Identifying star clusters
The Pleiades, the Hyades and the Beehive cluster are all amazing in their own right but, to pose a straightfoward question: which is which?
Imagine that you're separated from any kind of star chart. So how, other than searching by Right Ascension (RA) or Declination (DEC), could one know where to look?
The Pleiades is also known by its Messier catalogue number, M45. Although nicknamed the Seven Sisters (map identifying stars of the Pleiades), M45 is actually formed by over 100 stars.
It's located at RA 3:47, DEC +24.07 with a visual brightness of 1.6, and apparent dimension of 110.0 arc minutes.
The Hyades, also known as Melotte 25 can be found at RA 4:27, DEC +16 with a visual brightness .5 magnitude and apparent dimension of 330 arc minutes.
Both the Pleiades and the Hyades are open star clusters in the constellation Taurus (the Bull) and can been seen with the naked eye.
Locate the Seven Sisters perched on the Bull's right shoulder. They lie about 4 degrees from the elliptic so are frequently occulted by the Moon and other planets (which proves to be great observer eye candy!). There is quite a bit of nebulosity located within the Pleiades, especially around the brightest stars.
I enjoyed watching them appear to sit on the mountain summit where I lived outside Mexico City. Greek mythology relates the story of the Pleiades placed in the sky while seeking refuge from Orion’s nonstop pursuit.
The Hyades, 150 light years away from Earth, outlines the Bull’s face and is a loose V-shaped cluster of white stars. It's less populated and younger than the Pleiades. While the central star group is about 10 light years in diameter the outer group is spread over 80 light years. Astronomical studies of the sky show the Hyades moving eastward in the sky toward Betelgeuse in Orion.
It should be noted that the bright red star, Aldebaran (Alpha Tauri) - the eye of the Bull - is not a member of this cluster despite lying in the Hyades field.
It is thought that the Hyades may have a common origin with the Beehive cluster (located in the constellation Cancer), due to their similarities in proper motion and age.
The Beehive cluster lies at RA 08 : 40.1, DEC +19 : 59, with a visual brightness of 3.7 magnitude and apparent dimension of 95.0 arc minutes. Known also as Praesepe (Latin for manger), it's Messier No. is M44. When observing it, look for the eclipsing binary star TX Cancri. Epsilon Cancri is also an eye catcher and worth spending some time to find
Open clusters - NGC objects 4755, 265, 290 & 6231
Why Open Clusters are Important
Stars in an open cluster make excellent laboratories in the study of the evolution of stars. Born at the same time and from the same stellar gas cloud, they all move in the same direction. Although they may have different masses, they have same "star DNA".
Southern Sky Jewels
NGC 4755 is located at R.A.: 12:53.6 and Dec: -60:20
This cluster was discovered by Nicolas Louis de Lacaille from an observing station in South Africa during his stay there from 1751 to 1752. Herschel called NGC 4755 "the jewel box" and described it as "a casket of variously coloured precious stones". Kappa Crucis is a large and luminous aging supergiant orange star easily distinguishable against its sapphire colored companions. The three brightest blue stars are of magnitude 5.7 and are easy to locate.
This is an open cluster estimated to be between 7 and 10 million years old. This jewel box appears to be "falling" into the Coal Sack in the constellation Crux. Visible from southern latitudes and located east of the Southern Cross, it contains over 100 stars. It is also known as Caldwell 94, and the Kappa Crucis Cluster. There are at least 50 bright stars in this cluster located within a 10 arc minute diameter. The five brightest stars range in magnitude from 5.7 to 8.3 with the brightest star in the center of the cluster identified as a red supergiant comparable to Betelgeuse.
A Pair of Bright Sparkling Magellanic Gemstones
NGC 265 is located at R.A.: 0:47.3 and Dec: -73.29
NGC 290 is located at R.A.: 0:51.3 and Dec: -73:09
This pair of open clusters were born of the same intersteller gas cloud and are loosely held together by gravity. They are both bright (12 magnitude) compact open clusters located in the Small Magellanic Cloud spanning a distance of 65 light years across and located 200,000 light years in distance from the Blue Planet in the Constellation Tucana. Both open clusters consist of mainly young, hot white stars along with red supergiants, some day to meet their explosive death as supernovae.
