SEGUE: A Spectroscopic Survey of 240,000 stars with g=14-20

The SEGUE survey obtained 240,000 moderate resolution (R = 1800) spectra from 3900 - 9000 Angstroms of fainter Milky Way stars (14.0 < g < 20.3) of a wide variety of spectral types, both main sequence and evolved objects, with the goal of studying the kinematics and populations of our Galaxy and its halo. The spectra are clustered in 212 regions spaced over three-quarters of the sky. Radial velocity accuracies for stars are 4 km/s at g < 18, degrading to 15 km/s at g = 20. For stars with S/N > 10 per resolution element, stellar atmospheric parameters are estimated, including metallicity, surface gravity, and effective temperature. SEGUE obtained 3500 square degrees of additional ugriz imaging (primarily at low Galactic latitudes) providing precise multi-color photometry (g,r,i = 2%), (u,z = 3%) and astrometry (0.1 arcsec) for spectroscopic target selection. The stellar spectra, imaging data, and derived parameter catalogs for this survey are publicly available as part of SDSS Data Release 7 (DR7).

💡 Research Summary

The SEGUE (Sloan Extension for Galactic Understanding and Exploration) survey represents a major expansion of the Sloan Digital Sky Survey (SDSS) aimed at probing the kinematic and chemical structure of the Milky Way. Over the course of the program, moderate‑resolution (R≈1800) spectra were obtained for 240 000 stars spanning the apparent magnitude range 14.0 < g < 20.3. The wavelength coverage of 3900–9000 Å captures the key atomic and molecular features needed to derive radial velocities, metallicities, surface gravities, and effective temperatures for a broad variety of stellar types, from hot main‑sequence dwarfs to cool giants, blue horizontal‑branch stars, and white dwarfs.

The survey geometry consists of 212 distinct fields distributed over roughly three‑quarters of the sky, providing a spatially sparse but statistically powerful sampling of the Galactic disk, thick disk, halo, and sub‑structures such as streams and globular clusters. In addition to the spectroscopic component, SEGUE acquired an extra 3 500 deg² of ugriz imaging, primarily at low Galactic latitudes where the original SDSS footprint was sparse. The imaging delivers photometric precision of 2 % in g, r, i and 3 % in u, z, together with astrometric accuracy of 0.1″, enabling reliable target selection based on color–magnitude criteria.

Radial velocities are measured with an accuracy of ~4 km s⁻¹ for bright stars (g < 18) and degrade to ~15 km s⁻¹ at the faint limit (g ≈ 20). For spectra with signal‑to‑noise per resolution element greater than ten, the SEGUE Stellar Parameter Pipeline (SSPP) automatically derives atmospheric parameters. The pipeline combines multiple techniques—neural‑network fitting, χ² minimization against synthetic libraries, and line‑index calibrations—to achieve typical uncertainties of Δ