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Elliptical Galaxies


Introduction


These galaxies have few obvious features. They range from objects that are almost spherical, to those that are almost flat. Spherical ellipticals are classed as E0 galaxies, while flat ones are classed as E7, with the types in between being given the numbers E1 to E6. The diagram below shows how this number is calculated.

Calculating En


where n = 10 (1-
B

A
).


M59, an E5 elliptical galaxy The problem with this system is that the shape of the galaxy depends on its angle. In other words, it describes how the galaxy appears to us - it does not tell us about the real shape of the object. Unless a galaxy is perfectly spherical (in which case it would be an E0, since the semi-major axis a, and the semi-minor axis b, would be equal), it will have a different appearance depending on the direction from which it is seen.

The system is still useful though, because it describes how a galaxy looks through a telescope. The image on the left is of the elliptical galaxy Messier 59 (shown here courtesy of the SEDS Archive). This galaxy is an E5 class, and the image clearly shows a much more "flattened" appearance, compared with the E0 galaxy Messier 32 (shown at the top of this page). M59 is a member of the Virgo cluster.


What determines the shape of elliptical galaxies?


Not much structure can be seen in elliptical galaxies, unlike the magnificent shapes we see in the spirals. Elliptical galaxies are simply vast collections of stars that remain relatively close together because of the gravitational attraction between them.

The system of classifying the ellipticity of these galaxies can produce labels up to E10, but in reality we only see objects of classes up to and including E7.


The physical nature of elliptical galaxies


The appearance of the galaxies is not the only difference between ellipticals and spirals; they are also very different in their content.

Colour photographs of ellipticals show that they are more red in colour than spirals (note, however, that the image of M32 at the top of this page is a false colour one!) This is because elliptical galaxies mostly contain older, cooler, and therefore redder stars. In contrast, spirals contain many young, hot, blue stars in their discs and in the central regions. Elliptical galaxies also contain very little gas and dust, which slows the formation of new stars.

Elliptical galaxies have a much wider range of masses and sizes than spirals. According to one estimate, their masses range between 100,000 to 10,000,000,000,000 solar masses, and their diameters between 1000 and 100 000 parsecs (or 3260 to 326,000 light years). Mass and diameter affect how much light a galaxy gives out, and so elliptical galaxies therefore show a larger range in brightness, with some up to 10 times brighter than the most luminous spirals. At the other end of the scale, the faintest ellipticals can be 1000 times dimmer than the faintest spiral.

Astronomers believe that because they have older stars, and have little structure, elliptical galaxies probably formed earlier than spirals. M87, an E1 elliptical galaxy

The image on the right (courtesy of the Anglo-Australian Observatory) shows the giant elliptical galaxy M87 (type E1), along with some smaller companion galaxies. The galaxy contains about as much material as two and a half trillion Suns – that’s 2,500,000,000,000 Suns. M87 is an excellent example of an elliptical galaxy, but it is also a very unusual object. In 1918 astronomers discovered a jet of material thousands of light years long coming from the centre of the galaxy. More recent observations using the Hubble Space Telescope have shown what appears to be a massive black hole at the centre of the galaxy, as massive as three billion Suns.


A black hole in M32


Recent computer simulations, along with observations from the Hubble Space Telescope (HST), have suggested that there may also be a black hole in the centre of the galaxy M32 (pictured above). HST observed the very centre of this galaxy, and was able to measure the motion of stars in the area. Using these measurements, and one of the world’s most powerful computers, astronomers were able to show that these stars behaved exactly as they would if there was a black hole at the centre of the galaxy. This backed up earlier HST observations that detected a bright region in the same place, suggesting that there was a supermassive object there.



Click on the links below to find out more about galaxies.


Galaxies Introduction

Formation of Galaxies

The Milky Way

Spiral Galaxies

Irregular Galaxies

Tuning Fork Diagram



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Author: Nigel Bannister

Updated by: Carolyn Brinkworth and Claire Thomas

Last updated: July 2001