Apparent magnitude is a measure of the brightness of a star or other astronomical object observed from the Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's light caused by interstellar dust along the line of sight to the observer.
Absolute magnitude is a measure of the luminosity of a celestial object, on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it were viewed from a distance of exactly 10 parsecs, without extinction of its light due to absorption by interstellar matter and cosmic dust. By hypothetically placing all objects at a standard reference distance from the observer, their luminosities can be directly compared on a magnitude scale.
The parsec is a unit of length used to measure the large distances to astronomical objects outside the Solar System. One parsec is approximately equal to 31 trillion kilometres, or 210,000 astronomical units, and equates to about 3.3 light-years. A parsec is obtained by the use of parallax and trigonometry, and is defined as the distance at which one astronomical unit subtends an angle of one arcsecond. This corresponds to approximately 648000/π astronomical units, i.e. 1 pc = 1 au/tan(1"). The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun. Most of the stars visible to the unaided eye in the night sky are within 500 parsecs of the Sun.
Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object.
Stellar parallax is the apparent shift of position of any nearby star against the background of distant objects. Created by the different orbital positions of Earth, the extremely small observed shift is largest at time intervals of about six months, when Earth arrives at opposite sides of the Sun in its orbit, giving a baseline distance of about two astronomical units between observations. The parallax itself is considered to be half of this maximum, about equivalent to the observational shift that would occur due to the different positions of Earth and the Sun, a baseline of one astronomical unit (AU).
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. 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 of known intensity and colour, photometers can measure the brightness or apparent magnitude of celestial objects.
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.
The Malmquist bias is an effect in observational astronomy which leads to the preferential detection of intrinsically bright objects. It was first described in 1922 by Swedish astronomer Gunnar Malmquist (1893–1982), who then greatly elaborated upon this work in 1925. In statistics, this bias is referred to as a selection bias or data censoring. It affects the results in a brightness-limited survey, where stars below a certain apparent brightness cannot be included. Since observed stars and galaxies appear dimmer when farther away, the brightness that is measured will fall off with distance until their brightness falls below the observational threshold. Objects which are more luminous, or intrinsically brighter, can be observed at a greater distance, creating a false trend of increasing intrinsic brightness, and other related quantities, with distance. This effect has led to many spurious claims in the field of astronomy. Properly correcting for these effects has become an area of great focus.
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.
Psi Tauri, which is Latinized from ψ Tauri, is a solitary star in the zodiac constellation of Taurus. It has a yellow-white hue and is visible to the naked eye with an apparent visual magnitude of +5.22. The distance to this system, as determined using an annual parallax shift of 36.2 mas as seen from the Earth, is 90 light years. It is drifting further away with a radial velocity of +9 km/s.
Photographic magnitude is a measure of the relative brightness of a star or other astronomical object as imaged on a photographic film emulsion with a camera attached to a telescope. An object's apparent photographic magnitude depends on its intrinsic luminosity, its distance and any extinction of light by interstellar matter existing along the line of sight to the observer.
101 Aquarii is a star in the equatorial constellation of Aquarius. 101 Aquarii is the Flamsteed designation, although it also bears the Bayer designation b3 Aquarii. The combined apparent visual magnitude of the pair is 4.71, which is bright enough to be seen with the naked eye from the suburbs. The distance of this star from Earth is estimated as 290 light-years based upon parallax measurements.
In astronomy, the distance to a visual binary star may be estimated from the masses of its two components, the size of their orbit, and the period of their orbit about one another. A dynamical parallax is an (annual) parallax which is computed from such an estimated distance.
Spectroscopic parallax is an astronomical method for measuring the distances to stars.
HD 143361 is a star in the southern constellation Norma. With an apparent visual magnitude of 9.20, this star is too dim to be seen with the naked eye. It is close enough to the Earth that its distance can be determined using parallax measurements, yielding a value of roughly 224 light-years.
Theta 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.
69 Herculis is a binary star system in the northern constellation Hercules. It has the Bayer designation e Herculis, while 69 Herculis is the Flamsteed designation. This object is visible to the naked eye as a faint, white-hued star with a combined apparent visual magnitude of 4.63. The distance to this system can be estimated from parallax measurements, which yields a range of 175 light years. It is moving closer to the Earth with a heliocentric radial velocity of −10 km/s.
HD 125442 is a single star in the southern constellation of Lupus. Its apparent visual magnitude is 4.78, which can be seen with the naked eye. The distance to HD 125442, as determined from its annual parallax shift of 22.1 mas, is 147 light years.
24 Lyncis is a single star in the northern constellation of Lynx. It is visible to the naked eye as a dim, white-hued star with an apparent visual magnitude of 4.96. The distance to this star, as determined from its parallax measurements, is around 274 light years. It is moving further away from the Earth with a heliocentric radial velocity of about +9 km/s.
HD 95370 is a single star in the southern constellation of Vela. With an apparent visual magnitude of 4.37, it can be viewed with the naked eye. The distance to this star can be determined from its annual parallax shift of 17.2 mas, yielding a value of 190 light years. It is moving closer to the Earth with a heliocentric radial velocity of −3.5 km/s.
