Photometry (astronomy)
In astronomy, photometry, from Greek photo- ("light") and -metry ("measure"), is a technique used in astronomy that is concerned with measuring the flux or intensity of light radiated by astronomical objects.[1] This light is measured through a telescope using a photometer, often made using electronic devices such as a CCD photometer or a photoelectric photometer that converts light into an electric current by the photoelectric effect. When calibrated against standard stars (or other light sources) of known intensity and colour, photometers can measure the brightness or apparent magnitude of celestial objects.
The methods used to perform photometry depend on the wavelength region under study. At its most basic, photometry is conducted by gathering light and passing it through specialized photometric optical bandpass filters, and then capturing and recording the light energy with a photosensitive instrument. Standard sets of passbands (called a photometric system) are defined to allow accurate comparison of observations.[2] A more advanced technique is spectrophotometry that is measured with a spectrophotometer and observes both the amount of radiation and its detailed spectral distribution.[3]
Photometry is also used in the observation of variable stars,[4] by various techniques such as, differential photometry that simultaneously measures the brightness of a target object and nearby stars in the starfield[5] or relative photometry by comparing the brightness of the target object to stars with known fixed magnitudes.[6] Using multiple bandpass filters with relative photometry is termed absolute photometry. A plot of magnitude against time produces a light curve, yielding considerable information about the physical process causing the brightness changes.[7] Precision photoelectric photometers can measure starlight around 0.001 magnitude.[8]
The technique of surface photometry can also be used with extended objects like planets, comets, nebulae or galaxies that measures the apparent magnitude in terms of magnitudes per square arcsecond.[9] Knowing the area of the object and the average intensity of light across the astronomical object determines the surface brightness in terms of magnitudes per square arcsecond, while integrating the total light of the extended object can then calculate brightness in terms of its total magnitude, energy output or luminosity per unit surface area.
Magnitudes and colour indices[edit]
Modern photometric methods define magnitudes and colours of astronomical objects using electronic photometers viewed through standard coloured bandpass filters. This differs from other expressions of apparent visual magnitude[7] observed by the human eye or obtained by photography:[4] that usually appear in older astronomical texts and catalogues.
Magnitudes measured by photometers in some commonplace photometric systems (UBV, UBVRI or JHK) are expressed with a capital letter, such as "V" (mV) or "B" (mB). Other magnitudes estimated by the human eye are expressed using lower case letters, such as "v", "b" or "p", etc.[16] E.g. Visual magnitudes as mv,[17] while photographic magnitudes are mph / mp or photovisual magnitudes mp or mpv.[17][4] Hence, a 6th magnitude star might be stated as 6.0V, 6.0B, 6.0v or 6.0p. Because starlight is measured over a different range of wavelengths across the electromagnetic spectrum and are affected by different instrumental photometric sensitivities to light, they are not necessarily equivalent in numerical value.[16] For example, apparent magnitude in the UBV system for the solar-like star 51 Pegasi[18] is 5.46V, 6.16B or 6.39U,[19] corresponding to magnitudes observed through each of the visual 'V', blue 'B' or ultraviolet 'U' filters.
Magnitude differences between filters indicate colour differences and are related to temperature.[20] Using B and V filters in the UBV system produces the B–V colour index.[20] For 51 Pegasi, the B–V = 6.16 – 5.46 = +0.70, suggesting a yellow coloured star that agrees with its G2IV spectral type.[21][19] Knowing the B–V results determines the star's surface temperature,[22] finding an effective surface temperature of 5768±8 K.[23]
Another important application of colour indices is graphically plotting star's apparent magnitude against the B–V colour index. This forms the important relationships found between sets of stars in colour–magnitude diagrams, which for stars is the observed version of the Hertzsprung-Russell diagram. Typically photometric measurements of multiple objects obtained through two filters will show, for example in an open cluster,[24] the comparative stellar evolution between the component stars or to determine the cluster's relative age.[25]
Due to the large number of different photometric systems adopted by astronomers, there are many expressions of magnitudes and their indices.[10] Each of these newer photometric systems, excluding UBV, UBVRI or JHK systems, assigns an upper or lower case letter to the filter used. For example, magnitudes used by Gaia are 'G'[26] (with the blue and red photometric filters, GBP and GRP[27]) or the Strömgren photometric system having lower case letters of 'u', 'v', 'b', 'y', and two narrow and wide 'β' (Hydrogen-beta) filters.[10] Some photometric systems also have certain advantages. For example, Strömgren photometry can be used to measure the effects of reddening and interstellar extinction.[28] Strömgren allows calculation of parameters from the b and y filters (colour index of b − y) without the effects of reddening, as the indices m 1 and c 1.[28]
Software[edit]
A number of free computer programs are available for synthetic aperture photometry and PSF-fitting photometry.
SExtractor[37] and Aperture Photometry Tool[38] are popular examples for aperture photometry. The former is geared towards reduction of large scale galaxy-survey data, and the latter has a graphical user interface (GUI) suitable for studying individual images.
DAOPHOT is recognized as the best software for PSF-fitting photometry.[31]
There are a number of organizations, from professional to amateur, that gather and share photometric data and make it available on-line. Some sites gather the data primarily as a resource for other researchers (ex. AAVSO) and some solicit contributions of data for their own research (ex. CBA):