Hydrus is a small constellation in the deep southern sky. It was first depicted on a celestial atlas by Johann Bayer in his 1603 Uranometria. The French explorer and astronomer Nicolas Louis de Lacaille charted the brighter stars and gave their Bayer designations in 1756. Its name means ‘male water snake,’ as opposed to Hydra, a much larger constellation that represents a female water snake. It remains below the horizon for most Northern Hemisphere observers.
Irregular in shape, Hydrus is bordered by Mensa to the southeast, Eridanus to the east, Horologium and Reticulum to the northeast, Phoenix to the north, Tucana to the northwest and west, and Octans to the south; Lacaille had shortened Hydrus’ tail to make space for this last constellation he had drawn up. Covering 243 square degrees and 0.589% of the night sky, it ranks 61st of the 88 constellations in size. In the equatorial coordinate system, the right ascension coordinates of the constellation lie between 00h 06.1m and 04h 35.1m, while the declination coordinates are between −57.85° and −82.06°. As one of the deep southern constellations, it remains below the horizon at latitudes north of the 30th parallel in the Northern Hemisphere, and is circumpolar at latitudes south of the 50th parallel in the Southern Hemisphere. Hydrus culminates at midnight around the 26th of October.
Hydrus shown by Johann Bode in his Uranographia (1801). The object labelled Nubecula Minor, at centre, is the Small Magellanic Cloud. Part of the Large Magellanic Cloud, Nubecula Major, is visible in the bottom right corner.
Hydrus was one of the twelve constellations established by the Dutch astronomer Petrus Plancius from the observations of the southern sky by the Dutch explorers Pieter Dirkszoon Keyser and Frederick de Houtman, who had sailed on the first Dutch trading expedition, known as the Eerste Schipvaart, to the East Indies. It first appeared on a 35-cm (14 in) diameter celestial globe published in 1598 in Amsterdam by Plancius with Jodocus Hondius. The first depiction of this constellation in a celestial atlas was in the German cartographer Johann Bayer’s Uranometria of 1603. De Houtman included it in his southern star catalogue the same year under the Dutch name De Waterslang, ‘The Water Snake,’ it representing a type of snake encountered on the expedition rather than a mythical creature. The French explorer and astronomer Nicolas Louis de Lacaille called it l’Hydre Mâle on the 1756 version of his planisphere of the southern skies, distinguishing it from the feminine Hydra. The French name was retained by Jean Fortin in 1776 for his Atlas Céleste, while Lacaille Latinised the name to Hydrus for his revised Coelum Australe Stelliferum in 1763.
[http://stars.astro.illinois.edu/sow/hyi-t.html]
[http://www.shoalhavenastronomers.asn.au/constellations/hydrus/]
Keyzer and de Houtman assigned 15 stars to the constellation in their Malay and Madagascan vocabulary, with a star that would be later designated as Alpha Hydri marking the head, Gamma the chest and a number of stars that were later allocated to Tucana, Reticulum, Mensa and Horologium marking the body and tail.
Located at the northern edge of the constellation and just southwest of Achernar is Alpha Hydri, a white sub-giant star of magnitude 2.9, situated 72 light-years from Earth. Of spectral type F0IV, it is beginning to cool and enlarge as it uses up its supply of hydrogen. It is twice as massive and 3.3 times as wide as our sun and 26 times more luminous. A line drawn between Alpha Hydri and Beta Centauri is bisected by the south celestial pole.
Beta Hydri, the brightest star in Hydrus, is a yellow star of apparent magnitude 2.8, lying 24 light- years from Earth. It has about 104% of the mass of the Sun and 181% of the Sun’s radius, with more than three times the Sun’s luminosity. The spectrum of this star matches a stellar classification of G2 IV, with the luminosity class of ‘IV’ indicating this is a subgiant star. As such, it is a slightly more evolved star than the Sun, with the supply of hydrogen fuel at its core becoming exhausted. It is the nearest subgiant star to the Sun and one of the oldest stars in the solar neighbourhood. Thought to be between 6.4 and 7.1 billion years old, this star bears some resemblance to what the Sun may look like in the far distant future, making it an object of interest to astronomers. It is also the closest bright star to the south celestial pole.
