From the February 2018 issue

Why are globular clusters not considered galaxies? How far apart are their stars, and do their magnetic fields affect each other? Can a cluster’s stars have planets, and would you need sunglasses to travel through one?

Martin Heuer St. Petersburg, Florida
By | Published: February 28, 2018

Between their low masses and their orbit around other galaxies, globular clusters, like M22, don’t fall into the galaxy classification.

ESA/Hubble and NASA
Globular clusters aren’t considered galaxies because they are gravitationally bound to and orbiting galaxies like the Milky Way, and they have relatively small masses. When comparing the two, a typical globular cluster might contain a mass of 100,000 Suns, whereas the Milky Way has nearly 1 trillion solar masses. In other words, the Milky Way Galaxy contains 10 million times more mass than a typical globular cluster.

The stars in a globular cluster are 50 times closer to each other than the stars in our solar neighborhood. To put this in perspective, in a typical globular star cluster, we’d likely find stars separated by a distance 5,000 times greater than the Sun is from Earth. But even at these distances, the magnetic fields of individual stars would have little effect on each other.

If you were flying through the core of a globular cluster, you likely wouldn’t need sunglasses because even at these close separations, individual stars would still appear dimmer than our Moon. But given that you’d have many stars in close proximity to each other, there would be nearly 1,000 stars brighter than the planet Venus in the nighttime sky of any fictitious planet near the center of a globular cluster. Their combined light would add up to roughly the light of a Full Moon.

In the center of a globular cluster, conditions aren’t favorable for the formation of planets because the tidal forces of passing stars could destroy any protoplanetary disks. If a planet did somehow form, it would still find itself in danger, since the occasional close passage of a star would likely disrupt the planet’s orbit, flinging the planet into interstellar space.

Brian Murphy 

Professor of Physics and Astronomy, 
Butler University, Indianapolis