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Zeta-tagged Stars

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All known stars have a scientific name to identify them. Some have a letter from the Greek alphabet combined with the genitive form of the constellation. This is known as the 'Bayer designation' after the German astronomer Johann Bayer (1572-1625) who devised the system for his 1603 star atlas Uranometria. Bayer assigned the brightest star in each constellation the 'alpha' tag, the second-brightest was the 'beta' star, followed by gamma, delta, epsilon and all the way to omega. Zeta (ζ) is the letter for (roughly - Bayer did make a few mistakes in his calculations) the sixth-brightest star of any given constellation, so even the smallest in area, Crux, has a zeta star. As there are 88 official constellations we couldn't possibly cover them all, so here are just a few extraordinary stars tagged zeta.

Zeta Ophiuchi

Runaway star zeta Ophiuchi

Shining at +2.5 magnitude, zeta Ophiuchi is the third-brightest star in the serpent-bearer constellation Ophiuchus. The blue giant is a runaway star, travelling at 24km a second. Astronomers muse that it was the victim of a dramatic divorce; when its more massive binary companion went supernova, zeta Ophiuchi was kicked out of the system by the massive blast. As it ploughs through interstellar space it is creating a shockwave which, although invisible to the human eye, has been imaged in the infrared by the Spitzer Space Telescope, allowing us to marvel at its beauty. Its previous companion is now a solitary pulsar, a rapidly spinning neutron star which will probably last for eternity. Its ejected former partner won't fare so well; zeta Ophiuchi is still burning hydrogen but is losing mass in its wake, so it won't be a runaway star forever.

Image of runaway star zeta Ophiuchi provided by NASA

Zeta Orionis

A diagram of Orion by Gnomon.

Scottish astronomer Williamina Fleming identified red emission nebula IC 434, the Horsehead Nebula, in Orion in 1888, describing it thus:

A large nebulosity [IC 434] extending nearly south from zeta Orionis for about 60 minutes. More intense and well marked on the following side, with a semi-circular indentation 5 minutes in diameter 30 minutes south of zeta. Good plates of this region show this object [the Horsehead Nebula], and it has been used here as a test for some time.

Zeta Orionis is better known by its Arabian name Alnitak ('girdle'), one of the three stars which form the asterism Orion's Belt. From the Northern Hemisphere Alnitak is the lowest star and appears on the left of the other two. Viewed from the Southern Hemisphere, Orion is rotated by 180°, so Alnitak is on the other side of Orion's waist.

Zeta Scorpii

Zeta Scorpii is a line-of-sight double star; zeta2 Scorpii is an orange giant star some 150 light years away but zeta1 Scorpii is much further away, estimated at up to 5,700 light years distance! The only reason we can see zeta1 Scorpii is because it is one of the most luminous stars that we know of in our galaxy, probably a million times our Sun's luminosity, and up to 60 times the mass. Zeta1 Scorpii is a blue hypergiant of the same LBV (luminous blue variable) class as eta Carinae, which is expelling so much stellar material that it has formed its own nebula. Stars which are as massive as this live fast and die young; they only exist for a few million years before exploding in a spectacular death called a supernova.

Zeta1 Scorpii and zeta2 Scorpii are part of open star cluster NGC 6231, enigmatically named the Northern Jewel Box. Sir Patrick Moore (1923-2012) included the Northern Jewel Box in his catalogue for amateur astronomers (The Caldwell Catalogue) as Caldwell 76.

Zeta Ursae Majoris

A diagram of Ursa Major showing the Plough asterism.

Zeta Ursae Majoris is more commonly known as Mizar, one of seven stars forming possibly the most easily recognised asterism, which has a different name depending upon your location and culture. In the UK this star grouping is called 'the Plough', in the USA it is known as 'the Big Dipper', German people call it Großer Wagen ('Great Cart'), Native Americans know it as the hunting party (of the bear), and it is known as Saptarshi ('the seven rishes' - sages or wise men) by Hindu communities. Johann Bayer made an exception to his nomenclature rule in the case of these seven stars; he designated them in order that they appeared in the distinctive asterism so their magnitudes don't match their Greek-letter order, but they are the seven brightest stars within Ursa Major 'the Great Bear'. The brightest star at +1.7 magnitude is tagged epsilon (Alioth), followed by alpha (Dubhe) at +1.8 magnitude, eta (Alkaid) just a little dimmer at +1.85 magnitude, then Mizar (zeta) is 4th-brightest at +2.23 magnitude. Mizar was the first-identified double star in history (in 1650). Thanks to technological advances we now know that it is a multiple star system comprising four gravitationally-bound white dwarfs which are themselves circumorbited by Alcor (80 Ursae Majoris1) and its binary companion, making it a sextuplet stellar system.

