Sextans

Article written: 16 Jan , 2009
Updated: 24 Dec , 2015
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Located just south of the ecliptic plane, the small, dim constellation of Sextans was originally introduced in the 17th century by astronomer Johannes Hevelius. It covers 314 square degrees of sky and ranks 47th in constellation size. Sextans has 3 primary stars in its asterism and 28 Bayer Flamsteed designated stars within its confines. It is bordered by the constellations of Leo, Hydra and Crater. Sextans is visible to all observers located at latitudes between +80° and ?80° and is best seen at culmination during the month of April.

There is one annual meteor shower associated with Sextans which occurs during the daytime. The Sextantids begin their activity on or about September 9 and last through October 9 of each year with the peak date occurring on or about September 27. This daytime radio meteor stream can produce up to three or four per hour at maximum rate.

Since Sextans is considered a relatively “new” constellation, it has no mythology associated with it – only the object which it represents. Its original name – Sextans Uranae – is Latin for the astronomical sextant, an instrument which Johannes Hevelius made frequent use of in his stellar observations. Although the constellation is very faint, its angles do resemble this particular tool with which the ancient astronomer measured and charted star positions and it was adopted as the constellation Sextans by the International Astronomical Union as one of the 88 modern constellations.

Let’s begin our binocular tour with its brightest star – Alpha – the “a” symbol on our map. Just barely visible to the unaided eye and standing right on the celestial equator, Alpha Sextantis shines 122 times brighter than our Sun and is about 3 times larger. Little wonder it appears so dim, considering that its about 285 light years from Earth! At an estimated 300 million years old, Alpha is nearing the end of its hydrogen fusing lifetime and is about to become an orange giant star – one with its pole pointed right at us. Take note of Alpha’s position in the sky… Because thanks to Earth’s nutation, it was 7 arc seconds more to the north a century ago!

Now, shift your attention towards Beta – the “B” symbol. Beta is a a blue-white B-type main sequence dwarf star located about 345 light years from our solar system. While it looks very ordinary… It isn’t. Beta is a Alpha 2 Canum Venaticorum variable star – one that varies its magnitude ever so slightly just about every 15 days or so.

Ready to go to the telescope? Then aim it at Gamma – the “Y” symbol on our chart. Gamma Sextantis is a triple star system approximately 262 light years from Earth. Its two primary components, A and B, are approximately 0.38 arcseconds apart or approximately 30 Astronomical Units With apparent magnitudes of +5.8 and +6.2 this close proximity means you better have a big telescope and some super resolution to pull this pair apart! However, orbiting the binary star pair at a distance of 36 arcseconds, or roughly a hundred times farther out, is Gamma Sextantis C, a 12th magnitude companion that is also gravitationally bound to the system. Faint… But far enough away to be seen!

Before you give up on Sextans, be sure to turn your telescope or big binoculars towards NGC 3115 (RA 10 : 05.2 Dec -07 : 43). With a magnitude of 9 and more than 8 arc minutes of size, the “Spindle Galaxy” is sure to please everyone! This lenticular galaxy was discovered by William Herschel on February 22, 1787. At about 32 million light-years away from us, it might not look large in the eyepiece, but in reality it is several times bigger than our own Milky Way Galaxy. In 1992, a supermassive black hole was observed in NGC 3115 – the largest found to that date. With an estimated mass of 2 billion times the mass of the Sun, astronomers have kept a close eye on activity since its discovery. The galaxy itself appears to be comprised of mostly old stars and the growth of the black hole hasn’t increased in size since it was first observed.

The Chandra X-Ray Telescope has maintained its vigil and according to its press releases: “This is the best black hole candidate that is massive enough to have powered a quasar.”

These findings strengthen the popular view that quasars – the brightest objects in the Universe – are powered by accretion onto massive black holes. Quasars can be seen farther away than any other object. In many cases, their light has been traveling toward us for most of the age of the Universe. Therefore we see quasars as they were long ago. As a result, astronomers can infer how the quasar population evolved with time. They find that quasars were numerous when the Universe was 1/4 of its present age. Now they have mostly died out. So dead quasars should be hiding in many nearby galaxies. Quasar energies imply that the dead remnants should have masses of a billion Suns. The discovery of a supermassive black hole is a crucial confirmation of the black hole accretion theory of quasars.

Ironically, NGC 3115 is otherwise undistinguished. It’s name comes from its listing as object number 3115 in J. Dreyer’s “New General Catalog” of nebulae and star clusters, published in 1888. The galaxy is visible in moderate-sized amateur telescopes as a faint fuzzy patch in the constellation Sextans, The Sextant. But at a distance of 30 million light years, NGC 3115 is more than ten times farther from us than Andromeda or M32. In reality, it is several times bigger than our own Milky Way. But its stars are mostly old, it contains virtually no gas, and little is going on now apart from the stately orbits of its stars. In particular, its nucleus is extremely inactive. The growth of the black hole and the nuclear activity that it feeds are over, unless additional stars wander too close to the center. Whenever that happens, the nucleus is expected to experience a brief but energetic rebirth.

Although these findings support our general picture of quasars, they also highlight a number of unresolved issues. “We have only a very speculative idea of how supermassive black holes form,” Richstone said. “The processes that control their feeding, make them shine, and later turn them off are also poorly understood.” Finding nearby black holes is crucial to further progress. NGC 3115 provides a billion-solar-mass example.”

Sources:
Wikipedia
University of Illinois
Chart Courtesy of Your Sky.


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