Inastronomy, alight curve is agraph of thelight intensity of acelestial object or region as a function of time, typically with themagnitude oflight received 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 duringoccultation events. The study of a light curve and other observations can yield considerable information about the physical process that produces such a light curve, or constrain the physical theories about it.
Graphs of theapparent magnitude of 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 asCepheids have extremely regular light curves with exactly the same period, amplitude, and shape in each cycle. Others such asMira variables have somewhat less regular light curves with large amplitudes of several magnitudes, while thesemiregular variables are less regular still and have smaller amplitudes.[1]
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.[2] It may also show theeccentricity of the orbit anddistortions in the shape of the two stars.[3] 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.[4]
Light curves fromsupernovae can 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 supernovae have light curves with a sharpmaximum and gradually decline, whileType II supernovae have 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.[5]
Inplanetary science, a light curve can be used to derive therotation period of aminor planet,moon, orcomet nucleus. From theEarth there 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 (theamplitude of 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.[6] This allows astronomers to infer information about the shape and spin (but not size) of asteroids.
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 parameterU ranges from 0 (incorrect) to 3 (well-defined):[7]
A trailing plus sign (+) or minus sign (−) is also used to indicate a slightly better or worse quality than the unsigned value.[7]
Theoccultation light 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 achord across the occulting body.
Circumstances where the transitions are not instantaneous are;
The observations are typically recorded usingvideo equipment and the disappearance and reappearance timed using aGPS disciplined Video Time Inserter (VTI).
Occultation light curves are archived at theVizieR service.[9]
Periodic dips in a star's light curve graph could be due to anexoplanet passing 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 transit method.
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 imagestarspots or asteroid surfacealbedos.[10][11]
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 effect as 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-N2 is a microlensing event that may have been due to a star in theAndromeda Galaxy that has anexoplanet.[12]