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SEGUE Glossary

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For general SDSS terminology please see the SDSS glossary. Terms specific to SEGUE can be found below.

Bright plate One of two plates centered on a particular direction in the sky. Each bright plate is exposed for about 45 minutes (similar to SDSS survey).
Bulge Galactic component of high stellar density at the center of the Milky Way.
CAS Catalog Archive Server -- provides access to the object catalogs and related data from the Sloan Digital Sky Survey.
CWD Cool white dwarf -- white dwarf star with temperature below about 5000K. CWDs exhibit a unique spectral signature due to collisionally-induced absorption.
dwarf galaxy Small, low luminosity galaxy. SEGUE/SDSS has discovered several new dwarf galaxies in orbit about the Milky Way (see SEGUE publications).
Faint Plate One of two plates centered on a particular direction in the sky. Each faint plate is exposed for about 1.5 hours.
globular cluster Gravitationally bound clusters of hundreds of thousands of stars, found in orbit about the Milky Way. SEGUE is targeting several globular clusters as part of its special plate category.
Halo The Milky Way is embedded in an enormous halo of dark matter, which extends far beyond the visible portions of the Galaxy.
l-color Principle component color: l = -0.436u + 1.129g - 0.119r - 0.574i + 0.1984
line index Line index, or equivalent width, is a measure of the intensity of an absorption line in the spectrum. The SSPP measures 77 atomic and molecular lines, which are used in estimating stellar parameters. Line indices are calculated from SEGUE spectral data by integrating a continuum normalized flux over the specified wavelength region of each line band. A list of indices for DR7 SEGUE SSPP can be found on
metallicity Metallicity is a measurement of the heavy element (anything heavier than Helium) abundance in a star. It is often expressed in the relative abundance of iron to hydrogen, normalized to the solar metallicity on a logarithmic scale: [Fe/H]. Stars with solar metallicity have [Fe/H] = 0; a star with [Fe/H] = -1 has one-tenth solar metallicity. Details of SEGUE methods to determine metallicities can be found in Lee et al. 2007a.
open cluster A cluster of stars which is assumed to have formed from the same giant molecular cloud. Open clusters are loosely bound gravitationally, and are at approximately the same distance from us, with similar chemical compositions. SEGUE has observed the open cluster NGC 2420.
P1(s) color Principle component color: Color perpendicular to s-color (and thus perpendicular to the stellar locus).
P1(s) = P1 = 0.91(u-g) + 0.415(g-r) -1.280
photometric metallicity Estimation of metallicity from photometric data.
photometric parallax Method for estimating the distance to stars based on their colors and apparent luminosity.
pipeline see SSPP
Principle Component Color Analysis Parametrization of stellar locus in color space, which can be used to separate stellar populations based on properties such as metallicity, surface gravity or temperature. See Newberg & Yanny 1997 ApJS 113, 89; Lenz, D., Newberg, H., Rosner, R., Richards, G. & Stoughton, C. 1998 ApJS 119, 121; and Helmi, Ivezic et al 2003 ApJ 586, 195.
Principle Component classification of galaxies Automated method for objective classification of large numbers of galaxies using spectral data. See for example Yip et al 2004 and referenences therein.
proper motion Observed motion of an object on the sky; often given in arcsec/year for stars. SEGUE compares the positions of objects with their position in previous surveys such as USNO-B to derive proper motions. See Munn et al. 2004
Pop I Traditional classification of stars: Pop I stars are relatively young and metal-rich. The Sun is a member of this category.
Pop II Pop II stars are older, more metal-poor than Pop I. Pop II include the oldest observed stars.
Pop III Pop III are metal-free or extremely low metallicity stars proposed to represent the first stars in the Universe.
radial velocity The observed motion of an object along the line-of-sight. Radial velocities are inferred from a shift in the spectral features. See SEGUE radial velocity determination or Yee et al 2007a for details.
resolution A measure of how well details can be distinguished. SEGUE/SDSS spectral resolution, λ/ δλ ≈ 1800.
