The Hitchhiker's Guide to Elliptical Galaxies
Elliptical galaxies are ellipsoidal agglomerations of stars,
which usually do not contain much interstellar matter, and look
smoothly like small wads when viewed through a telescope.
Some disk galaxies without much structure can hardly be
distinguished from elliptical galaxies and thus are sometimes
misclassified.
Elliptical galaxies are unlike spiral galaxies and hence unlike
our own Milky Way Galaxy.
Ellipti-what?
The most popularly used classification of galaxies is due to
Hubble (1925) and according to this categorization, there are
two major groups: the spiral and elliptical galaxies, but there
are also lenticulars and irregulars.
Before hitchhiking to and through elliptical galaxies, one must
first familiarize themselves with all the other types of
galaxies.
Spiral coils in space
Spirals like our own galaxy, fall into several classes depending
on their shape and the relative size of their bulge or how they
curve.
Spiral galaxies are characterized by the presence of gas in the
disk which means star formation remains active at the present
time, hence the younger population of stars. Spirals are usually
found in the low density galactic field where their delicate
shape can avoid disruption by tidal forces from neighboring
galaxies.
The egg in space
Ellipticals on the other hand are placed in sub categories
depending on their degree of ellipticity. They have a uniform
luminosity and are similar to the bulge in a spiral galaxy, but
with no disk. The stars are old and there is no gas present.
Ellipticals are usually found in the high density field, at the
center of clusters.
Irregular Lentils
The last two other types of galaxies are called Lenticular and
Irregular. Lenticulars also possess both a bulge and a disk, but
they have no spiral arms. There is little or no gas and so all
the stars are old. They also appear to be an intermediate.
Irregulars on the other hand are small galaxies, with no bulge
and an ill-defined shape. Spots in the universe
Galaxies are like islands in the Universe, made of stars as well
as dust and gas clouds. They come in different sizes and shapes.
Galaxies are not only distinct in shape, they also vary in size:
some may be as "light" as a stellar globular cluster in our
Milky Way (i.e. they contain about the equivalent of a few
million Suns) while others may be more massive than a million
Suns.
Presently, more than half of the stars in the Universe are
located in massive spheroidal galaxies.
One of the main open questions of modern astrophysics and
cosmology is how and when galaxies formed and evolved starting
from the primordial gas that filled the early Universe.
In the most popular current theory, galaxies in the local
Universe are the result of a relatively slow process where small
and less massive galaxies merge to gradually build up bigger and
more massive galaxies.
In this scenario, dubbed "hierarchical merging", the young
Universe was populated by small galaxies with little mass,
whereas the present Universe contains large, old and massive
galaxies, the very last to form in the final stage of a slow
assembling process.
If this scenario were true, then one should not be able to find
massive elliptical galaxies in the young universe. Or, in other
words, due to the finite speed of light, there should be no such
massive galaxies very far from us. And indeed, until now no old
elliptical galaxy was known beyond a radio-galaxy that was
discovered almost ten years ago.
And so the mystery of the elliptical galaxy continues. Continue
hitchhiking through galaxies to understand things better and
whatever happens, remember not to panic.