Glossary

Definitions of astronomical terms used throughout ab astris. Written for students beginning their astronomy journey.

Core Concepts

Variable Star— A star whose brightness changes over time. This variation can be caused by pulsation (the star expanding and contracting), rotation (starspots moving in and out of view), or eclipses (another object passing in front).
  • Brightness changes are measured in magnitudes
  • Periods range from minutes to years
  • Over 2 million known variable stars cataloged in VSX
Lightcurve— A graph showing how a star's brightness changes over time. The x-axis is time (often in days), and the y-axis is brightness (in flux or magnitudes).
  • Raw lightcurves show all data points from observations
  • Phase-folded lightcurves stack data at the same point in each cycle
  • The shape of a lightcurve reveals the type of variability
Period— The time it takes for a variable star to complete one full cycle of its brightness variation. For eclipsing binaries, this is the orbital period. For pulsating stars, it's the pulsation cycle.
  • Measured in days (or hours for short-period variables)
  • Ultra-short period: less than 0.1 days (2.4 hours)
  • Periods help classify the type of variable star
Amplitude— The amount by which a star's brightness changes during its variation cycle. Larger amplitudes mean bigger brightness swings and are easier to detect.
  • Measured in magnitudes (mag)
  • Typical range: 0.01 to 2+ magnitudes
  • Lower amplitudes require higher-quality data to detect
Magnitude— A logarithmic measure of brightness. Lower numbers mean brighter objects (a magnitude 1 star is 100x brighter than magnitude 6). The scale is inverted from what you might expect.
  • Apparent magnitude: how bright something appears from Earth
  • The Sun is magnitude -27, full Moon is -13
  • TESS typically observes stars between magnitude 8 and 16
Flux— The amount of light received from a star, measured in physical units. Unlike magnitudes, higher flux means brighter. TESS measures flux in electrons per second.
  • Normalized flux of 1.0 means average brightness
  • A transit might show flux dropping to 0.99 (1% dip)
  • Flux is often converted to magnitudes for publication

Pulsating Variable Stars

Stars that physically expand and contract, causing brightness changes.

DSCT (Delta Scuti)— Short-period pulsating stars found on or near the main sequence. They pulsate in multiple modes simultaneously, creating complex lightcurves.
  • Periods: 0.02 to 0.3 days (30 minutes to 7 hours)
  • Amplitudes: typically 0.003 to 0.9 mag
  • Spectral types A-F (white to yellow-white stars)
  • Often show multiple periods superimposed
HADS (High-Amplitude Delta Scuti)— Delta Scuti stars with unusually large brightness variations. They typically pulsate in a single radial mode, making their lightcurves more regular.
  • Amplitudes: greater than 0.3 magnitudes
  • More regular lightcurves than normal DSCT
  • Rare compared to low-amplitude Delta Scuti
GDOR (Gamma Doradus)— Gravity-mode pulsators with longer periods than Delta Scuti stars. The pulsations are driven by convection in the outer layers.
  • Periods: 0.3 to 3 days
  • Amplitudes: typically less than 0.1 mag
  • Often found in the same stars as DSCT (hybrid pulsators)
  • Important for asteroseismology studies
RR Lyrae (RR, RRAB, RRC)— Old, metal-poor pulsating stars used as standard candles to measure cosmic distances. Found in globular clusters and the galactic halo.
  • Periods: 0.2 to 1 day
  • Amplitudes: 0.3 to 2 magnitudes
  • RRAB: fundamental mode (asymmetric lightcurve)
  • RRC: first overtone (more sinusoidal)
  • Absolute magnitude ~0.6, useful for distance measurement
CEP (Cepheid)— Luminous pulsating supergiants with a famous period-luminosity relationship. Classical Cepheids are key distance indicators for galaxies.
  • Periods: 1 to 100+ days
  • Amplitudes: 0.1 to 2 magnitudes
  • Brighter Cepheids have longer periods
  • Used to measure distances to nearby galaxies
SR (Semi-Regular)— Giant and supergiant stars that pulsate with periods that can vary or be poorly defined. Their lightcurves show less regular patterns.
  • Periods: 20 to 2000+ days
  • Often cool red giants (M-type)
  • May have multiple periods or cycle-to-cycle variations

Eclipsing Binary Stars

Two stars orbiting each other, causing brightness dips when one passes in front of the other.

