It seems like the only things I have seen in the evening sky recently have been clouds. Although the garden plants are happy with the rain, my dogs are unhappy about the thunder.
Whenever the sky does clear, this is a good time of year to look for globular clusters, often called just globulars. Last month, I mentioned Omega Centauri being visible just above the southern horizon. While this is the biggest, others that are farther from the horizon might be easier to see.
The name “globular,” coined by William Hershel, is very descriptive of their appearance – a globe (or I like to think of just a glob) of stars. They contain hundreds of thousands of stars in a very tight grouping that is typically only a few score light years across. There are fewer than 200 of these clusters known around the Milky Way, and unless you are familiar with specific ones, they all look very similar.
With binoculars, or low-power telescopes, they look like little cotton balls. And with these modest instruments, you can see most of the Milky Way’s globular clusters that happen to be above the horizon. The bigger your telescope, the more stars you can resolve toward the center of a cluster.
Globular clusters are ancient. Generally comprising what are called Population II stars (old ones with fewer heavy elements), there are no longer any luminous, blue, main sequence stars, and the remaining distribution of star colors is noticeably redder than that found in open clusters in the spiral arms.
These clusters came into existence while rest of the Milky Way was still forming. Rather than being in the disk, like most of the rest of the matter, these clusters reside in a spherical halo. Unlike objects in the disk, which have a mostly orderly rotation about the galactic center, the orbits of globular clusters are random.
It was the three-dimensional distribution of globular clusters that Harlow Shapely used to find the center of the Milky Way. His 3D map showed a spherical distribution, but our solar system was not anywhere near the center. Instead, the center was in the direction of the constellation Sagittarius. The core of the Milky Way rising just after dark is why this is a good time to look for these clusters.
Globular clusters are not unique to our galaxy. All large galaxies are observed to have globular clusters in a halo.
I am sure that some of you with dark-adapted eyes could see several of these globulars with your naked eye, but binoculars and a star chart can make this task easier. Here are some globular clusters that have convenient pointer stars, so might be easier to find than others:
- Omega Centauri is still just above the southern horizon. Because this is by far the brightest globular cluster in either hemisphere, it is worth seeking a clear view in that direction.
- M13 is the bright cluster in Hercules. It is right along one of the edges of the “keystone” asterism of Hercules’ torso. The constellation is on a line between the very bright stars Arcturus, near the zenith, and Vega, a bit to the north of east.
- Scorpius is along the southern horizon, rising just before the Milky Way. Antares is the bright, reddish star making the heart of the scorpion. Just to the right of Antares is M4, another fairly easy globular to find.
- Once the brightest part of the Milky Way has risen, look for Sagittarius and the asterism, or pattern of stars, that looks like a teapot. The Milky Way is the steam coming up out of the teapot spout. Just above and to the left of the topmost star in the lid of the teapot is M26, one of the brighter and easier globular clusters to find.
- Venus is still dominating the western sky after sunset. I rarely look at Venus through a telescope because it is so bright, but doing so while it is still light can be rewarding. There is very little structure that can be seen on its cloudy surface, but a small telescope or even some binoculars can show the phases of Venus. Right now, you should see something similar to a first quarter moon, but while the moon in this phase is waxing toward full, Venus will be waning to a crescent as it passes between the Earth and the sun.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory.