Photometric-standard stars are a series of stars that have had their light output in various passbands of photometric system measured very carefully. Other objects can be observed using CCD cameras or photoelectric photometers connected to a telescope, and the flux, or amount of light received, can be compared to a photometric-standard star to determine the exact brightness, or stellar magnitude, of the object.
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.
The scale is logarithmic and defined such that each step of one magnitude changes the brightness by a factor of the fifth root of 100, or approximately 2.512. For example, a magnitude 1 star is exactly 100 times brighter than a magnitude 6 star. The brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values.
Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude. The apparent magnitude (m) is the brightness of an object as it appears in the night sky from Earth. Apparent magnitude depends on an object's intrinsic luminosity, its distance, and the extinction reducing its brightness. The absolute magnitude (M) describes the intrinsic luminosity emitted by an object and is defined to be equal to the apparent magnitude that the object would have if it were placed at a certain distance from Earth, 10 parsecs for stars. A more complex definition of absolute magnitude is used for planets and small Solar System bodies, based on its brightness at one astronomical unit from the observer and the Sun.
The Sun has an apparent magnitude of −27 and Sirius, the brightest visible star in the night sky, −1.46. Apparent magnitudes can also be assigned to artificial objects in Earth orbit with the International Space Station (ISS) sometimes reaching a magnitude of −6.Outline of astronomy
The following outline is provided as an overview of and topical guide to astronomy:
Astronomy – studies the universe beyond Earth, including its formation and development, and the evolution, physics, chemistry, meteorology, and motion of celestial objects (such as galaxies, planets, etc.) and phenomena that originate outside the atmosphere of Earth (such as the cosmic background radiation).Telluric contamination
Telluric contamination is contamination of the astronomical spectra by the Earth's atmosphere.Theta Crateris
Theta Crateris (θ Crateris) is a solitary star in the southern constellation of Crater. It is a photometric-standard star that is faintly visible to the naked eye with an apparent visual magnitude of 4.70. With an annual parallax shift of 11.63 mas as seen from Earth, it is located around 280 light years from the Sun. At that distance, the visual magnitude of the star is diminished by an extinction factor of 0.07 because of interstellar dust.This is a B-type main sequence star with a stellar classification of B9.5 Vn, where the 'n' suffix indicates "nebulous" absorption lines due to rapid rotation. It is spinning with a projected rotational velocity of 212 km/s, giving the star an oblate shape with an equatorial bulge that is an estimated 7% larger than the polar radius. The star has 2.79 times the mass of the Sun and around 3.1 times the Sun's radius. With an age of about 117 million years, it is radiating 107 times the solar luminosity from its outer atmosphere at an effective temperature of 11,524 K.
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