EA (Algol-type / Detached)— Eclipsing binaries where both stars are well-separated and spherical. The lightcurve is flat between eclipses with sharp, distinct dips.
  • Stars do not touch or distort each other
  • Primary eclipse: dimmer star passes in front
  • Secondary eclipse: brighter star passes in front
  • Eclipse depths can differ significantly
EB (Beta Lyrae / Semi-Detached)— Eclipsing binaries where one star fills its Roche lobe and transfers mass to its companion. Continuous brightness variation between eclipses.
  • Stars are close enough to distort each other
  • Lightcurve never completely flat
  • Mass transfer often visible as gas streams
  • Eclipse depths are more similar than EA
EW (W Ursae Majoris / Contact)— Eclipsing binaries where both stars share a common envelope and are in contact. The lightcurve varies continuously with nearly equal eclipse depths.
  • Both stars touch and share outer layers
  • Short periods: typically 0.2 to 0.8 days
  • Primary and secondary eclipses nearly equal depth
  • Continuously varying brightness (no flat sections)

Rotating Variable Stars

Stars that vary because dark or bright spots rotate in and out of view.

ROT (Rotating Variable)— Stars showing brightness changes due to surface features (like starspots) rotating with the star. The period equals the rotation period.
  • Caused by starspots (like sunspots, but larger)
  • Periods match stellar rotation: hours to months
  • Lightcurve shape can change as spots evolve
  • Common in magnetically active stars
BY (BY Draconis)— Cool dwarf stars (K and M type) with chromospheric activity and starspots. Named after the prototype BY Draconis.
  • Late-type main sequence stars (orange/red dwarfs)
  • Amplitudes usually less than 0.5 mag
  • Often show flares in addition to rotation
  • Periods typically 1 to 20 days
RS (RS Canum Venaticorum)— Close binary systems with enhanced chromospheric activity. At least one component is a subgiant or giant with large starspots.
  • Stronger activity than single stars
  • Often show both eclipses and spot modulation
  • Can have dramatic flares
  • X-ray and radio emission common

Other Variable Types

CV (Cataclysmic Variable)— Binary systems containing a white dwarf accreting matter from a companion star. Known for dramatic outbursts and nova explosions.
  • White dwarf + red dwarf in close orbit
  • Dwarf novae: repeated outbursts of 2-5 magnitudes
  • Classical novae: thermonuclear explosions (8+ mag)
  • Often irregular or semi-periodic behavior
FLARE— Stars that show sudden, unpredictable brightenings due to magnetic reconnection events. Common in young or magnetically active stars.
  • Brightenings last seconds to hours
  • Can increase brightness by several magnitudes
  • Most common in M dwarfs (red dwarfs)
  • Related to stellar magnetic activity

Transit and Exoplanet Terms

Exoplanet— A planet orbiting a star other than our Sun. Over 5,000 confirmed exoplanets have been discovered, mostly by the transit method.
  • Hot Jupiters: gas giants orbiting very close to their stars
  • Super-Earths: rocky planets larger than Earth
  • Most exoplanets found by Kepler and TESS missions
Transit— The event when a planet passes in front of its host star as seen from Earth, causing a small, temporary dip in the star's brightness.
  • Typical depth: 0.01% to 1% (depending on planet size)
  • Duration: hours (depends on orbital period and geometry)
  • Requires orbital plane aligned with our line of sight
  • Repeats every orbital period
Transit Candidate— A potential exoplanet detection that shows transit-like signals but hasn't been confirmed. Many candidates turn out to be false positives.
  • Requires follow-up observations to confirm
  • False positives: eclipsing binaries, instrumental noise
  • TOI: TESS Object of Interest (official candidates)
BLS (Box Least Squares)— An algorithm designed to find periodic box-shaped dips in lightcurves, ideal for detecting planetary transits.
  • Searches for periodic, flat-bottomed dips
  • More sensitive than Lomb-Scargle for transits
  • Returns period, depth, and duration of best-fit transit
  • Can produce false positives from stellar variability
False Positive— A signal that looks like a planet transit but is actually caused by something else, such as an eclipsing binary or instrumental artifact.
  • Eclipsing binaries are the most common false positive
  • Background eclipsing binaries blended with target star
  • Odd/even eclipse differences indicate binary, not planet
  • ab astris uses 5-layer filtering to reduce false positives
SNR (Signal-to-Noise Ratio)— A measure of how clearly a signal stands out from the background noise. Higher SNR means more confident detections.
  • SNR > 7 typically required for confident transit detection
  • Depends on transit depth, star brightness, and data quality
  • ab astris shows SNR for each transit candidate

