This stunning picture from one of the instruments on board the Hubble Space Telescope has nothing to do with dark matter; it just gives some idea of how much stuff's `out there'. This picture covers an area of the sky one minute of arc square (about 1/700th of the area covered by the full moon) -- and almost everything you can see is a galaxy, most containing hundreds of thousands of millions of stars.
(more recently -- in March 2004 -- the Hubble Ultra Deep Field survey probed even more deeply an area about three minutes of arc square: links to images and story are here.)
The images, extracted from the Hubble Deep Field (HDF), were assembled from many separate exposures (342 frames total were taken, 276 have been fully processed to date and used for this picture) with the Wide Field and Planetary Camera 2 (WFPC2), for ten consecutive days between December 18 to 28, 1995.
Besides the classical spiral- and elliptical-shaped galaxies, there is a bewildering variety of other galaxy shapes and colors. The never before seen dimmest galaxies are nearly 30th magnitude, or nearly four billion times fainter than the limits of human vision.
Representing a narrow "keyhole" view stretching all the way to the visible horizon of the universe, the HDF image covers a speck of sky a tiny fraction the diameter of the full Moon. This is so narrow, just a few foreground stars in our Milky Way Galaxy are visible and are vastly outnumbered by the menagerie of far more distant galaxies.
Though the field is a very small sample of sky, it is considered representative of the typical distribution of galaxies in space because the universe, statistically, looks the same in all directions. The HDF will provide important clues to understanding the evolution of the universe. Some of the galaxies may have formed less than one billion years after the Big Bang.
This "true-color" view was assembled from separate images which were taken in blue, red, and infrared light. By combining these separate images into a single color picture, astronomers will be able to infer -- at least statistically -- the distance, age, and composition of galaxies. Bluer objects contain young stars and/or are relatively close, while redder objects contain older stellar populations and/or are farther away.
Credit: Robert Williams and the Hubble Deep Field Team (STScI) and NASA
The second picture is an enlargement of the bottom left quarter of the first. The two spiky things (one near the centre and one near the top) are stars; all the rest are galaxies.
PHOTO NO.: STScI-PRC96-24b
Small portion of the Hubble Deep Field image -- the deepest view of the universe taken with NASA's Hubble Space Telescope. Arrow points to a very faint galaxy that appears to be more distant than any known previously. Other galaxies in the image are at smaller distances.
Credit: Ken Lanzetta and Amos Yahil (State University of New York at Stony Brook), and NASA
The first picture in the series can be seen to be made up of 13 separate squares; the above picture covers approximately the top left quarter of the centre-right square in the bottom row -- about 1/200000th of the area of the moon.