Blizzards Of Suns — Two Spectacular Globular Clusters For Summer Skywatchers

The globular cluster NGC 6441 orbits the Milky Way Galaxy 13,000 light-years from its core. Tens of thousands of stars are held together through their mutual gravities in an enormous sphere or globulus, Latin for a small sphere. Click for a larger version. ESA/Hubble & NASA, G. Piotto

The last thing anyone’s thinking about right now is snow but that may be what comes to mind when looking at this photo. That’s the globular cluster NGC 6441 in Scorpius. So many stars are crowded together it looks like an oncoming blizzard. Like snowflakes, each of the cluster’s stars differs from the other though all were born together inside an enormous cloud of gas and dust some 13 billion years ago.

NGC 6441 is easy to find located right next to the 3rd magnitude naked-eye star G Scorpii. Use a small telescope to spot the 7.8 magnitude cluster. Stellarium

NGC 6441 located in the tail of Scorpius the scorpion it’s about 13,000 light years from the center of the Milky Way Galaxy. Astronomers estimate that together the stars have 1.6 million times the mass of the Sun, making NGC 6441 one of the most massive and luminous globular clusters in the Milky Way. It only appears modestly bright in the sky because it’s 44,000 light-years from Earth and partially obscured by clouds of cosmic dust. But a small 3-inch telescope will show it with ease.

If you study the image you’ll notice lots of red stars. Many of these are modest sized stars that were once just like the sun. Over 13 billion years time they’ve evolved into red giants, bloated gasbags 100 to 1,000 times larger than today’s sun. Our star is on the same evolutionary path and will expand into a red giant in about 5 billion years.

Stars like the sun (lower left) fuse hydrogen into helium for much of their lives, first in the core (1) and then in a shell around the core (2 and 3). This causes the star’s outer layers to balloon outward (3). Finally, the core becomes hot enough for helium to fuse into carbon (4). Thomas Kallinger, University of British Columbia and University of Vienna

Stars shine and radiate heat by fusing hydrogen into helium atoms in their cores. Eventually the star uses up its core hydrogen and begins fusing hydrogen in a shell around the core. Helium made in the reaction continues to accumulate in the core which contracts through gravity and becomes extremely hot. Shell burning combined with the heat radiating from the star’s searing core cause its outer layers to inflate, and the star becomes a red giant.

You’ll also see blue stars. They’re not younger since all the stars in the cluster were born around the same time. Many are likely blue stragglers that are either binary stars in the process of merging into single, larger stars or stars that have already done so.

Use the bright, white star Vega, which stands halfway up in the eastern sky around 10:30 p.m. local time in early June, to point you to the asterism called the Keystone, located two outstretched fists above and right of the star. Then point your binoculars or telescope about halfway down along the right side. Stellarium

There are about 150 known globular clusters in the Milky Way. Globular clusters contain some of the first stars to be produced in a galaxy but exactly how they evolved is still fuzzy. Speaking of which, that’s what many globulars look like in small telescopes and binoculars — fuzzy spots! While you’ll need a telescope to spot NGC 6441, you can easily see another cluster in binoculars called the Great Globular of Hercules, when the bright moon is absent from the sky. A small telescope will show faint sprinkles of stars around the cluster’s edges while an 8-inch or larger instrument will transform it into hundreds of tiny, crowded points of light. I look at it all the time, probably 10 times a year. The abundance of stars here overwhelms the senses. It’s that amazing.

Some 300,000 stars reside in this starry metropolis 22,000 light years from Earth known as M13. They’re gravitationally bound to each other as they travel together in an enormous loop around the core of the Milky Way galaxy. N.A. Sharp, REU program, NOAO, AURA, NSF

Also called M13 (the 13th object in 18th century astronomer Charles Messier’s catalog), it’s magnitude 5.8 and faintly visible to the unaided eye from a dark sky site. Through binoculars it looks like a fuzzball with a brighter center located about a third of the way down the upper (western) side of the trapezoid-shaped asterism called the Keystone of Hercules. We talked about how to use brilliant Vega to find the Keystone asterism in a previous post. I’ve also included a map for you here.

Have at it. And if you need additional help finding this cluster or its buddy NGC 6441 just ask.