Missions and Catalogs

TESS— Transiting Exoplanet Survey Satellite. NASA mission launched in 2018 to search for exoplanets around nearby bright stars using the transit method.
  • Observes nearly the entire sky in 27-day sectors
  • Optimized for bright, nearby stars (mag 4-13)
  • 2-minute cadence for priority targets
  • Has discovered thousands of exoplanet candidates
Kepler— NASA space telescope (2009-2018) that revolutionized exoplanet science by staring at one patch of sky for 4 years.
  • Discovered 2,700+ confirmed exoplanets
  • Observed ~150,000 stars continuously
  • Proved small rocky planets are common
  • K2: Extended mission observing along the ecliptic
Gaia— ESA mission mapping 1.8 billion stars with unprecedented precision. Provides positions, distances, motions, and variability data.
  • DR3: Third data release with variability classifications
  • Provides parallax (distance) measurements
  • Classifies millions of variable stars
  • ab astris uses Gaia for cross-validation
VSX— AAVSO International Variable Star Index. The most comprehensive catalog of known variable stars, with over 2 million entries.
  • Maintained by amateur and professional astronomers
  • ab astris checks VSX to identify known variables
  • New discoveries can be submitted to VSX
  • Includes variable type, period, and amplitude
ExoFOP— Exoplanet Follow-up Observing Program. NASA database tracking TESS Objects of Interest (TOIs) and their follow-up observations.
  • Central repository for TESS planet candidates
  • Tracks disposition: confirmed, false positive, candidate
  • ab astris cross-checks transit candidates against ExoFOP
TIC (TESS Input Catalog)— The catalog of stars observed by TESS. Each star has a unique TIC number used as its identifier.
  • Contains ~1.7 billion objects
  • Includes stellar parameters: temperature, radius, etc.
  • TIC IDs are the primary identifiers in ab astris

ab astris Terms

Enrichment— The process of combining data from multiple TESS sectors to improve period detection. Multi-sector analysis provides longer time baselines and more data points.
  • Stitches lightcurves from all available sectors
  • Longer baseline helps break period aliases
  • Cross-validates with Kepler/K2 and Gaia
  • Tracks whether enrichment improved the detection
TRUE Discovery— A variable star meeting all four criteria for high-confidence detection: variable, periodic, not in existing catalogs (VSX NEW!), and confirmed by enrichment.
  • 1. is_variable_detection = true
  • 2. is_periodic_discovery = true
  • 3. VSX recommendation = "new_submission"
  • 4. enrichment_improved = true
Pristine Discovery— A TRUE Discovery with zero matches in any astronomical catalog. These are the most valuable finds - completely unknown variable stars.
  • Meets all TRUE Discovery criteria
  • Zero VizieR catalog matches within search radius
  • Marked with a gem icon in the discoveries table
Cross-Validation— Checking a TESS period detection against independent observations from other missions (Kepler/K2, Gaia) to confirm the period is real.
  • Different missions have different sampling and systematics
  • Matching periods across missions rules out aliases
  • Gaia also provides independent variability classification
Period Alias— A false period detection caused by the sampling pattern of observations. TESS's 27-day sectors can create aliases at specific frequencies.
  • Common aliases: 1 day, 0.5 day, 27 days
  • Multi-sector analysis helps identify aliases
  • Cross-validation with other missions breaks aliases
Lomb-Scargle Periodogram— An algorithm that finds periodic signals in unevenly-spaced data. It calculates how well sine waves of different frequencies fit the data.
  • Standard method for period detection in astronomy
  • Handles gaps in data (unlike Fourier transform)
  • Returns power spectrum showing likely periods
  • FAP (False Alarm Probability) indicates significance

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