Starlight is the light emitted by stars. [1] It typically refers to visible electromagnetic radiation from stars other than the Sun, observable from Earth at night, although a component of starlight is observable from Earth during daytime.
Sunlight is the term used for the Sun's starlight observed during daytime. During nighttime, albedo describes solar reflections from other Solar System objects, including moonlight, planetshine, and zodiacal light.
Observation and measurement of starlight through telescopes is the basis for many fields of astronomy, [2] including photometry and stellar spectroscopy. [3] Hipparchus did not have a telescope or any instrument that could measure apparent brightness accurately, so he simply made estimates with his eyes. He sorted the stars into six brightness categories, which he called magnitudes. [4] He referred to the brightest stars in his catalog as first-magnitude stars and those so faint he could barely see them as sixth-magnitude stars.
Starlight is also a notable part of personal experience and human culture, impacting a diverse range of pursuits including poetry, [5] astronomy, [2] and military strategy. [6]
The United States Army spent millions of dollars in the 1950s and onward to develop a starlight scope, that could amplify starlight, moonlight filtered by clouds, and the fluorescence of rotting vegetation about 50,000 times to allow a person to see in the night. [6] In contrast to previously developed active infrared system such as sniperscope, it was a passive device and did not require additional light emission to see. [6]
The average color of starlight in the observable universe is a shade of yellowish-white that has been given the name Cosmic Latte.
Starlight spectroscopy, examination of the stellar spectra, was pioneered by Joseph Fraunhofer in 1814. [3] Starlight can be understood to be composed of three main spectra types, continuous spectrum, emission spectrum, and absorption spectrum. [1]
Starlight illuminance coincides with the human eye's minimum illuminance (~0.1 mlx) while moonlight coincides with the human eye's minimum colour vision illuminance (~50 mlx). [7] [8]
One of the oldest stars yet identified - oldest but not most distant in this case - was identified in 2014: while "only" 6,000 light years away, the star SMSS J031300.36−670839.3 was determined to be 13.8 billion years old, or more or less the same age as the universe itself. [9] The starlight shining on Earth includes this star. [9]
Night photography includes photographing subjects that are lit primarily by starlight. [10] Directly taking images of night sky is also a part of astrophotography. [11] Like other photography, it can be used for the pursuit of science and/or leisure. [12] [13] Subjects include nocturnal animals. [11] In many cases starlight photography may also overlap with a need to understand the impact of moonlight. [11]
Starlight intensity has been observed to be a function of its polarization.
Starlight becomes partially linearly polarized by scattering from elongated interstellar dust grains whose long axes tend to be oriented perpendicular to the galactic magnetic field. According to the Davis–Greenstein mechanism, the grains spin rapidly with their rotation axis along the magnetic field. Light polarized along the direction of the magnetic field perpendicular to the line of sight is transmitted, while light polarized in the plane defined by the rotating grain is blocked. Thus the polarization direction can be used to map the galactic magnetic field. The degree of polarization is on the order of 1.5% for stars at 1,000 parsecs' distance. [14]
Normally, a much smaller fraction of circular polarization is found in starlight. Serkowski, Mathewson and Ford [15] measured the polarization of 180 stars in UBVR filters. They found a maximum fractional circular polarization of , in the R filter.
The explanation is that the interstellar medium is optically thin. Starlight traveling through a kiloparsec column undergoes about a magnitude of extinction, so that the optical depth ~ 1. An optical depth of 1 corresponds to a mean free path, which is the distance, on average that a photon travels before scattering from a dust grain. So on average, a starlight photon is scattered from a single interstellar grain; multiple scattering (which produces circular polarization) is much less likely. Observationally, [14] the linear polarization fraction p ~ 0.015 from a single scattering; circular polarization from multiple scattering goes as , so we expect a circularly polarized fraction of .
Light from early-type stars has very little intrinsic polarization. Kemp et al. [16] measured the optical polarization of the Sun at sensitivity of ; they found upper limits of for both (fraction of linear polarization) and (fraction of circular polarization).
The interstellar medium can produce circularly polarized (CP) light from unpolarized light by sequential scattering from elongated interstellar grains aligned in different directions. One possibility is twisted grain alignment along the line of sight due to variation in the galactic magnetic field; another is the line of sight passes through multiple clouds. For these mechanisms the maximum expected CP fraction is , where is the fraction of linearly polarized (LP) light. Kemp & Wolstencroft [17] found CP in six early-type stars (no intrinsic polarization), which they were able to attribute to the first mechanism mentioned above. In all cases, in blue light.
Martin [18] showed that the interstellar medium can convert LP light to CP by scattering from partially aligned interstellar grains having a complex index of refraction. This effect was observed for light from the Crab Nebula by Martin, Illing and Angel. [19]
An optically thick circumstellar environment can potentially produce much larger CP than the interstellar medium. Martin [18] suggested that LP light can become CP near a star by multiple scattering in an optically thick asymmetric circumstellar dust cloud. This mechanism was invoked by Bastien, Robert and Nadeau, [20] to explain the CP measured in 6 T-Tauri stars at a wavelength of 768 nm. They found a maximum CP of . Serkowski [21] measured CP of for the red supergiant NML Cygni and in the long-period variable M star VY Canis Majoris in the H band, ascribing the CP to multiple scattering in circumstellar envelopes. Chrysostomou et al. [22] found CP with q of up to 0.17 in the Orion OMC-1 star-forming region, and explained it by reflection of starlight from aligned oblate grains in the dusty nebula.
Circular polarization of zodiacal light and Milky Way diffuse galactic light was measured at wavelength of 550 nm by Wolstencroft and Kemp. [23] They found values of , which is higher than for ordinary stars, presumably because of multiple scattering from dust grains.
