Saturday, 25 May 2013

Stargazing Introduction: Four Basic Tours

I've been starting to do a little work with schools and planetariums related to navigating your way around the night sky.   I've often found that this sort of introductory information is hard to come by, so I've tried to collate a few sources and provide some simple diagrams (mostly because they'll help me give a few star tours!).  Here are a few stargazing tours for novices without telescopes or binoculars, biased towards mid-latitudes in the northern hemisphere.  The star charts were created by annotating the star images obtained from the Neave Interactive Planetarium, set for 10pm in July in the UK for the first three tours, and December for the final tour.

Tour One:  The Plough and the North Pole

[Ursa Major, Ursa Minor, Bootes, Leo]

The Plough is not a constellation by itself, but an asterism within the Great Bear, Ursa Major.  It's one of the easiest things to find, and always visible in our sky throughout the year, so we'll use it as our starting point.  The myth goes that Zeus was lusting after a young woman names Callisto, making is wife Hera so jealous that she turned Callisto into a bear.  Callisto's son, Arcas, mistakenly tried to kill the bear, so Zeus turned him into the Little Bear and cast them both skywards for their own protection, forming Ursa Major and Ursa Minor.  M51, also known as the Whirlpool Galaxy, is just to the south of the saucepan handle.

The 'saucepan' shape provides pointer stars to other objects.  Starting from the two stars defining the right hand edge of the saucepan (Merek to the lower right, Dubhe to the upper right), extend a line 'upwards' from the pan (5x the distance between Merek and Dubhe) to reach Polaris, the pole star, and the tip of Ursa Minor (the Little Bear).  Polaris is directly above the Earth's north pole, so all stars in the night sky appear to rotate around that one point as the Earth spins on its axis.

Returning to the Plough, we can now use the 'handle of the saucepan' as a pointer (the stars Mizar and Alkaid), directing us to Arcturus, the brightest star in the northern hemisphere in the constellation Bootes, the herdsman.  Bootes may have been responsible for driving the oxen that the Greeks thought to represent the Plough.  The constellation looks like a large kite extending up from the horizon, and Arcturus is an orange giant 37 light years from Earth.

Finally, if we drill a hole in the bottom of the saucepan, and let the liquid run out, it'll hit the head of the Leo the Lion, and the brightest star Regulus (a blue-white star 78 light years away marking the heart of the Lion), leading to the mnemonic: "A hole in the bowl will leak on Leo".  Leo was the Nemean Lion killed by Hercules during one of his twelve labours, and cast into the sky, but looks to me like a coathanger bent out of shape.    The Leonid meteor show November 14-15 comes from this direction.

Tour Two:  Cassiopeia's Court

[Cassiopeia, Cepheus, Andromeda, Perseus]

As before, we start from the Plough asterism in Ursa Major, finding our way to Polaris (the pole star) using the pointers Merek and Dubhe.  But now we continue on in the same direction that you took from the Plough to Polaris, for around the same distance again, and you'll come to the distinctive 'W' that makes up the constellation of Cassiopeia.   The vain Queen was cast into the sky by Poseidon as punishment for boasting that she was more beautiful than the Nereids.  Cassiopeia lies within the Milky Way, so contains many deep sky objects.

Cassiopeia can also be used as pointer stars, although the directions aren't quite as obvious as for tour one.  From the right hand side of the 'W', follow upwards towards the north to find Cepheus (Cassiopeia's husband and King of Ethiopia).  Cassiopeia's punishment also extended to Ethiopia too, as Poseidon commanded the sea monster Cetus to attack.

Now extend the same line down (southwards) to find the feet of Andromeda (Cassiopeia's princess daughter).  Cepheus was told that the only way to save his kingdom from attack by Cetus was to sacrifice his daughter, so Andromeda was chained to a rock to be eaten by Cetus.   Andromeda is intimately linked to the brightest star in the Great Square of Pegasus.  The Andromeda Galaxy (M31) is the closest spiral galaxy in the Milky Way and can be seen within the 'A' shape of Andromeda.

Finally, one of the central bars of the 'W' leads to the head of Perseus to the southeast.  Perseus is the hero of our story, saving Andromeda from the sea monster Cetus using the head of Medusa to turn the monster to stone.  Perseus became Andromeda's husband.  The variable star Algol within Perseus is said to be Medusa's eye, and lies 93 light years away.  The constellation is connected to Auriga to the east, and the Perseids' meteor shower (August 9-14) originates from this constellation each year.

Tour Three:  The Summer Triangle

[Cygnus, Lyra, Aquila, Hercules and Pegasus]

The summer triangle consists of three bright stars, each marking a constellation, that are readily visible in the summer skies.

