BL Boötis (abbreviated to BL Boo) is a pulsating star in the constellation

Boötes Boötes ( ) is a constellation in the northern sky, located between 0° and +60° declination, and 13 and 16 hours of right ascension on the celestial sphere. The name comes from , which comes from 'herder, herdsman' or 'plowman' (literally, 'o ...

. It is the prototype of a class of anomalous

Cepheids A Cepheid variable () is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period (typically 1–100 days) and amplitude. Cepheids are important cosmi ...

which is intermediate in the H-R diagram between the type I

classical Cepheid Classical Cepheids are a type of Cepheid variable star. They are young, population I variable stars that exhibit regular radial pulsations with periods of a few days to a few weeks and visual amplitudes ranging from a few tenths of a magnitude u ...

s and the

type II Cepheid Type II Cepheids are variable stars which pulsate with periods typically between 1 and 50 days. They are population II stars: old, typically metal-poor, low mass objects. Like all Cepheid variables, Type IIs exhibit a period-luminosity relationshi ...

s. It varies from magnitude 14.45 to 15.10 over 0.82 days. It is located 4 arcminutes from the centre of (and assumed to be a member star of) the

globular cluster A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting ...

NGC 5466 NGC 5466 is a globular cluster of stars in the constellation Boötes. It was discovered by German-British astronomer William Herschel on May 17, 1784, and catalogued as H VI.9. This large, dim cluster has an apparent visual magnitude of 9.2 and ...

. Its variability was first noted in 1961 by Russian astronomer Nikolaĭ Efimovich Kurochkin, who gave it 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 ...

BL Boötis. However, he thought it was 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 ...

. It was subsequently thought to be an

RR Lyrae variable RR Lyrae variables are periodic variable stars, commonly found in globular clusters. They are used as standard candles to measure (extra) galactic distances, assisting with the cosmic distance ladder. This class is named after the prototype a ...

by T.I. Gryzunova in 1971. Robert Zinn confirmed it was a member of the globular cluster and found it was too blue to be an

. He gave it the name V19 within the cluster. He calculated its

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

to be around 1.56 times and its

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

to be around 278 times that of the Sun; its absolute magnitude is -1.27. The

spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...

has been compared to an A2 or A3

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 on the basis of its K lines and A9 or F0 on the basis of its hydrogen lines. The discrepancy is due to a strong deficiency in metals, around 100 times lower than that of 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 ...

. Despite the spectral classification, it is not thought to be a main-sequence star, at least not a normal one. It is larger and more luminous than its spectrum would indicate, comparable to a

horizontal branch The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha proc ...

star but more massive than comparable stars on the horizontal branch. The pulsations are thought to be in the first overtone. BL Boötis has been designated the prototype of a rare class of variable star known as an

anomalous Cepheid A Cepheid variable () is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period (typically 1–100 days) and amplitude. Cepheids are important cosmi ...

or BL Boötis variable. These stars are somewhat similar to

Cepheid variable A Cepheid variable () is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period (typically 1–100 days) and amplitude. Cepheids are important cosmi ...

s, but they do not have the same relationship between their period and luminosity. Their periods are similar to the ab subtypes of RR Lyrae variables; however, they are far brighter than these stars. Anomalous Cepheids are metal poor and have masses not much larger than the Sun's, on average, 1.5

solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...

es. The origin of these stars is uncertain, but thought to possibly be from the merger of two stars. Detailed examination of the spectrum of BL Boötis with the Keck-1 telescope at the W. M. Keck Observatory showed that its effective (surface) temperature is around 6450 K at minimum light. It also showed that the chemical composition was consistent with ageing metal-poor (Population II) stars and hence cast doubt on the origin as a result of a stellar merger. The radial velocity is lower than would be expected if it were from a stellar merger.

References

{{DEFAULTSORT:BL Bootis Boötes Bootis, BL Cepheid variables A-type stars