Theta Coronae Borealis, Latinized from θ Coronae Borealis, is a

binary star A binary star 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 using a telescope as separate stars, in ...

system in the constellation

Corona Borealis Corona Borealis is a small constellation in the Northern Celestial Hemisphere. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and remains one of the 88 modern constellations. Its brightest stars form a s ...

. It shines with a combined

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's lig ...

(V band) of 4.13. There are two components: Theta Coronae Borealis A with an

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...

of about 4.2, while Theta Coronae Borealis B lies around 1 arcsecond distant and has an apparent magnitude of 6.29. The system is located around 375

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 101 ...

s from Earth, as estimated from its parallax of 8.69 milliarcseconds. It is estimated to be 85 million years old, with the primary star expected to remain on the

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

burning its core hydrogen for another 75 million years and the secondary around 500 million years. Both stars will cool and expand once their core hydrogen is exhausted, becoming

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

s. The brighter component, Theta Coronae Borealis A, is a blue-white star that spins extremely rapidly—at a rate of around 393 km per second. This rapid spinning is thought to be the cause of a gaseous disk that surrounds the star: such stars are known as Be shell stars, recognizable because the gas radiates emission lines that give a characteristic pattern in the star's spectrum. Of

spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting th ...

B6Vnn, Theta Coronae Borealis A is around six times as massive as the Sun and has four times the diameter. It has a surface temperature of around 14910 K. In 1970, it faded by 0.7 magnitude, becoming 50% fainter. The cause for this is unknown, but thought possibly due to ejection of dust that obscured the star's light. Theta Coronae Borealis B is a white main sequence star of spectral type A2V that is around 2.5 times as massive as the Sun and located 86

astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbi ...

s from the primary star, the two stars taking an estimated 300 years to orbit around a common

centre of gravity In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...

References

{{DEFAULTSORT:Theta Coronae Borealis Corona Borealis B-type main-sequence stars Binary stars Be stars Shell stars Coronae Borealis, Theta Coronae Borealis, 04 Durchmusterung objects 5778 076127 138749