Cassiopeia - Constellation Guide

Cassiopeia

Cassiopeiae

Cas

The Seated Queen



Introduction

Cassiopeia is a prominent northern constellation named after Queen Cassiopeia, the wife of King Cepheus of Ethiopia. In Greek mythology, the Queen was arrogant and extremely boastful about her beauty. Legend has it she claimed both her and her daughter, Andromeda, were more beautiful than all the Nereids the nymph-daughters of the sea god Nereus. This brought the wrath of the ruling god of the sea Poseidon who decided to destroy the kingdom.

After consulting a wise oracle, the only way the King and Queen could stop Poseidon from carrying out his threat was to scarify Andromeda. The princess was left helplessly chained to a rock at the sea edge, awaiting her fate at the hands of Cetus, the sea monster. However, just in time, the hero Perseus arrived to save Andromeda and in the process killed the sea monster. Although Andromeda lived to marry Perseus, Poseidon deemed that Cassiopeia should not escape punishment and banished her to the sky forever, tied to the chair of torture!

The constellation is one of the original 48 plotted by second century astronomer Ptolemy and remains as one of the 88 modern constellations. It's one of the most recognisable constellations due to the distinctive W shape of its five brightest stars. This asterism forms part of the chair and consists of γ Cas (mag.(v) +1.6 -> +3.0), Schedar (α Cas - mag. +2.24), Caph (β Cas - mag. +2.28), Ruchbah (δ Cas - mag. +2.66) and ε Cas (mag. +3.35). Variable star γ Cas can peak at magnitude +1.6 and when it does it's easily the brightest member but currently it hovers around magnitude +2.15.

Since located right bang at the centre of the rich northern Milky Way, Cassiopeia is full of wonderful deep-sky objects. It contains over a dozen bright open clusters visible in small scopes. In addition, there are some beautiful double stars and interesting variable stars. At Cassiopeia's southern end there are three faint galaxies, two of which are dwarfs and members of the M31 Group. For larger scopes there are four faint nebulae within range.

From most northern temperate latitudes, Cassiopeia is circumpolar and therefore visible all year round. It appears highest in the sky during October, November and December. From Southern Hemisphere latitudes, the constellation appears low above the northern horizon or never even rises at all. Positioned on the opposite side of the North Celestial Pole is another famous constellation, Ursa Major, the Great Bear.

Cassiopeia is bordered by Andromeda to the south, Perseus to the southeast and Cepheus to the north. The famous supernova outburst of 1572, observed by Tycho Brahe, occurred near the star kappa (κ) Cassiopeiae. The remains of another supernova, which erupted around 1660, but went unseen at the time forms the brightest extrasolar radio source in the sky, known as Cassiopeia A. It lies near open cluster M52.

Cassiopeia Star Chart - pdf format (credit:- freestarcharts)

Interesting Stars

Bright Star, Variable Star, Double Star

Gamma Cassiopeiae (γ Cas) - a remarkable blue giant variable star known as a shell star. Rotating at high speed it's partly unstable and ejects rings of material at irregular intervals. This behaviour causes it to vary unpredictably between magnitudes +1.6 and +3.0. It currently shines at magnitude +2.15 and therefore marginally the constellation's brightest star.

Gamma Cassiopeiae is the middle star of the W and is located 613 light-years from Earth. Unusual for a bright star it has no traditional Arabic or Latin name. However, in Chinese it has the name Tsih meaning the whip. It was also given the nickname Navi by American astronaut Gus Grissom, who used it for navigational purposes.

The star is a spectroscopic double with an orbital period of about 204 days. It's also a very challenging optical double with a faint 11th magnitude yellow-white (F6) dwarf companion, separation 2.1 arc seconds. Splitting the pair requires decent aperture, steady seeing conditions and quite high magnifications. A minimum 250mm (10-inch) scope is recommended.

Gamma Cassiopeiae is 70,000 times more luminous than the Sun and is surrounded by two faint reflection / emission nebulae, IC 59 and IC 63.

Bright Star, Double Star

Schedar (alpha Cassiopeiae - α Cas) - is a magnitude +2.24 orange giant star, located 228 light-years distant. It has a wide (mag. +8.9) unrelated companion that's visible at low magnifications in small telescopes. The separation is 65 arc seconds. A couple of centuries ago observations suggested that Schedar was variable in nature, but recent measurements have determined no such variations.

Bright Stars

Caph (beta Cassiopeiae - β Cas) - is the far western star of the W asterism. It's a white (F2) Delta Scuti type variable that shines at magnitude +2.28 and almost identical in brightness to Schedar. Although variable, the range is very small and changes in brightness can't readily be noticed with the naked eye. At peak brightness Caph reaches magnitude +2.25, at minimum slightly dimmer at magnitude +2.31. The period is only 2.5 hours.

