Galaxies What is a galaxy? • A collection of gas, dust and billions of stars held together by gravity (e.g. Earth and our solar system are part of the Milky Way Galaxy) • They are scattered throughout the universe • They vary greatly in size and shape The “Discovery” of Galaxies At the beginning of the 20th century, what we now call spiral galaxies were referred to as “spiral nebulae” and most astronomers believed them to be clouds of gas and stars associated with our own Milky Way. The breakthrough came in 1924 when Edwin Hubble was able to measure the distance to the “Great Nebula in Andromeda” (M 31, at right) and found its distance to be much larger than the diameter of the Milky Way. This meant that M 31, and by extension other spiral nebulae, were galaxies in their own right, comparable to or even larger than the Milky Way. (NOAO/AURA Photo) Edwin P. Hubble (1889-1953) Types of Galaxies In 1926 Edwin Hubble classified galaxies according to their shape: • Elliptical (shape similar to a football) – Most common – Composed mainly of old stars and have very little interstellar gas or dust • Spiral (shape similar to a flat pinwheel) – Arms are composed of a lot of gas and dust and young blue stars (I.e. star formation is ongoing) Types of Galaxies (Cont’d) • Irregular (no particular shape) – Mixture of young and old stars embedded in gas and dust – Smaller and less common than spiral or elliptical Lenticular (intermediate between an elliptical galaxy and a spiral galaxy) – Lenticular galaxies are disc galaxies (like spiral galaxies) which have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. As a result, they consist mainly of aging stars (like elliptical galaxies). The dust in most lenticular galaxies is generally found only near the nucleus and generally follows the light profile of the galaxies' bulges. Because of their ill-defined spiral arms, if they are inclined face-on it is often difficult to distinguish between them and elliptical galaxies. Types of Galaxies I. Spirals Spiral galaxies are so-named because of the graceful shapes of arms emanating from a bright central nucleus. Spirals are classified according to how tightly or loosely wound the arms are, and it turns out that the brightness of the central nucleus is correlated to the tightness of the arm. The galaxies M 104 (below) and M 51 (right) respectively show tightly and loosely wounds. Notice the effects of dust in both galaxies. (NOAO/AURA Photos) Central Region of the Spiral Galaxy M 51 (Hubble Space Telescope Image) Barred Spiral Galaxies The spiral galaxies M 91 (left) and M 109 (right) have bars across their nuclei from which spiral arms unwind. In virtually all spirals (barred or not) the galaxies rotate such that the spiral arms trail behind in the rotation. The Milky Way is thought to be a barred spiral galaxy. (NOAO/AURA Photos) Types of Galaxies II. Ellipticals Elliptical galaxies lack spiral arms and dust and contain stars that are generally identified as being old. The elliptical galaxies M 32 (below) and M 110 (right) show varying degrees of ellipticity. (NOAO/AURA Photos) Types of Galaxies III. Irregulars Irregular galaxies lack any specific form and contain stars, gas and dust generally associated with a youth. The irregular galaxy at right is the Large Magellanic Cloud, a satellite of the Milky Way located about 180,000 light years from the sun. The LMC is about 60,000 light years across. The bright reddish feature in the upper right is the “Tarantula Nebula” a region of star formation in the LMC. (NOAO/AURA Photo) Galaxy Clusters • Most galaxies are not alone in the vast expanse of space, but are connected to one or more other galaxies by gravity • These collections of galaxies are known as galaxy clusters and they too appear to be organized into larger “superclusters” Unusual Galaxies • Quasars – Brightest objects in the universe – Lie at the edge of the observable universe – Give off 100 times more energy than Milky Way – Energy provided by black hole?!