Libra, Scorpius & Sagittarius
Capricornus, Aquarius & Pisces
The Zodiacal Sky
Bright Stars and Deep-Sky Objects in
Aries, Taurus, Gemini
and surrounding Constellations
by Martin J Powell
Star map showing the region of the night sky around the constellations Aries, Taurus and Gemini (click on the thumbnail for the full-size version, 132 KB). The star map applies to observers in the Northern hemisphere (i.e. North is up); Southern hemisphere observers should invert the map (South up) to obtain a correct orientation. The faintest stars on the map have an apparent magnitude of about +4.8. A printer-friendly version of this map is available here (67 KB). Co-ordinates of Right Ascension (astronomical longitude) and Declination (astronomical latitude) are marked around the border of the chart. The celestial equator (declination = 0°) is shown in dark blue and the ecliptic (the apparent path of the Sun, which the Moon and planets follow closely) is shown in dark yellow (to see where these celestial bodies are currently positioned, refer to the Naked-Eye Planets page). Also included on the star map are numerous deep-sky objects (objects beyond our Solar System) which can be seen through binoculars and small telescopes; these are detailed in the main text below.
This article describes the brighter stars and deep-sky objects in the constellations Aries, Taurus, Gemini and their surrounding constellations (covering the region of the zodiacal band from Right Ascension = 1 hour through to 8 hours). All of the brighter stars can be seen with the naked-eye under clear, dark skies, however most of the deep-sky objects require the assistance of binoculars and/or small telescopes. The fascinating origins and mythologies lying behind the zodiacal constellations will also be discussed.
The zodiac constellations Aries, Taurus and Gemini occupy what is arguably the most interesting region of the entire night sky. Rich in bright stars, asterisms, star clusters and nebulae, the beginner astronomer is well advised to become familiar with the constellations surrounding Orion and Taurus since these are ideal signposts to other constellations. Orion itself contains convenient stellar pointers towards the brightest stars in Taurus and Canis Major. A line drawn North-westwards through Orion's Belt leads one to the orange-red star Aldebaran in Taurus, the Bull. Extending the line of the Belt in the opposite direction (South-eastwards) leads one to the brightest star in the night sky, Sirius in Canis Major, the Greater Dog. Finally, a curved line extended North-eastwards from Rigel (at the South-western corner of Orion) through Mintaka (the Western star in the Belt) and Betelgeuse leads one to the star Alhena in South-western Gemini.
The six brightest stars on the star chart are (in order of descending brightness):
Note that the brightest star of all - the aforementioned Sirius ( CMa or Alpha Canis Majoris, mag. -1.4) - is not included in the above listing since, with a declination of -16°.7, it is positioned too far South of the celestial equator to appear on the chart (the Southern cut-off declination of the chart is about -7°). Sirius does however dominate this region of the sky and it cannot be overlooked in any discussion of the constellations here.
Three of these stars - namely Sirius, Procyon and Betelgeuse - form a near-equilateral triangle in the night sky which is sometimes referred to as The Winter Triangle in the Northern hemisphere because these stars are prominent in Northern hemisphere winter evenings (i.e. summer evenings in the Southern hemisphere).
Procyon, in the constellation Canis Minor (the Lesser Dog) is the eighth brightest star in the night sky and the fourth brightest Northern celestial hemisphere star. Procyon is a Greek name meaning 'before the dog', referring to the fact that it rises before the 'Dog Star', i.e. Sirius ( Canis Majoris, mag. -1.4, the brightest star in the night sky). Canis Minor contains little of interest for the binocular or telescopic observer, except for a few double stars. Procyon itself is interesting in that it has a companion white dwarf star (Procyon B), of very high density, orbiting the primary star (Procyon A) every 41 years. Procyon B is 11th magnitude and lies too close to Procyon A to be detectable in anything but professional telescopes; indeed, it was not discovered until 1896. Procyon is 11.4 light years distant (where 1 light year = 63,240 Astronomical Units), meaning that it takes light 11.8 years to traverse the distance, and is one of the nearest stars to the Earth. Sirius is however closer still, at 8.6 light years and co-incidentally it also has a white dwarf companion. The Eastern section of Canis Minor is shown on the Cancer-Leo-Virgo star map.
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Aries, the Ram, is a small but historically important zodiac constellation. Its ovine origin is, however, surprisingly obscure. The Babylonians (ca. second millennium BC) are generally considered to have invented the association, however its origins may be much older. To the ancient Egyptians the ram was represented by Amon, king of the gods, who was often portrayed as a ram or a man with a ram's head. They may have called this constellation 'srt' (pronounced 'srat') meaning 'the sheep', though there is some doubt about its exact placement in the night sky. The white star Sheratan ( Arietis or Beta Arietis, mag. +2.6) was referred to by the Egyptians as 'The Goose's Head' in their list of 'hour-stars' which marked out the path of the Moon. Although morphologically a ram and goose are not alike, the Goose and Ram were the two sacred animals of the god Amun of Thebes, hence to the Egyptians they represented the same divine power. All twelve of the currently-recognised zodiac constellations do not appear in a Babylonian text until the late Seleucid period (ca. 1st century BC). 'The Ram' may therefore have been an Egyptian and not a Babylonian invention, dating perhaps from the third millennium BC.
Aries the Ram depicted in Johann Bayer's classic star atlas Uranometria from 1603 (click on thumbnail for larger version, 13 KB). This is a hand-coloured version of the original print, dating from 1641. The blue lines represent the ecliptical band (Image: Albion Prints)
In Mesopotamia the ram was also venerated as a cult figure, often appearing symbolically alongside Ea (or Enki), the god of water. Cuneiform tablets known as The 36 Stars (ca. 1100 BC) and MUL.APIN (ca. 1000 BC) list the 18 constellations of the Babylonian zodiac and a ram does not appear among them. Instead, there is a small asterism named 'lu.unga', meaning a 'hireling' (i.e. a 'hired man' or farm worker). The word 'lu' means 'sheep' and the constellation was associated with the Sumerian shepherd Dumuzi; it probably comprised the stars Hamal (see below) and Sheratan. Dumuzi (later known as Tammuz) was a god sent to the Underworld but allowed to revive for six months each year; he may thus have been considered an appropriate custodian of the spring equinox, which the constellation marked at the time. He was never illustrated by the Babylonians and this probably explains the lack of references to a ram in this part of the zodiac. The First Persian Empire (ca. 550 BC to 330 BC) called the constellation 'Varak' (the Lamb), as mentioned in the Bundahishn, the Zoroastrian story of Creation. To the ancient Greeks (ca. 3rd century BC), who called the constellation 'Krios', it represented a mythological golden-fleeced ram, for which Jason and his Argonauts embarked on their famous voyage.
