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Apparent magnitude is a measure of the brightness of a star or other astronomical object observed from 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.
Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical objects.
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 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.
In astronomy, extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Interstellar extinction was first documented as such in 1930 by Robert Julius Trumpler. However, its effects had been noted in 1847 by Friedrich Georg Wilhelm von Struve, and its effect on the colors of stars had been observed by a number of individuals who did not connect it with the general presence of galactic dust. For stars that lie near the plane of the Milky Way and are within a few thousand parsecs of the Earth, extinction in the visual band of frequencies is roughly 1.8 magnitudes per kiloparsec.
In astronomy, magnitude is measure of the brightness of an object, usually in a defined passband. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.
TZ Arietis is a red dwarf in the northern constellation of Aries. With a normal apparent visual magnitude of 12.3, it is too faint to be seen by the naked eye, although it lies relatively close to the Sun at a distance of 14.6 light-years. It is a flare star, which means it can suddenly increase in brightness for short periods of time.
Theta2 Sagittarii, Latinized from θ2 Sagittarii, is a solitary star in the zodiac constellation of Sagittarius. It is faintly visible to the naked eye with an apparent visual magnitude of +5.30. The star is progressing in the general direction of the Sun with a radial velocity of −17.60 km/s. Based upon an annual parallax shift of 20.62 mas as seen from Earth, it is located around 158 light years from the Sun. The star is drifting closer with a radial velocity of −17.6 km/s.
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.
The UBV photometric system, also called the Johnson system, is a photometric system usually employed for classifying stars according to their colors. It was the first standardized photometric system. The apparent magnitudes of stars in the system are often used to determine the color indices B−V and U−B, the difference between the B and V magnitudes and the U and B magnitudes respectively.
A color–color diagram is a means of comparing the colors of an astronomical object at different wavelengths. Astronomers typically observe at narrow bands around certain wavelengths, and objects observed will have different brightnesses in each band. The difference in brightness between two bands is referred to as color. On color–color diagrams, the color defined by two wavelength bands is plotted on the horizontal axis, and the color defined by another brightness difference will be plotted on the vertical axis.
Arlo Udell Landolt was an American astronomer known for his widely used photometric standards.
HD 135438 is a K-type giant star in the northern constellation of Boötes. With an apparent magnitude of 6.0, it lies about 650 light years away.
HD 149382 is a hot subdwarf star in the constellation of Ophiuchus with an apparent visual magnitude of 8.943. This is too faint to be seen with the naked eye even under ideal conditions, although it can be viewed with a small telescope. Based upon parallax measurements, this star is located at a distance of about 246 light-years from the Earth.
In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. The lower the color index, the more blue the object is. Conversely, the larger the color index, the more red the object is. This is a consequence of the logarithmic magnitude scale, in which brighter objects have smaller magnitudes than dimmer ones. For comparison, the whitish Sun has a B−V index of 0.656 ± 0.005, whereas the bluish Rigel has a B−V of −0.03. Traditionally, the color index uses Vega as a zero point.
64 Draconis is a single star in the northern circumpolar constellation of Draco, located 452 light years away. It has the Bayer designation of e Draconis; 64 Draconis is the Flamsteed designation. The object is visible to the naked eye as a dim, red-hued star with an apparent visual magnitude of 5.27. It is moving closer to the Earth with a heliocentric radial velocity of −36 km/s, and it is predicted to come as close as 204 ly in around 4.3 million years.
SW Lyncis is a binary or possibly a multiple-star system in the northern constellation of Lynx, abbreviated SW Lyn. With a combined apparent visual magnitude of 9.58, it is too faint to be visible to the naked eye. The system is located at a distance of approximately 970 light years based on parallax measurements, and is drifting further away with a net radial velocity of about +32 km/s.
UX Orionis is a variable star in the constellation of Orion. It is a Herbig Ae star, located about 1000 light years from the Earth. At its brightest it is a magnitude 9.5 object, so it is too faint to be seen with the naked eye. UX Orionis is the prototype of the UX Orionis class of variable stars, which are young stellar objects that exhibit large, irregular changes in visual band brightness. UX Orionis was discovered by Henrietta Swan Leavitt.
References
- ↑ Landolt, A.U. (1 July 1992). "UBVRI photometric standard stars in the magnitude range 11.5-16.0 around the celestial equator". The Astronomical Journal. 104: 340–371. Bibcode:1992AJ....104..340L. doi:10.1086/116242.
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