Destiny is not an absolute

Yes, I know, this blog post has an awkward title. So what am I talking about here? Look at this older sketch that I made of the bright open cluster M35 in Gemini:

 

It's a treat, isn't it? Just look at those dozens of twinkling stars that under a really dark sky are already visible to the naked eye. Obviously you've also noticed that faint little blob, just below the drawing's centre. Now let's zoom into that with the binoscope:

 

It's another star cluster, called NGC2158. Is it related to big and bright M35, you might wonder? No, not at all! As I explained in my previous blog post appearances may deceive! M35 lies 2.800 lightyears away from us; to reach NGC2158 you'd have to travel 11.000 lightyears! All right, you might already have guessed that from the first sketch. But there's more! M35 is a typical middle-aged cluster, some 100 million years old, that is now slowly breaking up under the gravitational pull of our galaxy. Soon all of these bright and young stars will go their own way. NGC2158 on the other hand is surprisingly old for an open star cluster, it's age being estimated at 2 billion years! So why hasn't this cluster broken up as well? Isn't the force of gravity an absolute? Yes, of course it is. But first of all, NGC2158's much further away from our galaxy's gravitational centre and therefore it is influenced less by it. What's more, NGC2158's an extremely rich and compact cluster. Even at 285x with my gigantic telescope I wasn't able to resolve all the stars within it. Well, I could have taken the telescope to 500x and then perhaps I would have resolved more stars, but I liked the view as it was. So many stars packed together in such a confined space, means that their mutual gravitational pull outweighs the gravitational pull of our galaxy and therefore they've managed to stay together for such a long time. Many stars in this cluster have already come of age and some have even evolved into red giants, as opposed to the young, blue stars in M35. 

So you see, sometimes an unavoidable destiny can be avoided.