Hamal ( Ari or Alpha Arietis, mag. +2.0) is the leading star in Aries; the name derives from the Arabic 'Al Rs al Hamal' ('the head of the sheep'), the abridged version (Al Hamal) having been their name for the constellation as a whole. Hamal is a yellow giant star which lies at a distance of 85 light years from Earth.
An interesting multiple star in Aries is Mesarthim (or Mesartim), also referred to as Ari (Gamma Arietis). It is a triple star, of which the two brighter components (1 Ari and 2 Ari) can be easily split in small telescopes at low magnification. They are blue-white and shine at almost the same magnitude (+4.6 and +4.7). The two stars are 204 light years distant and take over 5,000 years to orbit each other. Very conveniently, they are orientated precisely North-South, separated by 7".5 (i.e. 7.5 arcseconds, where 1 arcsecond = 1/3600th of a degree). This is much too close to be resolved by the naked-eye and it is too difficult for all but the highest-powered binoculars. Mesarthim is widely considered to be one of the prettiest double stars in the night sky. Mesarthim was found to be a double star by the English scientist Robert Hooke in 1664, at which time such multiple star systems were considered very rare.
Castor ( Gem)
Mesarthim ( Ari)
Sirius ( CMa)
Multiple tar Systems described in the text, recorded by the author with a video camera pointed into the eyepiece of an 8-inch aperture Schmidt-Cassegrain telescope in 2005. The separations and orientations of the multiple stars are shown approximately to scale (North is up and East to the left)). Sirius (the brightest star in the night sky) is technically a double star although its companion (Sirius B, mag +8.5) is currently too close to its primary to be resolved in most amateur telescopes.
In classical times, Aries was the first zodiac constellation to rise at dawn at the start of the Spring season, marking the start of the new year in ancient Greece. It subsequently became the first sign of the modern astrological zodiac (replacing the Pleiades in Taurus, which had marked the starting point of all the most ancient zodiacs). Today the position from which astronomers measure star and planet positions is often called the First Point of Aries and it is usually denoted by the Ram symbol (). It marks the point in the zodiac at which the Sun is positioned at the Vernal Equinox (i.e. the Spring equinox in the Northern hemisphere) when the Sun crosses the celestial equator heading Northwards. On modern-day star charts it marks 0 hours in Right Ascension and 0° of ecliptic longitude (from which planetary positions are measured in their orbits). Around 1,000 years BC the First Point of Aries was positioned in its namesake constellation. However, an effect called precession of the equinoxes causes the First Point of Aries to drift Westwards over time, making one complete revolution of the zodiac in about 25,770 years. In the intervening 2,000 years or so since the astrological charts were drawn up, its position has drifted from Aries into Pisces and it is now positioned a little to the South-east of the Circlet of Pisces (its location is marked by the symbol in this chart). Precession is caused by the slow 'wobbling' of the Earth in space, an effect much like the wobbling of a spinning top or gyroscope as it winds down. It causes not only the First Point of Aries to move but also the position of the celestial pole to move in a circular motion over time (today the North celestial pole is positioned near the star Polaris but this has not always been the case). The First Point of Aries is slowly drifting towards Aquarius, the Water Carrier, which it will enter around the year 2600.
Being the oldest domesticated animal, it is no surprise that a bull should find a representation among the stars. The origin of Taurus, the Bull, lies more certainly in Babylonia. Cylinder-seals dating from the fourth millennium BC often depict a variety of animals - including bulls, lions and dogs - fighting each other or being fought by gods or other local heroes. The earliest known story relating to Taurus appears in the Epic of Gilgamesh, a Mesopotamian poem dating from ca. 2100 BC. Tablet six tells the story of how Gilgamesh, king of Uruk (in modern-day Iraq), together with Enkidu, a wild man created by the gods, kill the Bull of Heaven which Ishtar (the Babylonian goddess of love and fertility) has sent in order to punish Gilgamesh for spurning her advances. The relationship between the poem and the constellation is confirmed because of the bull's appearance in The 36 Stars and MUL.APIN tablets referred to above. Both tablets refer to the V-shaped Hyades star cluster in Taurus (see below) by the name 'gud.an.na', meaning 'Bull of Heaven'. It was also the constellation in which the spring equinox Sun was positioned, marking the Sumerian new year. It seems likely, however, that the direction in which the Bull's face and horns faced in Babylonian times was in the opposite direction to that with which we are familiar today, facing Westwards rather than Eastwards. This change in direction of posture seems to have taken place in classical Greece.
However the tale which is more commonly related to Taurus comes from Greek mythology. Zeus (the ancient Greek king of the Gods) attempts to seduce Europa (a Phoenician woman of high lineage) by transforming himself into a tame white bull. Europa becomes enamoured with the bull, Zeus then reveals his true identity and subsequently Europa becomes the first queen of Crete and together they sire three children. In this myth, Zeus re-creates the shape of the bull ('Tauros') amongst the stars. The Romans absorbed this colourful tale into their own mythology, replacing Zeus with their own equivalent god Jupiter.
Gilgamesh and Enkidu kill the Bull of Heaven whilst Ishtar tries to prevent them. From a late Babylonian cylinder seal dating from the 7th century BC (click on the thumbnail for a larger version, 12 KB) (Image: The Trustees of the British Museum)
Marking the eye of Taurus, Aldebaran ( Tau or Alpha Tauri, mag. +0.8) is the brightest star in the constellation and it appears amongst the Hyades star cluster (though it is not actually part of it). Aldebaran is a red giant star with a diameter some 45 times that of our Sun, lying 65 light years away. To the naked-eye Aldebaran is notably orange, in contast to the orange-red tint of Betelgeuse. The name Aldebaran is thought to have derived from the Arabic Al Dabaran meaning 'the Follower' because its appearance follows the rising of the Pleiades cluster. An alternative theory, proposed in the early 20th century, is that the name means 'the forecaster', after Nabu, the Assyrian and Babylonian god of wisdom and writing. As keeper of the Tablets of Destiny, he forecasted the destiny of each individual, with the power to lengthen or shorten his or her lifespan; his name derives from the Semitic root 'nb', meaning 'to call or announce'. Aldebaran was known as the 'Star of the Tablet' in Babylonian times, which incidentally also became its name in China.
