Elliptical Galaxies: Cosmic Ovals
Messier 87: A massive elliptical galaxy in the constellation Virgo.
Elliptical galaxies are one of the three main morphological classifications for galaxies (alongside spiral and irregular galaxies) in the de Vaucouleurs system. They are characterized by their smooth, featureless, ellipsoidal shape, lacking the prominent spiral arms seen in spiral galaxies.
These galaxies represent a vast range of shapes, from nearly spherical (E0) to highly flattened (E7). The "E" denotes "elliptical," and the number indicates its degree of ellipticity, calculated from its apparent flattening on the sky. An E0 galaxy appears spherical, while an E7 galaxy is very elongated.
Key Characteristics of Elliptical Galaxies:
- Shape: Smooth, oval, or ellipsoidal, ranging from spherical to highly flattened.
- Structure: Lack distinct spiral arms, central bars, or significant disks. They consist primarily of a bulge and a halo.
- Stellar Population: Dominated by older, redder stars (Population II stars). They contain very little gas and dust, and thus, have very low rates of ongoing star formation.
- Motion: Stars orbit the galactic center in random orientations, more like a swarm of bees than the ordered rotation seen in spiral galaxies.
- Size: They vary greatly in size, from dwarf ellipticals with only a few million stars to giant ellipticals containing trillions of stars, often found at the centers of galaxy clusters.
Formation and Evolution
The formation of elliptical galaxies is thought to be primarily the result of galaxy mergers. When two large galaxies, particularly spiral galaxies, collide and merge, the violent gravitational interactions can disrupt their ordered structures, randomize stellar orbits, and trigger bursts of star formation that quickly consume any available gas. Over time, this process can transform the merging galaxies into a single, larger elliptical galaxy. This is particularly common in the dense environments of galaxy clusters, where galaxy interactions are more frequent.
The lack of gas and dust in elliptical galaxies means that they are not actively forming new stars. Their stellar populations consist mainly of older, redder stars that have long since exhausted their supply of lighter elements. This contributes to their overall reddish or yellowish appearance, contrasting with the bluer hues often seen in the spiral arms of star-forming galaxies.
Notable Examples
- Messier 87 (M87): A supergiant elliptical galaxy and the central galaxy of the Virgo Cluster. It's famous for its prominent jet of plasma ejected from its central supermassive black hole.
- Messier 49 (M49): Another large elliptical galaxy in the Virgo Cluster.
- Centaurus A (NGC 5128): While often classified as a peculiar or lenticular galaxy, it has an elliptical core and exhibits features suggesting a past merger.
Elliptical galaxies, despite their apparent simplicity, play a crucial role in the evolution of the universe. They are the most massive galaxies and often reside at the heart of cosmic structures, influencing their surroundings through their gravitational pull and active galactic nuclei.
Discovering more about elliptical galaxies helps us piece together the grand narrative of cosmic evolution and the dynamic processes that shape the universe we inhabit.