Globular Clusters are some of the most fascinating objects in the night sky, as well as being spectacular deep-sky targets for telescopes of all sizes.
One of the best globular clusters in the sky for viewing, NGC 6752 was mentioned in last week’s BINTEL Newsletter. (You can read it here.) A few folks asked me about some background info on globular clusters, so here’s a quick introduction.
Globular Cluster NGC 6752, first observed by James Dunlap from Parramatta in 1826.
You might have heard of star clusters or even seen them through a telescope. These are collections of stars that are bound together by gravity and orbiting each, other or moving through their host galaxy in the same direction. There are two main types. One is an open cluster, a loosely bound collection of young stars ranging from a few hundred to possibly a few thousand. They’re usually found in the outer arms of a spiral galaxy, in the same plane as the galaxy itself. This is typically where stars are formed from vast clouds of interstellar gas and dust. Stars tend to like company and are created in groups. It’s estimated that over 80% of stars are in binary systems with many with three or ever more members. Our Sun, orbiting the Milky Way by itself is a bit of a loner. Our nearest stellar neighbour, Alpha Centauri, is a three-star system.
Over time, these open clusters from the same stellar nurseries will drift further apart become an ever more distant. Spread-out open clusters are called stellar associations or sometimes moving groups. Open clusters spread are apart over time by the motion of the members of the cluster themselves being fast enough to escape the gravitational pull of other members or, by interacting with other stars or clouds of gas as they move around through the Milky Way.
Our Sun was born in a nebula along with other stars. It was likely part of an open cluster that’s now drifted apart. There have been multiple studies to investigate what nearby stars were created in the same nebula with the Sun and possible Solar siblings have been found.
As open clusters are collections of stars of that have been recently born, in astronomical terms, they’re quite young. If you’re viewing an open cluster, then you’re not looking at an extremely old deep-sky object. Probably the most famous example of an open cluster in the southern skies is the Jewel Box cluster (NGC 4755) visible with just your eyes as a hazy patch near the Southern Cross, and the Pleiades (M 45) cluster. This has been known since ancient times and connected to stories of “The Seven Sisters” which are widely spread among first Australians.
Open cluster The Pleiades (M 44) imaged by Matt Dodds using a Celestron RASA 8 and posted to the BINTEL Society Facebook group.
We’ll dive deeper into open clusters in a later article, when we look into the life cycles of stars.
Globular clusters on the other hand are ancient, contain vastly more stars and are usually found in a halo around the outskirts of galaxies. They’re held together by gravity so great that they’re going to hang around for possibly the life of their host galaxy or even longer! ****
When observed through a telescope, globular clusters appear as a sparkly ball of stars, so closely bound, they seem to be touching each other. The first deep-sky object to be identified as a globular cluster was M22 or NGC 6652, in 1655. This is in the constellation of Sagittarius. This contains more than 83,000 stars and weighs about as much 500,000 times the weight of the Sun.
We’re lucky in the southern hemisphere to have in our skies some of the brightest and largest globular clusters in the Milky Way. The most famous of these is Omega Centauri. This appears to our eyes as a faint star, although it’s a bit “fuzzy”. Omega Centauri was known to ancient peoples and cataloged as a “star” but wasn’t known as anything of major interest until it was observed through early telescopes in the 17th century. Even with these primitive instruments, it was quickly found to be unlike any other star in the night sky.
Instead of being a single point of light, it was revealed to be an uncountable number of individual stars.
Adam Charter posted his image of Omega Centauri taken with a DWARF II to the BINTEL Society Facebook page. You can see how highly concentrated the more numerous stars are compared the image above.
We know of around 150 globular clusters in the Milky Way with a good chance there’s a few more that are obscured by the thick bulge of dust and gas of our home galaxy. Our nearest large galactic neighbour, the Andromeda Galaxy (M 31) has some 470 or so* globular clusters. The number of known open clusters in the Mily Way is around 1,000, with likely many times this number hidden as well.
Where did globular clusters come from?
While our understanding of star formation in a galaxy is generally agreed on, how globular clusters emerge is still not fully clear. There’s still a lot to learn!
Carlos Taylor posted his image of the bright globular cluster 47 Tucanae to the BINTEL Society Facebook Group. He used a 10″ Sky-Watcher Quattro Newtonian reflector.
