This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Star Clusters

by Dr Jamie Love Creative Commons Licence 1997 - 2011

A condensing nebula, like the Great Nebula of Orion, can produce many stars. These are called star clusters.
Most star clusters contain a few dozens to a few hundred stars and are referred to as open clusters because you can see gaps and thus make out individual stars. However, there are some places in the Galaxy where there have been very large nebulas that went on to form very large star clusters containing thousands and perhaps millions of stars. These huge star clusters are so packed with stars that they have an overall glow making it impossible to see most individual stars. Their huge numbers and gravity cause the cluster to form a globular shape so they are called globular clusters.

The smaller and simpler open clusters eventually drift apart as they are disturbed by the gravity of other non-cluster stars passing near. The globular clusters, however, maintain their shape and can contain very old stars. Globular clusters are rarer than open clusters and, as we will discuss in a future lesson, globular clusters tend to orbit way from the disc of stars that form most of our Galaxy.

Is there a globular cluster I can see?

Maybe.

M3 is a globular cluster but its magnitude is only 6.4 so you really need binoculars to find it. This image is of a section of the northeast part of the night sky soon after sunset on April 1st and it shows magnitudes as low as 6.5, so it may appear a bit confusing. You see the Little Dipper at the top of this image and the Big Dipper near the center. You can follow the curve of the handle from the Big Dipper to "arc to Arcturus" and easily identify this bright star. (It's light orange in color.) Once you've found Arcturus scan to the west and look for a very dim spot of light in the neighborhood of the circle I have drawn. That's M3. It is below the sensitivity of my eyes (even on a great night) but it can be found with binoculars and it's a nice site in a small telescope.

Don't knock yourself out trying to find M3. It's not easy especially without optical aid. There are several other globular clusters and I'll show you another one (slightly brighter) in a couple months.

Is there an open cluster I can see?

Yes! There are many easily found open clusters. I'll show you how to find some of them.

The easiest to find is the Pleiades in the shoulder of TAURUS.

Just extend the line that you used to find Aldebaran a bit further and add a small amount of southerly tilt to it once you pass Aldebaran. You'll come to a curious clump of stars with a collective magnitude of about 1.6 but they will appear dimmer than that because the light is "shared" among many individual stars scattered in a group. This famous star cluster is often called "The Seven Sisters" because on a clear night a person with average vision can make out seven stars in Pleiades. In fact, there are hundreds of stars in this cluster. Unlike the Great Nebula of Orion, you don't need a perfectly black night or pristine skies to see the Pleiades.

However, to get the most out of them you'll want to try for perfect conditions. Use averted vision to see it more clearly. Use a pair of binoculars and you'll see it even better!

The Pleiades captured the eyes of the Ancients. Homer (NOT Simpson ) referred to them in Odyssey and they're mention three times in the Bible. Other cultures have their own name for the Pleiades. Old German and English names refer to the cluster as a hen with chicks. The Ancient Japanese called it "Subaru" and the modern Japanese named a car after it!

This photo was taken through a telescope at the Mount Wilson Observatory. Your binoculars will show the stars well but not the colors nor the nebula. As you can see here, these stars are all bluish-white and are therefore very hot. They are also surrounded by a great deal of nebular dust and gas that reflects the light from the stars. (And, blue is reflected best because of the size of the dust grains.) The large amount of "nebulosity" is a good indication that this is a young cluster. Also, you'll recall that an open cluster like this will not maintain its shape for billions of years because it will eventually be torn apart by other stars.
Calculations estimate that the Pleiades star cluster is about 100 million years old and in about 250 million years they will have drifted so far apart that they will no longer be considered a cluster. At that time these stars will "behave" as individuals, each with its own path through the sky.

Currently, the Pleiades are about 380 light-years away and the entire cluster is about 13 light years across.

There's evidence (based on a technique called "infrared astronomy") that the Pleiades contain several brown dwarfs, so the Pleiades are a perfect example of stars and brown dwarfs in their early years.

While we are in this neighborhood I must tell you about the Hyades because you've probably already seen them!

The Hyades open star cluster makes up most of the "V" in the face of TAURUS. Aldebaran is not a member of the Hyades cluster - it just happens to lie along our line of sight to it. Indeed, Aldebaran is only 68 light-years away but most stars of the Hyades cluster are twice that far from us. The Hyades are made of many stars of reasonable magnitude, some as bright as 3.5. My green circle covers most of the Hyades, but some members extend outside this boundary. This is a very good example of an open cluster and it is a wonderful part of the sky to check out with some magnification.

The Hyades are older than the Pleiades so you won't find any nebulosity among the stars of the Hyades cluster. The stars in the Hyades have had longer to drift apart so you see them as more spread out than the Pleiades. (Also, the Hyades are closer than the Pleiades and that makes the Hyades seem more separated than the more distant Pleiades. It's a line-of-sight effect.)

Are all open clusters in TAURUS?

No, no, not at all! Lets' move away from TAURUS and show you some clusters in new constellations.

Praesepe is an open star cluster in CANCER, the Crab.
It's a lot easier to find CANCER if you first find LEO, the Lion.

I know of two ways to find LEO.
One way to find him is to imagine a line from Rigel to Procyon and extended that line beyond Procyon almost as far as the distance you just traveled. You'll come to the brightest star in LEO, Regulus.
Another way to find LEO and Regulus uses the Big Dipper. Drop an imaginary line from the two stars on the handle side of the bowl (Megraz and Phad) and you will crash into LEO's head and on to Regulus.

Notice that we are moving further away from ORION and will begin concentrating on constellations that will require different "pointers" as guide posts. LEO is a nice transition so learn to find Regulus both ways.

Regulus is a bright (magnitude 1.3) bluish (B-type) star 85 light-years away and around 130 times brighter than the sun.

The constellation of LEO is pretty obvious. Regulus is tucked into LEO's front paws and the head of LEO forms a great arc that looks like a question mark with Regulus as the dot at the bottom. LEO's body extends eastward and his hind legs are tucked under to form a nice triangle shape. The bright star Denebola forms the most posterior part of LEO's hindquarters and is at the base of the lion's tail. It doesn't take a lot of imagination to see him!

CANCER (The Crab) lies midway between LEO and GEMINI.

CANCER is an extremely dim constellation - its brightest star has a magnitude of only 3.5. On a very clear moonless night you can see the 5th and 6th magnitude stars that together give the impression of a multi-legged "thing" that the Ancients imagined as a crab.
(Some amateur astronomers say it looks like a dim, miniature ORION. I say some amateur astronomers should get more sleep! )
This image shows only the stars down to a magnitude of 4 so there is no way to see anything interesting except Praesepe and a few stars.

I've added some lines to help orient you, but there is nothing here that looks like a crab!

Praesepe was well known to the Ancients. Hipparchus (second century BC) referred to it as "a little cloud". Of course, it isn't a cloud but a cluster of about 350 stars over 500 light-years away and distributed across a dozen light-years of space.

Unlike the Pleiades, Praesepe has no detectable nebulosity so we assume it is more mature in its development.
Star formation is still going on in the Great Nebula of Orion, and perhaps in the Pleiades, but here in Praesepe it has ceased.
The age of the Praesepe star cluster has been estimated at about 400 million years.

This image was photographed by Sven Kohle and Till Credner of Bonn University using a 180 mm f/2.8 telephoto lens.

What's that bright, orange star below Regulus? It really sticks out because it seems to be all alone.

Yes, that's Alphard and it has the nickname of the "Solitary One" because it's the only bright star in that part of the sky. You can also find it by extending a line from Castor through Pollux and continuing it until you come to the first really bright star.

Alphard is a K-type star about 85 light-years away and 115 times more luminous than the Sun. It's the brightest star in the constellation of HYDRA, the Water Snake. Don't confuse the constellation of HYDRA with the star cluster called Hyades!
HYDRA is a good example of a constellation of dim stars (except Alphard) that wind along forming no particular shape. Let's face it, you can make a snake out of any group of stars! At least the HYDRA has a nice group of stars forming its head (underneath CANCER's "feet").