At the tip of Taurus' Northern horn is the star Nath, also known as Alnath or El Nath ( Tau or Beta Tauri, mag. +1.7). It is a giant blue star 142 light years from Earth. The name means 'the butting one', as befits a Bull's horn. Nath is shared with the neighbouring constellation of Auriga, the Charioteer, where it is known by the designation Aur (Gamma Aurigae). Nath is one of only two stars in the night sky which has two designations, the other being Sirrah ( Pegasi or Andromedae) on the Pegasus/Andromeda border.
The star Tau (Lambda Tauri), some 370 light years away, is a type of variable star called an eclipsing binary. The star's brightness varies from +3.4 to +4.0 (a fairly small range but detectable to the naked-eye) in about 4 days (or to be precise, 3.95295 days). Lambda Tauri is an eclipsing binary of the Algol type (of which, see below). The two stars are separated in space by about 15 million kms (10 million miles).
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Taurus features many interesting objects for the binocular and telescope user. Perhaps its most famous feature is the Pleiades star cluster Messier 45 (M45 or NGC 1435), also known as 'The Seven Sisters' after the seven daughters of Atlas and Pleione in Greek mythology. It is perhaps the most referenced star group in the history of world culture. In ancient Egyptian times, the Pleiades were referred to as 'Thousands' in their list of 'hour-stars' used as 'Moon-markers' (as described above when discussing Aries). The Babylonians knew them as 'Zappu' ('tuft of hair'), presumably referring to the bull's neck. The Chinese called them 'Mao' ('The Cock'), one of the 28 Chinese animal-based asterisms ('Hsiu') which followed the line of the celestial equator and which possibly date from the 24th century BC.
Around 700 BC, the Greek poet Hesiod referred to the Pleiades as 'Plein' ('The Sailing Stars') because their dawn rising took place during May, at the beginning of the navigating season. The cluster was also important to their agricultural calendar. In his famous agricultural poem 'Works and Days', Hesiod wrote:
"Begin your harvest when the Pleiads come
To rising, and your ploughing when they set."
The Arabs named them 'Al Thurayy', meaning 'the chandelier', a name which has also been linked with Hathor, the Egyptian goddess of the sky. Hathor was often depicted as a cow or a woman with a cow's head, and when she took the form of the 'Seven Hathors', she was associated with the Pleiades.
The Pleiades are the only asterism which is unambiguously mentioned in the Hebrew Bible, appearing twice in the Book of Job (ca. 6th century BC) and once in the Book of Amos (ca. 7th century BC). The original Hebrew word translated into 'Pleiades' is 'Kimah', which apparently means 'the heap'.
In India (ca. 3rd century BC) the group was called 'Krittika', meaning 'razor' or, metaphorically, 'dividing line', i.e. where the zodiac was considered to begin at that time. The Sanskrit name of the cluster ('Bahulah') means 'very many' and seems the most likely original name of the cluster in the Indian subcontinent.
In Mesoamerica the Mayans (1st millennium AD) called the cluster 'Tzab-ek' meaning 'Rattlesnake's Tail' and the star group formed the basis of their sacred calendar, the 'Tzolk'in'. The Aztecs identified the cluster with 'Tianquiztli', the 'Marketplace', as recorded in the Florentine Codex (16th century AD). The midnight transit of the Pleiades near the zenith determined the date of the Aztecs' most important feast day, the 'Binding of the Years', a fire ceremony which took place every 52 years.
Resembling a mini Big Dipper in outline, the six brightest stars in the group are easily seen by most people although the seventh (Pleione) can prove a little difficult. Its brightest member Alcyone (mag. +2.8) is assigned a Bayer letter of its own ( Tau or Eta Tauri). The Pleiades are relatively young blue giant stars (around 100 million years old) which are embedded in clouds of interstellar dust. In fact, the clouds are moving through the cluster and they are not the source material of the cluster stars themselves. The nebulae glow because the light from the cluster reflects from them (i.e. they are reflection nebulae). Disappointingly for amateur astronomers the nebulosity - which is brightest around the star Merope - is barely visible through amateur instruments. Telescopes of about 200 mm (8 in) aperture or larger are required to detect haziness around the cluster's four brightest stars, along with their bluish-white coloration. The binocular and telescopic view of the cluster is spectacular, however, revealing many dozens of stars which are well below naked-eye magnitude. Because the cluster extends across an area of about 120' (i.e. 120 arcminutes, where 1 arcminute = 1/60th of a degree) it is best appreciated when seen through wide-field binoculars. The cluster is about 440 light years from Earth and contains at least 500 stars.
The Pleiades are the most spectacular open cluster in the night sky, however they are not well seen from high Southern hemisphere latitudes; at 45° South, for example, they reach a maximum altitude (as they cross the due North point) of only 20° above the horizon. Many consider the cluster IC 2602 in the constellation of Carina, the Keel, to be an equally spectacular Southern hemisphere equivalent - hence its common name of the Southern Pleiades.
The Pleiades (M45) Open Star Cluster in North-western Taurus (click on thumbnail for larger version, 19 KB). This approximates the view as seen through binoculars. Click here (28 KB) to reveal the star names. North is up and East is to the left.
The Hyades cluster (also known as Melotte 25) is a very large and 'loose' cluster when compared to the other star clusters in the night sky. Named after the Greek mythological daughters of Atlas and Aethra (half-sisters of the Pleiades), it is a V-shaped pattern of stars defining the Bull's face, with Aldebaran ( Tau) in the position of the Bull's eye. The name Hyades is derived from the ancient Greek word 'hyein' meaning 'To Rain' because their appearance in the night sky coincided with the rainy spring season. The Romans named them 'Suculae' meaning 'The Little Pigs', which appears to have been a mistranslation of the Greek word 'hys', meaning 'a sow'.