Northern Latitude Jewel Box
NGC 6231 is located at R.A.: 16:54.0 and Dec.: -41.48
This cluster is located in the southern part of Scorpius, half a degree north of Zeta Scorpii. NGC 6231 was discovered by Giovanni Battista Hodierna, a Sicilian Roman Catholic priest, born in 1597, who taught mathematics and astronomy. It is a relatively young cluster, considered to be only about 3.2 million years old.
Although it is listed as a northern latitude target, it may be difficult to see due to its location in Scorpius. It is bright enough to be seen without visual aids under good seeing conditions. Visually, NGC 6231 is similar to the Pleiades (M45) with central bright white giant and supergiant stars ranging in magnitude from -7 to -3.5 and glimmering as sparkling diamonds against a dark velveteen sky, some as spectroscopic binaries. If the Bull's Pleiades and the Scorpion's open cluster (NGC 6231) were in the same part of the sky, the Scopion's cluster would be brighter.
Constellations - Canis Major
Canis Major is found south east of Orion. An easy way to locate the constellation is to locate the three stars that make up Orions Belt and follow the stars down in a south westerly direction until you come to the next bright star. This is Sirius in Canis Major, its the brightest star in the constellation..
South of Sirius, the open cluster M41 can be found. Like all open clusters, it contains a few hundred young stars and has no particular shape.
Together the stars are just bright enough to be seen with the naked eye on a clear night with no moon. With binoculars they appear as a faint smudge and in a telescope, the cluster is easily seen, covering about as much sky as the full moon does.
As the cluster appears low in the sky from northern latitudes light pollution reduces the splendor of this cluster but its still worth a look.
Sirius also has a companion star, known as Sirius B. Sirius B was the first 'white dwarf' to be discovered.
|Sirius||-1.4||8 L/Y||Has white dwarf companion Mag 8.5|
|Name||Mag / Seperation||Comments|
|Sirius||-1.4 & 8.5 / variable||Binary system with an orbital period of 50 years. In the mid 1990's the apparent separation was at its minimum so a powerful telescope is required to split the pair.|
Constellations - Cassiopeia
Cassiopeia is marked by its W-shape in the sky, with stars ranging from magnitude 2 to 3.5 marking each turn in the W. Like Ursa Major Cassiopeia is circumpolar and can be seen all year round from northern latitudes.
Cassiopeia lies in the Milky Way, so many objects like nebulae (clouds of gas and dust) and star clusters lie within it. Scanning Cassiopeia with binoculars will reveal many of them though not the nebulae. Using a telescope is even more revealing.
The Open Cluster M103 is quite easy to find as are the double stars, try locating M52 by using the star hopping technique. The objects listed below show up well in a 6" telescope though some are quite small or sparsely populated as in the case of M103.
|M52||6.9||Open cluster 100+ stars|
|M103||7.4||Open cluster 25 stars|
|NGC663||7.1||Open cluster 80 stars|
|NGC457||6.4||Open cluster 80 stars|
|2.2||780 L/Y||Variable star between 1.6 and 3.2|
|Name||Mag / Seperation||Distance|
|3.4 & 7.5 / 12.2"||480 L/Y|
|4.6 & 6.8 / 2.4"||840 L/Y|
Find your way to Cassiopeia
Constellations - Cygnus
Cygnus is situated in the Milky Way. Its brightest stars mark the Northern Cross. Cygnus, with the bright star Deneb in the swan's tail, appears high in the summer sky. The three bright stars Deneb, Vega (in the constellation of Lyra), and Altair (in the constellation of Aquila) mark the Summer Triangle. Altair is about 40 degrees from Deneb and Vega.
Albireo, the bright star at the head of the Cygnus, is an excellent example of a telescopic double star. Even with binoculars, you can see that it consists of two stars of different colours, one is very orange the other bluish white. You must take a look at them.
As Cygnus lies in the Milky Way it is full of variable stars (stars who's brightness varies over a period of time) and the have been several novae in the boundaries of this constellation - the last was seen in 1976.