Zeta Pegasi

Blue giant star zeta Pegasi holds the name Homam, which means 'high spirit'. It has a pulsation frequency of once every 23 hours, so it is classed as an SPB (slowly pulsating B-type) variable star. Homam is not gravitationally-bound to any other star or grouping; it evolved alone. There are two stars which appear close to Homam from our vantage point, but the relationship is merely line-of-sight. There have been no extrasolar planets detected (up to the time of writing, 2014) and there's no circumstellar disc around Homam, so there's no possibility of any planets forming to provide a family for this lone star. Besides its zeta tag by Bayer, Homam bears a Flamsteed designation of 42 Pegasi and it happens to be the 'zeta of these zetas' – that is, it's the 6th-brightest of the 11 stars in this Entry.

Zeta Puppis

Zeta Puppis, known as Naos ('ship'), is one of the hottest stars known at 42,000°C, and it is the closest blue supergiant to our Solar System at just over a thousand light years distant. Despite its zeta designation, Naos happens to be the brightest star in the constellation Puppis 'the Stern' at magnitude +2.2. This is because the stars in Puppis used to belong to the now-demoted constellation Argo Navis, which honoured the Argo, the fabled vessel of Jason and his Argonauts. Argo Navis was one of the original 48 constellations of Ptolemy's time. In the 18th Century Argo Navis was broken up into three separate 'modern' constellations, Carina 'the Keel', Vela 'the Sail' and Puppis, by French astronomer Nicolas Louis de La Caille2 (1713-62).

Zeta Tucanae

Zeta Tucanae is a yellow-white dwarf star about 28 light years3 away, which makes it a close neighbour in galactic terms. Its mass and radius are virtually identical to our own Sun, but zeta Tucanae is hotter and more luminous. This star has a debris disc calculated at approximately 2.3 AU4, which means that planets could be forming there. Zeta Tucanae was the subject of a 2006 paper called Astrobiologically Interesting Stars Within 10 Parsecs of the Sun5 by Brazilian astronomers G Porto de Mello, EF del Peloso and L Ghezzi.

Zeta Cancri

Zeta Cancri has the common name Tegmen, which means 'crab's shell'. The zeta Cancri system comprises at least four stars, categorised zeta Cancri A, B, C and D. Zeta Cancri A and B are yellow-white dwarfs, C is a yellow dwarf similar to our own Sun and D is thought to be a red dwarf, possibly a binary pair. From our vantage point zeta Cancri looks like one point of light; it takes a powerful telescope to separate them. Due to its position on the ecliptic6, zeta Cancri is interesting to the field astronomer because there are times when it can be occulted by the Moon and other Solar System members.

Zeta Geminorum

Zeta Geminorum is a supergiant Delta Cepheid-type variable star, which means that it belongs to a class, named after the prototype delta Cephei, of special stars which help astronomers determine distance. Nowadays the star is a member of Gemini 'the twins', although it still bears the Arabian name Mekbuda, meaning 'curled up lion's paw', as it was once part of an ancient constellation depicting a lion at rest.

Zeta Canis Majoris

Zeta Canis Majoris is part of the 'great dog' constellation Canis Major. The star has an Arabian name Furud meaning 'bright, single one'. It marks a toe on the back right paw of the canine star pattern. Furud is not a lone star but is actually a binary system; the top dog is a hot, blue variable star more than seven times the mass and over 3,000 times more luminous than the Sun. Furud's stellar companion has not been directly observed but astronomers can tell there's another star tugging on the primary due to the distortions in its spectrum.

Zeta Reticuli

Zeta Reticuli is a wide binary star system residing in this obscure southern constellation. The two yellow dwarfs orbit each other in a period of about 3,750 years. There's nothing particularly remarkable about this star pairing unless you delve into the realm of UFOs, close encounters and alien abduction conspiracy theories, where zeta Reticuli and, in particular, grey-skinned 'Zetans', are the stuff of legend. But that's another story.

1This is a Flamsteed designation, devised by the first Astronomer Royal, John Flamsteed (1646–1719), who catalogued the main stars of each constellation by number, eg 55 Cancri.2La Caille's posthumous catalogue Coelum Australe Stelliferum described 14 new constellations and 42 nebulous objects among almost 10,000 southern stars from information garnered on a 1751–54 expedition to the Cape of Good Hope.3A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.4AU means astronomical unit, the distance between Earth and the Sun – 150 million kilometres or 93 million miles.5One parsec is equivalent to 3.26 light years, so ten parsecs equates to 32.6 light years.6The ecliptic is the apparent path the Sun takes across the sky, viewed from Earth.

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