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Elliptical Galaxies


Introduction


These galaxies have few distinguishing 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. In general, an elliptical galaxy is given the label En, where the number n is determined by the galaxy shape. 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 classification is its complete dependance on the orientation of the galaxy with respect to the Earth. In other words, it describes how the galaxy appears to us - it does not tell us much about the true physical properties 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 according to the direction from which we observe it. Nevertheless, the En system is useful for describing 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?


There is very little structure visible in elliptical galaxies, unlike the magnificent forms 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. Unlike the spirals there is no well defined rotation axis, so the stars in the galaxy show a wide range of orbital paths around the centre. The motions of stars are not entirely random, however. There is a degree of uniformity in the direction of these motions, and this determines the overall shape of the galaxy.

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


Superficial appearance is not the only difference between elliptical and spiral galaxies. They are distinctly 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 old 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 restricts the formation of new stars.

Elliptical galaxies exhibit a much wider range of masses and sizes than their spiral counterparts; 100,000 to 10,000,000,000,000 solar masses according to one estimate, and diameters of between 1000 and 100 000 parsecs (or 3260 to 326,000 light years). 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.

The lack of structure and an old stellar population have led astronomers to believe that the elliptical galaxies formed earlier than spirals. M87, an E1 elliptical galaxy

The image on the right (courtesy of the AAO) shows the giant elliptical galaxy M87 (type E1), along with some smaller companion galaxies. The galaxy is estimated to contain material equivalent to two and a half trillion 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. In addition, astronomers studying the radio "noise" from M87 found that the galaxy was extremely energetic. Subsequent 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 theoretical 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 a black hole was present in the galactic centre. This backed up earlier HST observations that detected a bright region in the centre of the galaxy, suggesting the presence of a supermassive object.



Galaxies Introduction

Formation of Galaxies

The Milky Way

Spiral Galaxies

Irregular Galaxies

Tuning Fork Diagram

Galaxies to Observe with the Faulkes Telescope



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Author: Nigel Bannister

Updated by: Carolyn Brinkworth and Claire Thomas

Last updated: July 2001