Light curve

This article is about astronomical graphs of brightness variations. For photosynthetic response graphs, seeLight curve (botany).

Light curve of the asteroid201 Penelopebased on images taken on 6 October 2006 atMount John University Observatory.Shows just over one fullrotation,which lasts 3.7474 hours.

Inastronomy,alight curveis agraphof thelight intensityof acelestial objector region as a function of time, typically with themagnitudeoflightreceived on they-axis and with time on thex-axis. The light is usually in a particular frequency interval orband.

Light curves can be periodic, as in the case ofeclipsing binaries,Cepheid variables,other periodic variables, andtransitingextrasolar planets;oraperiodic,like the light curve of anova,cataclysmic variable star,supernova,microlensing event,or binary as observed duringoccultationevents. The study of the light curve, together with other observations, can yield considerable information about the physical process that produces it or constrain the physical theories about it.

Variable stars

Main article:Variable star

Light curve ofδ Cepheishowingmagnitudeversus pulsationphase

Graphs of theapparent magnitudeof a variable star over time are commonly used to visualise and analyse their behaviour. Although the categorisation of variable star types is increasingly done from their spectral properties, the amplitudes, periods, and regularity of their brightness changes are still important factors. Some types such asCepheidshave extremely regular light curves with exactly the same period, amplitude, and shape in each cycle. Others such asMira variableshave somewhat less regular light curves with large amplitudes of several magnitudes, while thesemiregular variablesare less regular still and have smaller amplitudes.

The shapes of variable star light curves give valuable information about the underlying physical processes producing the brightness changes. For eclipsing variables, the shape of the light curve indicates the degree of totality, the relative sizes of the stars, and their relative surface brightnesses.It may also show theeccentricityof the orbit anddistortions in the shapeof the two stars.For pulsating stars, the amplitude or period of the pulsations can be related to the luminosity of the star, and the light curve shape can be an indicator of the pulsation mode.

Supernovae

Main article:Supernova

Comparativesupernovatype light curves

Light curves fromsupernovaecan be indicative of the type of supernova. Although supernova types are defined on the basis of their spectra, each has typical light curve shapes.Type I supernovaehave light curves with a sharpmaximumand gradually decline, whileType II supernovaehave less sharp maxima. Light curves are helpful for classification of faint supernovae and for the determination of sub-types. For example, the type II-P (for plateau) have similar spectra to the type II-L (linear) but are distinguished by a light curve where the decline flattens out for several weeks or months before resuming its fade.

Planetary astronomy

Main article:Planetary science

Inplanetary science,a light curve can be used to derive therotation periodof aminor planet,moon,orcometnucleus. From theEarththere is often no way to resolve a small object in theSolar System,even in the most powerful oftelescopes,since the apparent angular size of the object is smaller than one pixel in the detector. Thus, astronomers measure the amount of light produced by an object as a function of time (the light curve). The time separation of peaks in the light curve gives an estimate of the rotational period of the object. The difference between the maximum and minimum brightnesses (theamplitudeof the light curve) can be due to the shape of the object, or to bright and dark areas on its surface. For example, an asymmetrical asteroid's light curve generally has more pronounced peaks, while a more spherical object's light curve will be flatter.This allows astronomers to infer information about the shape and spin (but not size) of asteroids.

Asteroid lightcurve database

Light curve quality code

TheAsteroid Lightcurve Database(LCDB) of the Collaborative Asteroid Lightcurve Link (CALL) uses a numeric code to assess the quality of a period solution for minor planet light curves (it does not necessarily assess the actual underlying data). Its quality code parameterUranges from 0 (incorrect) to 3 (well-defined):

• U= 0 → Result later proven incorrect
• U= 1 → Result based on fragmentary light curve(s), may be completely wrong.
• U= 2 → Result based on less than full coverage. Period may be wrong by 30 percent or ambiguous.
• U= 3 → Secure result within the precision given. No ambiguity.
• U= n.a. → Not available. Incomplete or inconclusive result.

A trailing plus sign (+) or minus sign (−) is also used to indicate a slightly better or worse quality than the unsigned value.

Occultation light curves

Further information:Occultation § Occultations by minor planets

Light curve of the asteroid 1247 Dysona occulting 4UCAC 174-171272, showing instantaneous disappearance and reappearance. Duration is 6.48 seconds.

Theoccultationlight curve is often characterised as binary, where the light from the star is terminated instantaneously, remains constant for the duration, and is reinstated instantaneously. The duration is equivalent to the length of achordacross the occulting body.

Circumstances where the transitions are not instantaneous are;

• when either the occulting or occulted body are double, e.g. adouble starordouble asteroid,then a step light curve is observed.
• when the occulted body is large, e.g. a star like Antares, then the transitions are gradual.
• when the occulting body has an atmosphere, e.g. the moonTitan

The observations are typically recorded usingvideoequipment and the disappearance and reappearance timed using aGPSdisciplined Video Time Inserter (VTI).

Occultation light curves are archived at theVizieRservice.

Exoplanet discovery

Periodic dips in a star's light curve graph could be due to anexoplanetpassing in front of the star that it is orbiting. When an exoplanet passes in front of its star, light from that star is temporarily blocked, resulting in a dip in the star's light curve. These dips are periodic, as planets periodically orbit a star. Many exoplanets have been discovered via this method, which is known as theastronomical transitmethod.

Light curve inversion

Light curve inversion is a mathematical technique used to model the surfaces of rotating objects from their brightness variations. This can be used to effectively imagestarspotsor asteroid surfacealbedos.

Microlensing

Main article:Gravitational microlensing

Microlensing is a process where relatively small and low-mass astronomical objects cause a brief small increase in the brightness of a more distant object. This is caused by the smallrelativistic effectas largergravitational lenses,but allows the detection and analysis of otherwise-invisible stellar and planetary mass objects. The properties of these objects can be inferred from the shape of the lensing light curve. For example,PA-99-N2is a microlensing event that may have been due to a star in theAndromeda Galaxythat has anexoplanet.

External links

• The AAVSO online light curve generatorArchived2020-12-21 at theWayback Machinecan plot light curves for thousands of variable stars
• The Open Astronomy Catalogshave light curves for several transient types, including supernovae
• Lightcurves: An Introductionby NASA's Imagine the Universe
• DAMITDatabase of Asteroid Models from Inversion Techniques