Theta1 Orionis B
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Theta1 Orionis B (θ1 Orionis B), also known as BM Orionis, 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 ...
containing at least five members. It is also one of the main stars of the
Trapezium Cluster The Trapezium or Orion Trapezium Cluster, also known by its Bayer designation of Theta1 Orionis (θ1 Orionis), is a tight open cluster of stars in the heart of the Orion Nebula, in the constellation of Orion. It was discovered by Galileo Galile ...
, with the others being A, C, and D. The primary is an
eclipsing variable 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 ...
and one of the youngest known eclipsing binary systems.


Variability

θ1 Orionis B varies in brightness and has been given the
variable star designation In astronomy, a variable-star designation is a unique identifier given to variable stars. It extends the Bayer designation format, with an identifying label (as described below) preceding the Latin genitive of the name of the constellation in whic ...
BM Orionis. Every 6.47 days, it drops from
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
7.90 to a minimum of magnitude 8.65 for 8–9 hours. It was quickly classified as an eclipsing variable showing total eclipses of the brighter component, an Algol-type variable. In between the primary eclipses, there are slight brightness variations attributed to reflection effects, and a shallow secondary eclipse of less than a tenth of a magnitude. Although the
light curve In astronomy, a light curve is a graph (discrete mathematics), graph of the Radiance, light intensity of a celestial object or region as a function of time, typically with the magnitude (astronomy), magnitude of light received on the ''y''-axis ...
appears straightforward, it shows variations in the shape of the eclipse from cycle to cycle and the properties of the eclipsing component cannot easily be reconciled with the light curve.


Mini-cluster

θ1 Orionis B has been resolved into four stars. Conventionally, the brightest star is known as B1, while the companions are known as B2, B3, and B4. B2 and B3 are only just over 0.1" apart, and the two are 0.9" from B1. B2 is approximately two magnitudes fainter than B1, and B3 another magnitude fainter. In between, B4 is 0.6" from B1 and five magnitudes fainter. The brightest component, B1, is known to be an
eclipsing binary A binary star or binary star system is a Star system, system of two stars that are gravity, 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 ...
and its unresolved companion is generally called B5. A third component of the eclipsing system has been proposed to account for unusual variations in the timing of the eclipses, but is not yet widely accepted. The unseen companion is likely to be a
pre-main-sequence star A pre-main-sequence star (also known as a PMS star and PMS object) is a star in the stage when it has not yet reached the main sequence. Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope o ...
with an age of between 10,000 and 100,000 years, making it one of the least-evolved stars known. As of 2013, the pair were considered to be the youngest known eclipsing binary. The stars making up θ1 Orionis B are gravitationally bound, but their configuration is likely to be unstable and will eventually decay. Only the close B1/B5 binary will remain after a few million years.


Properties

θ1 Orionis B1 is a hot
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 ...
star with a spectral type of B1. Its spectroscopic companion B5 is estimated to have a spectral type of G2 III from observations during the total eclipses. The unusual and changeable eclipses are thought to be caused by a translucent disc surrounding the secondary star. It is seen nearly edge-on and variations in its opacity cause differences in the light curve shape.


References

{{DEFAULTSORT:Theta1 Orionis B Orion (constellation) B-type main-sequence stars Algol variables 5 Orionis, BM 037021 J05351611-0523068 Orionis, Theta1B Orionis, 41, B
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