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.