U Sagittae is an eclipsing binary

star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or '' galaxy'', although, broadly speak ...

in the northern

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 origins of the e ...

of Sagitta. It has been actively studied since its discovery in 1901. The maximum apparent visual magnitude of this system is 6.50, which is near the lower limit of visibility to the naked eye. The system is located at a distance of approximately 868

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 1012 ...

s from the Sun based on

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...

measurements, but is drifting closer with a

radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...

of −17 km/s. It is positioned about 2° from the middle of the

Collinder 399 Brocchi's Cluster (also known as Collinder 399, Cr 399 or Al Sufi's Cluster) is a asterism of six stars in an apparent row, across 1.3° of the night sky and four others, in the south of the constellation Vulpecula, thus near Sagitta. Its nick ...

asterism, but lies much further away than the purported member stars. The

variable Variable may refer to: * Variable (computer science), a symbolic name associated with a value and whose associated value may be changed * Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...

nature of this system was discovered by German astronomer Friedrich Schwab in 1901. He determined it to be an Algol variable with a brightness that decreased by 2.1 magnitudes during an

eclipse An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ce ...

. A single-lined spectroscopic orbit was published in 1916 by Mary Fowler using spectrograms taken from the Allegheny Observatory. With a larger set of observations, D. H. McNamara published a revised orbit in 1951. In 1959, hydrogen lines from the system were found to show a systematically different radial velocity compared to other lines from the two stars. This is a

spectroscopic binary A binary star is a system of two star, 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 using a telescope as separa ...

system in a near circular orbit with a period of 3.38 days. It is a

semidetached binary 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 wh ...

and the brightest fully eclipsing Algol variable. During the primary eclipse the brightness of the system drops to magnitude 9.28, while the secondary eclipse lowers the magnitude to 6.71. Observations of the system minima over a period of more than 80 years do not indicate a significant change in the orbital period. The orbital plane is inclined at an angle of 89° to the line of sight from the Earth, so it is being viewed from edge-on. The system contains circumstellar gas with a temperature of that is streaming between the stars. This forms an intermittent accretion disk around the primary. The primary component is a B-type main-sequence star with a

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...

of B7.5V. It has 4.6 times the mass and 3.9 times the radius of the Sun. The star is rotating with a

projected rotational velocity Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface. The rotation of a star produces an equatorial bulge ...

of 100 km/s. It is radiating 48 times the luminosity of the Sun from its

photosphere The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...

at an

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

of 13,300 K. The gas stream from the secondary is causing an increase in ultraviolet emission from this star from an impact hot spot. The secondary has a class of G4 III–IV, indicating it is the more evolved member of the pair. It was originally the primary component of the system before expanding and transferring much of its mass to the present day primary. The secondary has filled its Roche lobe and is shedding mass at an estimated rate of ·yr⁻¹. It has 1.7 times the mass of the Sun and 5.5 times the Sun's radius. This star is rotating slightly faster than synchronous with the orbit, showing a projected rotational velocity of 73 km/s. It is radiating 2.7 times the luminosity of the Sun from its photosphere at an effective temperature of 5,500 K.

References

Further reading