This set of HST images of galaxies shows elliptical galaxies along the top row and spiral galaxies in the rows beneath. From left to right, they are increasingly more distant and so seen as the universe was at progressively earlier times.
The typical elliptical and spiral galaxies on the far left illustrate the two main 'classical' types of galaxy seen in the universe at the present epoch, about 14 billion years since the Big Bang. Both are within a few tens of millions of light years. Elliptical galaxies contain older stars, while star formation continues vigorously in the disks of spirals.
The center left column shows galaxies in a rich cluster when the universe was about two thirds its present age (about 9 billion years old). The elliptical galaxy at the top resembles its present day descendants. By contrast, some spirals have a 'frothier' appearance, with loosely shaped arms.
Distinctive spiral structure appears even more vague and disrupted in the galaxies shown in the center right column. These date from the time when the universe was nearly one third its present age (about 5 billion years old). These spirals are not as symmetrical as the ones we see today and contain irregular lumps where bursts of star, formation are taking place. However, even this far back, the shape of the elliptical galaxy in the top box is clearly recognizable.
The galaxies in the far right column existed when
the universe was nearly one tenth its present age (2 billion years old).
The distinction between spirals and ellipticals is much less clear. Nevertheless,
the object in the top frame has all the appearance of a mature elliptical
galaxy. This implies that ellipticals formed remarkably early in the universe,
while spiral galaxies took much longer to form.
Camera: WFPC2
Credit: A. Dressler (Carnegie Institution
of Washington), M. Dickinson (STScl), F. D. Macchetto (ESA and STScl),
M. Giavalisco (STScI), and NASA