The cluster comprises about 200 stars over a 5° area of the sky - too large to be appreciated through telescopes, binoculars providing a much more encompassing view. The stars of the cluster lie at a distance of about 150 light years from Earth, however Aldebaran is much closer (65 light years) so it is clearly not a true member of the group - Aldebaran is simply a foreground star which happens to be in our line-of-sight with the cluster. The Hyades are around 790 million years old (much older than the Pleiades) and astronomers believe that it was formed from the same nebulous cloud as the Beehive Cluster (M44) in Cancer, the Crab (see the Cancer-Leo-Virgo page), the two being of roughly the same age. The Hyades is the nearest and brightest open cluster to the Earth, however it is slowly increasing its distance from us, moving in the general direction of the star Betelgeuse in Orion.
The Hyades contains several double stars, a couple of which are discernible with the naked-eye. Positioned about half-way along the Southern arm of the V-shape, 1 Tauri and 2 Tauri are a very wide pair, separated by 337" (5'.6). Shining at magnitudes +3.4 and +3.8, the stars are white and yellow respectively. 1 Tauri is also the brightest member of the genuine cluster.
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Close to the tip of the Bull's Southern horn, about 1° North-west of the star Tau (Zeta Tauri, mag. +3.0) is the remnant of a supernova (a particularly large star which has exploded). The Crab Nebula (M1 or NGC 1952) is 6,500 light years distant and has been an historically important object to astronomers studying stellar evolution. In 1054 AD, Chinese court astronomers of the Sung Dynasty recorded the star's explosion, although of course they had no knowledge of its true nature. They referred to it as a 'guest star' and it shone so bright that it could be seen in daylight for 23 days! The Ancient Pueblo People of North America may also have recorded the event - in the form of a pictograph (rock painting) - at Chaco Canyon in New Mexico. The Crab Nebula is roughly oval in shape and has been expanding ever since the explosion, a process which has been carefully monitored by modern-day astronomers. At its centre is the stellar remnant of the explosion, known as a pulsar (the first such object to be discovered) though it is 16th magnitude and only visible in large telescopes. The pulsar (known as PSR 0531 +21) rotates 30 times a second and is a powerful emitter of radio waves, X-rays and gamma-rays. Despite its colourful history, the Crab Nebula is today a rather disappointing sight when seen through amateur instruments. Binoculars only show a faint spot of 8th-magnitude light, whilst telescopes reveal a grey, wispy, irregular outline, measuring just 6' across. Telescopic observers will usually find that a nebula filter enhances the detail somewhat.
Astronomical Symbolism at the Ancient Pueblo People's settlement of Chaco Canyon in New Mexico, USA (click on thumbnail for larger version, 6 KB). Below the handprint are a crescent Moon and a star, thought to be a depiction of the supernova of 1054 AD which resulted in the Crab nebula supernova remnant that we see in the night sky today (Image: Pomona College)
Positioned above the 'head' of the Bull, between its two horns, is the open cluster NGC 1647, which has an integrated magnitude +6.4 (i.e. the combined magnitude of all its stars compressed into a single point of light). This is a loose cluster of about 120 or so stars across an area of about 45', with a relatively bright pair of 9th magnitude stars close to its centre. Given the faintness of the stars it contains, the cluster is unspectacular in binoculars and is best seen through telescopes.
Gemini's leading stars Castor and Pollux are named after the twins of Queen Leda of Sparta in ancient Greek mythology; here the constellation was known as 'Didumoi'. The names of the Twins were not consistently identified amongst classical writers. The first account, named Phaenomena and written by Greek poet Aratus in the 3rd century BC, does not mention them by name, whilst Eratosthenes (3rd century BC) named them as Castor and Polydeuces. Both Hyginus (1st century BC) and Ptolemy (2nd century AD) named them as Apollo and Heracles. The Latin translation of 'the Twins' is Gemini or Castores, the Romans having considered them the patrons of sailors. The reasoning behind this appears to be the fact that the angular separation of the two stars is 4°.5, providing a useful reference for measuring angular distances in the night sky.
Like Aries, the constellation may have had an Egyptian origin. In ancient Egypt the star Alhena ( Gem or Gamma Geminorum - see below) was referred to as the 'Beginning of The Twins' in their list of 'hour-stars' marking the Moon's path across the night sky. In later centuries throughout the Middle East, the constellation was known as the 'Two Stars', consisting only of Castor and Pollux, the other stars of what we now call Gemini having been included in Orion, the Hunter. In Babylonia, where the constellation was named 'ma.tab.ba.gal.gal' ('Great Twins'), the story behind the name may have lain in the friendship between the aforementioned Gilgamesh and Enkidu. Alternatively, they may have represented the twin brothers Lugal-Irra and Meslamtaea, who were aspects of the twin-god Nergal, god of the Sun and the lower world.
Castor and Pollux depicted on a votive plaque from the ancient city of Tarentum in Italy (click on the thumbnail for a larger image, 11 KB) (Image: History for Kids)
Castor and Pollux lie at distances of 52 light years and 34 light years from Earth, respectively. The constellation's Western half is set against the backdrop of the Milky Way, so this region contains several open star clusters (groups of young stars within our galaxy which have been formed from the same nebulous cloud) and gaseous nebulae (interstellar clouds of gas and dust). The whole constellation also features numerous double stars and multiple stars.
The brightest star in Gemini is Pollux ( Gem or Beta Geminorum, mag. +1.1), which is positioned 4°.5 to the South-east of Castor. Through binoculars, Pollux appears orange-yellow compared to the blue-white coloration of its nearby 'twin'. Today Pollux is evidently the brighter of the two stars although its Beta () designation would suggest that it is second-brightest. Some astronomers have suggested that Castor has faded since it was given the Alpha () designation in 1603, when German lawyer and amateur astronomer Johann Bayer first assigned Greek letters to each of the brighter stars in the night sky for his ground-breaking star atlas Uranometria. However, it seems that stellar brightness was not the only factor which Bayer used when allotting the letters to the stars. The positioning of the stars within a constellation was also considered, hence in some instances stars were lettered in a West-to-East direction, regardless of their apparent brightness (eg. the stars in the Big Dipper, also known as the Plough). In addition to this we must remember that there were no accurate measures of stellar brightnesses in Bayer's day - only rough estimates by naked eye - hence 'mistakes' will undoubtedly have crept in to his lettering system.
