Cambridge Cosmology: Galaxies

Spiral and Elliptical Galaxies

The classification of galaxies due to Hubble (1925) remains the most popular in use today. There are two main categories, the spiral and elliptical galaxies, but also lenticulars and irregulars.

Spirals

like our own galaxy, fall into several classes depending on their shape and the relative size of the bulge: ordinary spirals are labelled either Sa-d,m while those which have developed a bar in the interior region of the spiral arms are SBa-d,m. Spiral galaxies are characterized by the presence of gas in the disk which means star formation remains active at the present time, hence the younger population of stars. Spirals are usually found in the low density galactic field where their delicate shape can avoid disruption by tidal forces from neighbouring galaxies.

M100 and NGC1365 (AAO)

Ellipticals

are placed in the categories E0-7 depending on their degree of ellipticity. They have a uniform luminosity and are similar to the bulge in a spiral galaxy, but with no disk. The stars are old and there is no gas present. Ellipticals are usually found in the high density field, at the centre of clusters.

The giant elliptical galaxy M87 (AAO)

Lenticulars

are labelled S0 and, although they possess both a bulge and a disk, they have no spiral arms. There is little or no gas and so all the stars are old. They appear to be an intermediate.

Sombrero galaxy NGC4549 (AAO)

Irregulars

are small galaxies, labelled Irr, with no bulge and an ill-defined shape. The Magellenic clouds are examples.

The Large Magellenic Cloud (AAO)

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