There are 88 official constellations (plus several pseudo-constellations, like the "Big Dipper"). Only the really advanced astronomers know them all or can define them without maps, so don't expect to learn them all here. Instead, I want to introduce you to the obvious constellations. HYDRA is only important because it contains Alphard.
[OK, HYDRA also has some interesting dim objects including a star cluster that is probably derived from the same nebula that formed the Praesepe. But let's keep this simple, OK? It's a BIG universe and you should start with the easy and obvious stuff.]
Some constellations are just not worth memorizing at this point but you will want to know that they are there. Here's some more examples of unremarkable constellations you may not want to learn. I'm presenting them here to show you that the sky is full of constellations even though we may not learn them all.

Above the large constellation of HYDRA are three small constellations.

SEXTANS, the Sextant, has no star brighter than 4.5 magnitude and is only of interest because you learned that a sextant is an object for measuring angles in the sky. The angle made by SEXTANS' three brightest stars is about 120o, or a third of a circle.
CRATER, the Cup, is supposed to be a goblet of wine (huh !? ) and it is so dim and boring you might easily miss it.
On the other hand, CORVUS (The Crow) is easy to identify because it's four brightest stars (all above magnitude 3) make an obvious four-sided figure with nothing else in the neighborhood to mistaken it.

(I've labeled the bright star, Spica, which we will discuss in a subsequent lesson.)

In this lesson you've learned about star clusters and how to find some. In the process, I taught you a few more constellations.

Because stars start inside nebula it's understandable that a young nebula, without any stars in it, will be of the dark variety, while an older nebula, old enough to have had time for stars to form, will often be illuminated by its "children". This group of children form a star cluster. If the family is fairly small, due to a relatively small nebula, the cluster will be open and eventually the members of the family will drift apart as gravitational forces from other stars and star clusters pull on them. We observe that the Pleiades stars are close to each other and there is still a lot of nebular material in the neighborhood. The Pleiades is an example of a relatively young (100 million years old) open cluster. We believe Praesepe is older (400 million years old) because we observe no nebulosity in the region and calculations based upon gravitational properties indicate that the members of this cluster have been drifting apart for a longer period of time. The Hyades are about as old as Praesepe but because the Hyades are much closer, their dispersion is more obvious.
A globular cluster is a huge family of stars, probably formed from a huge nebula, whose own collective gravity is so massive that they will not drift apart. Globular clusters are more remote than open clusters and require telescopes to see them well.

This month try to find the Great Nebula of Orion, the Pleiades and Praesepe. They won't look as nice as the photos here but at least you can see them. Try for an especially clear night, use averted vision and use binoculars if you can. If you're in the mood to hunt a more elusive object try to find the globular cluster called M3. (Note that moonless nights are the best time to find dim objects.) Imagine what is happening inside ORION's nebula, the masses swirling and colliding as gravity brings them together. Imagine how there may be protostars forming there and perhaps some are on their way to a T Tauri stage right now! Imagine the Pleiades soaking up their remaining nebula and drifting apart to become more like Hyades and Praesepe.
Then practice your star and constellation identification. You should easily find the stars of LEO, with Regulus as its "star star". You may not be so fortunate in finding all of CANCER, but be aware of where it is located and be able to find Praesepe. You should easily spot Alphard in HYDRA. It's not easy to figure out which stars make out HYDRA's body but you should be able to see it's "head". While you're in the neighborhood, find CORVUS. It's pretty easy, compared to the dim looking CRATER and SEXTANS.

Next month I'll teach you about the stars not found on the Main Sequence and you'll learn about the death of stars. However, before you can learn how stars die you must have a better understanding of the what makes them "alive" and that has to do with the physics of a star. Feel free to take a break now but be sure to come back and learn about star physics.




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.