In 2006 an extrasolar planet (or exoplanet, i.e. a planet beyond our own Solar System) was discovered orbiting Pollux using a Doppler-shift technique. Known variously as Pollux b, Beta Geminorum b or HD 62509 b, the planet has a mass of almost three Jupiters and orbits Pollux at a distance of 1.64 AU in a period of 590 days.
Gemini's second-brightest star Castor ( Gem or Alpha Geminorum, mag. +1.6) is a telescopic double which is part of a complex sextuple star system. The main star (Castor A, mag. +1.9) has a companion (Castor B, mag. +3.0) about 5" to its North-east; small telescopes will be able to split the two. Larger telescopes reveal a third star (Castor C or YY Geminorum) of ninth magnitude, some 70" away. Each of these three stars has a companion which is not visible in amateur telescopes. Castor B orbits Castor A in a period of 445 years; as seen from the Earth, their separation has been slowly increasing since 1968 when they were closest together. The two components will therefore be increasingly easy to view through telescopes until they reach their furthest distance apart in the year 2065.
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Gemini's Westernmost bright star is 1 Geminorum (mag. +4.1). Sir William Herschel discovered Uranus in the vicinity of this star in March 1781; the blue-green planet was then situated just across the border in Taurus.
A short distance North-east of 1 Geminorum is the open star cluster M35 (NGC 2168). It can be glimpsed with the naked-eye on a dark, clear night as a small, misty patch of light. The cluster has an apparent diameter about the size of the Full Moon and it contains over 400 stars, lying at a distance of about 2,800 light years from Earth. Binoculars show a bright, elliptical glow with several stars, the cluster appearing somewhat 'doughnut-shaped' since there are considerably fewer stars at its centre. Small telescopes fitted with wide-field (low magnification) eyepieces show curving chains of stars from 8th to 12th magnitude. M35 is one of the more spectacular open star clusters in the night sky and astronomy writers have variously described its appearance as 'rich', 'beautiful', 'outstanding' and 'marvelous'!
The star Eta Geminorum ( Gem, mag. +3.1v), also known as Propus or Tejat Prior (meaning 'forward foot') is both a double star and a variable star. It has a close companion star of 8th magnitude which orbits the primary star once every 500 years; medium or large telescopes are required to resolve the pair. Tejat Prior is a red giant which fluctuates in brightness between mags. +3.1 and +3.9 over the course of about 230 days. Mu Geminorum ( Gem, mag. +3.2v) or Tejat Posterior ('latter foot') is also a double star, having a ninth-magnitude companion positioned some 2' to the South-east of the primary, visible in small telescopes. The secondary star itself has a tenth-magnitude companion which requires larger telescopes to resolve. Mu Geminorum was known to the ancient Chinese as 'ching', meaning 'a well'. It was in the constellation of the Tapir, one of their 28 animal-based asterisms.
Another double star in Gemini is Wasat ( Geminorum, mag. +3.5), a name which is Arabic for 'middle'. The primary star has a magnitude of +3.5 and is creamy-white and its companion star (mag. +8.2) is orange. This pair should be easy to split, however the great brightness difference between the two stars makes them difficult to split in smaller telescopes. Wasat is 59 light years from the Earth and its companion dwarf star takes 1,200 years to orbit around it. Astronomer Clyde Tombaugh discovered Pluto near Wasat in February 1930, while he was examining photographic plates of the region which had been taken between January 23rd and 29th of that year.
The star Alhena ( Gem or Gamma Geminorum, mag. +1.9), once called 'The Guard of the Pleiades', is a blue-white star some 85 light years from Earth. Its name derives from the Arabic for 'brand' or 'mark', presumably meaning a mark on one of the Twins' feet.
Mebsuta ( Gem or Epsilon Geminorum) is also a double star, with components of magnitudes of +3.0 and +9.0. They are separated by a very wide 110" (more than twice the apparent width of the planet Jupiter at its closest). The two stars can easily be resolved in small telescopes but the companion star is rather too faint for most binoculars. Mebsuta is a yellow supergiant lying at a distance of 900 light years from Earth. The name is Arabic for 'outstretched arm', which is a little strange given the modern day depiction of the Gemini figure, where it marks the groin of the Western twin.
Another interesting double star is Mekbuda (Gem or Zeta Geminorum) also known as 43 Geminorum. The brighter component is a variable star, pale yellow in colour and 1200 light years away, its magnitude varying between +3.6 and +4.2 over a period of just over 10 days. It is a Cepheid variable, a type of supergiant star which pulsates at regular intervals, changing its size, temperature and spectral type in the process. Just over 2' to its North is a magnitude +7.6 star; the two stars are an easy pair to view in binoculars. This star is physically unrelated to Mekbuda; it is a chance line-of-sight from the Earth which causes the two stars to appear close together. Mekbuda does, however, have a genuine binary companion of magnitude +10.5, which is too close to its primary to be resolved in amateur telescopes.
In Western Gemini, not far from the border with Taurus, is one of the most spectacular open clusters in the night sky. M35 (NGC 2168) can be glimpsed with the naked-eye on a dark, clear night as a small, misty patch of light. Lying at a distance of about 2,800 light years from the Earth, it contains over 400 stars and has an apparent diameter about the size of the Full Moon. Binoculars show a bright, elliptical glow whilst small telescopes at low magnification show curving chains of stars from 8th to 12th magnitude, the cluster appearing somewhat 'doughnut-shaped' since there are significantly fewer stars at its centre.
Away from the zodiacal band of constellations, Cetus, the Whale, frequently sees the Moon and planets cutting across its North-western corner (around 0 hours 26 minutes in Right Ascension) because the ecliptic passes just to the North of it (through Pisces). In order to enter Cetus, a planet must be more than about 10' (0°.16) South of the ecliptic when it passes this point of the zodiac (about 7° in ecliptic longitude, i.e. 7° East of the First Point of Aries). In February 2011, for example, Jupiter was positioned about 1°.1 South of the ecliptic at this point of the zodiac and therefore entered Cetus, remaining there for about 12 days before returning to Pisces.