Alpha Hydri is the second brightest star in the constellation of Hydrus. It is readily visible to the naked eye with an apparent visual magnitude of +2.9. It is sometimes informally known as the Head of Hydrus. This should not be confused with Alpha Hydrae (Alphard) in the constellation Hydra. Alpha Hydri is one of only three stars in the constellation Hydrus that are above the fourth visual magnitude. This star can be readily located as it lies to the south and west of the prominent star Achernar in the constellation Eridanus.
Alpha Hydri is located at a distance of about 71.8 light-years (22.0 parsecs) from Earth. This subgiant star is 80% larger and twice as massive as the Sun, with a stellar classification of F0 IV. It is about 810 million years old and is radiating 32 times the Sun’s luminosity from its outer atmosphere at an effective temperature of 7,077 K. Alpha Hydri emits X-rays similar to Altair.
In the southeastern corner of the constellation is Gamma Hydri, a red giant of spectral type M2III located 214 light-years from Earth. It is a semi-regular variable star, pulsating between magnitudes 3.26 and 3.33. Observations over five years were not able to establish its periodicity. An ageing star, it is around 1.5 to 2 times as massive as our Sun, yet has expanded to have about 60 times the Sun’s diameter. It shines with about 655 times the luminosity of our Sun.
Eta Hydri is an optical double, composed of Eta1 and Eta2. Eta1 is a blue-white main sequence star of spectral type B9V that was suspected of being variable, and is located just over 700 light-years away. Eta2 has a magnitude of 4.7 and is a yellow giant star of spectral type G8.5III around 218 light-years distant, which has evolved off the main sequence and is expanding and cooling on its way to becoming a red giant. Calculations of its mass indicate it was most likely a white A-type main sequence star for most of its existence, around twice the mass of our Sun. A planet, Eta2 Hydri b, greater than 6.5 times the mass of Jupiter was discovered in 2005, orbiting around Eta2 every 711 days at a distance of 1.93 astronomical units (AU).
Three other systems have been found to have planets, most notably the Sun-like star HD 10180, which has seven planets, plus possibly an additional two for a total of nine- as of 2012 more than any other system to date, including the Solar System:
The planetary system around the Sun-like star HD 10180 (artist’s impression)
Astronomers using ESO’s world-leading HARPS instrument have discovered a planetary system containing at least five planets, orbiting the Sun-like star HD 10180. The researchers also have tantalizing evidence that two other planets may be present, one of which would have the lowest mass ever found. This would make the system similar to our Solar System in terms of the number of planets (seven as compared to the Solar System’s eight planets). Furthermore, the team also found evidence that the distances of the planets from their star follow a regular pattern, as also seen in our Solar System.
“We have found what is most likely the system with the most planets yet discovered,” says Christophe Lovis, lead author of the paper reporting the result. “This remarkable discovery also highlights the fact that we are now entering a new era in exoplanet research: the study of complex planetary systems and not just of individual planets. Studies of planetary motions in the new system reveal complex gravitational interactions between the planets and give us insights into the long-term evolution of the system.”
The team of astronomers used the HARPS spectrograph, attached to ESO’s 3.6-metre telescope at La Silla, Chile, for a six-year-long study of the Sun-like star HD 10180, located 127 light-years away in the southern constellation of Hydrus (the Male Water Snake). HARPS is an instrument with unrivalled measurement stability and great precision and is the world’s most successful exoplanet hunter.
Thanks to the 190 individual HARPS measurements, the astronomers detected the tiny back and forth motions of the star caused by the complex gravitational attractions from five or more planets. The five strongest signals correspond to planets with Neptune-like masses- between 13 and 25 Earth masses- which orbit the star with periods ranging from about 6 to 600 days. These planets are located between 0.06 and 1.4 times the Earth-Sun distance from their central star.
“We also have good reasons to believe that two other planets are present,” says Lovis. One would be a Saturn-like planet (with a minimum mass of 65 Earth masses) orbiting in 2200 days. The other would be the least massive exoplanet ever discovered, with a mass of about 1.4 times that of the Earth. It is very close to its host star, at just 2 percent of the Earth-Sun distance. One ‘year’ on this planet would last only 1.18 Earth-days.
“This object causes a wobble of its star of only about 3 km/hour- slower than walking speed- and this motion is very hard to measure,” says team member Damien Ségransan. If confirmed, this object would be another example of a hot rocky planet, similar to Corot-7b.
