcos was a targeted survey, so no new stars were expected to be discovered).
Another, more informative, comparison is illustrated using the preset selection expressions.
| Selection Expressions for the Nearby Stars | ||
| Alias | Partiview Command | Description |
| eye | thresh appmag -2 6.5 | Select those stars visible to the unaided eye in the night sky |
| lowlum | thresh absmag 10 25 | Select the intrinsically faint stars |
| hip | thresh hipid 0 130000 | Select those stars with a Hipparcos ID |
| nonhip | thresh hipid < 0 | Select all stars without a Hipparcos ID |
Let's view the number of stars that are bright enough to see with our unaided eye using the eye selection expression. Type see eye and 577 of the 7,031 stars will remain. Perform the same exercise on the stars data group (activate the stars group and type see eye) and over 8,600 stars will remain. This tells us that over the course of one year there are about 8,600 “naked eye” stars in the night sky of both hemispheres. Only 577 of them are within 100 light-years. Therefore, about 6,500 nearby stars, right in our own astronomical backyard, remain invisible without the use of a telescope.
To see the intrinsically dim stars, we can use the lowlum selection expression. Now we have over 4,600 dim stars in view. These are primarily cool, red stars that are ubiquitous throughout the Galaxy.
While this catalog presents a more accurate representation of the local stellar density, fainter objects remain and astronomers are beginning to catalog them.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott