Galaxy classification

Edwin Hubble’s monumental discoveries revealed that the universe consists of a variety of vast galaxies that exist far beyond our Milky Way galaxy.

Edwin Hubble can be ranked as one of the great astronomers, whose discoveries are as important as those of Nicolaus Copernicus and Galileo Galilei.

In 1919, he began working at the Mount Wilson Observatory in California. A few years later, he used the newly built 2.5-metre (100-inch) Hooker Telescope to study Cepheid variable stars located in spiral nebulas. From these observations, he found that they must exist far beyond the Milky Way. In 1925, he presented his finding that these nebulas are galaxies in their own right, proving that the Milky Way is just a small part of the universe as opposed to the existing view that nothing existed beyond it.

Another major discovery was that the spectra of the light emitted by galaxies shifted towards the blue end of the spectrum if they were moving towards the Earth and that those moving away from the Earth showed a shift to the red end of the spectrum. From studying 18 galaxies, Hubble determined that the further away the galaxy is the faster it is moving away from us, which became known as Hubble’s Law. From his measurements he reasoned that 2 billion years ago the universe began in a single burst of energy and has been moving away from this point of origin ever since. Today, it is thought the universe began 13.77 billion years ago but this was an important foundation for the influential Big Bang theory.

In 1926, Hubble created his morphological classification of the different types of visible galaxies. He divided galaxies into three major groups that he arranged in his famous Hubble tuning fork diagram. Elliptical galaxies represent over half the visible galaxies and are given the prefix E, while a number from 0 to 7 denotes how round or oval they are. Giant ellipticals exist at the centre of galactic clusters that can contain billions of stars, though there are also dwarf elliptical galaxies that contain a few hundred thousand stars.

SPIRAL GALAXIES

Spiral galaxies are equally divided between spirals with a central bulge (S types) and spirals with a central bar (SB types). Stars form in the spiral arms and centres of these galaxies. A further designation running from a to c was given to indicate how tightly or loosely wound the arms are. A ‘d’ type represents galaxies that have luminous, fragmentary and loosely wound arms, and an ‘m’ type is for galaxies with no bulge and irregular appearance.

LENTICULAR GALAXIES

Lenticular (S0 type) galaxies do not have spiral arms but do have a bright central bulge of stars like the spiral galaxies. Unlike the spirals they do not have significant amounts of stars forming inside them.

IRREGULAR GALAXIES

Hubble identified a further group of irregular galaxies. He designated two types, the asymmetric Irr I that lacks a spiral or bulge structure that contains young stars, and Irr II, asymmetric galaxies that have no clearly visible stars or clusters.

CLASSIFYING GALAXIES

Although lenticular and elliptical galaxies are often called ‘early type’ galaxies, and spirals look more perfectly formed, Hubble was at pains to point out that his scheme did not show the evolutionary path of galactic formation. Instead, the diagram should be used as a guide to the visual appearances of the galaxies as seen from Earth.

Hubble’s system is not perfect because it depends on a subjective assessment of which category a galaxy fits into. Another problem is that it only deals with the galaxies as two-dimensional objects and does not fully represent their true three-dimensional structures. French astronomer Gérard de Vaucouleurs found several shortcomings with the scheme. In 1959, he extended it to include ring-like galactic structures and he gave numerical values to the different classes of galaxy. Certainly, advances in astronomy have provided more information about the structure of galaxies and more complex classification systems exist, but Hubble’s system remains a useful guide to understanding their appearance and basic structures.

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