Hercules … and You
Hercules has long been recognized as a human figure and symbol of awesome strength, even though in the sky he’s upside down and kneeling. Few people today would see a man first off. There is a quadrangle with trails of stars off of each corner. The upper trails, the legs, are shorter – that explains the kneeling part – and the lower arms are extended far south due to his holding of various weapons, a la Orion. We better focus on finding the quadrangle, also known as the Keystone.
Find Bootes which appears to have a tall kite shape. From the top of the kite look to the east (left). There is a tight U-shaped constellation known as the Northern Crown (Corona Borealis). A bit further east is the Keystone. It is wedged between the Crown and the bright star Vega in another small constellation called Lyra.
The alpha star Ras Algethi is found far south of the Keystone. The chart above shows that the simplest way to “hop” there is to imagine a line running south from the northeast corner of the Keystone (pi Hercules), through Sarin (delta Herc) and about that much further south. Ras Algethi, an Arabic name, ironically seems to sport Christmas colors out of season. Most telescopic observers see a deep red and somewhat greenish pair. There are no green stars per se, but some appear that way in contrast to a near neighbor. With your unaided eye you may detect a shimmer or a twinkle on occasion, but this star is one worth borrowing a telescope to see.
On the leading or western edge of the Keystone, on a good seeing night, one should detect a slightly fuzzy star. This is one of the most spectacular sights in the Northern Hemisphere’s sky. It is a group of hundreds of thousands of stars which Charles Messier catalogued as M13. Imagine the biggest, brightest diamond cluster ring available on any home shopping channel. Yes, it’s another reason to borrow, buy, or steal a scope.
Credit: Raysastrophotograhy. Creative Commons Attribution-Share Alike 4.0 International license.
M13 is 25,000 light years distant. You might think that, if we can see it naked eye, it must be a gargantuan object, and you would be correct. It’s difficult to pin down the size of the cluster because of the thinning out of stars around the perimeter. Where do we draw the line? M13 also has curved trails of stars – think Milky Way arms – fanning out in several directions. Despite all these complications we’ll just use Burnham’s best estimate of 100 light years in diameter.
A Trick of Scale. All the fine photos of M13 depict a tremendous jumble of stars. Why doesn’t gravity just pull this mess together in one huge black hole or a mega-nova? As a matter of fact many of the larger clusters are surmised to have black holes at their centers, but to answer the question, Burnham suggests imagining a scale model. Let the stars of M13 be represented by upwards of a million grains of sand. Distribute them in a sphere some 300 miles in diameter. That’s the distance across Pennsylvania from Philly to Pittsburgh. Depending on how near the center, where the stars are closer to one another, incredibly, each grain of sand would be, on average, between one and three miles from its nearest neighbor! The universe is vast, dear reader.
We’ll do more with the M13 dimensions in projects below.
… and You
I. The coinage honoring our big strong guy Hercules carries important historical associations. See if you can dig out the full stories of:
a. The labor of the cattle, the Straits of Gibraltar, and the phrase “[Ne] plus ultra.” Look here. b. The thirty pieces of silver paid to Judas. c. The legendary Pieces-of-Eight from pirate lore. Look here.
II. M13 and other globular clusters allowed astronomer Harlow Shapely in the 1920s to make an eye-opening estimate of the size of our own Milky Way. He noticed that the clusters were more numerous in the part of the sky toward Scorpius and Sagittarius, where the Milky Way center is thought to be. He theorized that the center of distribution of these MW hangers-around might agree with the galaxy’s center.
a. How big is the Milky Way? Look here. b. How far are we from the center of our galaxy? c. How far is M13 from the center?
III. About that trick of scale. Let’s accept 100 light years as the diameter of M13.
a. How many miles is that? b. We have two man-made satellites that have actually left our solar system. How many miles distant are they?
Do a Google search “light years to miles.” It will show that one light year is almost six trillion miles, so M13 is about 600 trillion miles across.
The Voyager I probe, at 148 AU (astronomical units) distant remains in radio contact with us. One AU is the Earth-Sun distance of 93 million miles, so 148 of these puts Voyager I at a couple of billion miles away. Voyager launched in 1977, or 43 years ago. If the probe had been launched then from the center of M13, it would be just one part in 100,000 from leaving the cluster.
Sources: Burnham’s Celestial Handbook, Audubon Field Guide