Sextans – the Sextant

Sextans – the Sextant
Sextans is a southern constellation invented by Johannes Hevelius in the 17th century to represent his astronomical sextant. Its stars are dim, but it's rich in deep sky objects.

The Polish astronomer Johannes Hevelius (1611-1687) was one of the most prominent astronomers of his time. He invented ten constellations, seven of them still in use. Even though Hevelius was born after telescopes were invented, he preferred to use his sextant to measure star positions. (The picture depicts him observing with his wife Elisabetha.)

As well as an observer and celestial cartographer, Hevelius was also an instrument maker. Sextans commemorates the sextant that was lost when a fire swept through Hevelius's observatory, destroying his instruments, books and papers.

Stars and planets
Three stars form the triangular outline of the sextant. Sextans is one of the dimmest constellations, but its stars may be visible without binoculars in good viewing conditions. In fact, they only look dim because they're so far away.

The brightest star, Alpha Sextantis, is a young white giant star. Its mass is three times that of the Sun, and it's 120 times more luminous. Alpha's surface temperature is close to 10,000°C, but the Sun's is less than 6000°C. Alpha is 290 light years away, nearly three times the distance of Leo's brightest star Regulus. However, Alpha would look as bright as Regulus if it were at the same distance.

Gamma Sextantis is almost the same distance from us as Alpha. It's a binary system made up of two bright stars. Although it's much less luminous than Alpha, it's notably brighter than the Sun. There's also a third star that may be gravitationally bound to make a Gamma triple system.

Beta Sextantis is a hot young blueish variable star. Although hotter and brighter than Alpha and Gamma, it appears dimmer because it's about 400 light years away. It's 180 times more luminous than the Sun and has a surface temperature of around 15,000°C.

As of April 2019, there were five stars in Sextans with a total of seven confirmed exoplanets, all of them gas giants. One of the planets, WASP-43b, is the size of Jupiter but twice as massive. It's also so close to its star that a year lasts just over 19 hours. Tidally locked, the planet always keeps the same side to its star.

Interestingly, WASP-43b's atmosphere has been studied by a team using the Hubble Space Telescope. They found winds blowing around the planet at the speed of sound. The temperature is 500°C on the night side and 1500°C on the day side. Although it's too hot for clouds to form, the team found water vapor in the atmosphere.

Deep sky objects
One of the best known deep sky objects in Sextans is the NGC 3115 (the Spindle Galaxy), discovered in 1787 by William Herschel. What he saw as a nebula is actually a lenticular galaxy. These galaxies look like lenses as they're seen edge-on. NGC 3115 is 30 million light years away, with a disk, but no spiral arms. It's an old galaxy with old stars, not having enough material for new star formation. It's several times bigger than the Milky Way and has a supermassive black hole at its center with the mass of a billion Suns. [Image: Image credit: X-ray: NASA/CXC/Univ. of Alabama/K. Wong et al; Optical: ESO/VLT]

NGC 3169 and NGC 3166 are two spiral galaxies – both discovered by William Herschel in 1783. They're about 70 million light years from us and 50,000 light years apart. NGC 3169 is just under the bright star Regulus in the photo. It's distorted because of its gravitational interaction with NGC 3166. You can see that its spiral arms have been pulled out into what are known as tidal tails. Eventually, the two galaxies will merge. [Image: Adam Block]

Sextans A and Sextans B are two irregular dwarf galaxies that may be distant members of our own Local Group of galaxies. An irregular galaxy has a peculiar shape that doesn't look like either an elliptical galaxy or a spiral. For example, from Earth Sextans A looks square. [Image: D. Hunter, Z. Levay]

Cosmos Redshift 7 (CR7) is one of the oldest and most distant galaxies known. It's 12.9 billion light years away. The light from this galaxy has been travelling for so long that we're now seeing the galaxy in its youth. It contains stars that formed soon after the Big Bang when the Universe was only 800 million years old. Soccer football fans may realize that the galaxy's nickname CR7 was inspired by Portuguese footballer Cristiano Ronaldo who is known as CR7.

Observation at several wavelengths of one of the most distant known galaxy clusters, CL J1001+0220, shows the hot gas associated with such clusters, and a very high star formation rate at the cluster center. A galaxy may contain billions of stars, so a cluster of galaxies is an impressive structure.

Even more impressive is a proto-supercluster in Sextans, discovered in 2018. It's known as Hyperion and is a group of young galaxies undergoing gravitational collapse to become a supercluster. A supercluster is a group of galaxy clusters. We're now 13.8 billion years after the Big Bang. This baby supercluster, some 300 million light years wide, formed 2.3 billion years after the Big Bang.

“Hubble reveals most detailed exoplanet weather map ever”
“Astronomers find largest-yet galaxy proto-supercluster”

You Should Also Read:
Johannes Hevelius
What Is a Galaxy
Searching for Exoplanets

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