BY Draconis is a
multiple star system
A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. It may sometimes be used to refer to a single star. A large group of stars bound by gravitation is generally called a ''st ...
in the
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The first constellati ...
Draco
DRACO (double-stranded RNA activated caspase oligomerizer) is a group of experimental antiviral drugs formerly under development at the Massachusetts Institute of Technology. In cell culture, DRACO was reported to have broad-spectrum efficacy aga ...
, consisting of at least three components. Components A and B are
main sequence
In astronomy, the main sequence is a classification of stars which appear on plots of stellar color index, color versus absolute magnitude, brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or d ...
stars,
[ and form a close ]binary star
A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...
system with a short orbital period
The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...
of only 5.98 days. Their individual spectroscopic
Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum.
Spectrosc ...
classifications are dK5e and dK7e.[ They form the prototype of a class of ]variable star
A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...
s known as BY Draconis variable
BY Draconis variables are variable stars of late spectral types, usually K or M, and typically belong to the main sequence. The name comes from the archetype for this category of variable star system, BY Draconis. They exhibit variations in thei ...
s.
The third component (C) is, by comparison, widely separated from the A-B pair by an angular distance
Angular distance or angular separation is the measure of the angle between the orientation (geometry), orientation of two straight lines, ray (geometry), rays, or vector (geometry), vectors in three-dimensional space, or the central angle subtende ...
of 17 arcsecond
A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...
s, which corresponds to 260 AU at the estimated distance of this star system—where an AU is the average distance from the Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
to the Sun. Component C is an M5 class red dwarf
A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...
star. There may be a fourth component to the system, orbiting with a ≤1000‑day period, responsible for the eccentricity of the 5.98-day orbit, but this has not been visually confirmed.[
The variability of BY Draconis is caused by activity in the stellar ]photosphere
The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...
called starspot
Starspots are stellar phenomena, so-named by analogy with sunspots.
Spots as small as sunspots have not been detected on other stars, as they would cause undetectably small fluctuations in brightness. The commonly observed starspots are in gene ...
s, which are comparable to sunspot
Sunspots are temporary spots on the Sun's surface that are darker than the surrounding area. They are one of the most recognizable Solar phenomena and despite the fact that they are mostly visible in the solar photosphere they usually aff ...
s on the Sun
The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
, in combination with rapid rotation that changes the viewing angle of the activity relative to the observer. This variation has an average periodicity of 3.8285 days, but the brightness also changes over the course of several years—depending on the level of surface activity. Most observers believe that the primary star (A) is responsible for the variability as the secondary produces only a third of the total luminosity from the system. However, the spots may occur on both stars. Unlike the Sun, these spots may occur in the polar regions of the stars.
References
External links
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{{DEFAULTSORT:BY Draconis
Draco (constellation)
BY Draconis variables
K-type main-sequence stars
Draconis, BY
234667
0719
Durchmusterung objects
091009
Flare stars
TIC objects
Triple star systems
Emission-line stars