The North American Nebula lies within Cygnus, it's very dim but is reported to be easy to photograph. The nebula needs a telescope with aperture larger than 6" to see it.
|Deneb||1.2||1800 L/Y||70,000 times as luminous as the Sun.|
|Name||Mag / Seperation||Comments|
|Alberio||3.1 & 5.1 / 34.4"||Best example of a double star, with one star shining red the other blue, magnificent.|
Star Profile: Fomalhaut
Here's a tricky question for astronomers everywhere: what's the 17th brightest star in the sky?
The answer (if you hadn't guessed it from the title of the article!) is Fomalhaut known also, rather curiously, as the 'First Frog' or alternatively 'The Lonely One'. It lies in the constellation of the Southern Fish (Pisces Austrinus), which actually looks like a stylistic fish lying on its back drinking in the waters of knowledge from Aquarius.
Fomalhaut is a bluish white star, younger than our Sun and is located about 25 light years away. It's surrounded by a warped disk formed from icy dust particles similar to that around Vega, Beta Pictoris and Denebola.
Fomalhaut may hold secrets of a forming planetary system, its dusty ring is similar to the disk generated by the Solar System's Edgeworth-Kuiper Belt.
So where did this star get its name and how do you pronounce it? Well, 'FUM-al-HUT' in Arabic means fish's mouth, and I am told it is pronounced 'foma-low' in English. This makes it easy for me to remember since Fomalhaut hangs low in the south autumn sky.
It is a suspected variable star and has been categorized under many names:TYC6977:1267:1 HR8728, Hip113368, HD216956, SAO191524, and LTT9292, just to name a few.
Mr. Thomas Jefferson Jackson See reported a companion red-orange dwarf TW PISCIS (CG31978, SAO214197, TYC7505:100:1), in 1897.
Another K5 dwarf star known as LTT8273 may be an optical companion. This particular star is originally thought to be a member of a now dispersed star cluster including Fomalhaut, Vega and Castor. Fomalhaut's future may entail evolving into a white dwarf in a billion years. (Don’t wait up!)
Has Fomalhaut's dusty disk coalesced into planets around the star? Theories point to this possibility. Rumour also has it that our Solar System may have looked like Fomalhaut's dusty system four billion years ago. I think you will enjoy observing this star - it's more than just a small glimmer in the southern sky.
Constellations - Pegasus
Pegasus is marked by "the Great Square of Pegasus", fours stars in the horse's body that form a square whose sides are each over 10° degrees (the width of one fist, held up to the sky) across.
You can find the Square of Pegasus by following the line from the pointers of the Big Dipper through Polaris and then twice as far on Polaris' other side. One of the stars (Alpheratz) is actually now over the boundary into the constellation Andromeda. The other three stars are in Pegasus. All are between 2nd and 3rd magnitude. There are no brighter stars in the square.
Pegasus is home to few faint galaxies and a Globular Cluster (M15). M15 is easy to see in binoculars though no detail can be seen. A telescope shows a little detail around the edges on a night with good visibility.
You'll need to be pretty experienced at observing variables to spot the difference that Scheat displays varying in brightness by just half a magnitude! Good hunting.
|Scheat||2.4 - 2.9||178 L/Y||Semi variable|
|Name||Mag / Seperation||Comments|
|Matar||2.9 & 9.9 / 90.4"||9.9 star has faint companion|
|Scheat||2.4 - 2.9||35 - 40 days|
Constellations - Ursa Major
Ursa Major, the Big Bear, is one of the most familiar constellations. For those of us at northern latitudes, it can be seen all year as the constellation is circumpolar. (Although its orientation about the pole star will change over the course of the year.)
The star in the middle of the Big Dipper's handle is a double star, you can see them with the naked eye if the sky is clear and your eyesight is good. The brighter of the two stars is known as Mizar, and the fainter one is Alcor. It's easy to spot this double star using a pair of binoculars.
In a telescope, Mizar turns out to be double itself so we have a three star system. All the main stars listed below except Duhbe and Alkaid are part of a moving cluster though you'll have to wait many thousands of years to notice any shift in position!
|Mizar||2.1||59 L/Y||An easy double|
|Name||Mag / Seperation||Comments|
|Mizar||2.3 & 4.0 / 14"||Makes naked eye pair with Alcor|
Find your way around Ursa Major (Big Dipper)