SSPP SEGUE Stellar Parameter Pipeline. SSPP uses a combination of techniques to extract reliable estimates of stellar parameters from the SEGUE spectroscopic data.
s-color Principle component color: s-color is a linear combination of (u-g), (g-r) and (r-i) that runs parallel to the stellar locus.
s = -0.249u + 0.794g - 0.555r + 0.234
S/N Signal to noise ratio.
Spheroid Sometimes referred to as the stellar halo, the spheroid is composed mainly of older stars and is one of the main components of the Galaxy. The exact shape and density distribution of the spheroid is still not known (see for example Newberg & Yanny 2005) although in general the density falls off more steeply than the dark matter halo.
Stellar stream Several streams of stars have been identified within the halo of the Galaxy. These streams correspond to disrupted satellites of the Milky Way. SEGUE has already been instrumental in mapping the substructure within the Galactic halo; finding new streams and identifying their progenitors. A better understanding of stellar streams and tidal debris within the halo will lead to a more complete picture of the shape of the halo and the merger history of the Galaxy. See the SEGUE publications page for several recent references.
surface gravity Surface gravity = log g.The surface gravity of a star is related to its absolute luminosity class. Stars with lower surface gravity are, in general, larger (giant stars), whereas stars with higher surface gravity are in general, small, more compact (dwarf) stars. Estimates of log g are calculated from stellar spectra via the SSPP.
Target Selection Objects in the SEGUE photometric database are selected for spectral observation based on a specific target selection algorithm.
Target type Segue targets objects for spectral observation using the following abbreviations:
AGB Asymtotic Giant Branch candidate star
BHB Blue Horizontal Branch candidate star
CVR Cool White Dwarf candidate star
FTO F turnoff candidate star
GAL SDSS main survey or LRG GALAXY candidate
GD G Dwarf candidate star
HOT HOT standard star (main SDSS Survey, g-r < 0)
KD K Dwarf candidate star
KG K Giant candidate star
LOW Low metallicity candidate star
MD M Dwarf candidate star
PHO Photometric Standard star (usually brighter F dwarf)
QA Quality Assurance target (in SEGUE, duplicates another fiber)
QSO SDSS main survey QSO candidate
RED Reddening Standard star (usually fainter F dwarf)
ROS may be F/G dwarf or MS/WD or low lat target
SER Serendip. Manual (1. globular/open cluster 2. High Prop motion)
STA Main SDSS survey STAR (not GAL or QSO or Standard)
SKY Sky fiber, should have no object flux
WD White Dwarf candidate
Teff Effective temperature: Teff is estimated from the spectral data, and also from g - r colors (see Yee et al 2007a.)
Thick Disk One of the major stellar components of the Galaxy, the thick disk (with a scale height on the order of 1 kpc) extends beyond the thin disk but lies well within the dark matter halo. The thick disk contains roughly 10% of the mass of the thin disk, and its stars are in general older and more metal-poor. SEGUE will greatly extend our understanding of the shape, kinematics, composition and origin of the thick disk. SEGUE will also target the thick disk/halo boundary and substructure.
Thin Disk The thin disk (scale height ~ 0.3 kpc) contains the vast majority of the stars in the Milky Way. SEGUE's low latitude observations will help to disentangle the stars of the thin and thick disk. SEGUE will also study the warp and flare of the outer portion of the Galactic disk.
vGSR Velocity (in km/s) corrected to the Galactic Standard of Rest, assuming a solar velocity of
16.6 km/s toward α(J2000.0) = 17h49m58.7, δ(J2000.0) = +28o07'04" and a motion of the local standard of rest of 220 km/s toward α(J2000.0) = 21h12m01.1, δ(J2000.0) = +48o19'47" .
velocity dispersion Veocity dispersion σ is a measure of the spread in velocities of individual objects within a larger structure. σ is often used to refer to the velocity dispersion of an integrated galaxy spectrum, which probes the gravitational potential of the galaxy. SEGUE will also study the velocity dispersions of Galactic components and substructures.
w-color Principle component color:
w = -0.227g+0.792r-0.567i+0.040
wavelength coverage SEGUE and SDSS spectroscopy covers the wavelength range from 3,800 Å - 9,200 Å.


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