Delta Scuti type variable stars are subgiant or main sequence stars between 1.5 and 2.5 times more massive that the Sun and within the spectral class range A0 to F5. They are nearing the end of their hydrogen fusion lifetime and as a result exhibit a slight pulsating effect, resulting in a small brightness variation. Of the known Delta Scuti variables, only Altair (α Aqr - mag. +0.77) is brighter than Caph.

Caph is also a spectroscopic binary star with a faint companion. The orbital period is 27 days but little else is known about the companion. The system is located 54 light-years from Earth.

Ruchbah (Delta Cassiopeiae - δ Cas) - is another slightly variable star in the W. It's an Algol-type eclipsing binary star that fluctuates between magnitudes +2.68 and +2.74 over a period of 759 days. Ruchbah is located 99 light-years distant and telescopically appears as single blue-white star. It's the second most eastern star of the W.

Epsilon Cassiopeiae (ε Cas) - mag. +3.35, is a blue-white B type giant star located 440 light-years from Earth. It forms the eastern star of the W and is also the faintest of the five.

Epsilon Cassiopeiae has the traditional name Segin and is more than 2,500 times more luminous than the Sun.

Multiple Stars, Double Stars

Achird (Eta Cassiopeiae - η Cas) - is a beautiful double star for small telescopes. It consists of a magnitude +3.4 yellow G2 star, similar to the Sun, and a fainter orange-red K class dwarf secondary star of magnitude +7.5. They are separated by 13 arc seconds. A small 80mm (3.1-inch) scope at 80x magnification will easily split them. Both stars are classified as RS Canum Venaticorum variables. They fluctuate very slightly (0.05 magnitudes) due to active chromospheres.

At a distance of 19.4 light-years from Earth, Achird is the nearest Cassiopeia star to us.

Iota Cassiopeiae (ι Cas) - is an impressive triple star located 142 light-years from Earth. A 100mm (4-inch) scope at about 120x magnification easily reveals a white primary star of magnitude +4.6 along with a wide fainter red star of magnitude +8.5, separated by 7.3 arc seconds. More difficult, but also visible is the closer yellow star of magnitude +6.9 (separation 2.8 arc seconds from the primary).

The triple system may just be a chance alignment. However, the main star is a spectroscopic binary.

Psi Cassiopeiae (ψ Cas) - another multiple star in Cassiopeia. Although not as impressive as iota Cassiopeiae, it's a nice target for medium size scopes. It consists of a magnitude +4.7 orange primary star with a magnitude +9.0 wide companion, which itself is a double. The separation between the primary and faint pair is 20 arc seconds. The faint pair consists of white stars of magnitudes +9.4 and +10.0, separated by 2.9 arc seconds. The primary itself has a very faint close companion (14th mag.) making this a quadruple system. It's 193 light-years distant.

Although not visually spectacular, the three brightest members can be spotted in medium size scopes. A 200mm (8-inch) scope at about 200x magnification should show them all.

Sigma Cassiopeiae (σ Cas) - is located 1 degree southwest of open cluster NGC 7789. Sigma Cassiopeiae consists of magnitude +5.0 and +7.1 components, separated by 3 arc seconds. It's a challenging double for owners of 80mm (3.1-inch) scopes, but relatively easy with a small increase in aperture. With a magnification of about 150x, a 150mm (6-inch) scope will easily split them and on nights of good seeing. Smaller scopes should also do the job.

Sigma Cassiopeiae is located 1,500 light-years distant.

Variable Stars

Rho Cassiopeiae (ρ Cas) - is a rare yellow hypergiant and one of the most luminous stars in the galaxy. Rho Cassiopeiae is half a million times more luminous than the Sun and has a diameter of 630 million kilometers. To give an idea of it's enormous size, if the star were at the centre of our Solar System it would be large enough to completely swallow Mercury, Venus, Earth, Mars and almost reach the orbit of Jupiter!

Rho Cassiopeiae is classified a semi-regular pulsating variable star with a minimum magnitude of +6.2 and a maximum magnitude of +4.1. The period is approximately 320 days and it can be seen with the naked eye when towards the brighter end of the range. It's located 8,200 light-years from Earth.

Open cluster NGC 7789 is positioned 1.5 degrees southeast of rho Cassiopeiae.