Messier 77 Seyfert Galaxy in the constellation of Cetus (click on thumbnail for larger version, 7 KB) (Image: Rancho Del Sol Observatory/NASA)
The star Menkar ( Cet or Alpha Ceti, mag. +2.5) has been given the honour of an Alpha () listing although it is not the brightest star in the constellation. At magnitude +2.0, Deneb Kaitos or Diphda ( Cet) is the brightest, though with a declination of -18° it is positioned too far South to appear on the star chart. In Arabic, Menkar means 'nose', which together with Deneb Kaitos (the 'tail of the whale') define the front and rear of the animal.
Cetus' most famous star is Mira ( Cet or Omicron Ceti), a variable star which was nicknamed 'the wonderful star' by German astronomer Johannes Hevelius in 1642. It is a red giant long-period variable, 418 light years distant, ranging in magnitude from around +2.0 to +10.1 over a period of about 332 days. In other words, it is naked-eye for some of the time but only visible through telescopes and binoculars for the majority of the time (the naked-eye limiting magnitude typically being from +6.0 to +6.5). The large brightness variation is caused by the star's dramatically changing size and luminosity. It is an old, dying star whose outer gaseous shell pulsates between 300 and 400 times the diameter of the Sun. Observers may wish to try estimating Mira's current brightness by comparing it with the surrounding stars (of steady brightness) shown on the American Association of Variable Star Observers (AAVSO) comparison star sequence chart. Although it is a red giant, Mira's pulsation also causes it to change colour and temperature, such that its reddish tint is not always obvious to the Earthbound observer. Indeed, binoculars will often show it to have a yellow tint. Mira also has a close, small companion star (Mira B, also called VZ Ceti) which is itself variable, separated from Mira A by 0".5 and only visible in large telescopes whenever Mira is around minimum brightness.
Variable Star lightcurves (click on thumbnails for larger versions; 9 KB, 12 KB and 18 KB) for the stars Beta Persei (Algol) on the left and Omicron Ceti (Mira) in the centre. On the right is a Mira comparison star sequence chart for estimating Mira's magnitude when seen through telescopes and binoculars (Mira itself is positioned at the centre of the chart, marked by the circle and cross). Star magnitudes are shown beside each star without the decimal point, hence '85' indicates magnitude +8.5 (Image sources: Algol lightcurve from the Southwest Research Institute; Mira lightcurve from AAVSO; Mira star chart generated from AAVSO's Star Plotter facility, where the original black-&-white version can be obtained).
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Mira was the first variable star to be discovered (by Dutch astronomer David Fabricius in 1596) and when other similar types of stars were later found they became known as Mira stars. Examples include R Aqr in Aquarius, S Car in Carina, Cyg (Chi Cygni) in Cygnus, R Hya in Hydra and R Leo in Leo. Charts of these stars and a list of 'easy to observe' variables can be found on the AAVSO website.
Straddling the celestial equator in North-eastern Cetus, the galaxy M77 (NGC 1068) requires a telescope of 150mm (6 inches) aperture or greater to see any detail, although at magnitude +8.8 it is a little brighter (and its apparent size a little smaller) than M74 (NGC 628), its Piscean counterpart positioned not far to the North-west. Alternatively known as the Cetus A Galaxy, M77 is also easy to find, being just 1° South-east of the star Cet (Delta Ceti, mag. +4.1). Through binoculars it appears somewhat stellar with a bright core and a surrounding circular haze. Lying at a distance of 50 million light years it is the most distant of the Messier objects and perhaps the most distant object which is visible in binoculars. M77 is a type of spiral galaxy known as a Seyfert galaxy, whose active nucleus makes it appear brighter than most spiral galaxies seen at this distance. It is also a radio source and astronomers believe that it has a supermassive black hole at its centre.
Orion is arguably the most recognisable and interesting constellation in the night sky, having seven stars brighter than 2nd magnitude and numerous multiple and variable stars. One such multiple star is Theta 1 Orionis (1Ori), best known as The Trapezium. Positioned near the centre of Orion's Sword, it is embedded within the much-celebrated M42 or Orion Nebula (of which, see below). The four stars appear as one to the naked-eye but they are easily separated in small telescopes, having magnitudes of +5.1, +6.7, +6.7 and +8.0. The hot blue stars in this system are around 30,000 years old (relatively recent on a cosmological timescale), their ultraviolet radiation illuminating the nebula around them. Telescopes of greater than 100 mm (4 inches) aperture show an additional two stars of magnitude +11.1 and +11.5 close by. Some 2.2 arcminutes to the South-east of Theta 1 Orionis is Theta 2 Orionis (2Ori), a bright double star of magnitudes +5.1 and +6.4 which are separated in an East-West direction by a wide 52" - easily seen in binoculars. The entire Theta Orionis system is about 1,300 light years away and comprises at least ten stars.
Sigma Orionis ( Ori) is a visually-spectacular quintuple star system, with two bright components visible through binoculars (mags. +3.8 and +6.6) but three more are visible in telescopes. This is an example of a multiple star system whose components have various colours. Sigma itself appears blue-white, the +6.6 star appears bluish and the +7.2 star appears reddish. More difficult to see (because of the glare of Sigma Orionis) is a 10th-magnitude star nearby.
The Trapezium Star Cluster lies at the heart of the Orion Nebula (M42) (click on thumbnail for larger version, 4 KB). The four stars are embedded inside a bright fan-shaped region of nebulosity. Lines of shock-waves from the star formation have accumulated along the nebula's South-eastern side, giving it a distinct edge (upper right). Just to the right of the Trapezium is a dark intrusion called the Fish's Mouth.
