KW Sagittarii is a

red supergiant Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) and a stellar classification K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelg ...

star, located approximately away from 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 the direction of the constellation Sagittarius. It is one of the largest known stars, with a diameter about 1,000 times larger than the Sun. If placed at the center of the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

, the

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

's surface would engulf

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

, coming close to

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

's orbit.

Variability

In 1942, Henrietta Hill Swope listed KW Sagittarii as a

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 ...

. It varies erratically in brightness over a range of about two magnitudes. It is classified as a

semiregular variable In astronomy, a semiregular variable star, a type of variable star, is a Red giant, giant or supergiant of intermediate and late (cooler) spectral type. It shows considerable periodicity in its light changes, accompanied or sometimes interrupted b ...

, although the listed period of 670 days is poorly defined. The peculiar cool spectrum has led to comparisons with symbiotic variables, but it is no longer considered to be a cataclysmic binary.

Distance

A distance of 2,420 parsecs is based on the assumption of membership on the Sagittarius OB5 association. The parallax derived from the

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of ...

mission is negative so doesn't give much information about the distance except that it is likely to be large. The Gaia Data Release 2 parallax is and implies a distance of around . The Gaia result carries a significant statistical

margin of error The margin of error is a statistic expressing the amount of random sampling error in the results of a Statistical survey, survey. The larger the margin of error, the less confidence one should have that a poll result would reflect the result of ...

, as well as an indicator that the astrometric excess noise is far beyond acceptable levels so that the parallax should be considered unreliable. A 2021 study published a photogeometric distance of 2,159 pc to KW Sgr, using a parallax published by Gaia DR3 (the successor of Gaia DR2). Data about this star can be see
here

Characteristics

KW Sagittarii is classed as a luminous cool supergiant and varies its spectral type between M0 and M4. A 2005 study led by Levesque, using a MARCS model, calculated a high luminosity of for KW Sgr and consequently very large radius of based on the assumption of an effective temperature of at a distance of . The star was then described as among the four

largest Large means of great size. Large may also refer to: Mathematics * Arbitrarily large, a phrase in mathematics * Large cardinal, a property of certain transfinite numbers * Large category, a category with a proper class of objects and morphisms (or ...

and most luminous galactic red supergiants, which includes V354 Cephei, KY Cygni and

Mu Cephei Mu Cephei ( Latinized from μ Cephei, abbreviated Mu Cep or μ Cep), also known as the Garnet Star, is a red supergiant star in the constellation Cepheus. It appears garnet red and is located at the edge of the IC 1396 nebula. It is a 4 ...

. More recently, KW Sagittarii was calculated to have a lower bolometric luminosity around and a radius around was based on the measured angular diameter and luminosity.

Notes

References

External links