Sextans constellation lies in the southern sky, near the celestial equator. It represents the astronomical sextant. Sextans is the 47th constellation in size, occupying an area of 314 square degrees. It is located in the second quadrant of the southern hemisphere (SQ2) and can be seen at latitudes between +80° and -90°. The neighboring constellations are Crater, Hydra and Leo.
[http://www.constellation-guide.com/constellation-list/sextans-constellation/]
Sextans above the coils of Hydra, illustrated in the Atlas Coelestis of John Flamsteed (1729)
A faint constellation south of Leo, introduced by the Polish astronomer Johannes Hevelius in 1687 under the name Sextans Uraniae to commemorate the instrument with which he measured star positions, and which was destroyed along with other instruments in a fire at his observatory in 1679.
Hevelius had continued to make naked-eye sightings with his sextant throughout his life, even though he used telescopes for observing the Moon and planets; it was perhaps to demonstrate the keenness of his eyes that he formed Sextans out of such faint stars, as he also did with another of his inventions, Lynx.
[http://www.ianridpath.com/startales/sextans.htm]
Constellation of Sextans and the head of Hydra
[https://www.davidmalin.com/fujii/source/Sex.html]
[http://www.dibonsmith.com/sex_con.htm]
Sextans as a constellation covers a rather dim, sparse region of the sky. It has only one star above the fifth magnitude, namely Alpha Sextantis.
Alpha Sextantis is the brightest star in the constellation. It is a white A-type giant with an apparent magnitude of +4.48. It is approximately 287 light years from Earth. Alpha Sextantis is considered an informal ‘equator star,’ as it lies less than a quarter of a degree south of the celestial equator. In 1900, it was 7 minutes of arc north of the equator. As a result of movement of the Earth’s axial tilt, it crossed over to the Southern Hemisphere in December 1923.
[https://en.wikipedia.org/wiki/Alpha_Sextantis]
Gamma Sextantis is a triple star system. It is approximately 262 light years from Earth. The combined apparent magnitude of the system is +5.07. The system is composed of a close binary star, Gamma Sextantis A and B, which is given the stellar classification A1. The two component stars are approximately 0.38 arcseconds apart, or approximately 30 Astronomical Units, and have apparent magnitudes of +5.8 and +6.2. They complete one orbit every 77.6 years. Orbiting the binary star, at a distance of 36 arcseconds, roughly a hundred times farther out, is Gamma Sextantis C, a 12th magnitude companion.
[https://en.wikipedia.org/wiki/Gamma_Sextantis]
Beta Sextantis is a variable star, a blue-white B-type main sequence dwarf. It is approximately 345 light years from Earth. It is classified as an Alpha2 Canum Venaticorum variable and its brightness varies from magnitude +5.00 to +5.10. The period of this variability is unclear but may be around 15.4 days.
[https://en.wikipedia.org/wiki/Beta_Sextantis]
Delta Sextantis is also a blue-white B-type main sequence dwarf with an apparent magnitude of +5.19. It is approximately 300 light years from Earth.
[https://en.wikipedia.org/wiki/Delta_Sextantis]
Epsilon Sextantis is a yellow-white F-type giant with an apparent magnitude of +5.25. It is approximately 183 light years from Earth.
[https://en.wikipedia.org/wiki/Epsilon_Sextantis]
Generic rendering of LHS 292 flare star
LHS 292 is a red dwarf in the constellation Sextans. It is far too faint to be seen with the unaided eye and requires a large amateur telescope to be seen visually. It lies relatively close to the Sun at a distance of about 14.8 light years. It is a flare star, which means it can suddenly increase in brightness for short periods of time.
[https://en.wikipedia.org/wiki/LHS_292]
HD 86081
HD 86081 is a yellow-white dwarf star in the constellation Sextans south of the celestial equator. It is bigger and more massive than our Sun at 1.22 and 1.21 solar units respectively. It has temperature of 6028 K and luminosity of 1.75 solar. The star began being monitored in November 2005 and its first planet, HD 86081 b, was discovered on April 17, 2006. HD 86081 b is a gas giant exoplanet that orbits close to its host star, completing its orbit for only 2.1375 days. With such a short orbit, it belongs to the class of exoplanets known as hot Jupiters.
