Sloan Stars
Sloan Stars
| Group Name | sloanstars |
| Reference | Sloan Digital Sky Survey
John Bochanski & Suzanne Hawley (U Washington), Kevin Covey (Harvard CfA), Neill Reid (Space Telescope Science Institute), Andrew West (UC Berkeley), Sloan Digital Sky Survey Collaboration |
| Prepared by | Brian Abbott (AMNH/Hayden) |
| Labels | No |
| Files | sdssMstars.speck |
| Dependencies | halo.sgi |
| Census | 1,000,000 M stars |
In the Digital Universe, the stars data set contains stars for which relatively accurate distances exist. It is not an accident that these are located close to Earth. However, these stars represent a mere drop in the bucket when we consider the few hundred billion stars in the Milky Way.
The Sloan stars are a sampling of one million stars from the halo of our Galaxy. Because these stars are so far away, astronomers cannot use trigonometric parallax to determine their distance. Rather, they must use the starlight itself, its photometric properties, to obtain a distance to the star (see “Parallax and Distance” for a detailed discussion on these techniques). While this distance is not as accurate as a geometrically obtained trigonometric parallax distance, it is informative and, frankly, the best technique astronomers have for normal stars at this distance.
The Sloan Digital Sky Survey's primary goal is to map galaxies and quasars that lie beyond the Milky Way (see the Sloan galaxies and the Sloan quasars in the Extragalactic chapter). They intend to accomplish this over one-quarter of the sky using a dedicated, 2.5-meter telescope located in Arizona. A byproduct of this project is the detection of millions of stars, 28 million of which are cool, M dwarf stars. This data group contains 1 million randomly sampled stars from this larger, 28-million-star data set.
M dwarfs, also known as red dwarfs, are relatively small, cool stars. They have less than half the Sun's mass and emit less than 10% of the Sun's luminosity. Their surface temperature is about 3,500 Kelvin (3,200o C or 5,800o F), compared to the Sun's 5,800 Kelvin. M stars are so dim that even those close to the Sun remain invisible to the unaided eye.
Taking a census by number, M dwarfs comprise about 75% of all stars in the Galaxy. One reason for this is that they have incredibly long lifetimes. The Sun, a G dwarf, has a lifetime of around 10 billion years. After this time, its primary source of fuel, hydrogen, is exhausted and the star enters into the final stages of its existence. M dwarfs live for tens of billions and perhaps even trillions of years, meaning most M stars that were ever created in the Galaxy are still around. While the hotter, brighter, more massive stars come and go, the cooler, red dwarfs remain.
The Bow Tie Effect
By now, you may have noticed that these stars, as seen from outside the Galaxy, appear to form the shape of a bow tie whose center is at the Sun. These wedges describe the patches of sky observed by the telescope. If you return home to the Sun, you will see where these stars were observed in the night sky. The stars are not everywhere, but in select patches in the sky. As you move away from the Sun, you can see these patches transform from a 2-D view to a 3-D distribution. If the telescope were to observe the entire sky, we would see a spherical distribution of stars around the Sun, rather than the triangular shapes presently seen.
Galactic Structure
Astronomers study these stars to understand the structure of our Galaxy. Because we are in the midst of the Milky Way's disk, it is difficult to grasp the true shape and configuration of our own Galaxy.
From a point where you see all these stars (you may have to brighten them), notice the contours of this data set. Stars at the “edge” of the data set, i.e., those stars farthest from the Sun, form a circular arc. Conversely, the higher density region closer to the Sun has an asymmetric shape. If you do not see this shape, jump directly to this point:
jump -9000 12000 1500 -50 -80 -100From this vantage point, you will see a distinct increase in the stellar density parallel to the Galactic disk and in the direction of the Galactic core. This rising profile reflects the vertical structure of the Milky Way. You may also notice a small increase in density in the lower half of the data, less pronounced because there is less data in this part of the survey.
Astronomers use 3-D star counts to realize the structure of the Galaxy. While we call the Milky Way our home, we still know remarkably little about the overall structure of the galaxy. Knowledge of these star counts in various locations allow us to map the disk, bulge, and halo, and better understand the Galaxy we live in.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott