Hubble Photo of Globular Cluster NGC 6441, One of the Most Massive in the Milky Way

The Hubble Space Telescope has delivered another outstanding image. This one is of NGC 6441, a massive globular cluster in the constellation Scorpius. It’s one of the most massive ones in the Milky Way, and the stars in it have a combined mass of 1.6 million solar masses.

NGC 6441 is a gorgeous visual spectacle. It’s also of great scientific interest; it hosts four pulsars, an abnormally high number of variable stars, and has a rather high metallicity for a globular cluster. And it’s host to several Type II Cepheid stars, which is unusual for a cluster with high metallicity.

It was discovered in 1826 by Scottish astronomer James Dunlop, who also published “A Catalogue of Nebulae and Clusters of Stars in the Southern Hemisphere observed in New South Wales” in 1828. Those were challenging times for observers, and only about half of the objects he discovered were actually real. The remainder were artifacts of the telescopes of the time.

But NGC 6441 is real, and it’s only one of the Milky Way’s globular clusters, which number about 150. Globular clusters are found in a galaxy’s halo. They’re gravitationally bound formations, with greater stellar density in their cores.

A labelled illustration of a spiral galaxy. Globular clusters reside in the galactic halo. Image Credit: University of Oregon.

As of now, astronomers don’t have a detailed understanding of how globular clusters form. Do the stars in a cluster form in a single generation? Or are they spawned in multiple generations, covering hundreds of millions of years?

In many globular clusters, the stars are in the same stage of evolution, which suggests that they did indeed form in a single generation of starbirth. But other clusters contain distinct populations of stars that seem to be in different stages of stellar evolution. One study from 2008 suggested that globular clusters interacted with giant molecular clouds (GMCs) causing a round of starbirth, which created a separate population of younger stars than the rest of the cluster.

A sky map showing the location of NGC 6441. Image Credit: Roberto Mura / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Astronomers think that globular clusters are the oldest objects in the galaxy. Observations show no active starbirth in them, backing that viewpoint.

NGC 6441 also hosts a planetary nebula named JaFu2. That’s unusual for a globular cluster. Out of the approximately 150 globular clusters in the Milky Way, there are only four planetary nebulae.

But these are the scientific details.

Sometimes it’s nice to just enjoy an image, let the imagination wander, and remember what makes space so enticing in the first place.

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Evan Gough

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