Mapping the Milky Way & Environs

Previous astronomical surveys such as the Sloan Digital Sky Survey have enabled us to draw an ever-finer picture of the Milky Way and its neighborhood, called the Local Group. For example, survey data have revealed numerous stellar streams in our galaxy’s halo, as well as dozens of small dwarf galaxies.

Data from the LSST will further refine the image of our home galaxy, in the process redefining it as a laboratory where studies of galaxy growth and evolution can take place on a grander scale. 

All told, the LSST will catalog more than 10 billion stars. With the telescope’s ability to observe in the near infrared, it can observe stars in the interstellar dust layer, providing information on the structure of star-forming regions within our Galaxy’s disk. In standard surveying mode the LSST will be able to detect RR Lyrae variables and classical novae almost to the distance of the Andromeda galaxy. RR Lyrae and some stellar novae are "standard candles," whose absolute magnitude, or intrinsic brightness, can be determined. Comparing how bright they are to how bright they appear gives an accurate estimate of their distances. With that knowledge the extent and structure of the galactic halo can be mapped. 

The LSST will inventory numerous stars beyond the presumed edge of the galaxy's halo, measure their distribution by chemical makeup throughout most of the halo, and determine their motions beyond the thick disk/halo boundary, enabling the most detailed studies ever of our home galaxy, the Milky Way. 

With LSST data we can: 

• Conduct the faintest ever search for galaxy satellites and intergalactic stars over much of the Local Group.
• Conduct high-resolution studies of the distribution of stars in the outer halo based on certain properties, such as chemical make-up and the star's proper motion, or direction of travel. For example, the chemistry and proper motion of a sample of about 200 million F/G main-sequence stars (our sun is a G star on the main sequence) will be detected out to a distance of 100 kpc.
• Create deep and highly accurate color-magnitude diagrams for over half of the known globular clusters, including tangential velocities from proper motion measurements.
• Map the chemical composition, motions and spatial profile of the Sagittarius Dwarf tidal stream, a stream of stars the Milky Way is pulling off the neighboring Sagittarius Dwarf Galaxy.
• Map the Milky Way's halo and the Local Group out to 400000 parsec, using RR Lyrae stars as cosmic yardsticks.
• Study areas of greater and lesser gravitational pull in the Milky Way that reveal how matter is distributed.
• Conduct detailed studies of variable star populations using highly accurate multicolor light curves for a sample of at least 50 million variable stars. 
• Study rare, faint, fast-moving objects that could reveal the lower limits of mass for star formation, or even be candidates for free-floating planets.
• Determine accurate distances to extremely faint objects to learn just how faint a star can be and still be a star. 
• Separate halo M sub-dwarfs from disk M dwarfs, two classes of faint star that resemble each other, by subtle differences in color.
• Create a complete census of the solar neighborhood to a distance of 100 parsecs.
• Search for exoplanets: the data set may include a mini survey of 600 deg² of sky which will collect about 40 hour-long sequences of 200 observations each over a four-month period. The main survey would include an additional 800 observations over 10 years of these same fields. With about 10 million or more stars in the sample (depending on where the fields are placed), this would provide an excellent material to study the planet frequency as a function of stellar type and distance from the galactic plane.
• Create a complete census of Asymptotic Giant Branch stars, which are stars like our sun that have entered their red giant phase.
• Create a complete census of faint populations in nearby star forming regions using color and variability selection.
• Conduct high-resolution three-dimensional studies of interstellar dust.