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A neutrino is a light electrically-neutral elementary particle, specifically a fermion (i.e., with a half-integer spin, like quarks and nucleons) that is also a lepton (as are electrons). Neutrinos are thought to be very common (estimated to average 340 per cubic centimeter throughout the universe, for a total of 1.2×1089) but rarely interact with photons or "normal" matter, thus are virtually invisible and largely pass through solids such as the Earth. They are presumed to be ubiquitous: on the order of a million are presumed to be passing through your body at any moment.
Three types of neutrinos are known (aka neutrino flavors, neutrino families, neutrino species). It is also known that they come in three masses, but you cannot determine a specific neutrino's type and its mass as well (an uncertainty imposed by quantum mechanics in a manner analogous to the Heisenberg uncertainty principle). Some constraints are known regarding the ratios of the three masses, and it has been established that each mass is very likely to be less than 0.12 eV/c2. Cosmology-related astronomical observations have as a goal, to determine neutrinos' mass, given the resulting effect on the universe.
Speculation exists that there might be a fourth species, currently termed a sterile neutrino, sterile in that it does not interact with other particles in the manner currently used to investigate neutrinos. Also, that there might be more than one, e.g., three more species that are sterile. Some theories suggest it (or one of the three) could have sufficient mass to constitute dark matter, i.e., a WIMP.
Neutrinos below a certain mass are classified as light (light neutrinos). Given the upper limit established on the mass of observed neutrinos, all those observed fall in this category. If there exist more massive neutrino species, these would be considered heavy neutrinos. Cosmological observation is considered to have eliminated a middle ground, i.e., there is a gap between the masses of known neutrinos and the possible heavy neutrinos.
Some particle interactions (e.g., created in particle accelerators) and some observed cosmological data are dependent upon the count of light neutrino species. The results of their measurement and analysis produce a likely number of neutrino species, termed the effective number of neutrino species (Neff). For example, a particular experiment and analysis might produce the result 2.9. Along with error bars, this might suggest 3 as the number.
Neutrino astronomy uses large ground-based detectors (neutrino observatories) aiming to detect the interaction between a neutrino and some substance acting as a scintillator. They are typically underground or under water to reduce confounding detections of other particles, and are large to offer lots of opportunity for the extremely rare interactions. An example is IceCube at the South Pole. Most of the neutrinos detected are from the Sun, neutrino astronomy offering a window into solar core processes. Neutrinos were detected coincident with SN 1987A light reaching Earth, and there is an initiative to quickly detect the presence of any galactic supernova by this means. Blazars are another target for detection.
Neutrino cooling is the transfer of thermal energy out of an astronomical body through emission of neutrinos e.g., those produced by the Urca process within white dwarfs.