Be ye lads an’ lasses of valor? Do ye favor a quest fer treasure in the Celestial Sea? Then grab yer binoculars (or telescopes) and let’s weigh anchor on the next dark tide to quest for a king’s ransom of silver in the heart of Hercules. (Yes, my 4-year-old son has been playing Lego Pirates of the Caribbean a bit too much of late).
To find the object of our quest, we will need a clear dark night with either no moon, or only a less-than quarter moon in the sky. The next 10 days are just such a period, with the moon in its new phase (invisible, near the sun in the sky) on June 19th. The Great Cluster of Hercules is barely visible to the naked eye (from a very dark location on a perfect night), but will be seen easily in binoculars or a small telescope.
At about 9:30-10:30 p.m., face south and look directly up over your head. Three bright constellations dominate the June sky. Referring to the map I’ve attached to this blog, look for the Big Dipper to your upper right as you face south. This very commonly recognized constellation can be used as a signpost to find many of the other bright constellations in the sky.
Following the line formed by the two stars at the end of the bowl of the Dipper, we can find the North Star. Following the curve of the last three stars of the Dipper’s handle, we can “arc to Arcturus”, the brightest star in the constellation Bootes. But to complete today’s quest, we follow the line formed by the first two stars of the handle, cross above Bootes, and straight into the heart of Hercules, the greatest demi-god hero of Greek legend.
Before using the binoculars, be sure you have been in the dark for at least 10 minutes (and that you haven’t been looking into a bright light during that time, and that your flashlight has been off). Your eyes need that time to become adapted to the dark. They really need 20-40 minutes (depending on your age) to become completely adapted, but 10 minutes will be enough for this quest.
Hercules is a majestic and large constellation. Its core is formed by four stars in a trapezoid or keystone shape. Aim your binoculars at the bright star at the northwestern corner of the keystone (the upper right star looking at the map I’ve provided). Now slowly scan toward the lower bright star on the same side of the keystone. About 1/3rd of the way toward the lower star, you will find a fuzzy gray patch of light – this is the object of our quest.
One of the first lessons I give to new astronomers is to be sure to take plenty of time observing objects that you see with telescopes or binoculars. Your eyes, and your mind, need time to interpret what they are seeing. Generally, the longer you look at an astronomical object, the more detail you will be able to see. So when you do find the Great Cluster, give it its due — be sure to look steadily at it for at least 60 seconds, and hopefully much longer.
At first glance, the Great Cluster will appear as a soft gray cloud. As you look longer, you should be able to tell that the center of that cloud is brighter than the outer edges. Looking longer, or with a larger pair of binoculars or a telescope, you may be able to tell that what you are seeing is a vast pile of individual stars – you may be able to see isolated stars around the outer edge of the cluster.
The Great Cluster of Hercules is a globular star cluster, so called because it is a vast ball of stars, all gravitationally bound to one another. Although it is not possible to count the number of stars in this cluster, it is estimated to have several hundred thousand stars, and fills a volume of space 145 light years in diameter. The light that travels through your binoculars and into your eyes as you observe this object has travelled though space for over 25,000 years (that is, it is 25,000 light years from Earth).
The globular clusters we observe from Earth are not in the Milky Way galaxy, but rather orbit our galaxy in a colossal spherical halo centered on the core of our galaxy. These clusters formed at the same time as our galaxy, approximately 11 billion years ago. The brightest stars within globular clusters are red giant stars nearing the end of their lives as they exhaust the hydrogen upon which their fusion reactions have run throughout their existence.
In the attachments I’ve included my best image of this gargantuan cluster of stars, as photographed through the 10” diameter telescope I use for the observation portion of my course in astronomy. To read more about the course, please see www.turnerclasses.com.
I’ve also included an incredible photograph taken by the Hubble Space Telescope of the Sombrero Galaxy, in which we can see several hundred of the globular clusters orbiting that galaxy. The Sombrero Galaxy has about 2000 globular clusters, about 10 times as many as have been discovered orbiting our own Milky Way galaxy.
I hope you have the opportunity to get out on a dark night over the next couple of months to see this wonder of the sky. Please let me know of your triumphant capture of this observational treasure!