Galaxies

A galaxy is made of billions of stars, dust, and gas all held together by gravity. Galaxies are scattered throughout the Universe. They vary greatly in size and shape. Not all galaxies look alike.

Types of Galaxies Galaxies come in a variety of shapes. In the 1920s Edwin Hubble was the first to study the morphology (shape) of galaxies. Using the 100-inch Hooker reflector telescope at Mount Wilson Observatory in California between , Hubble photographed numerous galaxies. He categorized these shapes or basic schemes as spiral, barred spiral, elliptical, irregular, and peculiar. This system was known as the Hubble morphological sequence of galaxy types.

Spiral Galaxies Some galaxies, like the M31 Andromeda Galaxy, appear as disks and have arms of stars and dust which appeared in a spiral pattern. These galaxies appear nearly uniform in brightness and in some the arms are more tightly wound around the galaxy. These are called spiral galaxies. Our Galaxy, the Milky Way, is an example of a spiral galaxy.

Barred Galaxies Barred spirals are spiral galaxies that have a straight bar across their centre from the ends of which the spiral arms emerge. The arms of spiral galaxies contain gas and dust from which new stars are still forming.

Elliptical Galaxies Elliptical galaxies contain old stars and very little gas. They include the most massive galaxies known, containing a trillion stars. At least some elliptical galaxies are thought to be formed by merges between spiral galaxies.

Irregular Galaxies A fourth type of galaxy observed by Hubble was neither spiral nor elliptical, but was irregular in shape. These galaxies were called irregular. An example of this is the Magellanic Clouds.

Peculiar Galaxies Finally, there are some galaxies that fit none of these descriptions. These are called peculiar galaxies, one example of which is Centaurus A

Astronomers now have decided that the morphology classification should consist of only two types of galaxies: the spiral and the elliptical. Barred spirals are a subclass of spirals. Irregulars may be either spiral or barred spiral. Peculiars are not fundamentally a different type. They are simply galaxies in the act of colliding; the collision distorts their shape and makes them appear “peculiar” Our own galaxy, the Milky Way, is about light years in diameter, and contains at least 100 billion stars. It is a member of a small cluster of the local group. The Sun lies between two spiral arms, about light years from the centre. Classifying Galaxies

Spiral galaxies are denoted by “S”, and barred spirals by “SB”. Letters “a”, “b”, “c” denote how tightly the spiral arms are wound, with “a” being most tightly wound. The Andromeda Galaxy is an Sb. Elliptical galaxies are denoted by “E”, with a number from 0-7 indicating how circular it appears. An example of this would be M87, which is an E0 galaxy. Irregulars, such as the Small Magellanic Cloud, are denoted by “Irr”. Peculiar galaxies, such as Centaurus A, are denoted by “P”. Classifying Galaxies

Hubble’s Classification of Galaxies

Formation of Galaxies Galaxies form from the clumping of primordial matter that arise after the Big Bang and grow under inflation. This clumping of primordial matter forms filaments, and galaxies form in knots along the filaments.

Spiral vs. Elliptical Galaxies The final type of galaxy depends on initial rate of star formation: If stars form quickly, then galaxy becomes elliptical. Stars form within initial distribution of gas, and follow their initial orbits. If stars form later, the gas has time to collapse into a disk. Most stars from within the disk. The galaxy becomes a spiral.

Formation via Galaxy Mergers In clusters, galaxies can pass close to one another. Galaxies can become distorted, and often merge. Mergers often lead to giant elliptical galaxies at the heart of large clusters.