Elliptical Galaxy

Elliptical Galaxies  are spherical or elliptical in shape. They lack the gasses required to form new stars, so they are almost entirely composed of old stars (Population II), giving them a reddish/yellowish color in contrast to the blue of spiral galaxies. Large elliptical galaxies usually have extensive systems of globular clusters surrounding them. Elliptical galaxies are thought to be the result of galactic collisions. 15% of galaxies are elliptical.

The smaller cousin of the Elliptical galaxy is the Dwarf Elliptical Galaxy: something in between an elliptical galaxy and a globular cluster. These dwarfs are often found as a satellite galaxy to a bigger galaxy.

The stars within the galaxy don’t orbit on a single plane, as in disk-shaped galaxies, but rather orbit randomly at varying inclinations. Profiles such as the Sersic Profile and Einasto profile can mathematically calculate the intensity and density of the galactic bulge, but these are rather complex and require you to work out things such as the scale height and central intensity of your elliptical galaxy, which is overkill and not very relevant overall. All that you really need to know is that the stellar density of the galaxy increases as you get closer to the galactic bulge.

Examples of elliptical galaxies include Messier 49, Messier 87, Messier 89 and Maffei 1.

Classification
The Hubble Classification System denotes ellipticals with the letter E, and dwarf ellipticals with dE. A number from 0 to 7 follows this, and describes the shape of the elliptical galaxy, with 0 describing a spherical galaxy and 7 describing a cigar-shaped galaxy.

If you know the semi-major and semi-minor axes of the elliptical, you can calculate its "E number" as En  where: where  a  is the semi-major axis and  b  is the semi-minor axis of the ellipse.

About 1/10 of ellipticals have shell structures, where stars in the galaxy’s halos are arranged in concentric shells. This is a property exhibited only by ellipticals.

Physical Properties
Elliptical galaxies have the broadest mass and radius range of any galaxy type, ranging from 50 parsecs to over 50 kiloparsecs and from 107 solar masses to almost 1013 solar masses.

Each elliptical galaxy has a supermassive black hole at its centre, and, just as in spiral galaxies, the mass of this black hole (M) is related (via the M-sigma relation) to the stellar velocity dispersion (𝜎) of the galaxy. The only major difference is that in the case of spirals, the relation calculates only the 𝜎 of the bulge, whereas the relation applies to the entirety of an elliptical galaxy. For dwarf elliptical galaxies, this value is around 80 km/s, whereas for normal elliptical galaxies, this value is about 200 km/s.

As in the case of spiral galaxies, the M-sigma relation is: Where M is expressed in solar masses and σ in km/s.

If you wish, you can calculate just how many stars your galaxy has by calculating the volume of your galaxy and applying it to this formula:



Where ρs is stellar density. The stellar density of the Milky Way's bulge is about 2.882 stars per light year cubed (99.993 stars per cubic parsec). As one gets further away from the centre of the galaxy, the stellar density diminishes.

You can use this calculator to calculate the volume of the galaxy.

Habitability
In terms of galactic habitability, elliptical galaxies actually seem to be more suitable for both simple and complex life. Old stars are good candidates for having planetary systems and this, combined with 1/10 the supernova rate of our own galaxy, creates a rich environment for life.

Habitable planets would not, however, be found in the galactic bulge due to lethal amounts of radiation and the high rate of star collisions (similar to the galactic bulge of spiral galaxies). One should therefore define how large the bulge is in respect to the rest of the galaxy, in order to establish the habitable zone of the galaxy.

From the perspective of a planet inside an elliptical galaxy, the night sky would not look too different from one viewed from within a spiral galaxy; however, no galactic band would be visible.

Elliptical galaxies are also rather boring to map, compared to the beautiful spiral arms of a spiral galaxy or the interesting configurations of an irregular or peculiar galaxy.

Worldbuilding in Practice
"The target for the next wave of intergalactic colonisation is the fairly typical and unassuming elliptical galaxy Inti, located in the Thebes cluster. With a semi-major axis of about 60,000 light years and a semi-minor axis of about 48,000, Inti is a standard E2 elliptical galaxy with about 2.1 hundred trillion (2.1 x 1014) stars. Inti is average in almost every way, and would be completely glossed over by astronomers were it not for two reasons." "The first is that Inti has a comparatively tiny core compared to the its halo. This means that it is very habitable.""Secondly, it is orbited very closely by ISG 23, a dwarf spiral galaxy which would be rather cheaply and quickly accessed from Inti.""ISG 23 is not Inti’s only companion, however. Inti is orbited by at least 37 objects: including ISG 23, ISG 3 and ISG 14 (both dwarf irregulars) and ISG 1 (an unusually bright globular cluster)."