Orion is home to numerous bright and dark nebulae and is one of the first constellations towards which newcomers to astronomy first turn their telescopes. To the South of Orion's Belt, in the Sword of Orion is the night sky's brightest and most spectacular gaseous nebula. The Orion Nebula (M42 or NGC 1976) is seen with the naked-eye as a grey, misty patch of light. Telescopes show it as pale blue-green whilst long-exposure photographs and CCD images reveal a pinkish-red coloration. It is an emission nebula, some 1,340 light years from Earth, which glows as a result of the ultraviolet radiation emitted by the new stars which are forming within it. These stars, together with the remarkable discovery of protoplanetary disks (or proplyds, i.e. new Solar Systems in the making) were famously revealed in various Hubble Space Telescope images during the 1990s. The primary illuminators of the nebula are the four stars which comprise the aforementioned Trapezium cluster. They are embedded within a fan-shaped lump of glowing nebulosity whose mottled texture is nicely revealed in moderately-sized telescopes. In 1967 infrared photographs of the region indicated the prescence of another star-forming region behind the Trapezium cluster, hidden from our view.
The Orion Nebula is impressive when viewed through almost any telescope or pair of binoculars, since it covers an area of sky roughly 1° by 1° (about the apparent diameter of two Full Moons). As a general rule, the larger the telescope aperture, the greater the amount of detail which is seen. Along with the many dozens of stars, there are dust clouds, curving bright arcs and dark intrusions to be seen aplenty. Before astro-photography became commonplace, amateur astronomers spent many hours at the telescope eyepiece making sketches of the nebula - a rewarding task even in today's high-tech age. Manual sketching of this kind trains the eye to see fine detail in nebulae which might otherwise be missed during a brief observing session.
A little to the North of The Trapezium and its associated nebula, separated from it by a darker dust lane called the Fish's Mouth, is a Northern extension of the nebula known by its own designation M43 (NGC 1982). Telescopes show it as a roundish glow some 12' across, with a 7th-magnitude variable star (NU Orionis) positioned offset from its centre. In long-exposure photographs M42 and M43 together appear rather like a bird in flight, flying towards the North-east, the two curving arcs of the nebula forming the leading edges of the wings and M43 the bird's head and beak.
Deep-Sky Objects in Taurus, Auriga and Orion (click on thumbnails for larger images; 20 KB, 16 KB and 6 KB) Three examples of deep-sky objects which can be seen in binoculars and small telescopes: (Left) the Crab Nebula (M1) in Taurus, imaged by the Hubble Space Telescope (Centre) the open cluster M37 in Auriga and (Right) the Orion Nebula (M42). The Crab Nebula requires large telescopes to begin to resolve detail in its structure, through which it appears as an unevenly bright 'S-shaped' glow. M37 is easily seen through most instruments, appearing much like the image here; the brightness of the stars varies depending upon the telescope aperture. The Orion Nebula appears much fainter and less colourful through telescopes than the image here suggests. The long exposure required to reveal the fainter extremities of the nebula has 'burned out' (over-exposed) the bright, central region where the Trapezium star cluster resides, so the individual stars are not visible (Image sources: M1 from NASA/ESA/ASU; M37 by Jan Wisniewski (SEDS.org); M42 from NOAO/AURA/NSF)
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A short distance North-east of Orion's Belt is another gaseous nebula, M78 (NGC 2068). This is another example of a reflection nebula, meaning that the light within the nebula is reflected by interstellar dust particles. M78 is the brightest emission nebula in the night sky (integrated magnitude +8.3) and small telescopes will show it as a faint, roundish, comet-like glow. At its centre is a double star of 10th magnitude, which is the light source of the nebula.
The constellation Monoceros, the Unicorn, is positioned between Procyon and Sirius. The fact that it is outshone by these two luminaries somewhat detracts from the fact that it contains a number of interesting star clusters and nebulae. Perhaps the most interesting of these is the Rosette Nebula (NGC 2237) which, like the Orion Nebula, is an emission nebula which glows by emitting light from the radiation of the young, hot stars contained within it. The Rosette is about 4,700 light years distant and appears much like a beautiful pink carnation in long-exposure photographs. Alas, it is too faint to be seen in binoculars and small telescopes; medium and large telescopes are only likely to reveal the nebula's brightest regions when seen under very dark skies. More obvious to binocular and small telescope observers will be the star cluster at the nebula's centre (NGC 2244) comprising numerous stars of sixth magnitude and fainter.
The Rosette Nebula (NGC 2237) is a diffuse nebula situated in the constellation of Monoceros (click on thumbnail for larger version, 25 KB). The star cluster NGC 2244 is at its centre (Image: Wikimedia Commons / John Lanoue)
Beta Monocerotis ( Mon) is an interesting example of a triple star system; in fact it is often described as the finest example in the night sky. To the naked-eye it appears as a single star of about magnitude +3.7, however its three components can be easily separated in small telescopes. The stars appear blue-white and have magnitudes of +4.7, +4.8 and +5.6. The two brighter stars (A and B) are separated by 7".3, the faintest of the three (C) being separated from its primary (B) by 2".8. Stars B and C orbit each other in a period of around 4,000 years whilst star A orbits the BC pair in a period of some 14,000 years! The triple system is about 700 light years from Earth.
In Northern Monoceros the Christmas Tree Cluster (NGC 2264) is an open star cluster with an associated nebulosity known as the Cone Nebula. The nebula is too faint to be seen in amateur telescopes, but the cluster's leading star, the fifth-magnitude blue supergiant S Monocerotis (or 15 Mon), situated at the Northern end (base) of the 'Tree', is easily seen in binoculars, along with around 20 other stars down to about ninth magnitude. The 'Christmas Tree' appears inverted to Northern hemisphere observers and upright to Southern hemisphere observers. Appropriately, the Christmas Tree Cluster crosses the observer's meridian (due South in the Northern hemisphere, due North in the Southern hemisphere) at local midnight around Christmas time.
In the Southern part of Monoceros is the open star cluster M50 (NGC 2323), about 2,600 light years distant. Its brightest stars can be seen in binoculars but a telescope is needed to resolve its 150 or so stars. The Eastern section of Monoceros is shown on the Cancer-Leo-Virgo star map.
