Does Size Matter?

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.

The large image shows a star field imaged by Dr. Michael Hilker, using the 2.5-meter Du Pont Telescope at Las Campanas, Chile. The inset boxes are close-ups made using HST of two ultra compact disk galaxies, courtesy Michael Drinkwater of the University of Queensland. (Click to biggenate.)
The large image shows a star field imaged by Dr. Michael Hilker, using the 2.5-meter Du Pont Telescope at Las Campanas, Chile. The inset boxes are close-ups made using HST of two ultra compact dwarf galaxies, courtesy Michael Drinkwater of the University of Queensland. (Click to exframulate.)

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.

Happy Darwin Day!

A Global Celebration of Science and Reason

Evolution: from Earth to the Stars
Evolution: from Earth to the Stars (based on the graphic at DarwinDay.org and a piece of free clip art found at a science education clip art site. Composite by Carolyn Collins Petersen)

This year is the 200th anniversary of the birth of Charles Darwin, the evolutionary biologist who first described biological evolution and natural selection with scientific rigor and insight. The folks over at DarwinDay.org have put together a website to help folks who want to celebrate Darwin’s Day — or just to learn a little bit more about this amazing person.

Why is Darwin’s work important?  He set the bar high for scientists of all disciplines — collecting data, analyzing what he found, and then proposing a set of ideas about what the data told him that tell us today where we came from and how life evolved on this planet — and why it evolved.  His work, published as “The Origin of Species” is one of the most important pieces of scientific work ever published.  He may not have intended to have his work stand for science and reason (and openness to new ideas in the face of entrenched fundamentalism), but today it does.  There are people who don’t like Darwin’s work, don’t agree with it, and argue against it on theological reasons. But, none of them has ever brought any scientific data up that disproves what Darwin found in nature. Many have tried, but their data haven’t withstood reasonable scientific scrutiny.  And, the scientific method is what reasonable and thinking people use to explain the physical processes that affect everything that happens in the universe.

So, celebrate Darwin’s birthday today and do a little reading about what he did and stood for.  You’ll be celebrating not just a man’s birthday, but his insights into humanity’s origins — and the institutions of science and reason that bring us his work.