Sunday, November 22, 2009

Asterisms

Update 1: For a new and improved version see the asterisms page
Update 2: For relations between asterisms and visibility around the year see the alignments page.

This is the chart I wanted when I was young; to get an overview of the night sky in general and work my way forward from there. For this I looked for traditionally known relations between the brightest stars in the sky, or asterisms, and put them all together on a chart.


Click for larger image

Some of the asterisms are constellations (Cassiopiea, Crux, Scorpius), some are parts of constellations (Big Dipper, Sickle, Teapot, Orions Belt and Sword) and some are traditionally known patterns of bright stars grouped into simple shapes (Pegasus Square, Summer Triangle, Great Diamond, Winter Hexagon), that invovle stars from many different constellations. And finally Argo Navis is an old constellation that was dismanteled into the smaller pieces Carina, Vela, Puppis and Pyxis becaus it was deemed yust too big. But for an asterism it is just fine. At least I hope so, because I don't have much experience with the southern sky, so I'm not sure it makes an easy to recognize pattern. I'm willing to stand corrected on this one.

I think the grouping of simple patterns along the celestial equator is no accident, for they align nicely with the seasons: The Summer Triangle and Winter Hexagon are even named after the seasons in which they are best visible. The Great Diamond is up in the spring and the Pegasus Square is best visible in the fall, completing the year. These seasonal alignments are of course from a northern perspective, for the southern viewpoint assume the opposite season. Amd 'visibility' here means that in every season the respective asterism is visible due south (or north from the southern hemisphere) at around midnight. This arrangement makes it possible to draw a text-diagram of the entire night sky (from the northern hemisphere):

'W'                Dipper          
    Square  Triangle   Diamond  Hexagon
            Teapot Hook  Cross Boat
That might make a useful mnemonic, but I'm not so sure ;).

Another neat alignment occurs along the Milky Way:

'W'  Triangle  Hook  Pointers  Cross  Boat  Hexagon,
which is no accident since obviously most bright stars should be where the concentration of all stars is highest.

Along with the asterisms I included some alignments of bright stars that point to other positions: The Southern Pointer of Alpha and Beta Centauri points to the Southern Cross, which in turn points to the celestial South Pole; the two hindmost stars of the Big Dipper pointing to the celestial North Pole. This one may be a bit confusing on the chart because in the projection I used, the entire upper and and lower edge of the map represent the celestial poles, and thus Polaris looks way out of alignment where the pointer stars should be pointing directly at it. But since the entire upper edge represents a single point, Polaris is right there at the end of the Northern Pointer in the real sky. Finally there is the alignment of Orion's Belt with Sirius; while it is certainly well known, nobody has ever dared to call it "The Leash" before, even though for me it sounds like a very logical thing to call it, since in the origin myth Canis Major is supposed to be Orion's hunting dog, along with Canis Minor.

There are much more known asterisms, (see here for a nice overview) but I restricted myself to those which are visible to the naked eye, because the map is meant to provide a quick and easy overview of the night sky, after all. Some of the entries overlap with others not shown or can be interpreted differently, this is mentioned in the notes below. I also included some of the brightest deep sky objects mentioned in the notes for reference.

The map projection I used here is Winkel tripel, which is said to be the best oveall compromise between angle-, area- and other distortions for a global view. From the look of the chart, I think I can agree with that. The chart contains all stars brighter than magnitude 7.0, stars brighter than 2.5 mag or contained in an asterism are labeled with their proper name or Bayer designation (greek letter).

For comparison I included a blank map of the sky with only the stars brighter than second magnitude named. See if you can find all the Asterisms:


Click for larger image

·✧·

All the member stars:

Asterism
Size1
Constellations Star Name
Big Dipper2
44°x22°
Ursa Major 1.79 α UMa 2.37 β UMa 2.44 γ UMa 3.31 δ UMa 1.77 ε UMa 2.27 ζ UMa 1.86 η UMa Dubhe
Merak
Phad
Megreth
Alioth
Mizar
Benetnash
Alpha and Beta UMa point towards Polaris, the celestial North Pole:
North Pointer Ursa Major
Ursa Minor
1.79 α UMa 2.37 β UMa 2.02 α UMi Dubhe
Merak
Polaris
The Queens W10
25°x7°
Cassiopeia 2.27 β Cas 2.23 α Cas 2.47 γ Cas 2.68 δ Cas 3.38 ε Cas Caph
Shedir

