Starlight
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.
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.
[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] 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] In many cases starlight photography may also overlap with a need to understand the impact of moonlight.
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.
Serkowski, Mathewson and Ford[15] measured the polarization of 180 stars in UBVR filters.
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.
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.
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.
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.
