Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.arcetri.astro.it/~palla/ANDREAIMMAGINI/Cosmologia/GalaxEvC.txt Дата изменения: Sun Mar 4 21:07:26 2001 Дата индексирования: Sat Dec 22 08:55:11 2007 Кодировка: Поисковые слова: spiral galaxy

PHOTO RELEASE NO.: STScI-PR94-52C
EMBARGOED UNTIL: 1:00PM (EST) DECEMBER 6, 1994


GALAXIES: SNAPSHOTS IN TIME

This sequence of NASA Hubble Space Telescope (HST) images of
remote galaxies offers tantalizing initial clues to the evolution of galaxies
in the universe.

[far left column]
These are traditional spiral and elliptical-shaped galaxies that make up
the two basic classes of island star cities that inhabit the universe we
see in our current epoch (14 billion years after the birth of the universe
in the Big Bang). Elliptical galaxies contain older stars, while spirals
have vigorous ongoing star formation in their dusty, pancake-shaped
disks. Our Milky Way galaxy is a typical spiral, or disk-shaped galaxy,
on the periphery of the great Virgo cluster. Both galaxies in this column
are a few tens of millions of light-years away, and therefore represent
our current stage of the universe s evolution.

[center left column]
These galaxies existed in a rich cluster when the universe was
approximately two-thirds its present age. Elliptical galaxies (top)
appear fully evolved because they resemble today's descendants. By
contrast, some spirals have a frothier appearance, with loosely shaped
arms of young star formation. The spiral population appears more
disrupted due to a variety of possible dynamical effects that result
from dwelling in a dense cluster.

[center right column]
Distinctive spiral structure appears more vague and disrupted in
galaxies that existed when the universe was nearly one-third its present
age. These objects do not have the symmetry of current day spirals
and contain irregular lumps of starburst activity. However, even this far
back toward the beginning of time, the elliptical galaxy (top) is still
clearly recognizable. However, the distinction between ellipticals and
spirals grows less certain with increasing distance.

[far right column]
These extremely remote, primeval objects existed with the universe was
nearly one-tenth its current age. The distinction between spiral and
elliptical galaxies may well disappear at this early epoch. However, the
object in the top frame has the light profile of a mature elliptical galaxy.
This implies that ellipticals formed remarkably early in the universe
while spiral galaxies took much longer to form.

Credit: A. Dressler (Carnegie Institutions of Washington),
M. Dickinson (STScI), D. Macchetto (ESA/STScI), M. Giavalisco
(STScI), and NASA