The Magnificent Seven
An open star cluster is a group of up to a few thousand stars that were formed at the same time. Star clusters are held together by gravitational attraction when young, but usually only survive for a few hundred million years. As they make their epic journey around our galaxy they gradually break up as they pass close to other clusters and clouds of gas.
More than 1,100 open star clusters have been discovered in the Milky Way. Here are my magnificent seven, over half of which can be seen with the naked eye from our Dark Sky Park under good conditions. They are visible in the south to south west during late winter and early spring evenings.
The Hyades is our nearest open star cluster – just 153 light years away. This makes it a prominent object that is easily seen with the naked eye. It can be found in the constellation of Taurus where its brightest stars form a “V” shape along with the still brighter Aldeberan. Aldebaran itself however is not part of the Hyades because it is much closer to Earth and merely happens to lie along the same line of sight.
Around a dozen stars are visible to the naked eye, but several dozen can be seen through binoculars. Because the Hyades span over five degrees of sky they look far better in binoculars than a telescope which cannot fit them all in!
The age of the Hyades is estimated to be about 625 million years. In England the cluster was known as the “April Rainers” from an association with April showers, as recorded in the folk song “Green Grow the Rushes O”.
The Pleiades are also easily visible with the naked eye, but at 444 light years are further away than the Hyades. They can be found to the north west of the Hyades. A prominent sight in winter and early spring, they have been know to cultures all around the world since antiquity.
The nine brightest stars of the Pleiades are named after the Seven Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygeta, Celaeno and Alcyone. These were the daughters of Atlas by Pleione.
The cluster has over 1,000 members and is dominated by hot blue and luminous stars that have formed within the last 100 million years.
M44 – The Beehive
To the naked eye the Beehive looks like a large area of nebulosity covering almost three times the diameter of the full moon. You can find it on the western side of the centre of the constellation of Cancer.
Ancient Greeks and Romans perceived this star cluster as a manger from which two donkeys are eating. The adjacent stars Asellus Borealis and Asellus Australis are the donkeys themselves.
Containing over 350 stars it lies a little further away then the Pleiades at a distance of 577 light years. Viewed through binoculars it springs to life, filling a good proportion of the field of view with dozens of stars sprinkled over a hazy background. A modest telescope reveals groups of bluish-white stars arranged in pairs and triplets. Their age and proper motion within our galaxy coincides with those of the Hyades, suggesting they may share similar origins.
At 3,900 light years Messier 35 is much further away than the Pleiades. It is nevertheless visible to the naked eye from our Dark Sky Park under good conditions. Look in the constellation of Gemini where it appears as a faint misty glow just above the end of Castor’s foot. It has around 400 member stars covering an area almost the size of the full moon.
Through 10 x 50 binoculars, the cluster appears as a hazy rectangle with around a dozen individual stars visible. An 80mm telescope reveals many more stars scattered across the face of the cluster.
Messier 37 is the richest and brightest of the three Auriga clusters and is about 4,500 light years from Earth. However it lies at the limit of naked eye visibility so binoculars will be needed. It can be found just to the south east of the “Pentagon” shape formed by the five main stars of Auriga.
A pair of 10 x 50 binoculars will reveal a large hazy patch of light. Through a small (100mm) telescope about a dozen or so tenth magnitude stars can be seen concentrated towards the centre of the group. The stars appear faint and surrounded by a misty haze, giving the impression of a sprinkling of diamonds or stardust!
M36 – The Pinwheel
Messier 36 is also at the limit of naked eye visibility and lies around 4,300 light years distant from Earth. It can be found to the north west of M37, just inside the Pentagon.
Through binoculars it appears as a small fuzzy patch of light which has at least 60 members. When viewed through larger binoculars or a small telescope, the fuzziness of M36 is transformed into a sprinkling of stars. An 80mm telescope at low / medium powers reveals about 15 or so bright stars scattered throughout the cluster. Most of them appear white or bluish white and are arranged in an “X” shape.
M38 – The Starfish
Although Messier 38 is too faint to be seen with the naked eye it is still an easy binocular target. Lying some 3,500 light years away from Earth, it can be found just south of the centre of the Pentagon.
When viewed through 10 x 50 binoculars, it appears large and misty with the brightest stars just about resolvable. A modest telescope will reveal many more individual stars. Some say that this cluster reminds them of a starfish!