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Starry sky crossed with the Milky Way and a meteor Perseid Meteor.jpg
Starry sky crossed with the Milky Way and a meteor

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-magnitudes stars, which were the brightest stars and those so faint he could barely see them were sixth-magnitude stars.:0" />

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]

Oldest starlight

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 would include 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]


Startlight 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.

See also

Related Research Articles

Polarization (waves) Property of waves that can oscillate with more than one orientation

Polarization is a property applying to transverse waves that 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.

Interstellar medium Matter and radiation in the space between the star systems in a galaxy

In astronomy, the interstellar medium (ISM) is the matter and radiation that exists 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.

Reflection nebula in astronomy clouds of interstellar dust

In astronomy, reflection nebulae are clouds of interstellar dust which might reflect the light of a nearby star or stars. The energy from the nearby stars is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust visible. Thus, the frequency spectrum shown by reflection nebulae is similar to that of the illuminating stars. Among the microscopic particles responsible for the scattering are carbon compounds and compounds of other elements such as iron and nickel. The latter two are often aligned with the galactic magnetic field and cause the scattered light to be slightly polarized.

Active galactic nucleus Compact region at the center of a galaxy that has a much-higher-than-normal luminosity

An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. Such excess non-stellar emission has 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.

Galactic Center Rotational center of the Milky Way galaxy

The Galactic Center is the rotational center of the Milky Way galaxy; it is a supermassive black hole of 4.100 ± 0.034 million solar masses which powers the compact radio source Sagittarius A*. It is 8.2 ± 0.4 kiloparsecs (26,700 ± 1,300 ly) away from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest.

In physics, the Faraday effect or Faraday rotation is a magneto-optical phenomenon—that is, an interaction between light and a magnetic field in a medium. The Faraday effect causes a rotation of the plane of polarization which is linearly proportional to the component of the magnetic field in the direction of propagation. Formally, it is a special case of gyroelectromagnetism obtained when the dielectric permittivity tensor is diagonal.

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.

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 Dust floating in space

Cosmic dust, also called extraterrestrial dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles measure between a few molecules and 0.1 µm. Cosmic dust can be further distinguished by its astronomical location: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust.

V1500 Cygni star

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.

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 located 42 light-years from Earth in the constellation Draco. With a magnitude of about 13 it is visible only through a large telescope. It is the smallest star known at this date at just under 2 kilometers diameter.

In astronomy, color–color diagrams are a means of comparing the apparent magnitudes of stars 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 then 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.

Quintuplet cluster open cluster in the constellation Sagittarius

The Quintuplet cluster is a dense cluster of massive young stars about 100 light years from the Galactic Center (GC). Its name comes from the fact it has five prominent infrared sources residing in it. Along with the Arches Cluster it is one of two in the immediate GC region. Due to heavy extinction by dust in the vicinity, it is invisible to optical observation and must be studied in the X-ray, radio, and infrared bands.

Lambda Cephei

Lambda Cephei is a fifth magnitude blue supergiant star in the constellation Cepheus, one of the hottest and most luminous visible to the naked eye.

69 Virginis is a single star in the zodiac constellation of Virgo, located about 259 light years away. It is visible to the naked eye as a faint orange-hued star with an apparent visual magnitude of 4.76, although it is a suspected variable that may range in magnitude from 4.75 down to 4.79. This object is moving closer to the Earth with a heliocentric radial velocity of −13 km/s. The light from this star is polarized due to intervening interstellar dust.

Cosmology Large Angular Scale Surveyor array of microwave telescopes

The Cosmology Large Angular Scale Surveyor (CLASS) is an array of microwave telescopes at a high-altitude site in the Atacama Desert of Chile as part of the Parque Astronómico de Atacama. The CLASS experiment aims to improve our understanding of cosmic dawn when the first stars turned on, test the theory of cosmic inflation, and distinguish between inflationary models of the very early universe by making precise measurements of the polarization of the Cosmic Microwave Background (CMB) over 65% of the sky at multiple frequencies in the microwave region of the electromagnetic spectrum.

V4650 Sagittarii star

V4650 Sagittarii (qF362) is a luminous blue variable star (LBV) in the constellation of Sagittarius. Located some 25,000 light years away, the star is positioned on the edge of a starburst cluster known as the Quintuplet cluster.


