Galaxies and the Expanding Universe
Types of Galaxy
Astronomers classify galaxies in three main groups: elliptical, spiral and barred spiral.
These galaxies range in shape from almost spherical (often referred to as lenticular galaxies), to a flattened lens shape. This group encompasses both the biggest and the smallest of the known galaxies in the Universe. The image is a colour composite photo of the central area of NGC 1316, a giant elliptical galaxy in the Fornax cluster of galaxies. Many dark dust clouds and lanes are visible. Some of the star-like objects in the field are globular clusters of stars that belong to the galaxy.
The distinguishing feature of spiral galaxies is the set of arms winding out from a central bulge. Most frequently, there are two arms, but sometimes there are more. This spectacular image of a large spiral galaxy is based on three exposures in ultra-violet, blue and red light, respectively. The colours of the different regions are clearly visible: the central areas contain older stars of reddish colour, while the spiral arms are populated by young, blue stars and many star-forming regions.
NGC 1365 is one of the most prominent barred galaxies in the sky. It is a supergiant galaxy with a diameter of about 200 000 light years.
A massive straight bar runs through this galaxy and contains the nucleus at the centre. It consists mostly of older stars that give a reddish colour to the bar.
The gravitational perturbation from the bar causes interstellar gas and dust clouds to form a pair of spiral arms that extend from the ends of the bar. Young luminous hot stars, born out of the interstellar clouds, give these arms a prominent appearance and a blue colour.
The bar and spiral pattern rotates clockwise, as seen from Earth. One full turn takes about 350 million years.
Under the gravitational grasp of a large gang of galaxies, called the Fornax cluster, the small bluish galaxy is plunging headlong into the group at 600 kilometres per second. NGC 1427A will not survive long as an identifiable galaxy passing through the cluster. Within the next billion years, it will be completely disintergrated, spilling its stars and remaining gas into intergalactic space within the Fornax cluster.
The spectacular barred spiral galaxy NGC 6872 that is shaped like an "integral sign". It is of type SBb and is accompanied by a smaller, interacting galaxy, IC 4970 of type S0 (just above the centre). The bright object to the lower right of the galaxies is a star in the Milky Way.
The upper left spiral arm of NGC 6872 is significantly disturbed and is populated by a plethora of bluish objects, many of which are star-forming regions. This may have been be caused by a recent passage of IC 4970 through it.