Globular clusters contain ancient stars, usually many billions of years old. It was thought that all their stars were formed at the same time from a single large gas cloud, but more recent studies have found that nearly all globular clusters have two or more groups (or “populations” as astronomers like to call them) which shows that at least for a brief period of time, there was enough gas and dust for another relatively brief round of star formation. They form an important tool for learning about the early stages of galaxy formation.
Unlike open clusters like the Pleaides, there’s no gas and dust in a globular cluster and this means no new stars are now being formed. What you’re viewing is light only from the stars themselves and not from illuminated gas clouds.
The old stars in globular clusters are Population II stars. This means they have a lower proportion of elements other than hydrogen and helium than Population I stars such as The Sun. ** The stars in Omega Centauri were already ancient when the Sun and planets in our Solar System came into being. On the other hand, early primates were already well established by the time the stars in the Jewel Box Cluster started to shine.
There’s some discussion about whether globular clusters either formed with galaxies, or are captured groups of stars. Some of the larger globular clusters around the edge of the Milky Way orbit it in the opposite direction compared to the other stars in the galaxy, pointing to them being possibly cores of smaller galaxies that came too close to the Milky Way and were captured.
One giant galaxy, M 87, contains trillions of stars and some 15,000 globular clusters. The smaller galaxies around M 87 seem to contain almost no globular clusters, pointing to them being stripped of them by its larger neighbour.
How tightly packed are globular clusters? Do stars ever collide in them?
Despite looking like stars are right on top of each other when viewed through a telescope, there’s still an enormous distance between each star in a globular cluster.
The average distance between stars in globular clusters is around one light year, with stars in some regions near the core separated by around a third of a light year. This is still vast in human terms. Both of NASA’s Voyager spacecraft have been travelling for 40 years, but would take them a further 6,300 years to reach the nearest star if it were travelling to the core of a globular cluster. Even with these distances, collisions between stars at the centre of globular clusters are possible. Whether collisions or mergers results in “core collapse” or black holes is still a topic for research.
Globular clusters are probably not likely places for planets to form. The lack of elements heavier than hydrogen and helium around their older stars means the materials needed for planets, especially rocky ones like the Earth, simply aren’t there***. If planets were to form, they’d probably be yeeted from the globular cluster quicker than a possum squatting in an orangutang cage in a Perth zoo.
What telescopes are good for viewing globular clusters?
Just about any telescope will reveal sparkling, jewel like views of globular clusters. The larger ones such as Omega Centauri or 47 Tucanae will even appear as clusters in binoculars.
You also don’t need high magnification. While fainter than the full Moon, some globular clusters are almost the same size in the sky.
A comparison of the sizes of the Moon and Omega Centauri in the night sky as viewed through an 8″ Dobsonian telescope and lower power eyepiece. Created with the BINTEL Setup Calculator.
Stepping up to 130mm (5″) telescope is will bring out the details in globular clusters. Again, don’t worry about high magnifications – concentrate on getting as much of the globular cluster in your field of view. As they’re ideal objects to observe visually, you don’t need expensive astro imaging gear. A few suggestions for telescopes that can easily locate and view globular clusters include:
- Celestron StarSense Explorer LT80 AZ
- Celestron StarSense Explorer DX 130
- Celestron StarSense Explorer DX 5
Here’s some of our favourite globular clusters.
(You might need to wait for some of these to be visible from your location)
Feel free to comment with your suggestions, and we’ll add them to the list.
Make sure you point your telescope at a globular cluster in the coming months. They’re an amazing sight and there’s nothing like staring at one of these sparkling balls on the edge of our galaxy and realising you’re looking at thousands, if not millions of ancient stars from the early days of the Universe.
20th of September 2023
PS: This short article touches on what’s a very complex area of astronomy and I’ve had to leave out a fair bit about globular clusters. Please let me know if there’s anything else you’d to discover about them
*Why “or so”? The number give is based an estimate of 460 +/- 70. You can read the full study here.
**Astronomers refer to elements other than hydrogen and helium as “metals”. During the life cycle of a star, other elements are produced and spread into the dust and gas clouds from which later generations of stars are formed. Stars of low “Metallicity”, like those in globular clusters, means they’re formed much earlier than Population I stars.
***The famous short story “Nightfall” by Isaac Asimov takes place in a globular cluster. Well worth a read.
****Globular clusters are tough because of the intense gravitational effects of their members. Largers ones may even survive the collision and merger of their host galaxies.