Sigma Sagittarii
Encyclopedia
Sigma Sagittarii (σ Sgr, σ Sagittarii) is the second brightest star system
in the constellation
Sagittarius
.
Its modern name Nunki is an Assyria
n or Babylonian name recovered by archaeologists and made public by R.H.Allen.
Nunki has an apparent magnitude
of +2.1 and belongs to the spectral classification B3. The total luminosity
of σ Sgr is 3300 times that of the Sun
while it has a mass of roughly 7 solar mass
es. The radius of the star is equivalent to 5 Suns and it has a surface temperature of 20,000 kelvin
s. Sigma Sgr has a magnitude +9.5 companion, Sigma Sagittarii B, located 5.2 arcminutes away. It may also have a closer companion.
Because it is close to the ecliptic
, Nunki can be occulted
by the Moon
and very rarely by planet
s. The last occultation of Nunki by a planet took place on November 17, 1981, when it was occulted by Venus
.
Furthermore, Nunki is the brightest star that can be principally occulted by an exterior planet.
However, only Mars can do this, and only rarely; the last time was on 3 September 423.
Star system
A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems.-Binary star systems:A stellar...
in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Sagittarius
Sagittarius (constellation)
Sagittarius is a constellation of the zodiac, the one containing the galactic center. Its name is Latin for the archer, and its symbol is , a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow...
.
Its modern name Nunki is an Assyria
Assyria
Assyria was a Semitic Akkadian kingdom, extant as a nation state from the mid–23rd century BC to 608 BC centred on the Upper Tigris river, in northern Mesopotamia , that came to rule regional empires a number of times through history. It was named for its original capital, the ancient city of Assur...
n or Babylonian name recovered by archaeologists and made public by R.H.Allen.
Nunki has an apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
of +2.1 and belongs to the spectral classification B3. The total luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
of σ Sgr is 3300 times that of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
while it has a mass of roughly 7 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es. The radius of the star is equivalent to 5 Suns and it has a surface temperature of 20,000 kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s. Sigma Sgr has a magnitude +9.5 companion, Sigma Sagittarii B, located 5.2 arcminutes away. It may also have a closer companion.
Because it is close to the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
, Nunki can be occulted
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...
by the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
and very rarely by planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s. The last occultation of Nunki by a planet took place on November 17, 1981, when it was occulted by Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
.
Furthermore, Nunki is the brightest star that can be principally occulted by an exterior planet.
However, only Mars can do this, and only rarely; the last time was on 3 September 423.