V509 Cassiopeiae - although only around a dozen yellow hypergiant stars are known in the Milky Way there is another example in Cassiopeia, V509. This star is slightly smaller and less luminous than Rho Cassiopeiae but still incredibly powerful and large in its own right. It varies between magnitudes +4.6 and +5.2 over a period of about a year.

R Cassiopeiae - is a long period variable star of the Mira-type that changes between magnitudes +4.7 and +13.5 over a period of 430.5 days. At its brightest it's visible to the naked eye and easily seen with binoculars. The star is positioned in the southwestern part of the constellation and is located 410 light-years from Earth.

RZ Cassiopeiae - is an Algol-type variable for binocular and small telescope observers. It varies between magnitude +6.2 and +7.7 over a period of 28.7 hours.

YZ Cassiopeiae - another Algol type variable for binocular and small telescope observers. The brightness variation (mag. +5.7 -to +6.1) is not as large as RZ Cassiopeiae and therefore more challenging. It has a period of 4.47 days.

YZ also has a faint magnitude +10.5 wide companion, separated by 36 arc seconds.

Deep Sky

Open Clusters

NGC 457 - at magnitude +6.4, NGC 457 is the brightest open cluster in Cassiopeia and one of the finest examples in the northern sky. Since not a Messier item, it's often overlooked and therefore not as well known as other bright clusters. However, it's a stunning object in its own right with the main stars arranged in curves resembling an Owl shape, hence the popular name the Owl Cluster.

NGC 457 is just beyond naked-eye visibility. The brightest star inside is Phi Cassiopeiae (φ Cas - mag. +5.0), which can be seen with the naked eye although it's not a true member. Together with another 7th magnitude foreground star they form the Owl's bright eyes. Binoculars easily show the two brightest stars that are immersed in a cloud of just about resolvable nebulosity. A small 80mm (3.1-inch) scope reveals tens of mainly white stars. The notable exceptions are the brightest two members, which appear yellow and blue in colour. With medium and larger scopes many dozens of stars are visible, spread over 20 arc minutes of apparent sky.

NGC 457 is located 7,900 light-years distant and is estimated to be 21 million years old. It's also known as Caldwell 13.

NGC 129 - is a beautiful mag. +6.5 cluster located midway between Caph and gamma (γ) Cas. This sparkling group contains at least 35 stars with the brightest members being of 8th magnitude and visible in binoculars. The cluster is a fine telescope object with many dozens of stars visible arranged in chains across a diameter of 21 arc minutes.

NGC 129 is 9,900 light-years distant and estimated to be 76 million years old. It was discovered by William Herschel on December 16, 1788.

NGC 7789 - is a superb open cluster for medium or large size telescopes. It's located 3 degrees southwest of Caph and about halfway between rho Cassiopeiae and sigma Cassiopeiae.

This open cluster is one of the richest in the sky, consisting of at least 300 faint stars spread across 15 arc minutes of sky. At mag. +6.7, it appears as a faint hazy patch of light in binoculars. Telescopically, NGC 7789 is a treat. A 200mm (8-inch) scope reveals dozens of faint stars, extending into the hundreds with larger instruments. It gives the appearance of a loosely populated globular cluster.

NGC 7789 is located 7,600 light-years distant.

NGC 663 - mag. +7.1, is a prominent open cluster well suited for telescopes. It contains at least 80 stars visible in amateur scopes spread over a diameter of 16 arc minutes with the brightest cluster members visible with binoculars. A 100mm (4-inch) scope reveals a bright rich open cluster, with many stars visible, immersed in a hazy glow. The cluster is striking in large scopes where dozens of stars can be seen across the face. It's more concentrated towards the centre.

NGC 663 is located 3 degrees northeast of Ruchbah and 3 degrees southwest of epsilon (ε) Cas. Next to it are clusters NGC 654 and NGC 659 and M103.

NGC 663 is located 7,000 light-years distant. It's also known as Caldwell 10.

IC 1805 - is a large scatted bright mag. +6.5 open cluster located in eastern Cassiopeia. Its positioned 8 degrees east of Ruchbah and about 4 degrees north-northeast of the Double Cluster (NGC 869/884) in Perseus. IC 1805 is located just over a degree directly west of NGC 1027 with small cluster IC 1848, 2.5 degrees southeast of IC 1805. This part of the sky is an excellent region for scanning with binoculars. Also surrounding the three clusters is a network of faint nebulosity that's difficult to detect visually even with large amateur scopes.

The cluster contains about 40 stars spread over 22 arc minutes. The brightest members are 8th magnitude. IC 1805 is visible in binoculars. It appears like a detached part of the Milky Way with the brightest stars resolvable. In small and medium scopes, it's a nice object that's fully resolvable.

