Have you ever looked up at the sky and seen the Milky Way? It might be hard to recognize it at first, given what it looks like: Nothing more than a streak of white and brown and black, flung across the heavens. Compare that to some of the breathtaking pictures you see of other galaxies and it doesn’t look like much. It doesn’t even look like a galaxy!
Here’s NGC 6744, a galaxy similar to the Milky Way:
This, on the other hand, is the Milky Way:
Why the difference? The Solar System — and the Earth, and us — is in the galactic disk, the plane-like main area of the Milky Way, where the spiral arms and most of the gas, dust, and stars lie. The reason that NGC 6744 looks so different is that it lies face-on, parallel to the Milky Way. We can see lots of galaxies at similar angles. Their centers, spiral arms, and stars are visible to us, but the center of the Milky Way is blocked from our view.
What if we could see the Milky Way face-on? Then we would be able to see it in its full glory, with its four arms curled around the center in a magnificent spiral. There are some stars that move about like this, called halo stars. Many are solitary, but others orbit in small clusters. There are also remains of dwarf galaxies in the halo, and stellar streams torn away from them.
There are, however, larger groups of stars out there: globular clusters. These are dense groups of old, old stars — some dating back 12 billion years ago! Star formation in globular clusters has long since ceased, so all that’s left is . . . stars. They’re packed closely together — so closely that stellar collisions can happen. Young stars called blue stragglers are thought to be the results of these collisions.
Now the fun stars: What would it be like to live in a globular cluster, with the Milky Way hanging in full glory every night? Could planetary systems form there? What about life? Now we get at the true heart of building a world.
First, let’s talk about timing. Remember that most of these stars are old. Star formation ended long ago, when all the gas and dust was used up. Our Sun, however, was born only 4.5 billion years ago. So if humans had begun evolving on an Earth-like world in a globular cluster, we would reach the equivalent of the present day some 7.5 billion years ago!
Could the Sun have even formed? Maybe, maybe not. Not all stars are alike. Stars in globular clusters are mainly Population II stars. All this means is that they have a certain content of “metals”, elements heavier than hydrogen and helium. Our Sun, however, is a Population I star. This means that it has a lot of metal — much more than Population II stars. There might not be enough heavy elements in globular clusters to form terrestrial planets like Earth. That could be a problem.
Another problem is that globular clusters are really, really, really dense, as I said before. Leonard (1989) gives some formulas to calculate collision rates. To save you the trouble of doing the math out, I’ll give you a hint: The rates are pretty high. Those calculations don’t even give formulas for near-misses that could strip away planets. It would be hard for the Solar System to survive long enough for me to write this blog post! Even scarier is the idea of core collapse, where stellar density at the core increases rapidly for a period of time.
Also, there are many binary systems in globular clusters, meaning that the Sun could either form with a partner or gain one through interactions — which would not be good for Earth! Planets can form in binary systems, but they may not always be habitable.
Can planets form in globular clusters? Absolutely. Sigurdsson et al. (2008) discuss the first planet discovered in a globular cluster. It’s not the nicest place to live, but it’s a planet nonetheless.
So what would it be like to gaze at the stars from a planet in a globular cluster?
It turns out that this was covered on Astronomy Stack Exchange, asked by user Alexey Bobrick. Another user, astromax, calculated that a supergiant, as far from the Sun as stars are from each other on average in a globular cluster, would appear as luminous as the Sun! Alexey Bobrick also found a simulated image from inside a globular cluster:
So in the inner parts of a globular cluster, it would actually be really hard to see the Milky Way! But on the outer edges, where it’s less dense and a bit nicer for a planet, things might be a little different.
Either way, nights would be a lot more interesting. . .