See M4, The Closest Globular Cluster. It’s So Easy!

Find the closest globular star cluster, M4, the next clear night. It’s visible with the slightest optical aid just to the right of red Antares in Scorpius about 2-3 fists up from the southern horizon. Created with Stellarium

What do amateur astronomers look at in their telescopes? Planets, double stars, asteroids, comets and deep sky objects. Deep sky is shorthand for galaxies, stellar birth clouds called nebulae and star clusters. You’re probably familiar with a few like the Orion Nebula, the Pleiades star cluster (a.k.a. the Seven Sisters) and Andromeda Galaxy.

Charles Messier at about age 40 circa 1770. Messier loved comets but his greatest legacy was his catalog of cosmic bonbons.  

Tonight, we’re going to look at a juicy one, an object known simply as M4, the fourth deep sky object in a catalog of 110 objects compiled by 18th century astronomer Charles Messier. Messier was obsessed with hunting for comets but kept running into other glowy stuff — galaxies, nebulae, etc. — that he mistook for his favorite fuzzies. To avoid confusion, he cataloged the comet imposters. Ironically, despite his passion for comet hunting and the discovery of 13 comets, his catalog has become the source of his fame today.  All beginning and amateur astronomers cut their teeth hunting up Messier objects, many of which comprise the brightest and best the deep sky has to offer.

M4 is a rich, spherical-shaped star cluster located 7,200 light years away in Scorpius. Even small telescopes will partially resolve the ball into individual stars. The  nearly vertical line of brighter stars mentioned later in the text is visible across the center of the cluster. Credit: Jim Misti

Summer brings tons of stuff to look at through the telescope because the brightest swath of the Milky Way rises in the east this time of year and remains well-placed for deep sky mining all season and into the next. One of the brightest and easiest Messier objects is the globular cluster M4 in the constellation Scorpius the Scorpion.

Globulars, pronounced GLOB-you-ler, are the stellar equivalent of a busy beehive. Containing anywhere from tens of thousands to 10 million stars, they’re densely packed into the shape of a sphere. Most globulars don’t rotate like the Earth does, and the stars move randomly, affected by the gravitational pull of its neighbors. But they all hang together as big popcorn balls in the outer halo of the Milky Way galaxy, taking eons to revolve above its core. We know of 152 of them in our galaxy and have spotted thousands more in others.

The closest at 7,200 light years and also one of the brightest is M4.  So easy to find, you simply have to identify the bright, red-orange star Antares in Scorpius. In mid-June, Antares is up 2 fists high in the south-southeastern sky around 10:30 p.m. Three fists up if you live in the southern U.S. While the cluster is visible as a faint spot with the naked eye under the darkest skies, even pair of 35mm binoculars will pop it into view just 1.3° west (right) of Antares.

This sparkling picture taken by the Hubble Space Telescope shows the center of globular cluster M4. We see a multitude of stars, each a colossal nuclear furnace. Credit: NASA/ESA

Find Antares and then focus the star sharply in binoculars. Now look a short distance just to its right and a little below. Assuming you’re observing from reasonably dark skies (not downtown!) you should see a fuzzy patch of haze like a star that doesn’t come to focus. That’s it! Binoculars aren’t powerful enough to resolve the cluster into individual stars. Instead they blend together into a uniform glow.

But the reality is the cluster spans some 50 light years and contains more than 20,000 stars. We see it as it was over 7,000 years ago about the time the wheel was invented and humans began experimenting with writing. Yes, it’s still there in 2017! Stars in globular clusters live lives measuring in many billions of years, so a few thousand hardly makes a difference.

All 110 Messier objects photographed by Rolando Ligustri.

Close and bright, M4 was the first globular cluster in which individual stars were resolved. Even a 6-inch telescope will show them heaped like tiny sugar crystals in the bowl of a spoon on moonless nights. I encourage telescope owners to shoot over to M4 after looking at Saturn. It’s a must-see deep sky object. When you bring up the magnification to 150x or higher, a row of brighter stars runs north-south across the center of the cluster. It looks like a zipper to my eyes.

Giant telescopes uncover additional treasures within this starry disco ball including the 12-13 billion-year-old white dwarfs (burned out stars), the oldest so far discovered in the galaxy and a city-sized neutron star spinning at the rate of 300 times a second and beaming powerful pulses of radiation into space.

It’s all there for you to see and ponder on breezy summer nights.