[https://en.wikipedia.org/wiki/HD_86081]
[https://en.wikipedia.org/wiki/HD_86081_b]
Sextans A and Sextans B are two dwarf irregular galaxies in Sextans:
Sextans A: A seemingly square galaxy
What’s bothering local galaxy Sextans A? A small dwarf irregular galaxy spanning 5 thousand light years across, Sextans A is located only 5 million light-years away. Named for its home constellation of Sextans, the ‘diamond in the rough’ structure relates to an ancient unknown event. 100 million years ago, something mysterious started a new wave of star formation in Sextans A’s center. Massive short-lived stars exploded in supernovae that caused more star formation and yet more supernovae, ultimately resulting in an expanding shell. Today, young blue stars highlight areas and shell edges high in current star formation, a shell that from our perspective appears roughly square. In the above picture, a bright orange star in our own Milky Way Galaxy appears superposed in the foreground.
[https://apod.nasa.gov/apod/ap981103.html]
Sextans B
[https://en.wikipedia.org/wiki/Sextans_B]
Sextans B (also known as UGC 5373 and DDO 70) is, like Sextans A, an irregular dwarf galaxy that may be part of the Local Group, or lie just beyond it. Sextans B is 4.44 million light-years away from Earth and thus is one of the most distant members of the Local Group, if it is indeed a member. It forms a pair with its neighbouring galaxy Sextans A. It is a type Ir IV-V galaxy according to the galaxy morphological classification scheme. Sextans B may also be gravitationally associated with the galaxies NGC 3109 and the Antlia Dwarf.
[https://en.wikipedia.org/wiki/Sextans_B]
NGC 3169 is a spiral galaxy in the constellation Sextans, gravitationally interacting with the neighboring galaxy NGC 3166:
Unraveling NGC 3169
Bright spiral galaxy NGC 3169 appears to be unraveling in this cosmic scene, played out some 70 million light-years away just below bright star Regulus toward the faint constellation Sextans. Its beautiful spiral arms are distorted into sweeping tidal tails as NGC 3169 (left) and neighboring NGC 3166 interact gravitationally, a common fate even for bright galaxies in the local universe. In fact, drawn out stellar arcs and plumes, indications of gravitational interactions, seem rampant in the deep and colorful galaxy group photo. The picture spans 20 arc minutes, or about 400,000 light-years at the group’s estimated distance, and includes smaller, dimmer NGC 3165 at the right. NGC 3169 is also known to shine across the spectrum from radio to X-rays, harboring an active galactic nucleus that is likely the site of a supermassive black hole.
[http://apod.nasa.gov/apod/ap130328.html]
NGC 3115 is an edge-on lenticular galaxy, with a super- massive black hole at its center:
Composite image of NGC 3115 from the Chandra X-ray Observatory and the Very Large Telescope.
NGC 3115 (also called the Spindle Galaxy or Caldwell 53) is a field lenticular galaxy in the constellation Sextans. The galaxy was discovered by William Herschel on February 22, 1787. At about 32 million light-years away from us it is several times bigger than the Milky Way. It is a lenticular (S0) galaxy because it contains a disk and a central bulge of stars, but without a detectable spiral pattern. NGC 3115 is seen almost exactly edge-on, but was nevertheless mis-classified as elliptical. There is some speculation that NGC 3115, in its youth, was a quasar.
NGC 3115 has consumed most of the gas of its youthful accretion disk. It has very little gas and dust left that would trigger new star formation. The vast majority of its component stars are very old.
In 2011, NASA’s Chandra X-ray Observatory examined the black hole at the center of the large galaxy. A flow of hot gas toward the supermassive black hole has been imaged, making this the first time clear evidence for such a flow has been observed in any black hole. As gas flows toward the black hole, it becomes hotter and brighter. The researchers found the rise in gas temperature begins at about 700 light years from the black hole, giving the location of the Bondi radius. This suggests that the black hole in the center of NGC 3115 has a mass of about two billion solar masses, supporting previous results from optical observations. This would make NGC 3115 the nearest billion-solar-mass black hole to Earth.
[https://en.wikipedia.org/wiki/NGC_3115]
Sextans is the location of the field studied by the COSMOS project, undertaken by the Hubble Space Telescope. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy (at z = 6.60) found in the constellation:
VLT discovers CR7, the brightest distant galaxy, and signs of Population III stars
Astronomers using ESO’s Very Large Telescope have discovered by far the brightest galaxy yet found in the early Universe and found strong evidence that examples of the first generation of stars lurk within it. These massive, brilliant, and previously purely theoretical objects were the creators of the first heavy elements in history- the elements necessary to forge the stars around us today, the planets that orbit them, and life as we know it. The newly found galaxy, labelled CR7, is three times brighter than the brightest distant galaxy known up to now.
Astronomers have long theorized the existence of a first generation of stars- known as Population III stars- that were born out of the primordial material from the Big Bang. All the heavier chemical elements- such as oxygen, nitrogen, carbon and iron, which are essential to life- were forged in the bellies of stars. This means that the first stars must have formed out of the only elements to exist prior to stars: hydrogen, helium and trace amounts of lithium.
These Population III stars would have been enormous- several hundred or even a thousand times more massive than the Sun- blazing hot, and transient- exploding as supernovae after only about two million years. But until now the search for physical proof of their existence had been inconclusive.
A team led by David Sobral has now used ESO’s Very Large Telescope (VLT) to peer back into the ancient Universe, to a period known as deionization, approximately 800 million years after the Big Bang. Instead of conducting a narrow and deep study of a small area of the sky, they broadened their scope to produce the widest survey of very distant galaxies ever attempted.
The team discovered- and confirmed- a number of surprisingly bright very young galaxies. One of these, labelled CR7, was an exceptionally rare object, by far the brightest galaxy ever observed at this stage in the Universe. With the discovery of CR7 and other bright galaxies, the study was already a success, but further inspection provided additional exciting news.
The instruments on the VLT found strong ionized helium emission in CR7 but- crucially and surprisingly- no sign of any heavier elements in a bright pocket in the galaxy. This meant the team had discovered the first good evidence for clusters of Population III stars that had ionised gas within a galaxy in the early Universe.
“The discovery challenged our expectations from the start,” said David Sobral, “as we didn’t expect to find such a bright galaxy. Then, by unveiling the nature of CR7 piece by piece, we understood that not only had we found by far the most luminous distant galaxy, but also started to realise that it had every single characteristic expected of Population III stars. Those stars were the ones that formed the first heavy atoms that ultimately allowed us to be here. It doesn’t really get any more exciting than this.”
Within CR7, bluer and somewhat redder clusters of stars were found, indicating that the formation of Population III stars had occurred in waves- as had been predicted. What the team directly observed was the last wave of Population III stars, suggesting that such stars should be easier to find than previously thought: they reside amongst regular stars, in brighter galaxies, not just in the earliest, smallest, and dimmest galaxies, which are so faint as to be extremely difficult to study.
Jorryt Matthee, second author of the paper, concluded: “I have always wondered where we come from. Even as a child I wanted to know where the elements come from: the calcium in my bones, the carbon in my muscles, the iron in my blood. I found out that these were first formed at the very beginning of the Universe, by the first generation of stars. With this discovery, remarkably, we are starting to actually see such objects for the first time.”
Further observations with the VLT, ALMA, and the NASA/ESA Hubble Space Telescope are planned to confirm beyond doubt that what has been observed are Population III stars, and to search for and identify further examples.
[http://www.eso.org/public/usa/news/eso1524/]
[https://en.wikipedia.org/wiki/Sextans]
No comments:
Post a Comment