What Dark Secret Dwells In This Cluster’s Heart?


Storm clouds drift across the northern sky just before sunset yesterday evening. The left side is still lit by the sun while the right is in shadow. Photo: Bob King

Just a few days ago, we looked at the constellation of Pegasus the Flying Horse, which features a large, diamond-shaped asterism called the Great Square of Pegasus. You’ll find the Square two outstretched fists below and to the right of the W of Cassiopeia in the northeastern sky at nightfall.


The Great Square is already climbing up the east-northeast sky at 9:30-10 o’clock. Just above the star Enif, the bright cluster M15 beckons binocular users. Maps created with Stellarium.

Dangling like a tasty morsel just beyond the horse’s nose is the spectacular globular cluster M15, the 15th entry in the Charles Messier’s catalog of clusters, nebulas and galaxies that he saw in his small telescope back in the mid-18th century. M15 is easily visible in 7×35 or 10×50 binoculars as a round, cottony fuzzball tucked into a small triangle of stars. You’ll notice right away that it looks different from its neighbors.


Once you find Enif, the nose of the horse, place it at the bottom of your binocular view. Now look up near the top for a tight group of stars, one of which will betray itself as M15 by its soft, fuzzy outline.

A 6-inch telescope begins to resolve some of the cluster’s tens of thousands of stars and clearly shows how dense and bright the cluster’s core is. In fact, M15 has one of the densest cores of any cluster in the Milky Way galaxy. While the cluster spans some 175 light years, half of all its starry matter is concentrated inside a sphere only 20 light years across at its center. The view of the night sky from a planet revolving around a star in the core must be packed with hundreds of stars brighter than Venus. Its residents would never know a dark night sky.


The gorgeous globular (GLOB-you-lurr) cluster M15 in Pegasus is resolved into a pile of stars when photographed through a telescope. Globulars are named for their globe-like appearance; they’re the chandeliers of the galaxy. Credit: Jim Misti

Scientists still aren’t sure why M15 has undergone "core collapse". Either the stars have settled at the center through gravity or else there’s a supermassive black hole hiding there drawing the stars into ever tighter orbits. If you do get a chance to see this globular cluster in a telescope, the brightest stars you’ll glimpse are some 1000 times brighter than our sun. M15 is a cluster of superlatives and one of amateur astronomers’ favorites.


You might be able to split Ganymede and Europa around 12:30 a.m. Central time but you’ll need very high power. Watch as the two moons separate slightly before the eclipse begins at 1:04 (below). The map shows Jupiter as you’d see it in a reflecting telescope with south at top and east to the right.

For more advanced observers, there’s an interesting eclipse happening after midnight tomorrow morning, Weds. Aug. 19. Every six years, Jupiter, the Earth and the sun are lined up so that we see the moons of Jupiter pass almost directly in front and behind one another as they orbit the planet. When a moon crosses in front of another moon astronomers call it an occultation. When the shadow of one moon hides another, that’s an eclipse. 


This view shows the planet and moons at 1:10 a.m. Central time when the eclipse is underway. I’ve drawn the outline of Europa to show you where it is — you won’t actually see it as a circle.

From 12:32-12:48 a.m. tomorrow, the moon Ganymede will occult Europa. Through a telescope, the two moons will be visible as two, closely-spaced separate dots at around midnight, but then merge into a single dot during the occultation. Things get more interesting just 15 minutes later. From 1:04-1:22 a.m., Ganymede will cast its shadow on Europa and make it disappear from view! The eclipse is 99 percent total.

The amazing video at right shows a partial eclipse of Ganymede by the moon Io. Philippine amateur astronomer Chris Go created it by combining multiple photos taken through his telescope.

Sounds like fun tonight if you’re willing to sacrifice sleep. If staying up is just not an option, don’t sweat it. There will be other eclipses and occultations visible in small telescopes later this summer when Jupiter will be up high during more convenient viewing hours. Please check back periodically for updates. You can also stay on top of the events by visiting this dedicated website.

If you do see the eclipse tomorrow, we’d love to hear how it went. Just use the Comments link below.

3 Responses

  1. Larry

    M15 was right there above Enif in the binos, quite nice. I brought out the SCT, the longer I stared the more individual stars started popping out from it. I moved down to Jove to watch the Io transit, it was my first real try at seeing a transit. The “Jupiter Tool” you mentioned a while back really helped. It took some fiddling with the eyepieces, but then at around 125x there was the shadow. It was right between the bands, and reminded me of looking at a pinhole in a paper plate.

  2. astrobob

    Glad you found M15 Larry! Like you, I also saw the Io shadow transit — smack in the center of the planet. I like your description. Did you also by chance see the Great Red Spot? It was directly south of Io and very pale pink.

  3. Larry

    I did see it there, really fun to look at. I had a bit of a hard time picking it out next to the equitoral band at first, but after a few minutes it started to become more apparent. For the heck of it I put the moon filter on my eyepiece, and that seemed to help a bit with the contrast on the bands, but I couldn’t really make out the shadow of Io then, and it didn’t seem to help much with the GRS.

Comments are closed.