Ruchbah
Segin
Orion's Belt
Orion 2.23 δ Ori 1.70 ε Ori 2.05 ζ Ori Mintaka
Alnilam
Alnitak
Orions Belt points toward west in the direction of Sirius:
.."The Leash"3 Canis Major -1.46 α CMa Sirius
Orion's Sword
3.5°
Orion 3.77 σ Ori 4.98 θ Ori 4.0 M424 2.77 ι Ori

Orion Nebula
Nair al Saif
Winter Hexagon5
48°x52°
Canis Major
Canis Minor
Gemini
Auriga
Taurus
Orion
-1.46 α CMa 0.38 α CMi 1.14 β Gem 7 0.08 α Aur 0.85 α Tau6 0.12 β Ori Sirius
Procyon
Pollux
Capella
Aldebaran
Rigel
Sirius and Procyon together with Betelgeuse form the Winter Triangle:
Winter Triangle
26°x24°
Canis Major
Canis Minor
Orion
-1.46 α CMa 0.38 α CMi 0.50 α Ori Sirius
Procyon
Betelgeuse
Great Diamond8
36°x50°
Bootes
Virgo
Leo
Canes Venatici
-0.04 α Boo 0.98 α Vir 2.14 β Leo 2.90 α CVn Arcturus
Spica
Denebola
Cor Caroli
The Sickle
9°x14°
Leo 1.36 α Leo 3.48 η Leo 2.01 γ Leo 3.43 ζ Leo 3.88 μ Leo 2.97 ε Leo Regulus
Al'dzhabkhakh
Algieba
Adhafera
Alshemali
Algenubi
Summer Triangle9
31°x36°
Lyra
Aquila
Cygnus
0.03 α Lyr 0.77 α Aql 1.25 α Cyg Vega
Altair
Deneb
Pegasus Square10
17°x14°
Pegasus
Andromeda
2.42 β Peg 2.49 α Peg 2.83 γ Peg 2.06 α And Scheat
Markab
Algenib
Alpheratz
Teapot11
15°x9°
Sagittarius 2.81 λ Sgr 2.70 δ Sgr 1.85 ε Sgr 2.60 ζ Sgr 3.17 φ Sgr The Handle: 3.32 τ Sgr 2.02 σ Sgr The Spout: 2.99 γ Sgr Kaus Borealis
Kaus Meridionalis
Kaus Australis
Ascella



Nunki

Nash
Fish Hook12
27°x23°
Scorpius 2.32 δ Sco 2.62 β Sco 2.89 σ Sco 0.96 α Sco 2.82 τ Sco 2.29 ε Sco 3.08 μ Sco 3.62 ζ Sco 3.33 η Sco 1.87 θ Sco 3.03 ι Sco 2.41 κ Sco 1.63 λ Sco Dschubba
Graffias
Alniyat
Antares
Alniyat




Sargas


Shaula
Southern Cross13
8°x6°
Crux 0.76 α Cru 1.25 β Cru 1.63 γ Cru 3.79 δ Cru Acrux
Mimosa
Gacrux
Alpha and beta Centauri point toward the Cross:
Southern Pointer Centaurus -0.27 α Cen 0.61 β Cen Rigel Kent
Agena
Argo Navis14
66°x46°
Carina
Vela
Puppis
Pyxis
The Hull: -0.72 α Car 1.68 β Car 3.32 ω Car 2.76 θ Car 2.25 ι Car 1.78 γ Vel 2.25 ζ Pup 4.40 ρ Pup 3.34 ξ Pup 4.50 κ Pup 2.70 π Pup 3.17 ν Pup The Sail: 1.96 δ Vel 2.50 κ Vel 3.54 φ Vel 2.69 μ Vel 4.83 q Vel 3.60 ψ Vel 2.21 λ Vel And the Compass: 3.90 β Pyx 3.68 α Pyx 4.01 γ Pyx
Canopus
Miaplacidus


Turaris
Regor
Naos

Asmidiske










Alsuhail

Raw data of the table above (txt)

Notes:

1) Greatest extension in east-west direction x north-south direction; in degrees. For linear Asterisms, length in degrees.

