Hubble Spots an Ancient Galaxy Near Us

So what was the early universe like? What were the early stars like? What about infant galaxies? It seems that the Hubble Space Telescope may have uncovered an object to help answer those questions. It was doing an observation of a globular cluster called NGC 6752. The cluster of stars is actually part of our Milky Way galaxy. It lies about 13,000 light-years away from us, in the direction of the constellation of Pavo. This tightly packed cluster contains well over a hundred thousand stars. The core of the cluster is a region about 1.3 light-years across and overcrowded with stars. This cluster is visible to the naked eye from a dark sky sight.

In the course of the observation, the telescope found another population of stars that’s somewhat hidden behind NGC 6752. This collection of stars is actually a “dwarf spheroidal” galaxy, called Bedin 1. The image shows this dim little dwarf (circled) in the lower left. The bright stars of the globular outshine Bedin 1 by quite a bit.

Why Isn’t Bedin 1 a Cluster?

So, one question about Bedin 1 is whether it’s really just another cluster of stars like the nearby globular. As I looked at the image, I thought about the difference between a globular cluster and a dwarf spheroidal. It turns out that the delineation between them isn’t always clear. Both types of objects have stars that are very old, and very often metal-poor. That last bit is important because the very youngest stars in the universe were also metal-poor. Not until the first generations of stars were born, lived their lives, and died did the universe become enriched with elements heavier than hydrogen and helium.

That’s because as stars live, they produce elements heavier than hydrogen and helium in their cores. At their deaths, those elements: carbon, nitrogen, oxygen, and so on, are spread out to surrounding space. The most massive stars are also implicated in the creation of such elements as iron, gold, and so on. Neutron stars are also involved in the creation of heavy elements when they collide with each other.

I mentioned that globular cluster stars are also very old. And, they, too, are often metal-poor. So, what’s the difference between a dwarf spheroidal and a globular? It turns out there’s more than metallicity to worry about. To figure out which one is a dwarf spheroidal and which is a cluster, astronomers have to study the motions of stars in both types of objects. Motions are affected by mass, and it turns out that we have to look at the amount of dark matter involved. A dwarf spheroidal is likely to have more dark matter, and hence, more mass, than a globular cluster.

What Does Bedin 1 Tell Us?

That brings me back to Bedin 1. It’s likely to be a dwarf spheroidal if it’s massive (meaning it has a large component of dark matter for its tiny size: only 3,000 light-years across. It’s also very dim. Of course, astronomers will continue to study and measure its mass.

So, what this means is: galaxies in the early universe were relatively metal-poor. And, dwarf spheroidals like Bedin 1 represent that early, “pristine” universe. The coolest thing about Bedin 1 is that it’s a remnant of that very early universe. Its stars hark back to a time when the universe was very young. And, it shows us what stars were like at that time.

Luckily, Bedin 1 hasn’t had a history of collisions with other galaxies. It has existed since early times as much as it is today. And, that gives astronomers a great look back in time to the infancy of the cosmos. It reveals what galaxies might have been like at a very young age. That was before the explosions of stars enriched them with elements that ultimately led to planets and life.