The newly discovered system of planets around HD 10180 is unique in several respects. First of all, with at least five Neptune-like planets lying within a distance equivalent to the orbit of Mars, this system is more populated than our Solar System in its inner region, and has many more massive planets there. Furthermore, the system probably has no Jupiter-like gas giant. In addition, all the planets seem to have almost circular orbits.
So far, astronomers know of fifteen systems with at least three planets. The last record-holder was 55 Cancri, which contains five planets, two of them being giant planets. “Systems of low-mass planets like the one around HD 10180 appear to be quite common, but their formation history remains a puzzle,” says Lovis.
Using the new discovery as well as data for other planetary systems, the astronomers found an equivalent of the Titius–Bode law that exists in our Solar System: the distances of the planets from their star seem to follow a regular pattern. “This could be a signature of the formation process of these planetary systems,” says team member Michel Mayor.
Another important result found by the astronomers while studying these systems is that there is a relationship between the mass of a planetary system and the mass and chemical content of its host star. All very massive planetary systems are found around massive and metal-rich stars, while the four lowest-mass systems are found around lower-mass and metal-poor stars. Such properties confirm current theoretical models.
[https://www.eso.org/public/news/eso1035/]
SMSS J031300.36-670839.3
SMSS J031300.36-670839.3 is a star in the Milky Way at a distance of 6,000 light years from Earth. With an age of approximately 13.6 billion years, it is one of the oldest stars known. Another star, HD 140283, is considered to be older, but there is uncertainty in values of its age. This makes SM0313 the oldest known star with an accurate determination of its age. The star formed only about 100 million years after the Big Bang, and has been shining for 13.6 billion years. The star’s very low upper limit of iron of less than one ten millionth the iron level of the Sun, suggests that it is one of the first Population II stars, formed from a gas cloud enriched by some of the very first (Population III) stars. SMSS J031300.36-670839.3 also has a much higher carbon supply compared to iron, more than a thousand times greater. Apart from hydrogen, which appeared in the Big Bang, the star also contains carbon, magnesium, and calcium which could have been formed in a low energy supernova, and indicates that the supernovae of the first generation of stars may not have been as powerful as previously thought.
[https://en.wikipedia.org/wiki/SMSS_J031300.36-670839.3]
Comparison of VW Hydri (T) during an outburst
[https://sites.google.com/site/ivansastronomicalprojects/project-definition/vw_hyi/vw_hyi_finder]
VW Hydri is a dwarf nova of the SU Ursae Majoris type, a star system that consists of a white dwarf and another generally cool star. It is one of the brightest dwarf novae systems in the sky. These systems are characterized by frequent eruptions and less frequent supereruptions. The former are smooth, while the latter exhibit short ‘superhumps’ of heightened activity. The white dwarf sucks matter from the other star onto an accretion disc and periodically erupts, reaching apparent magnitude 8.4 in superoutbursts, 9.0 in normal outbursts and remaining at magnitude 14.4 when quiet. Normal outbursts occur every 27.3 days and last for 1.4 days, while superoutbursts happen 179 days and last for 12.6 days.
[https://en.wikipedia.org/wiki/VW_Hydri]
NGC 1466
Located mostly in Dorado, the Large Magellanic Cloud extends into Hydrus. The globular cluster NGC 1466 is an outlying component of the galaxy. It has an integrated visual magnitude of 11.4, and spans an apparent size of 3.50 arc minutes. It has a reddening corrected distance modulus of 18.43±0.15, corresponding to a distance of 48.5 kpc. The cluster has a mass of about 140,000 times the mass of the Sun. It is an old cluster, having an estimated age of 13.1 billion years.
There are a total of 49 known and one candidate RR Lyrae variable stars in the cluster, as of 2011, with average periods between 0.591 days for RR Lyrae type ab and 0.335 days for RR Lyrae type c. Twelve other variables have been identified, including two long-period variables and a Cepheid variable.
