With the realm of the galaxies passing into the western sky and the Milky Way rising in the east, this week is a great time to explore the many globular clusters of the early summer sky.
Globular clusters are huge ancient clusters of stars. They contain 100,000 or more stars and date back to the formation of the Milky Way galaxy.
These star clusters orbit the central mass of the galaxy but are not confined to the plane of the galaxy. As a result they appear in a band just above and below the plane of the galaxy.
If you are out under a dark country sky in the northern hemisphere in early July, you will see the main band of the Milky Way galaxy rising from the southern horizon and crossing the eastern half of the sky.
The globular clusters lie in a band just to the right (west) of the Milky Way. There is a similar band on the southern side of the Milky Way. (This graphic shows how to spot the globular clusters in the sky).
Globular clusters are best viewed with magnifications around 100x in order to resolve them into stars. [More photos of globular clusters.]
The first globular cluster hunter
Noted French astronomer Charles Messier was the first to pay close attention to globular clusters. He began his famous catalog of deep sky objects in May 1764, and most of the first objects he catalogued were globulars.
Tonight you can follow in his footsteps.
On May 3, 1764 Messier logged an object he called Messier 3 (M3) as a "Nebula without star; center brilliant, gradually fading away; round. In a dark sky, visible in a telescope of 1-foot."
In Messier?s day, telescopes were described by their focal length, so a "telescope of 1-foot" had a focal length of only 300 mm, rather like the finder scope on a modern amateur telescope. This explains why Messier saw no stars in the cluster. It takes at least a 4-inch telescope to resolve it into stars.
Messier 3 is located in the starless spaces of Canes Venatici, and is most easily found by starhopping from the bright star Arcturus in Bootes.
Five nights later Messier observed Messier 4 (M4). It had been discovered earlier by Philippe Loys de Ch?seaux. Messier described it as a "cluster of very small stars; with an inferior telescope it appears more like a nebula."
This cluster is one of the treasures of the southern sky. Northern observers miss the best views of it because of its low altitude in northern skies. Look for it just to the right of the bright star Antares.
More clusters followed
Messier observed Messier 5 (M5) a couple of weeks later on May 23. Again he described it as "a fine nebula which I am sure contains no star. Round; seen well in a good sky in a telescope of 1-foot." In any good modern telescope, it is resolved into many, many stars.
M5 is located in the constellation Serpens (the Snake), an odd constellation because it is in two parts. This is because the Snake has been cut in two. Serpens Caput is the snake?s head and Serpens Cauda is the snake?s tail. It?s usually easiest to find M5 by star-hopping northward from Libra.
In late May 1764, Messier ventured into Ophiuchus and logged four more globular clusters on his list — all of which appeared to him to contain no stars, thanks to his tiny telescopes.
Ophiuchus is the Orion of the summer sky — a huge hero standing astride the sky and carrying half a snake in each hand: The two halves of Serpens.
M9 is down near his foot, where the constellation dips into the ecliptic and becomes the 13th constellation of the zodiac. M10, M12, and M14 form a chain in the middle of Ophiuchus, where Orion has his belt.
Riches of Hercules
Finally, on June 1, 1764, Messier observed the finest globular cluster in the northern sky, Messier 13 (M13) in the constellation Hercules.
Messier was not the discoverer of this cluster; that honor went to the famous British astronomer Edmond Halley in 1714. Halley said, "This is but a little patch but it shows itself to the naked eye when the sky is serene and the moon absent."
Once again, Messier saw no stars in M13.
The most notable feature of the Hercules constellation is the keystone pattern at its center, which is sometimes called by the less glamorous name "the flower pot."
This cluster is easily found just below the upper right corner of the flower pot. In a telescope, look for the propeller patterns which many people see in the central part of this rich cluster.
Many years later, in 1781, Messier returned to Hercules to observe a smaller globular cluster which Bode had discovered four years earlier. Once again Messier was using his 1-foot telescope and saw no stars.
We may wonder why Messier used such a small telescope for so many of his discoveries.
His main interest was in comets, not nebulae or clusters, and to find comets it?s necessary to sweep large areas of sky, so a low power, wide field telescope is preferable. Nowadays we are more interested in resolving these clusters into stars, and for this a moderately large aperture, say 8- to 10-inches, is preferable.
- Gallery — Globular Clusters in Space
- Beginner Astrophotographer Telescopes
- More Night Sky Features from Starry Night Education
This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions.