NGC 1027 - another fine binocular and small telescope open cluster is NGC 1027. Located in the same region of sky as IC 1805 and IC 1848, this cluster is similar in size and appearance to IC 1805 only slight fainter at mag. +6.7. It contains 40 members.

IC 1848 - mag. +6.5 is the last of our eastern Cassiopeia open clusters. Although not as impressive as its two neighbours, it contains only 10 stars and about half the size, the cluster is easily visible with binoculars and small scopes.

M 52 - is located towards the constellations western boundary. This is the brightest of Cassiopeia's two Messier clusters and a fine northern Milky Way cluster. Shining at magnitude +7.2, it's easily visible in binoculars appearing as a somewhat fan or V shaped haze with a prominent 8th magnitude yellow star positioned at the southwest corner. Small scopes reveal some fainter stars sprinkled across the diameter of the cluster. In larger scopes, M52 is a wonderful sight with dozens of stars visible.

In total, M52 contains 200 stars spread across 13 arc minutes of sky. It's located between 3,000 and 7,000 light-years distant and estimated to be 35 million years old. The emission nebula NGC 7635 (the Bubble Nebula) is one degree southwest of M52.

NGC 225 - mag. +7.0, is a loose open cluster consisting of 20 stars spread across 12 arc minutes of sky that's visible in binoculars. Telescopes reveal stars of similar brightness without any particularly great concentration.

NGC 225 was nicknamed the Sailboat cluster by Rod Pommier.

M 103 - is a small open cluster positioned 1 degree east of Ruchbah. It contains at least 40 members with a combined magnitude of +7.4. The stars are packed into a small area just 6 arc minutes across, which is equivalent to 1/5th the diameter of the full Moon. M103 is too faint to be seen with the naked eye but easily visible in binoculars, appearing like a faint wedge of light. Through a 100mm (4-inch) scope, its brightest four stars are resolvable and shaped like the Greek letter lambda (λ). With averted vision a nebulous triangular patch of light is revealed that extends beyond the brightest stars.

A 7th magnitude star (Struve 131) is the stand out star in the group although it's not a true member. This is an interloper, a star that's closer to us and just happens to be in the line of sight. Struve 131 is also a multiple star that's easily split in small scopes.

M103 is located 10,000 light-years distant. A few degrees east of it are open clusters NGC 654, NGC 659, Trumpler 1 and NGC 663. The latter is occasionally confused with M103.

NGC 659 - half a degree southwest of NGC 663 is magnitude +7.9 cluster NGC 659. This is small and compact with about 40 stars crammed into 5 arc minutes of apparent sky. It appears in binoculars as very small faint patch of light that's best seen with averted vision. Also visible in the same binocular field of view are NGC 663, NGC 654, Trumpler 1 and M103.

Through a 200mm (8-inch) scope, NGC 659 appears almost circular with at least 15 stars visible with direct vision and many more with averted vision. It's a wonderful sight.

NGC 654 - is located 0.75 degrees north of open cluster NGC 663. Like NGC 659, this is a small compact cluster with 60 stars across 5 arc minutes. It shines at magnitude +6.5, although often described as looking fainter than NGC 663. The dominant star is a magnitude +7.3 star towards the southeast, which is easily visible with binoculars. At 100x magnification, a 150mm (6-inch) scope reveals at least 40 stars configured in streams that look like a V shape.

Trumper 1 (Tr 1) - is the first entry in a catalogue of star clusters compiled by America astronomer Robert Trumpler during the 20th century. It's a small cluster of 20 stars spanning 4.5 arc minutes of sky. Trumpler 1 has a combined magnitude of +8.1 with member stars being of 10th magnitude or fainter. Although faintly visible in binoculars, it appears nothing more than a slightly denser clump of the surrounding Milky Way. Telescopes reveal a sprinkling of faint stars.

Nebulae

NGC 7635 (The Bubble Nebula) - is an emission nebula 0.5 degrees southwest of open cluster M52. It's an unusual nebula in that it's caused by gas expansion from the stellar wind of a massive hot magnitude +8.7 central star. At the same time the gas is excited causing it to glow.

Through a 200mm (8-inch) or 250mm (10-inch) telescope, NGC 7635 is visible on nights of good transparency and seeing. It appears as an extremely large faint shell of light surrounding the central star. The view is enhanced with averted vision and nebula filters can help. In total, NGC 7635 measures 15 x 8 arc minutes across although very large scopes are required to notice any elongation.

The Bubble Nebula was discovered in 1787 by William Herschel and is approximately 9,000 light-years distant. It's also known as Caldwell 11.