  1. 1 2 Robinson, Keith (2009). Starlight: An Introduction to Stellar Physics for Amateurs. Springer Science & Business Media. pp. 38–40. ISBN   978-1-4419-0708-0.
  2. 1 2 Macpherson, Hector (1911). The romance of modern astronomy. J.B. Lippincott. p.  191. Starlight astronomy.
  3. 1 2 J. B. Hearnshaw (1990). The Analysis of Starlight: One Hundred and Fifty Years of Astronomical Spectroscopy. CUP Archive. p. 51. ISBN   978-0-521-39916-6.
  4. Astronomy. Rice University. 2016. p. 761. ISBN   1938168283- via Open Stax.
  5. Wells Hawks Skinner – Studies in literature and composition for high schools, normal schools, and ... (1897) – Page 102 (Google eBook link)
  6. 1 2 3 Popular Mechanics – Jan 1969 – "How the Army Learned to See in the Dark" by Mort Schultz (Google Books link)
  7. Schlyter, Paul (1997–2009). "Radiometry and photometry in astronomy".
  8. IEE Reviews, 1972, page 1183
  9. 1 2 "Ancient Star May Be Oldest in Known Universe".
  10. Rowell, Tony (2 April 2018). Sierra Starlight: The Astrophotography of Tony Rowell. Heyday. ISBN   9781597143134 via Google Books.
  11. 1 2 3 Ray, Sidney (23 October 2015). Scientific Photography and Applied Imaging. CRC Press. ISBN   9781136094385 via Google Books.
  12. Ray, Sidney (2015-10-23). Scientific Photography and Applied Imaging. CRC Press. ISBN   9781136094385.
  13. Ray, Sidney (2015-10-23). Scientific Photography and Applied Imaging. CRC Press. ISBN   9781136094385.
  14. 1 2 Fosalba, Pablo; Lazarian, Alex; Prunet, Simon; Tauber, Jan A. (2002). "Statistical Properties of Galactic Starlight Polarization". Astrophysical Journal. 564 (2): 762–772. arXiv: astro-ph/0105023 . Bibcode:2002ApJ...564..762F. doi:10.1086/324297.
  15. Serkowski, K.; Mathewson and Ford (1975). "Wavelength dependence of interstellar polarization and ratio of total to selective extinction". Astrophysical Journal. 196: 261. Bibcode:1975ApJ...196..261S. doi:10.1086/153410.
  16. Kemp, J. C.; et al. (1987). "The optical polarization of the Sun measured at a sensitivity of parts in ten million". Nature. 326 (6110): 270–273. Bibcode:1987Natur.326..270K. doi:10.1038/326270a0.
  17. Kemp, James C.; Wolstencroft (1972). "Interstellar Circular Polarization: Data for Six Stars and the Wavelength Dependence". Astrophysical Journal. 176: L115. Bibcode:1972ApJ...176L.115K. doi:10.1086/181036.
  18. 1 2 Martin (1972). "Interstellar circular polarization". MNRAS. 159 (2): 179–190. Bibcode:1972MNRAS.159..179M. doi: 10.1093/mnras/159.2.179 .
  19. Martin, P.G.; Illing, R.; Angel, J. R. P. (1972). "Discovery of interstellar circular polarization in the direction of the Crab nebula". MNRAS. 159 (2): 191–201. Bibcode:1972MNRAS.159..191M. doi: 10.1093/mnras/159.2.191 .
  20. Bastein, Pierre; Robert and Nadeau (1989). "Circular polarization in T Tauri stars. II - New observations and evidence for multiple scattering". Astrophysical Journal. 339: 1089. Bibcode:1989ApJ...339.1089B. doi:10.1086/167363.
  21. Serkowski, K. (1973). "Infrared Circular Polarization of NML Cygni and VY Canis Majoris". Astrophysical Journal. 179: L101. Bibcode:1973ApJ...179L.101S. doi:10.1086/181126.
  22. Chrysostomou, Antonio; et al. (2000). "Polarimetry of young stellar objects - III. Circular polarimetry of OMC-1". MNRAS. 312 (1): 103–115. Bibcode:2000MNRAS.312..103C. CiteSeerX . doi:10.1046/j.1365-8711.2000.03126.x.
  23. Wolstencroft, Ramon D.; Kemp (1972). "Circular Polarization of the Nightsky Radiation". Astrophysical Journal. 177: L137. Bibcode:1972ApJ...177L.137W. doi:10.1086/181068.