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Far to the North of the ecliptic, in the constellation of Perseus, is the most celebrated binary star in the night sky. Algol ( Per or Beta Persei) is named after the Arabic 'Al Ghul' meaning 'the demon'. It marks the eye of a Gorgon named Medusa (from Greek mythology) which the hero Perseus slain; ancient star maps picture him holding Medusa's severed head. Algol is a type of variable called an eclipsing binary, meaning that its brightness change is caused by a large, darker companion star (Algol B) which is in orbit about the brighter (and smaller) primary star (Algol A). When Algol B passes in front of Algol A (as viewed from the Earth) the overall magnitude of Algol reduces. When they are separated, the combined light of the two stars make Algol appear brighter. Algol's magnitude varies from +2.1 to +3.4 in a period of 2 days, 20 hours and 49 minutes. For all except 10 hours of this period Algol shines at maximum brightness. The fall in brightness takes place in just 5 hours, as the dark companion moves in front of the primary star. Following minimum, Algol returns to its original brightness after a further 5 hours. The changing brightness of Algol can be observed by the naked-eye alone, hence it has often been called the 'winking star'. Its fainter component, however, cannot be resolved in telescopes since it orbits the primary at a distance of just 10.5 million kms (6.5 million miles). The dates and times at which Algol reaches its minimum (the 'Minima of Algol') can be found on various websites, eg. Sky & Telescope and a lightcurve is shown in the diagram above. Algol is positioned 93 light years from Earth and in the 1970s was one of the first stars discovered to be a source of X-ray emissions.
Some 5° WNW of Algol is the bright open cluster M34 (NGC 1039). It contains around 80 stars of magnitude +7 to +11 spread across an area equal to the apparent diameter of the Full Moon. Its integrated magnitude is +5.2, making it just visible to the naked-eye when seen under dark, light-pollution-free skies. In binoculars it is a nice and easy sight, the individual stars becoming just about resolvable. Through telescopes at low magnifications, the stars are rather concentrated in the centre, with lines of stars spreading outwards.
In neighbouring Auriga, the star Almaaz ( Aur or Epsilon Aurigae) is an eclipsing binary with a much longer period than Algol. Its usual magnitude is +3.0 but it drops to around +3.8 every 9,892 days (27 years), remaining in eclipse for 22 months. The most recent eclipse began in 2009 and at mid-eclipse (in 2010) astronomers were able to image the event using a network of four telescopes, concluding that the eclipse is caused by "a thin disc of opaque dust trailed by a massive and unseen companion" (see BBC News item). Almaaz is a hot supergiant lying some 2,040 light years distant. It forms an asterism (star pattern) with the stars (eta) Aurigae and (zeta) Aurigae known as 'The Kids', depicted on old star maps as young goats being held on the Charioteer's shoulder.
Three Messier open clusters in Auriga are shown on the star chart; M36 (NGC 1960), M37 (NGC 2099) and M38 (NGC 1912). They are easily seen through binoculars but they each have a very differing character. M36, also known by the names Pinwheel Cluster or, more rarely, The Frog, is a small, bright group of about 60 stars spread across 12' of sky; its integrated magnitude is +6.0. Seen through binoculars, it is the most prominent cluster of the three but it is best appreciated in low-power telescopic eyepieces. M37 is perhaps the best open cluster within Auriga's boundaries. The integrated magnitude of the cluster is +5.6 and its apparent diameter is 20'. It appears as a hazy patch in binoculars because its stars are so closely packed together. Telescopes above 100 mm (4 in) aperture reveal a rich field of around 1,800 faint stars, with a mag. +9.2 star at its centre. There is some disagreement about the colour of this star - it has been described by various observers as yellow, orange and red! M38, the Starfish Cluster, is a large but dim cluster whose stars can be partly resolved in binoculars. Through telescopes, a cruciform star pattern can be discerned from the 150 or so stars. The three clusters lie at an average distance of 4,200 light years from Earth and are seen against the backdrop of the Milky Way.
Also in Auriga, NGC 2281 is a much closer cluster (1,800 light years) and is rather less impressive than its Messier neighbours. About 30 stars can be seen through small telescopes in a roughly crescent shape, with four stars in a diamond formation at the cluster's centre. Larger instruments reveal around 40 stars within an area of about 20'. The cluster has been romantically named the Broken Heart Cluster.
M33 Triangulum Galaxy in the constellation of Triangulum (click on thumbnail for larger version, 13 KB) (Image: Hunter Wilson/Wikipedia).
Triangulum is a small but easily recognisable constellation situated just to the North of Aries. Its most famous feature is M33 (NGC 598), the Triangulum Galaxy. Although star atlases usually give a magnitude of around +5.7 (technically within naked-eye visibility) this galaxy is menacingly difficult to see because its apparent width is over 1° (about twice the apparent diameter of the Full Moon) and its surface brightness (i.e. its apparent magnitude allowing for the fact that it is spread over an area of the sky) is +14.1 (for more on surface brightness, see the Cancer-Leo-Virgo page). M33 is considered a good test of local dark sky conditions; if it is visible to the naked-eye, the sky is perfect for observing faint, extended objects such as galaxies and nebulae. Under such conditions, binoculars will show an elliptical glow, orientated NE-SW, whilst large aperture telescopes will begin to reveal the galaxy's spiral arms. The spiral structures so clearly visible in most observatory photographs are unfortunately very difficult to resolve visually in amateur telescopes. In practice, most observers will only detect the brightest regions of the galaxies (the central core and some mottled structure in the spiral arms) however long-exposure photographs and CCD images will detect a lot more detail.
At 2.9 million light years distant, M33 is a relatively close galaxy. If it were more conspicuous to the naked-eye it would beat M31 (the famous Andromeda Galaxy) to the title of 'the most distant object visible to the naked-eye'. Both M33 and M31 are members of the Local Group of galaxies, which contains around 30 galaxies (including our own spiral galaxy, the Milky Way) out to a distance of around 7.5 million light years. The Triangulum Galaxy is sometimes referred to as the The Pinwheel Galaxy, which confusingly is also the name given to the galaxy M101 (NGC 5457) in Ursa Major.
Andromeda itself contains many treasures for the binocular and telescopic user (the aforementioned M31 being one of them), however only the open cluster NGC 752 appears within the area of the star chart. With an integrated magnitude of +5.7, its 100 or so stars are spread across an area of 50'. It is easily seen through binoculars but because its stars are mostly around 9th magnitude the cluster is best seen through telescopes.
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