NGC 281 - is another faint emission nebula but easy to locate as it's 2.5 degrees directly east of Schedar. This large cloud covers an equatorial triangle of 7th magnitude stars. It can be seen with a 250mm (10-inch) scope at low magnifications on nights of good seeing and transparency.

IC 59 and IC 63 - Surrounding bright shell star gamma Cassiopeiae are two faint reflection/emission nebulae IC 59 and IC 63. Both are a difficult catch, they have extremely low surface brightness and the close proximity to second magnitude gamma doesn't help. It's possible to spot them with a 250mm (10-inch) scope from dark skies. A nebula filter and averted vision will help.

Galaxies

NGC 147 and NGC 185 - are two dwarf spheroidal galaxies located towards the constellation southern end close to the Andromeda boundary. Both galaxies are satellite galaxies of the Andromeda galaxy (M31) and therefore members of the Local Group. Neither has a particularly high surface brightness, although they can be spotted in 100mm (4-inch) scopes. Even with largest backyard scopes they appear mostly featureless.

NGC 147 and NGC 185 measure 13 x 8 arc minutes and 11 x 10 arc minutes respectively. They shine at magnitudes +9.3 and +9.1 respectively. NGC 147 is located about 2.5 million light-years distant with NGC 185 closer at 2.0 million light-years. The galaxies are objects 17 and 18 in the Caldwell catalogue.

NGC 278 - Three degrees south of NGC 185 is faint small elliptical galaxy NGC 278. In large backyard scopes this magnitude +11.5 galaxy appears compact and almost circular with a brighter core. It's apparent size is 2.2 x 2.1 arc minutes.

NGC 278 is located 40 million light-years from Earth.

Cassiopeia Data Table

Henry Draper Catalogue (HD) Hipparcos Catalogue (HIP) Bayer Flamsteed Struve Name RA (J2000) DEC (J2000) Visual Mag. Var. Var. Mag. Range Period (days) Double Sep. (arc secs) PA (deg.) Mag. Primary. Sec 5394 4427 Gamma Cas 27 --- Gamma Cas 00h 56m 43s 60d 43m 00s 2.15 Y 1.6 -> 3.0 irregular Y 2.1 248 2.2 / 11.2 3712 3179 Alpha Cas 18 --- Schedar 00h 40m 30s 56d 32m 14s 2.24 --- --- --- Y 65 280 2.2 / 8.9 432 746 Beta Cas 11 --- Caph 00h 09m 11s 59d 08m 59s 2.28 --- --- --- --- --- --- --- 8538 6686 Delta Cas 37 --- Ruchbah 01h 25m 49s 60d 14m 07s 2.66 --- --- --- --- --- --- --- 11415 8886 Epsilon Cas 45 --- Epsilon Cas 01h 54m 24s 63d 40m 12s 3.35 --- --- --- --- --- --- --- 4614 3821 Eta Cas 24 60 Achird 00h 49m 06s 57d 48m 55s 3.46 --- --- --- Y 12.9 317 3.4 / 7.5 15089 11569 Iota Cas --- 262 Iota Cas 02h 29m 04s 67d 24m 09s 4.46 --- --- --- Y AB 2.8 / AC 7.3 AB 230 / AC 114 A 4.6 / B 6.9 / C 8.5 8491 6692 Psi Cas 36 --- Psi Cas 01h 25m 56s 68d 07m 48s 4.72 --- --- --- Y AC 20 / CD 2.9 AC 128 / CD 253 A 4.7 / B 9.4 / C 10.0 224572 118243 Sigma Cas 8 3049 Sigma Cas 23h 59m 01s 55d 45m 18s 4.88 --- --- --- Y 3.0 326 5.0 / 7.1 224014 117863 Rho Cas 7 --- Rho Cas 23h 54m 23s 57d 29m 58s 4.51 Y 4.1 -> 6.2 320 --- --- --- --- 224490 118188 R Cas --- --- R Cas 23h 58m 25s 51d 23m 20s 4.70 Y 4.7 -> 13.5 430.5 --- --- --- --- 4161 3572 YZ Cas 21 --- YZ Cas 00h 45m 39s 74d 59m 17s 5.64 Y 5.7 -> 6.1 4.47 Y 36 160 5.7 / 10.6 217476 113561 V509 Cas --- --- V509 Cas 23h 00m 05s 56d 56m 43s 5.10 Y 4.6 -> 5.2 360 --- --- --- --- 17138 13133 RZ Cas --- --- RZ Cas 02h 48m 56s 69d 38m 03s 6.18 Y 6.2 -> 7.7 1.20 --- --- --- ---

Cassiopeia Sky Deep Sky Data Table