Starlight

Starlight is the

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

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

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]

Photography

Starlight intensity has been observed to be a function of its polarization.

Starlight becomes partially

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 ${\displaystyle q=6\times 10^{-4}}$, 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 ${\displaystyle p^{2}}$, so we expect a circularly polarized fraction of ${\displaystyle q\sim 2\times 10^{-4}}$.

Light from early-type stars has very little intrinsic polarization. Kemp et al.^[16] measured the optical polarization of the Sun at sensitivity of ${\displaystyle 3\times 10^{-7}}$; they found upper limits of ${\displaystyle 10^{-6}}$ for both ${\displaystyle p}$ (fraction of linear polarization) and ${\displaystyle q}$ (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 ${\displaystyle q\sim p^{2}}$, where ${\displaystyle p}$ 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, ${\displaystyle q\sim 10^{-4}}$ 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 ${\displaystyle q\sim 7\times 10^{-4}}$. Serkowski^[21] measured CP of ${\displaystyle q=7\times 10^{-3}}$ for the red supergiant NML Cygni and ${\displaystyle q=2\times 10^{-3}}$ 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 ${\displaystyle q\sim 5\times 10^{-3}}$, which is higher than for ordinary stars, presumably because of multiple scattering from dust grains.

See also

• List of brightest stars
• Purkinje effect
• Sunlight
• Moonlight

References