[https://en.wikipedia.org/wiki/NGC_1466]
NGC 602 is a region composed of an emission nebula and a young, bright open cluster of stars that is an outlying component on the eastern edge of the Small Magellanic Cloud:
NGC 602: Taken Under the ‘Wing’ of the Small Magellanic Cloud
The Small Magellanic Cloud (SMC) is one of the Milky Way’s closest galactic neighbors. Even though it is a small, or so-called dwarf galaxy, the SMC is so bright that it is visible to the unaided eye from the Southern Hemisphere and near the equator. Many navigators, including Ferdinand Magellan who lends his name to the SMC, used it to help find their way across the oceans.
Modern astronomers are also interested in studying the SMC (and its cousin, the Large Magellanic Cloud), but for very different reasons. Because the SMC is so close and bright, it offers an opportunity to study phenomena that are difficult to examine in more distant galaxies.
New Chandra data of the SMC have provided one such discovery: the first detection of X-ray emission from young stars with masses similar to our Sun outside our Milky Way galaxy. The new Chandra observations of these low-mass stars were made of the region known as the ‘Wing’ of the SMC. In this composite image of the Wing the Chandra data is shown in purple, optical data from the Hubble Space Telescope is shown in red, green and blue and infrared data from the Spitzer Space Telescope is shown in red.
Astronomers call all elements heavier than hydrogen and helium- that is, with more than two protons in the atom’s nucleus- ‘metals.’ The Wing is a region known to have fewer metals compared to most areas within the Milky Way. There are also relatively lower amounts of gas, dust, and stars in the Wing compared to the Milky Way.
Taken together, these properties make the Wing an excellent location to study the life cycle of stars and the gas lying in between them. Not only are these conditions typical for dwarf irregular galaxies like the SMC, they also mimic ones that would have existed in the early Universe.
Most star formation near the tip of the Wing is occurring in a small region known as NGC 602, which contains a collection of at least three star clusters. One of them, NGC 602a, is similar in age, mass, and size to the famous Orion Nebula Cluster. Researchers have studied NGC 602a to see if young stars- that is, those only a few million years old - have different properties when they have low levels of metals, like the ones found in NGC 602a.
Using Chandra, astronomers discovered extended X-ray emission, from the two most densely populated regions in NGC 602a. The extended X-ray cloud likely comes from the population of young, low-mass stars in the cluster, which have previously been picked out by infrared and optical surveys, using Spitzer and Hubble respectively. This emission is not likely to be hot gas blown away by massive stars, because the low metal content of stars in NGC 602a implies that these stars should have weak winds. The failure to detect X-ray emission from the most massive star in NGC 602a supports this conclusion, because X-ray emission is an indicator of the strength of winds from massive stars. No individual low-mass stars are detected, but the overlapping emission from several thousand stars is bright enough to be observed.
The Chandra results imply that the young, metal-poor stars in NGC 602a produce X-rays in a manner similar to stars with much higher metal content found in the Orion cluster in our galaxy. The authors speculate that if the X-ray properties of young stars are similar in different environments, then other related properties- including the formation and evolution of disks where planets form- are also likely to be similar.
X-ray emission traces the magnetic activity of young stars and is related to how efficiently their magnetic dynamo operates. Magnetic dynamos generate magnetic fields in stars through a process involving the star’s speed of rotation, and convection, the rising and falling of hot gas in the star’s interior.
The combined X-ray, optical and infrared data also revealed, for the first time outside our Galaxy, objects representative of an even younger stage of evolution of a star. These so-called ‘young stellar objects’ have ages of a few thousand years and are still embedded in the pillar of dust and gas from which stars form, as in the famous ‘Pillars of Creation’ of the Eagle Nebula. A labeled version shows the location of these young stellar objects (the small circle in the image above).
[http://chandra.harvard.edu/photo/2013/ngc602/]
PGC 6240, known as the White Rose Galaxy, is a giant spiral galaxy in the constellation of Hydrus, surrounded by shells resembling rose petals:
Hubble image of PGC 6240
The beautiful, petal-like shells of galaxy PGC 6240 are captured here in intricate detail by the NASA/ESA Hubble Space Telescope, set against a sky full of distant background galaxies. PGC 6240 is an elliptical galaxy approximately 350 000 000 light years away in the southern constellation of Hydrus (The Water Snake). It is orbited by a number of globular clusters that contain both young and old stars- thought to be a result of a galactic merger in the recent past.
[https://en.wikipedia.org/wiki/Hydrus]
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