Deneb is a bright star at the tail of Cygnus the Swan, a constellation lying in the plane of the Milky Way, and is easily recognisable due to the asterism known as the Northern Cross.  Deneb is a blue white supergiant 3200 light years away.  The beak is Albireo, a binary star of an orange giant and a blue-green star.  Transformations of Greek gods into swans seemed a common occurrence, with Zeus, Orpheus and Cycnus all having gone through the process!  The constellation contains the X-ray source Cygnus X-1, which is now thought to be caused by a black hole accreting matter in a binary star system.

Vega is in Lyra the Harp, a small constellation containing the 3rd brightest star in the northern hemisphere.  After the deal of the musician Orpheus, his lyre was thrown into a river but retrieved by an eagle from Zeus, to be placed into the sky.

The final star in the triangle is Altair, a bright star within Aquila the Eagle.  The eagle carried Zeus' thunderbolts.  Like the Swan, the Eagle lies within the plane of the Milky Way so is rich with deep sky objects.  Altair is one of the closest naked eye stars to Earth at a distance of only 17 light years. Interestingly, NASA's Pioneer 11 spacecraft (flew by Jupiter and Saturn in the late 1970s) is headed in that direction.

Both the Eagle and the Swan are flying in the direction of the Milky Way across the sky, and appear to be headed away from Cepheus (Cassiopeia's husband).  Extending a line from Altair through to Deneb shows you the way to Cepheus (see Tour Two).

A line at right angles to this, coming out of the centre of the Summer triangle, will lead you to the Great Square of Pegasus, the winged horse.  The square is made of four stars, alpha Peg, beta Peg, gamma Peg and alpha Andromedae, which it shares with Andromeda.  51 Peg features the first ever extrasolar planet to be discovered, and HD 2090458b provided the first evidence of water vapour from transit spectroscopy.  Pegasus carried Medusa's head to Polydectes, and was a bearer of thunder and lighting for Zeus.

A final line from Deneb to Vega will point in the direction of a trapezium of four stars making up the body of Hercules, known as the Keystone asterism.  Hercules is depicted as kneeling, praying to his father Zeus after winning a battle following his tenth labour.

Tour Four:  Orion's Hunting Ground

[Orion, Canis Major, Taurus, Gemini]

Orion is one of the brightest constellations in the winter sky, and its belt can be used as a starting point for some star-hopping.  The chart is depicted for 10pm in mid-December from northern mid-latitudes.  Orion lies on the celestial equator, and is depicted as a hunter with a bow and arrow.  Rigel, a blue white star; and Betelgeuse, a red supergiant, are the brightest stars in the constellation.  Orion was a supernaturally strong hunter in Greek mythology, son of Poseidon.  The three stars making up Orion's belt are Alnitak, Alnilam and Mintaka, and a sword hanging from the belt features the beautiful Orion Nebula (M42) 1344 light years away.

Follow the belt to the left (southeast), and we arrive at Sirius, the brightest star visible in our night sky and part of Canis Major.  Follow across the shoulders of Orion to the east and we find Procyon, part of Canis Minor.  Canis Major and Canis Minor were the two hunting dogs of Orion.  The Winter Triangle is made up of Sirius in Canis Major, Procyon in Canis Minor, and the red supergiant Betelgeuse, an asterism to rival the Summer Triangle.  Voyager 2 is slowly moving towards Canis Major.

Follow the belt to the right (northwest) and find the red giant Aldebaran, the brightest star in Taurus the bull.  Orion is typically depicted as fighting Taurus.  Several objects of interest lie in this constellation, including the Crab Nebula (M1, a supernova remnant), the Hyades and the Pleiades.  Keep on following the line through Taurus and you'll come to the Pleiades (M45), an open cluster of many stars, the seven most prominent giving the cluster its nickname.  Zeus took on the form of a white bull to abduct the Phoenician princess Europa, but the identification of Taurus goes back must further into our history due to its position in the zodiac.

Now follow the Hunter's right arm upwards through the red giant Betelgeuse towards Gemini, the twins, and the two stars Castor and Pollux.  Gemini is not in the plane of the Milky Way so features fewer deep sky objects, but is the origin of the Geminids meteor shower on December 13-14 each year. In Greek mythology, Pollux was the immortal son of Zeus and Leda, whereas Castor was the mortal son of Leda and the Spartan Kind, Tyndareus.  When Castor died, Pollux begged his father to give Castor immortality, and the two were united in the heavens.

Finally, a further asterism known as the Winter Hexagon can be found by connecting Sirius to Rigel, Aldebaran, Capella, Pollux and Procyon, with Betelgeuse roughly in the centre.

General Star Charts (Summer/Winter)

To give a sense of how all of these star hopping tours fit together, here's a star chart for the northern hemisphere constellations in mid-summer.
...and another for mid-winter.

Some Helpful Links

Superb Stargazing Live guide from the BBC:

Dave Snyder's 2003 guide to the constellations

Sky maps and planispheres from

A great beginners guide to constellations from

Visualisations of what the constellations really look like:

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