Galactic Cluster

Galactic Cluster

There are two kinds of star cluster. One type, known as Galactic clusters, have a loose or open structure. They contain anything from a few dozen to several hundred stars which travel together through space at speeds of many miles a second and which presumably had a common origin. Two conspicuous examples in Northern skies are the Hyades (the V-shaped group that forms the face of Taurus, the Bull) and the Pleiades, popularly called the Seven Sisters. The latter is particularly striking when seen through binoculars or a small telescope, for its brightest stars are huddled together in a space no larger than the apparent area of the full moon. The cluster also contains several hundred fainter stars, but owing to its distance, 400 light-years, most of these can be seen only in large telescopes.

NGC 700. North America Nebula.Image in public domain

Globular Clusters

The other kind of cluster takes the form of a hall-like swarm of at least several thousand stars, packed densely toward the center, and spread more thinly in the outer regions. In small telescopes these globular clusters look insignificant, but it is because they are so far away. The brightest, Omega Centauri, lies in Southern skies at a distance of about 22,000 light-years. To the unaided eye it looks like a hazy star, but in reality it consists of several hundred thousand stars spread over at least 200 light-years of space.

The brightest globular cluster in Northern skies is Messier 13 in Hercules, discovered by Edmond Halley in 1716. “This is but a little patch,” Halley wrote, “but it shows itself to the naked eye when the sky is serene and the moon absent.” Later, in the 18th century, William Herschel observed it through large reflecting telescopes and estimated that 14,000 were “cribb’d, cabined and confined “in Halley’s “little patch”. Modern studies have let astronomers to raise their number to at least 50, 000, and to suggest that it may surpass one million. The estimated distance is about 34,000 light-years.

Crab Nebula. Messier I, the Crab Nebula in Taurus. Image in public domain

Hercules Cluster. Messier 13, a globular star cluster in Hercules. Image: Bareket.astro.com

The Pleiades. Image in public domain