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Elliptical Galaxies

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Most galaxies in our Universe are spiral galaxies, much like our own Milky Way. However, a significant percentage, perhaps 15% or so, are what are known as elliptical galaxies.

General Characteristics of Elliptical Galaxies

As the name suggests, elliptical galaxies range from spherical in space, to more elongated - nearly geometry closer to that of a football.

Some are only a fraction the size of the Milky Way, while others are many times larger.

Elliptical galaxies also appear to have a large amount of dark matter, a fact that distinguishes their smallest constituents (the dwarf ellipticals) from simple star clusters.

Star Types and Star Formation

Elliptical Galaxies are noticeably absent of gas - the key component of star forming regions. Therefore the stars in these galaxies tend to be very old, with new star births a rarity.

Furthermore, these old stars tend to be yellow and red stars; which according to our understanding of stellar evolution means that they are smaller, dimmer stars. This also explains why star formation has ceased.

When many large stars are formed, they die quickly and redistribute much of their mass during a supernova event, leaving the seeds for new stars to be formed. But since smaller mass stars take tens of billions of years to evolve into planetary nebulae, the rate at which gas and dust is redistributed in the galaxy is very low.

And when the gas from a planetary nebula finally drifts into the intergalactic medium, there is typically not nearly enough around to begin forming a new star, unless there are several other stars nearby.

Formation of Elliptical Galaxies

Since star formation seems to have ceased, it is thought that a period of rapid formation must have happened early in the galaxy's history.

One theory is that elliptical galaxies may primarily form through the collision and merger of two spiral galaxies. The current stars of those galaxies would become intermixed, while the gas and dust would collide.

The result is that there would be a sudden burst of star formation, using up much of the available gas and dust.

Simulations of these mergers also show that the resulting galaxy would have a formation much like that of elliptical galaxies. This also explains why spiral galaxies seem to dominate, while ellipticals are more rare.

This would also explain why we don't see very many ellipticals when we survey the oldest galaxies we can detect. Most of these galaxies are, instead, Quasars - a type of active galaxy.

Elliptical Galaxies and Supermassive Black Holes

Some physicists have theorized that at the center of every galaxy, almost regardless of type, lies a supermassive black hole. While this is somewhat difficult to prove - even in galaxies where we don't directly "see" a black hole, that does not necessarily mean that one is not there - it seems that at least all (non-dwarf) elliptical (and spiral) galaxies that we have observer contain these gravitational monsters.

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