TW Piscis Austrini
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© Torben Krogh & Mogens Winther,
(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)
TW PsA is an orange-red dwarf
star, like Epsilon Eridani at
left center of meteor. (A Digitized
Sky Survey image of TW PsA
from the Nearby Stars Database
may become available.)
This star is located about 24.9 light-years (ly) away from our Sun, Sol, at the eastern edge (22:56:24.1-31:33:56.0, ICRS 2000.0) of Constellation Pisces Austrinus (or Australis), the Southern Fish -- southeast of Fomalhaut (Alpha Piscis Austrini) and northeast of Delta Piscis Austrini. Although smaller and much dimmer than Sol, some Humans may able to see TW Piscis Austrini (TW PsA) without a telescope in Earth's night sky. According to the SIMBAD Astronomical Database, it is also a flare star.
The proximity of TW Piscis to Fomalhaut was first reported in 1897 by Thomas Jefferson Jackson See (1866-1962). Subsequently, the two stars have been determined to be distant physical companions (D. Barrado y Navascues, 1998). However, another K5 dwarf (LTT 8273) later observed in proper motion studies is now believed to be an optical companion, although it also may be a remaining member of a low-density star cluster including Fomalhaut, Vega, and Castor that has gradually dispersed over hundreds of millions of years.
As a relatively bright star in Earth's night sky, TW PsA is catalogued as Harvard Revised (HR) 8721, a numbering system derived from the 1908 Revised Harvard Photometry catalogue of stars visible to many Humans with the naked eye. The HR system has been preserved through its successor, the Yale Bright Star Catalogue -- updated and expanded through the hard work of E. Dorrit Hoffleit and others. HR 8721 is also listed as HD 216803 in the Henry Draper (1837-82) Catalogue with extension (HDE), a massive photographic stellar spectrum survey carried out by Annie Jump Cannon (1863-1941) and Edward Charles Pickering (1846-1919) from 1911 to 1915 under the sponsorship of a memorial fund created by Henry's wife, Anna Mary Palmer. (More discussion on star names and catalogue numbers is available from Alan MacRobert at Sky and Telescope and from Professor James B. Kaler's Star Names.)
TW Piscis Austrini is a orange-red main sequence dwarf star of spectral and luminosity type K4-5 Vpe. This star may have around 81 percent of Sol's mass, 76 to 85 percent of its diameter (Pasinetti-Fracassini et al, 2001), and 12 to 13 percent of its visual luminosity. It may be only about 200 million years old (D. Barrado y Navascues, 1998).
The Yale Bright Star Catalogue's notes entry for HR 8721 indicate that TW PsA is a BY Draconi-type variable (Vogt et al, 1983) that varies in apparent visual magnitude from 6.44 to 6.49 over 10 days and shares common proper motion with Fomalhaut (HR 8728). As a flare star, TW Piscis Austrini is its variable star designation. Other useful star catalogue designations for the star include: TW PsA, HR 8721*, Gl 879, Hip 113283, HD 216803, CD-32 17321, CP(D)-32 6550, SAO 214197, and LTT 9283.
Hunt for Substellar Companions
Since TW Piscis Austrini is sort of like a distant cousin to Sol, some speculate whether it might just be bright enough to support Earth-type life on a planet lucky enough to orbit in its water zone. The distance from TW PsA where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.36 AU -- within Mercury's orbital distance in the Solar System. At that distance from the star, such a planet would have an orbital period of about 88 days -- about a fourth of an Earth year. Astronomers would find it very difficult to detect using present methods.
Life Around a Flare Star
Many dim, red (M) and some orange-red (K) dwarf stars exhibit unusually violent flare activity for their size and brightness. These flare stars are actually common because red dwarfs make up more than half of all stars in our galaxy. Although flares do occur on the Sun every so often, the amount of energy released in a Solar flare is small compared to the total amount of energy that Sol produces. However, a flare the size of a solar flare occurring on a orange-red dwarf star (such as TW Piscis Austrini) that normally has less than 14 percent of than Sol's luminosity would be more noticeable.
High resolution and jumbo images (Benz et al, 1998).
TW PsA is a flare star, like UV Ceti (Luyten
726-8 B) shown flaring at left. UV Ceti is an extreme
example of a flare star that can boost its brightness by
five times in less than a minute, then fall somewhat slower
back down to normal luminosity within two or three
minutes before flaring suddenly again after several hours.
Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a few minutes to reach peak brightness, and more than one flare can occur at a time. Moreover, in addition to bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000 times as many X-rays as a comparably-sized Solar flare on our own Sun, and so flares would be lethal to Earth-type life on planets near the flare star. Hence, Earth-type life around flare stars may be less likely because their planets must be located very close to dim orange-red dwarfs to be warmed sufficiently by star light to have liquid water (about 0.36 AU for TW Piscis Austrini), which makes flares even more dangerous around such stars. In any case, the light emitted by late orange-red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.
The following star systems are located within 10 light-years of TW Piscis Austrini.
|Star System||Spectra &|
|CD-23 17699||K5-M0 V||4.4|
|FK Aquarii AabB||M0-2 Ve |
|L 362-81||DA5 /VII||7.3|
|L 788-34||M4.5 V||7.4|
|LHS 1070||M5.5 V||8.4|
|LTT 20||M3.5 V||9.8|
|L 499-56||M3.5 V||10.0|
Up-to-date technical summaries on these stars can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS, and the Nearby Stars Database. Additional information may be available at Roger Wilcox's Internet Stellar Database.
Also known as Piscis Australis, Piscis Austrinus is supposed to represent a fish lying on its back, drinking in the waters pouring from the jars of Aquarius. Known since ancient times, the constellation may have been the original Constellation Pisces, referring to the Assyrian Fish God Dagon and the Babylonian God Oannes. In Arabic, it is the Constellation Al Hut al Janubiyy, the Large Southern Fish. For more information about the stars and objects in this constellation, go to Christine Kronberg's Pisces Austrinus. For an illustration, see David Haworth's Pisces Australis (or Austrinus).
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
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