Early stars formed

galactic ‘jam’

BY KULVINDER SINGH CHADHA

ASTRONOMY NOW

Posted: 13 February, 2009

Stars in the early Universe, particularly in ultra-compact dwarf galaxies (UCDs), could have been crammed a thousand times closer together than they are today, according to a team of scientists from the University of Bonn in Germany.

If the same situation applied in the Milky Way then, as an example, the closest star to the Sun, Alpha Centauri (which lies 4.2 light years away), would have ended up only 6.7 times further away from us than Pluto is. Team member Joerg Dabringhausen sums it up by saying, "Billions of years ago, UCDs must have been extraordinary. To have such a vast number of stars packed closely together is quite unlike anything we see today. An observer on a (hypothetical) planet inside a UCD would have seen a night sky as bright as day on Earth."

Image showing the location of two ultra compact dwarf galaxies. Image: Michael Hiker (Bonn University)/Michael Drinkwater (University of Queensland).

This situation is thought to have existed in UCDs, which were discovered in 1999. Ultra compact dwarfs are thought to be the dense cores of elliptical galaxies that collided, stripping away the remaining gas. This would have left an object 60 to 200 light years across and typically containing 100 million stars (for comparison, the Milky Way is 100,000 light years in diameter and contains between 200 – 400 billion stars). But UCDs have much higher masses than are observed from their stars’ light, which has led to the idea that they contain large amounts of unseen dark matter.

However, it’s unclear whether dark matter can accrue in sufficient quantities to explain what’s observed and Professor Pavel Kroupa and colleagues from Bonn University have a different angle on things. With so many stars crammed into such a confined space (as much as a million per cubic light year), mergers between them would have been highly likely to occur, like drops of water coalescing. This would have created enormous stars that devour their hydrogen fuel in a short space of time, before destroying themselves in supernova explosions.

These would leave behind dense neutron stars or black holes with powerful gravitational fields. And in a place with lots of supernovae, there would be lots of these objects. Effectively, UCDs could be likened to a galactic ‘jam’.

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