In electrodynamics, circular polarization of an electromagnetic wave is a polarization state in which, at each point, the electromagnetic field of the wave has a constant magnitude and is rotating at a constant rate in a plane perpendicular to the direction of the wave.
Polarization is a property of transverse waves which specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. A simple example of a polarized transverse wave is vibrations traveling along a taut string (see image); for example, in a musical instrument like a guitar string. Depending on how the string is plucked, the vibrations can be in a vertical direction, horizontal direction, or at any angle perpendicular to the string. In contrast, in longitudinal waves, such as sound waves in a liquid or gas, the displacement of the particles in the oscillation is always in the direction of propagation, so these waves do not exhibit polarization. Transverse waves that exhibit polarization include electromagnetic waves such as light and radio waves, gravitational waves, and transverse sound waves in solids.
In astronomy, the interstellar medium (ISM) is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field. Although the density of atoms in the ISM is usually far below that in the best laboratory vacuums, the mean free path between collisions is short compared to typical interstellar lengths, so on these scales the ISM behaves as a gas (more precisely, as a plasma: it is everywhere at least slightly ionized), responding to pressure forces, and not as a collection of non-interacting particles.
An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that the luminosity is not produced by stars. Such excess, non-stellar emissions have been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.
Messier 87 is a supergiant elliptical galaxy in the constellation Virgo that contains several trillion stars. One of the largest and most massive galaxies in the local universe, it has a large population of globular clusters—about 15,000 compared with the 150–200 orbiting the Milky Way—and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs, traveling at a relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.
The Faraday effect or Faraday rotation, sometimes referred to as the magneto-optic Faraday effect (MOFE), is a physical magneto-optical phenomenon. The Faraday effect causes a polarization rotation which is proportional to the projection of the magnetic field along the direction of the light propagation. Formally, it is a special case of gyroelectromagnetism obtained when the dielectric permittivity tensor is diagonal. This effect occurs in most optically transparent dielectric materials under the influence of magnetic fields.
In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable matter in the universe is either hydrogen or helium, and astronomers use the word "metals" as a convenient short term for "all elements except hydrogen and helium". This word-use is distinct from the conventional chemical or physical definition of a metal as an electrically conducting solid. Stars and nebulae with relatively high abundances of heavier elements are called "metal-rich" in astrophysical terms, even though many of those elements are nonmetals in chemistry.
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 lying near the plane of the Milky Way which are within a few thousand parsecs of the Earth, extinction in the visual band of frequencies is roughly 1.8 magnitudes per kiloparsec.
Polarization is an important phenomenon in astronomy.
Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves. Typically polarimetry is done on electromagnetic waves that have traveled through or have been reflected, refracted or diffracted by some material in order to characterize that object.
Cosmic dust – also called extraterrestrial dust, space dust, or star dust – is dust that occurs in outer space or has fallen onto Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 μm), such as micrometeoroids. Larger particles are called meteoroids. Cosmic dust can be further distinguished by its astronomical location: intergalactic dust, interstellar dust, interplanetary dust, and circumplanetary dust. There are several methods to obtain space dust measurement.
V1500 Cygni or Nova Cygni 1975 was a bright nova occurring in 1975 in the constellation Cygnus. It had the second highest intrinsic brightness of any nova of the 20th century, exceeded only by CP Puppis in 1942.
In astronomy, a polar is a highly magnetic type of cataclysmic variable (CV) binary star system, originally known as an AM Herculis star after the prototype member AM Herculis. Like other CVs, polars contain two stars: an accreting white dwarf (WD), and a low-mass donor star which is transferring mass to the WD as a result of the WD's gravitational pull, overflowing its Roche lobe. Polars are distinguished from other CVs by the presence of a very strong magnetic field in the WD. Typical magnetic field strengths of polar systems are 10 million to 80 million gauss. The WD in the polar AN Ursae Majoris has the strongest known magnetic field among cataclysmic variables, with a field strength of 230 million gauss.
AM Herculis is a binary variable star located in the constellation Hercules. This star, along with the star AN Ursae Majoris, is the prototype for a category of cataclysmic variable stars called polars, or AM Her type stars.
GRW +70 8247 is a white dwarf star located 42 light-years from Earth in the constellation Draco. With a magnitude of about 13 it is visible only through a large telescope.
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.
Gamma-ray burst emission mechanisms are theories that explain how the energy from a gamma-ray burst progenitor is turned into radiation. These mechanisms are a major topic of research as of 2007. Neither the light curves nor the early-time spectra of GRBs show resemblance to the radiation emitted by any familiar physical process.
Chandrasekhar Polarization is a partial polarization of emergent radiation at the limb of rapidly rotating early-type stars or binary star system with purely electron-scattering atmosphere, named after the Indian American astrophysicist Subrahmanyan Chandrasekhar, who first predicted its existence theoretically in 1946.
Chandrasekhar–Fermi method or CF method or Davis–Chandrasekhar–Fermi method is a method that is used to calculate the mean strength of the interstellar magnetic field that is projected on the plane of the sky. The method was described by Leverett Davis Jr in 1951 and independently by Subrahmanyan Chandrasekhar and Enrico Fermi in 1953. According to this method, the magnetic field in the plane of the sky is given by
BG Canis Minoris is a binary star system in the equatorial constellation of Canis Minor, abbreviated BG CMi. With an apparent visual magnitude that fluctuates around 14.5, it is much too faint to be visible to the naked eye. Parallax measurements provide a distance estimate of approximately 2,910 light years from the Sun.
Starlight astronomy.
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