The complex diversity of the gorgeous globular cluster M5

I love the fact that we can make changes to our minds after there is enough evidence.
This is exemplified by the globular clusters. Globulars are star-clusters that contain hundreds of thousands, or for the most powerful, millions, of stars packed into a volume roughly spherical, measuring from a few dozen to a few hundred light years.

They are absolutely stunning through the eyepiece. When I was a child, they were still one of my favorite objects to look at through the eyepiece. Because the core of a star globular is so dense, individual stars can be seen as they decrease in density.

When I was younger, I had read about them. The idea was that globulars were very old and that all stars formed simultaneously. This makes them excellent laboratories for studying stars. They are all the same age and located at the same distance from us. It removes two of the most complicated factors in trying to understand stars' behavior.

This is the fun part: It's not true. Astronomers began to discover stars in globulars which didn't appear as old as others as they improved their skills at studying them. It was possible to see two populations of stars in some globulars, sometimes one more than the other. Evidently, globulars can be more complex than we think.

A new paper now shows that M5 is even more complex than it appears: It seems to have at most three populations of stars.

The magnificent globular cluster M5, which is made up of hundreds of thousands stars orbiting one another in a densely packed ball, is a spectacular sight. Photo: Credit: ESA/Hubble & NASA

M5 is a beautiful globular, which is one of the oldest in the galaxy. It is located approximately 30,000 light-years from us. This Hubble image shows approximately 100,000 stars within it. You're only looking at its inner regions. It contains a lot of stars.

An analysis of the stars has revealed that there are two types of stars. The first group is quite normal. However, the second group seems to contain more carbon and less nitrogen than you would expect.

This is not an easy observation. By taking spectra and breaking down the light into thousands of wavelengths, we can measure the abundance of atoms in stars. (Think of this as color). Different atoms absorb different colors of light so you can see which colors are missing or diminished. We use these absorption features to determine the chemical abundances.

Because the stars are so close together, it is difficult to do this in a cluster. How can you make sure that you are measuring the same star as another?

The Hubble Space Telescope has captured a close-up of M5's core to show how evenly spaced the stars. Photo: Credit: ESA/Hubble & NASA

The astronomer used filters to replace spectra in their new work. These filters allowed for very specific colors of light, which we call narrow band filters. This enabled them to identify individual stars much more quickly and still measure their absorption by key molecules. They looked at red giant stars, which are similar to the Sun, but that have swollen and cooled down. The second population of stars with more nitrogen and less in them was divided into two groups. One was concentrated in the cluster's core, while the other was evenly distributed. The ones distributed evenly also had more nitrogen and less carb than those that were centrally concentrated.

What is the difference? The cluster may have had two waves star formation. It formed 12 billion years ago. They pass each other often because stars are tightly packed in globulars. They interact gravitationally and the result is that the more massive stars sink towards the center and the smaller ones rise toward the outskirts. This is what we call mass segregation and it's very similar to how heavier things sink and lighter things rise in air or water.

These massive stars explode in supernovae and create heavier elements, which are then hurled out of the cluster. This gas can contain more carbon and less nitrogen because of the way that massive stars create elements over their lifetimes.

The second generation of stars formed from the leftover gas in the cluster. They had more of this gas, so they had an increased level of nitrogen and a decreased amount of carbon. Stars formed in the core where this gas was plentiful got more nitrogen and less carbon. Stars that formed further out received less of this gas and were therefore more likely to get it. They are still more nitrogenated than the first generation, and less carbonated so to speak, but not at the same level as stars in the core.

This creates three populations: One old, one middle, and one in between that is more nitrogen-enriched and less carbon depleted.

This may sound complicated. Astronomers have long assumed that the globulars were simple laboratories for studying stellar phenomena. Based on years of evidence, our view has dramatically changed.

They are complex and diverse individuals that create a beautiful spectrum of objects worth studying. Understanding them is more difficult than you might think, but once you get the basics down, your fascination with their beauty and fascinating nature will be even greater.

Tip o’ the spectroscopic grating to AAS Nova. This has a more technical synopsis.