It Might for Galaxies in the Early Universe
We live in a region of the Milky Way Galaxy where our neighbors are pretty few and far between. The closest star is 4.5 light-years away and even if we had a good interstellar space ship that could travel at oh, say, a tenth the speed of light (roughly 300,000,000 meters per second), it would take centuries to get there. It would appear that living the galactic sticks is part and parcel of being in our spiral galaxy.
But, what if we lived in a galaxy that was perhaps 1/1000 the diameter of the Milky Way? Say something like an Ultra Compact Dwarf Galaxy? Such objects existed in the early universe and they had stars jam-packed together in a region only about 60 light-years across! The bright star Aldebaran in the constellation Taurus is around 60 light-years away. Imagine packing a whole galaxy into the space between us and Aldebaran!
So, does a galaxy’s size matter? It could, depending on what you want to know about it.
For one thing, if there were any planets around these stars (and that’s probably doubtful), the “light pollution” from all the nearby stars would drown out our view of the more distant ones. For another, these objects were most likely formed when more “normal” galaxies collided and mingled stars a few billion years after the Big Bang. That means that they’ve experienced some major transformations and evolution since then.
Actually, what really matters about these UCDs (as they are often referred to) is their masses. They seem to have way more matter than their starlight implies. Could they be full of massive stars or dark matter? Or something else?
A team of astronomers led by Professor Pavel Kroupa and graduate student Joerg Dabringhousen of the University of Bonn has been looking at UCDs to figure out what they’re made of. They think that each UCD was incredibly packed with stars — maybe as many as a million in each cubic light -year of space. For comparison, in our part of the Milky Way, that number is closer 1 star per cubic light-year. Think of cramming a million stars within a cubic light-year of the solar system. The sky would literally glow.
Having stars crammed together that closely in the UCDs means that over time, they could merge together to build hugely massive stars — the kind that live fast and die young as supernovae. What’s left of these massive stars at that point are superdense neutron stars or the occasional black hole. Both of which are incredibly dense without being bright.
So in today’s UCDs, much of their mass is sunk into these dark remnants that you can think of as fossils pointing the way to a more dramatic and active past. Billions of years ago they must have looked absolutely stunningly bright. And, if you were inside one, on a hypothethical planet, your sky would be nothing but stars making it as bright as a sunny day here on Earth.
I’m really happy than we are in a relative big galaxy. This make our civilization to be able to last longer (if we don’t kill ourself before something else does)
This raises a whole series of questions for me.
1. Why did UCDs only form in the early universe? Is it because the universe was more dense then?
2. If UCDs seem to have more matter than their starlight accounts for, could most of that matter be things like brown dwarfs? Why posit an unknown like dark matter?
3. How does a UCD compare in density to a globular cluster?
Good questions. Your second question is answered (sort of) in the article — but the authors pointed to the idea of the supermassive young stars in these UCDs dying in supernova explosions and leaving behind incredibly dense corpses (black holes, neutron stars) that don’t emit much (if any) light. That would explain why there’s less starlight than the mass implies. I don’t think they were saying it was “only” dark matter.
I don’t think we know for sure why we only see these in the early universe. But I’m not an expert in that area. And, I’d think that the UCD would be much denser than a typical globular cluster — but are you asking for a comparison to GCs of today, or of that epoch?
Here’s a link to the press release about the finding:
http://www.ras.org.uk/index.php?option=com_content&task=view&id=1556&Itemid=2