2) Most of the stars of the Big Dipper (except α and η UMa) are part of the Ursa Major Moving Cluster, the closest open cluster to the sun.

3) "The Leash" is my own invention and refers to the fact that it points from Orion's belt to the neck (i.e. α CMa) of his larger dog. Yes, I'm sirius. ;)

4) M42 is, of course, not a star but an emission nebula, the second brightest after the Eta Carinae Nebula.

5) If the hexagon is continued from Rigel (β Ori) to Betelgeuse (α Ori), we do not have a Hexagon containing a Triangle, but a large 'Heavenly G'. Orion, Canis Major, Canis Minor, Taurus as well as Lepus (the Hare) all originate from a common myth.

6) The fuzzy blob east of Aldebaran (α Tau) are the Pleiades, on of the brightest open star clusters. Even closer to Aldebaran, but visible only in a very dark sky are the Hyades, another bright open star cluster.

7) Gemini contains two almost equally bright stars, but Castor (β Gem) is a little bit brighter than Pollux (α Gem), so it gets casted.

8) Regulus (α Leo) is another bright star close by that is not part of the asterism. It is, however, the base of an arc of stars that make up the lion's head and is sometimes called 'The Sickle'. Within the Diamond lies the Coma Star Cluster, one of the oldest known open clusters, cataloged by Ptolemy.

9) The whole contellation Cygnus is also known as the Northern Cross. There are many more cruciform asterisms in the south (see below). The dark nebula that runs through Cygnus is sometimes called the Dark Rift

10) Pegasus, Andromeda, Perseus, Cassiopeia and Cepheus all originate from the ancient greek myth of Perseus Close to the Pegasus Square off in the Andromeda corner is M31, the Andromada Galaxy. Also in the area are M33, the Triangulum Galaxy; the Alpha Persei Moving cluster, another bright open star cluster close to the brightest star in Perseus, Mirfak (α Per), as well as the Double Cluster h & Chi Persei.

11) The most striking feature in this region is the center of the Milky Way, with too many deep sky objects to list here (see link). In the context of the asterism the Milky Way is supposed to be the steam coming out of the spout. A nice dark cloud feature close by is the Pipe Nebula, which in a dark enough sky looks like a, well, pipe with steam coming out of the bowl.

12) Antares lies in the very colorful Rho Ophiuchi Cloud, a nice collection of dark-, emission- reflection nebulae, open and globular clusters, a little bit of everything. Scorpius also contains M7, Ptolemy's Cluster, one of the brightest open clusters

13) An imaginary line along the longer vertical arm of the Southern Cross about 4.5 times the length of this arm points to the celestial south pole. Other bright objects around the south pole are the Large and Small Magellanic Clouds. Close to the latter lies 47 Tuc, the second brightest globular cluster after Omega Centauri. The Coalsack dark nebula is directly adjacent to the cross. In native Australian star lore it was the head of a giant Emu made up of dark clouds stretching along the Milky Way, with the body in Sagittarius.

14) More about Argo Navis The region contains two formations that may be mistaken for the Southern Cross: The upside down Diamond Cross with β (Miaplacidus), ω, ο and υ Carinae; and The False Cross, consisting of κ and δ Velorum (Markeb and Alhabor) plus &iota and ε Carinae (Aspidiske and Avior), which even has it's False Pointers with &zeta Pup (Naos) and &gamma Vel (Regor) Close to the False Cross lies the Eta Carinae Nebula, the brightest emission nebula in the sky.

·✧·

Asterism data from the great SAC Asterisms file (scroll down) from the Saguaro Astronomy Club.
Star data from the Hipparcos and Bright Star Catalog
Names from HD-DM-GC-HR-HIP-Bayer-Flamsteed Cross Index Deep Sky Object data mostly from SEDS
Star Charts by me, with Milky Way data from this fine application, sadly not developed anymore, it seems.

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