Photometric-standard stars are a series of star

Star

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s that have had their light

Light

Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 output in various passbands of photometric system

Photometric system

In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.The first known...

, measured very carefully. Other objects can be observed using CCD

Charge-coupled device

A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 cameras or photoelectric photometers connected to a telescope

Telescope

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

, and the flux

Flux

In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...

, or amount of light received, can be compared to a photometric-standard star to determine the exact brightness, or stellar magnitude, of the object.

The current set of photometric-standard stars for UBVRI photometry most often used by astronomers is that published by Arlo U. Landolt

Arlo U. Landolt

Arlo U. Landolt is an American astronomer. Landolt has worked principally in photometry and has published a number of widely used lists of standard stars. In 1995, he received the George van Biesbroeck Prize from the American Astronomical Society...

 in 1992 in the Astronomical Journal

Astronomical Journal

The Astronomical Journal is a peer-reviewed monthly scientific journal owned by the American Astronomical Society and currently published by Institute of Physics Publishing. It is one of the premier journals for astronomy in the world...

, vol. 104, no. 1, p. 340-371.