The Hubble sequence is a morphological classification process for galaxies invented by Edwin Hubble in 1936.[1] It is often known colloquially as the Hubble tuning-forkâ because of the shape in which it is historicallyin the past represented. Hubbleâs process divides galaxies in to three broad classes based on their visual appearance:
* Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity on the sky.
* Spiral galaxies consist of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge, which is similar in appearance to an elliptical galaxy. They are given the symbol S. Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol SB.
* Lenticular galaxies (designated S0) also consist of a bright central bulge surrounded by an extended, disk-like structure but, unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity.
The Hubble sequence is often represented in the type of a two-pronged fork, with the ellipticals on the left (with the degree of ellipticity increasing from left to right) and the barred and unbarred spirals forming the parallel prongs of the fork. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the prongs meet the âhandleâ.
These broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxy, such as irregular galaxies, which have no obvious regular structure (either disk-like or ellipsoidal).
To this day, the Hubble sequence is the most often used method for classifying galaxies, both in professional astronomical research and in amateur astronomy.
* Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity on the sky.
* Spiral galaxies consist of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge, which is similar in appearance to an elliptical galaxy. They are given the symbol S. Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol SB.
* Lenticular galaxies (designated S0) also consist of a bright central bulge surrounded by an extended, disk-like structure but, unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity.
The Hubble sequence is often represented in the type of a two-pronged fork, with the ellipticals on the left (with the degree of ellipticity increasing from left to right) and the barred and unbarred spirals forming the parallel prongs of the fork. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the prongs meet the âhandleâ.
These broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxy, such as irregular galaxies, which have no obvious regular structure (either disk-like or ellipsoidal).
To this day, the Hubble sequence is the most often used method for classifying galaxies, both in professional astronomical research and in amateur astronomy.