Glossary

Astronomical Terms

Right Ascension (RA) : Celestial equivalent of longitude. Measured in hours (0-24) or degrees (0-360).

Declination (Dec) : Celestial equivalent of latitude. Measured in degrees (-90 to +90).

Galactic Coordinates (l, b) : Coordinate system centered on the Milky Way. l = longitude, b = latitude.

Parallax : Apparent shift in star position as Earth orbits the Sun. Measured in milliarcseconds. Larger parallax = closer star.

Parsec (pc) : Distance unit. 1 parsec = distance at which 1 AU subtends 1 arcsecond ≈ 3.26 light-years.

Apparent Magnitude : Brightness as seen from Earth. Lower = brighter. Sun = -26.7, faintest naked-eye stars ≈ +6.

Absolute Magnitude : Brightness if object were at 10 parsecs. Allows comparison of intrinsic luminosity.

Photometric Band : Filter used for measurement. Common: U (ultraviolet), B (blue), V (visual), R (red), I (infrared), G (Gaia).

Spectral Type : Classification based on temperature. O B A F G K M (hot to cool). Sun = G2V.

Luminosity Class : Size category. I = supergiant, III = giant, V = main sequence (dwarf).

Proper Motion : Star’s apparent motion across the sky, measured in milliarcseconds per year.

Redshift (z) : Stretching of light wavelength due to expansion or motion. z = (λ_observed - λ_emitted) / λ_emitted.

Hubble’s Law : v = H₀ × d. Recession velocity proportional to distance.

Hubble Constant (H₀) : Rate of cosmic expansion. ~70 km/s/Mpc.

Transit Method : Detecting planets by dimming as they pass in front of their star.

Radial Velocity : Detecting planets by stellar wobble toward/away from us.

Habitable Zone : Orbital region where liquid water could exist on a planet’s surface.

Earth Mass / Jupiter Mass : Mass units. 1 Jupiter mass ≈ 318 Earth masses.

TAP (Table Access Protocol) : Standard protocol for querying astronomical databases.

ADQL : Astronomical Data Query Language. SQL-like syntax for TAP services.

SDSS (Sloan Digital Sky Survey) : Major optical survey of the northern sky.

Gaia : ESA mission measuring positions and motions of ~2 billion stars.

VizieR : CDS service hosting thousands of published astronomical catalogs.

ADR (Architecture Decision Record) : Document recording a significant technical decision, its context, and consequences.

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