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Zeta Sagittarii, 38 Sagittarii

Proper NameAscella
Bayer DesignationZeta Sagittarii
Flamsteed Number38 Sagittarii
HR (BSC)7194
Right Ascension19h 2m 37s
Declination-29° 52' 48"
Distance88 light years
27 parsecs
MagnitudeApparent: +2.6
Absolute: +0.4
Spectral ClassA2III white giant
Optimum VisibilityJuly
NotesThe Ascella system consists of two massive white stars. The name Ascella formally applies to the white giant Zeta Sagittarii A, while its companion, Zeta Sagittarii B, in s a white subgiant. The system also appears to contain a third member star, fainter than the other two and in a much more distant orbit.

One of five stars that together form the shape known as the Milk Dipper, running through the central parts of the constellation Sagittarius. Ascella lies on the fringes of the band of the Milk Way, in the same general line of sight as the Sagittarius Dwarf Galaxy and globular cluster M54. These structures lie outside the Milky Way Galaxy, thousands of light years beyond the star Ascella. In the distant past Ascella was a near neighbour of the Sun, lying just ten light years from the Solar System, though its movement through the Galaxy has carried to a point some eighty-eight light years away, a distance that continues to increase.

The name Ascella derives from the Latin for 'armpit', representing the upper part of one of the arms of the Archer Sagittarius. It may indeed not be the only star to be named 'armpit' - one interpretation of the name Betelgeuse derives from the Arabic for 'armpit of the central one' (though it should be noted that this etymology of Betelgeuse is open to significant dispute).

Ascella is a binary system consisting of two white stars, each rather more massive than the Sun. These two stars follow a mutual orbit that takes a little over twenty-one years to complete. This orbit is strongly elliptical, so that the distance between the two component stars varies between about ten and sixteen AU. At their closest approach to one another, their distance apart is comparable with that from the Sun to the planet Saturn.


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