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Stellar radio sources, radio source stars or radio stars are stellar objects that produce copious emissions of various
radio frequencies Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upper ...
, whether constant or pulsed. Radio emissions from stars can be produced in many varied ways.


Neutron stars

Pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
s, a type of
neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white ...
, are examples of radio stars. ''
Rotation-powered pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
s'' are, as the name suggests, powered by the slow-down of their rotation. The rotation powers a magnetic field, which generates the radio emissions. Not all rotation-powered pulsars generate their pulses in the radio spectrum. Some of them, from the
millisecond pulsar A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio, X-ray, and gamma ray portions of the electromagnetic spectrum. The leading theory for the origin of ...
s, generate X-rays instead. Aside from
radio pulsar A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
s and
X-ray pulsar X-ray pulsars or accretion-powered pulsars are a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity. The X-ray periods range from as little as a fraction of a second to as much as several m ...
s, there are also gamma ray pulsars, which are mostly
magnetar A magnetar is a type of neutron star with an extremely powerful magnetic field (∼109 to 1011 T, ∼1013 to 1015 G). The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.War ...
s. Some radio pulsars are also optical pulsars. Aside from pulsars, another type of neutron star is also characterized by radio emissions: the
rotating radio transient Rotating radio transients (RRATs) are sources of short, moderately bright, radio pulses, which were first discovered in 2006. RRATs are thought to be pulsars, i.e. rotating magnetised neutron stars which emit more sporadically and/or with higher pu ...
(RRAT). As suggested by the name, the radio emission is erratic.


Quasars

Quasar A quasar is an extremely Luminosity, luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a m ...
s (quasi-stellar radio sources) are not radio stars. They also emit radio frequencies, but from the effects of
supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions of times the mass of the Sun (). Black holes are a class of astronomical obj ...
s at the centre of
galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...
. Although they appear to be stars, they are not stars, but the hyperactive heart of a galaxy.


By other stellar objects

;Astrophysical masers Some late-type stars can produce
astrophysical maser An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum. This emission may arise in molecular clouds, comets, planetary atmospheres, stellar at ...
s from their atmospheres and beam out coherent bursts of microwaves. ;The Sun The
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, the nearest
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
to
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, is known to emit radio waves, though it is virtually the only regular star that has been detected in the radio spectrum, because it is so close. It is not considered a radio star because it is not a strong radio source. ;Main-sequence stars in general Some studies have found that main-sequence stars may extremely rarely emit radio waves. A 2009 survey found a maximum of 112 candidate radio stars cross-matching the
FIRST First or 1st is the ordinal form of the number one (#1). First or 1st may also refer to: *World record, specifically the first instance of a particular achievement Arts and media Music * 1$T, American rapper, singer-songwriter, DJ, and rec ...
and NVSS surveys, but estimated that 108 ± 13 of the samples are from "contamination" from background sources. They estimate that less than 1.2 in 1 million stars between an apparent magnitude of 15 and 19.1 emit more than 1.25 mJy in the 21-centimeter band. ;Fast radio bursts
Fast radio burst In radio astronomy, a fast radio burst (FRB) is a transient radio pulse of length ranging from a fraction of a millisecond to 3 seconds, caused by some high-energy astrophysical process not yet understood. Astronomers estimate the average FRB rel ...
s (FRB) are hypothesized to originate from extra-galactic sources. These bright, brief emissions of ~1 GHz radio occur at the rate of 104 per day across the sky, and no emission counterparts have been found in other bands. An alternative scenario is that FRBs are emitted as the result of
flare A flare, also sometimes called a fusée, fusee, or bengala in some Latin-speaking countries, is a type of pyrotechnic that produces a bright light or intense heat without an explosion. Flares are used for distress signaling, illumination, ...
activity on nearby stars within a
kiloparsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...
of the Sun. This would make it easier to explain the luminosity of these events. ;Red dwarfs In 2020, 10 days before reports about BLC1 – reported to be an apparent possible radio signal from
Proxima Centauri Proxima Centauri is a small, low-mass star located away from the Sun in the southern constellation of Centaurus. Its Latin name means the 'nearest tarof Centaurus'. It was discovered in 1915 by Robert Innes and is the nearest-kno ...
, astronomers reported "a bright, long-duration optical flare, accompanied by a series of intense, coherent radio bursts" from the nearest star to the Sun. They state that it constitutes the "most compelling detection of a solar-like radio burst from another star to date" and strongly indicates a causal relationship between these emissions. Like BLC1, the signal was recorded in April and May 2019. Despite this, their finding has not been put in direct relation to the BLC1 signal by scientists or media outlets as of January 2021 but implies that planets around Proxima Centauri and other
red dwarf ''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
s are likely to be rather uninhabitable for humans and other currently known organisms.


References

{{Neutron star Star types