Stars cheek by jowl in the early Universe

February 12, 2009
The background image was taken by Dr Michael Hilker of the University of Bonn using the 2.5-metre Du Pont telescope, part of the Las Campanas Observatory in Chile. The two boxes show close-ups of two UCD galaxies in the Hilker image. These images were made using the Hubble Space Telescope by a team led by Professor Michael Drinkwater of the University of Queensland.

( -- In the early Universe, some galaxies may have had stars packed together a hundred times more closely than in the present day, according to research by a University of Bonn team to be published in a paper in Monthly Notices of the Royal Astronomical Society.(

In our Galaxy, we are used to the idea that even the nearest stars are light years away from the Sun. But a team of scientists led by Professor Pavel Kroupa of the University of Bonn think things were very different in the early Universe. In particular, Ultra Compact Dwarf galaxies (UCDs), a recently discovered class of object, may have had stars a hundred times closer together than in the solar neighbourhood, according to calculations made by team member and PhD student Joerg Dabringhausen and presented in a paper in Monthly Notices of the Royal Astronomical Society.

UCDs were discovered in 1999. Although they are still enormous by everyday standards, at about 60 light years across, they are less than 1/1000th the diameter of our own Galaxy, the Milky Way. (In more familiar units, a light year is about 10 million million km). Astronomers believe that UCDs were created when more normal galaxies collided in the early Universe. But oddly, UCDs clearly have more mass than the light from the stars they contain would imply.

Up to now, exotic dark matter has been suggested to explain this ‘missing mass’, but this is not thought to gather in sufficient quantities within a UCD. In their paper Mr Dabringhausen, Professor Kroupa and their colleague Dr Holger Baumgardt present a different explanation.

The astronomers think that at one time, each UCD had an incredibly high density of stars, with perhaps 1 million in each cubic light year of space, compared with the 1 that we see in the region of space around the Sun. These stars would have been close enough to merge from time to time, creating many much more massive stars in their place. These more massive stars consume hydrogen (their nuclear fuel) much more rapidly, before ending their lives in violent supernova explosions. All that then remains is either a superdense neutron star or sometimes a black hole.

So in today’s UCDs, a good part of their mass is made up of these dark remnants, largely invisible to Earth-based telescopes but fossils of a more dramatic past.

Mr Dabringhausen comments, “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.”

Provided by Royal Astronomical Society

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not rated yet Feb 12, 2009
I've heard this idea before, but I still find it incredible. 1 MILLION stars per cubic light year. That's like sardines in a tin!
5 / 5 (1) Feb 13, 2009
I find it more disturbing that the dark matter that cannot be seen in these galaxies is calculated to be neutron stars, or in other words, ordinary Baryon matter.

Yet, over and over again, we are told that there can be no extra matter than what we can see. So gravitation effects has to be cased by some strange matter with stranger properties. Regardless that some people protest and ask "why?" the answer is always because there is more mass than we have calculated.

And in My Opinion, the answer should be "so what?" how reliable is your calculation anyway? I see re-estimates that involve orders of magnitude so forgive me if I don't jump on the "dark matter" bandwagon.

Much easier to adjust the mass to match the gravity and then start working at where the mass is and what it looks like. Meanwhile leave the dark matter out of mainstream until it becomes essential. We are a far cry from that position yet.
5 / 5 (2) Feb 13, 2009
I think they're just saying the dark matter alone cannot explain the mass in these galaxies. Although they are still assuming it's presence.
4.7 / 5 (3) Feb 13, 2009
jeffsaunders: "Dark Matter" is an all inclusive term for everything that we suspect is there, but can't see. It isn't just one thing. It includes cold gas, old (cool) brown dwarves, black dwarves (if they've had enough time to form, which they probably haven't), non-pulsar cold neutron stars, neutrinos, and finally non-binary inactive black holes (nothing nearby for them to affect, so we can't see them).

However, when all of that stuff is added up, it accounts for only a fraction of the total dark matter that we can see the effects of, based on gravitational interactions.

There have been ideas that maybe we just had the theory of gravity *slightly* wrong on large scales, but recent observations have made that hypothesis very unlikely, lending credence to the idea of some form of more exotic dark matter.

The most likely form of the irregular dark matter is some form of supersymmetric particle, IIRC.
not rated yet Feb 13, 2009
I don't believe it!!
1 / 5 (1) Feb 18, 2009
"The Real Explanation"

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