Alachua Astronomy Club: FirstLight Tenth Year -- Sugalski 1992 December

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Introduction & Dedication

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1992

Tenth Anniversary Edition: From December 1992 FirstLight

Blue Galaxies and the Size of the Universe

by Jan Sugalski
In the June, if 1992 edition of Physics Today, there is an article entitled ‘Mapping Dark Matter with Gravitational Lenses.’ If you look far enough into space, back to a time when the universe was one half to one third its present size, you will see a background of blue galaxies. This is true in any direction you look. The light from these background blue galaxies can be bent by the gravitation of dark foreground matter. Astronomers can determine a ot about the dark matter by seeing how it bends the light from these blue background galaxies.

I first heard about these blue galaxies on the six-part series called ‘The Astronomers’ which was run on PBS. Since then I have wondered – how can they be all blue? The article on gravitational lensing gave the answer.

It turns out that at the age of the universe we are seeing at the distance of the blue galaxies, star formation was much more active than it is now. Furthermore the abundance of new hot massive stars radiate very brightly in ultraviolet light. But at the distance of the blue galaxies, the red shift is large and has shifted the ultraviolet just into the visible spectrum at the blue end. Hence these galaxies appear blue to us.

In addition to this information, the article on gravitational lensing mentioned something which may help you appreciate the true size of the universe.

The most sensitive CCD detectors when used with the 4 meter telescope at Cerro Tololo in Chile has found more than 300,000 blue background galaxies per square degree of sky! You may pause here to catch your breath). To put this in perspective, the Uranometria 2000.0 star atlas catalogs a total of 332,556 stars and 10,300 non-stellar objects for a grand total of 342,856 objects. It is certainly possible then, in fact likely, that each square degree of sky contains as many galaxies as all the objects listed in the entire Uranometria 2000.0 atlas. Now consider that each of these galaxies contains about 100 billion stars! The mind boggles.

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