The Night Sky From Inside A Globular Cluster

An imaginary view of a winter night on a planet orbiting a star in the core of a globular cluster. Illustrations created with Stellarium; background image of M4 cluster by the Hubble Space Telescope/NASA/ESA

It’s fun to imagine what the sky would look like if you could plop yourself on a planet around another star. We’ve only scratched the surface, yet in fewer than 20 years, astronomers have discovered more than 750 new planets beyond those in our own solar system.

While I’d love to see a double sunrise and sunset like the fictional Luke Skywalker on the planet Tatooine, one of my favorite places to imagine a radically different sky from Earth’s is inside massive balls of stars called globular clusters. We know of about 150 in the Milky Way galaxy. Most are tens of thousands of light years away in the direction of the center of the galaxy in the constellation Sagittarius. Globulars are tightly bound by the gravity of their many stars into spherical shapes resembling hoards of bees.

The huge Omega Centauri cluster packs 10 million stars into a ball 230 light years across. It’s about 16,000 light years from Earth. Stars are only 1/10 of a light year apart in its core. Click photo to enlarge. Credit: Hubble/NASA/ESA

Most globular clusters contain several hundred thousand stars, but a really big one like Omega Centauri (visible from the southern U.S.) tops out around 10 million. Star densities inside the clusters are much higher than in the sun’s neighborhood. The closest star system to our sun, Alpha Centauri, is 4.3 light years away or some 25 trillion miles. Within the densely packed core of some globulars, star densities are phenomenal – up to 1000 stars stuffed into a cube 3 light years on a side.

Globular clusters form a sphere centered on the Milky Way galaxy’s center. Credit: Science Frontiers online

While so many stars whizzing about make it difficult for planets around stars to remain in stable orbits, we can safely assume planets are still possible. Think of the view on clear night. The sky from Earth contains at best a dozen bright first magnitude stars on a given night, all considerably fainter than Jupiter and Venus. Inside a globular, where stars are only tenths of a light year apart, there would be thousands of stars as bright as Venus and Jupiter. You could easily walk around (or dogsled) at night without the need of a moon to find your way. Some lucky life form must witness bejeweled nights in at least one of the many globulars that dot our skies. My feeble attempt above only hints at what we might imagine.

Because globulars are arrayed in a spherical cloud centered on the core of the Milky Way, they only begin to populate our evening sky as spring turns into summer and Sagittarius makes its appearance in the east. There are several out right now – the vanguard as it were – that you can spot in binoculars. And if you have a telescope, especially from 6-inches and up, you’ll can experience for yourself how fantastically starry-rich these objects are.

M3 is conveniently placed between the bright pink star Arcturus below the Dipper’s Handle and Cor Caroli, the brightest star in the Hunting Dogs, a small constellation near the Dipper.

One of the best in the northern sky is named M3, the 3rd entry in a catalog of deep sky objects compiled by 18th century French astronomer Charles Messier. It’s located in the constellation Canes Venatici the Hunting Dogs below the handle of the Big Dipper. The cluster is 33,900 light years away, spans some 200 light years and contains half a million stars.

To find it, face northeast around 9:30 – 10 p.m. local time and locate brilliant Arcturus by following the arc of the Big Dipper’s Handle. The cluster is located not quite halfway from Arcturus to Cor Caroli, an easy-to-see star to the right of the Handle.

In binoculars look for a fuzzy 6th magnitude blob with a brighter core just to the left or north of a 6th magnitude star. You can use the more detailed map below to pinpoint M3’s location. M3 is easy to see in typical binoculars from suburban and rural areas. Take your imagination for a ride the next clear night.

In this closeup map, start with Arcturus and “star-step” your way upward to M3. The distance between them is about 1 1/2 binocular fields of view.

7 Responses

  1. thomas s

    great stuff Bob. but would a planet inside a g. cluster ever experience “dark” . wouldn’t so many stars at such close range provide constate illumination, if not the equivalent of full daylight, at least a kind of twilight?

    1. astrobob

      Hard to say. Assuming it’s approximately equidistant from its neighbor stars, nights would be bright – lots and lots of Venuses – but not like true daylight. The star the planet would revolve around would be far brighter because it would be much closer.

  2. For simplicities sake , imagine a world orbiting a star on the OUTER edge of a globular cluster . As the seasons progress , occupants of that world would be treated to the blazing spectacle of their parent globular filling half the night sky ; while at other times of the ‘year’ an overhead view of the entire galaxy would stretch from horizon to horizon ; just take a minute and imagine that!!!!

  3. Bob Newsome

    Bob, I failed to find a contact page for this but . . . the caption under the simulated sky graphic indiacates the picture was created using Stellatium. I think you meant Stellarium. Just trying to help.

    Also, excellently done thank you!

  4. I did not know there existed Globular Clusters. How fantastic is all this! Imagine an inhabited planet in a solar system within a cluster! Imagine what the future holds for us as humans! If we do not destroy ourselves (and I hope we will not) how far will we get? What will we be able to see and experience! (March 3th, 2014)

    1. astrobob

      Hi Gerardo,
      The view would be incredible. Let us hope our far future descendants are able to enjoy it!

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