A guide to the night sky in the coming months part 3 - Open Clusters and star colours

If you purchased a telescope from us for the 2025 Summer of the Planets, welcome to the BINTEL family!

While Mars and Jupiter are still putting on a show in the evening sky even as we head into the autumn months, remember these delights are only a small part of what you can discover with a telescope. 

(Make sure you catch up Part 1 of this series where we chatted about misty patches of the sky called nebulae where stars are being formed.)

This week we'll explore open star clusters and feature what's perhaps the most beautiful object like this in the night sky - the Jewel Box Custer. 

Unlike the ancient globular clusters that are bound together for billions of years and we touched on last week, open clusters are far more youthful and represent collections of stars that will eventually part company with each other.

 Where to find the Jewel Box cluster on 15th of March around 10.00pm as seen from Sydney - it's near the "side" of the Southern Cross, shown with the two pointers Hadar and Alpha Centauri.

Back in part one we talked about the vast clouds of interstellar gas called nebulae. Some of these star forming region such the Great Orion Nebula or the Tarantula Nebula are spectacular in just about any telescope. Over time these clouds of gas collapse due to gravity. Their tightly packed and heated hydrogen gas molecules ignite as stars due to the atomic fusion. 

Once this happens, the outward radiation pressure from the stars formed in the nebula pushes the remaining gas outwards and spreads it further out into interstellar space, leaving behind groups of new stars. They are loosely bound together by gravity and generally start travelling around the galaxy in the same direction and speed.  Members of this group are also all about the same age to within a few million years or so - as once they ignite and disperse the interstellar gas in the nebula, it is no longer be able to continue to create new stars. 

This newish group of stars is called an open cluster. 

The shape of open clusters and how many stars are contained within it vary as they size and much gas was in each nebula varies. Basically, no two open clusters appear the same unlike globular clusters which all basically resemble tight balls of stars. 

What do open clusters look like?

What's possibly the best open cluster to view in a telescope is the Jewel Box cluster which is part of the Southern Cross constellation. It was called as "a superb piece of fancy jewellery" by astronomer John Herschel and the name stuck. 

This contains about 100 stars, many of which are visible even in binoculars. To your eyes alone, it can be spotted as a small, fuzzy patch of sky not far from the star in the left "arm" of the Southern Cross. Larger telescopes will show more and more stars and also highlight their vivid range of colours. 

If you've never seen a star cluster through a telescope, the Jewel Box is a must see! 

Another open cluster that's on the other side of the Southern Cross to the Jewel Box and located in the constellation of Centaurus is NGC 3766, which is sometimes called the Pearl Cluster. This is just on the edge of being able to be seen with your eyes under a dark sky, so grab some binoculars or any telescope to view it.  Another lovely collection of brightly coloured star that's well worth exploring. 

Moving further along the Milky Way that runs through this part of the sky is the famous open cluster, IC 2602 which is often called the Southern Pleiades. This cluster can be spotted with your eyes under dark skies. Its stars are more spread out across area much larger than a Full Moon and is about 45 million years old.

You'll find both of these open clusters are easily found by the Celestron StarSense Explorer system and apps like Stellarium can be used to help locate them.

The entire region is worth exploring with your telescope, especially if you can get away from the worst of the city lights when the Moon isn't is in the sky.

Globular clusters appear to around the edges of galaxies and in the sky, spread right across it. Why do open clusters seem to be within the Milky Way?

The Milky Way, especially the brighter central regions we see during the cooler months in the southern hemisphere, contain star vast regions of gas and dust which are denser in the galaxy's spiral arms. We're seeing these open clusters which are recently formed (in galactic terms) before they spread out further into the Milk Way.

How many open clusters are there in the Milky Way?

Our galaxy is home to about  known 1,000 open star clusters, but maybe even ten times this number are probably hidden by dust and gas clouds. We see open star clusters in many other galaxies where active star formation is taking place. 

This compares with 150 globular clusters in our galaxy, with the possibility of just a few more obscured and hiding from view.

Did the Sun have companion stars that were formed from the same nebulae? Where are they now?

Yes. There would have been numerous stars formed around the same time as the Sun from the same nebula. They have been tricky to pin down despite extensive searches through catalogues of stars. 

Astronomers have searched for stars of about the same age as the Sun (appox. 4.6 billion years) with the roughly the same chemical composition. They mightn't be the same size or colour as the Sun as they might have collected more or less of the material from our host nebula before they turned into stars.  The Sun has also completed over 20 entire orbits of the Milky Way and the stars it formed near it have spread throughout the galaxy.

Astronomers have discovered some potential siblings of the Sun.  At some 110 light years away, HD 162826 is in the constellation of Hercules. You would able to spot it in a telescope as it shines about magnitude 6.5. A second star.  HD 186302 in Pavo, was thought to also be a sibling of the Sun, although some have suggested its orbit around the Milky Way is different enough to rule it out from being formed with the Sun.

This is a complex area and there's a lot of ongoing research about our Sun's history. 

Did the Sun form with our nearest star, Alpha Centauri?

Almost anyone who's looked at up our southern skies would know the two pointers to the Southern Cross. The brightest of these is Alpha Centauri which is a three-star system of which two are easily visible separately in most telescopes. (It's also the home to the baddies in Netflix and sci-fi book series "The Three Body Problem".) 

Despite it being the closest star system to us, it doesn't seem to have been formed from the same nebula as the Sun. Both major component stars are older than the Sun. This rules the system our as stars formed in the same nebula are thought of roughly the same age and they also have different compositions - what astronomers call "metallicity". Read more about this here. 

What this means is the Alpha Centauri and our Solar System have simply wandered close to each other during respective orbits around the Milky Way.

If they're the same age, why do different members of an open cluster have different colours?

One thing you notice about stars when you view them through a telescope is just how vivid their colours are. It's something you can certainly see with your eyes but it takes a telescope to really appreciate them and these are especially on show in open clusters.

We'll go into the complex life cycles of star in an article shortly. The short version is that the colour of a star is related to the temperature the star is "burning" through its nuclear fuel and this is closely related to how much material in the star.  Stars that form in denser, rich clouds of gas in their nebulae will collect more material and by the time they "ignite" will be much larger and hotter. 

The Sun is a middle of the road star in terms of temperature, mass and life span. 

Would the Sun appear to be in a cluster from elsewhere in the Milky Way?

Surprisingly, the answer is no. 

Not only is our Sun is not only a rare single star system (well over 80% of stars are in multiple star systems, although roughly just under half of those of the same class as the Sun are in binary system), but by now members of its original star forming nebula have dispersed. The Sun would appear to be somewhat of a loner from other observers in the galaxy. 

To wrap up:

Star clusters are wonderful and stunning parts of the night sky. Many of these will quickly become old favourites that astronomers return to over and over again to observe while find ones to explore. 

Cheers,

Earl White

BINTEL 

21st February 2025