Notes 4-1: Galaxies 4/15/09

People have been observing the night sky since antiquity. A truly dark sky, like the one seen here, will reveal some amazing views.

The white “cloud” seen streaking across the sky is the Milky Way.

The Milky Way is the name given to our galaxy. A galaxy is a gravitationally bound group of stars separated by vast distances from other similar systems. What we are seeing appear as a white cloud in the night sky is in fact the light from billions of stars too distant and too numerous to make out as individual points of light. The Milky Way got its name long before people realized what they were observing. The name comes from the fact that it looks like a path of milk spilled across the night sky.

Our solar system lies near the edge of our galaxy. What you see below is a 360  panoramic of the Milky Way as observed from Earth. The bright spot in the middle is looking directly toward the center of our galaxy.

In 1750, an astronomer named Thomas Wright published "An Original Theory of the Universe" in which he explained the Milky Way by proposing that the stars were distributed in a thick disk -- the first step in the discovery that the Milky Way is a galaxy. In 1755, the great philosopher Immanuel Kant, inspired by Wright's ideas, proposed that the Milky Way was only one of many galaxies scattered throughout an infinite universe.

In 1781, Charles Messier published the Messier Catalogue of 110 "nebulae", of which 40 are galaxies. Thus, the brightest and biggest galaxies are often labeled by their number in the Messier Catalogue - e.g., M31, the Andromeda Galaxy. The Messier catalogue actually contains galaxies, star clusters and nebulae. In 1864, John Herschel published the General Catalogue of Nebulae and Clusters of Stars, which evolved into the New General Catalogue of more than 10,000 galaxies. Therefore, you will often see a galaxy labeled by its number in the New General Catalogue. For example, M31 is also called NGC224. Remember, at this time most astronomers believe that all of these objects are within the Milky Way galaxy. A few speculate that some of them are outside our own galaxy, but there is little evidence to support that.

From observations of globular clusters, American astronomer Harlow Shapley proved in 1918 that the globular clusters were distributed in a roughly spherical system, centered, not on the Sun, but at a point at a distance of some 15,000 parsecs. (Shapley overestimated the distance; we now know it is closer to 8,000 parsecs.) This was compelling evidence that the Sun is not at the center of the Milky Way. In 1920, Harlow Shapley and Heber Curtis held a famous debate about the nature of the “spiral nebulae” (the term for what are now known as galaxies at that time). Shapley argued that they were gas clouds belonging to the Milky Way; Curtis argued that they were other galaxies, as big as the Milky Way itself and at great distances.

Working at at the great new 2.4-m telescope at Mt. Wilson, California in 1924, astronomer Edwin Hubble began a systematic survey to measure the distances to spiral galaxies. Edwin Hubble and others found novae and Cepheid variable stars in nearby galaxies that allowed them to accurately measure the distances to these galaxies. In 1930, Shapley conceded that the spiral nebulae must really be other galaxies.

Galaxies represent the largest structures in the Universe. 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. Very small galaxies are often called "dwarf galaxies" and very large galaxies are often called "massive galaxies".

When telescopes led to the discovery of galaxies, astronomers observed many differences. In 1926, an astronomer named Edwin Hubble decided to classify the galaxies, grouping them according to some logical scheme. After considering different schemes, he decided to arrange or group them by shapes. He would classify them according to the way they looked. In science, the study of something according to its form or structure is called "morphology".g

Galaxies 3 main categories according to appearance Disk galaxies – cosmic frisbies Spiral Lenticular Elliptical galaxies – cosmic footballs Irregular galaxies – cosmic misfits

Spiral Galaxy - Spiral galaxies usually consist of two major components: First, a large flat disk which often contains a lot of interstellar matter and young open star clusters, which have emerged from the interstellar clouds. And, second, a large halo with a population of old star. Spiral galaxies are often arranged in striking spiral patters. They also contain an ellipsoidal shaped bulge on the top and bottom that consists of old stars often arranged in globular clusters with no interstellar matter.

Structure of a spiral galaxy What is at the center of the galaxy?

Structure of a spiral galaxy What is at the center of the galaxy? Because of the rates that objects in our galaxy rotate around the center it is thought that at the center there must be a very massive object, a giant black hole.

Structure of a spiral galaxy Halo - The Halo consists of the oldest stars known, including Globular Clusters, believed to have been formed during the early formation of the Galaxy. The halo is also filled with a very diffuse, hot, highly-ionized gas.

Structure of a spiral galaxy Disk – The disk of a spiral galaxy is a flattened, rotating system which contains intermediate to young stars as well as clouds of interstellar gas.

Structure of a spiral galaxy Nuclear Bulge – near the center of a spiral galaxy there is a large bulge of globular clusters orbiting the center of the galaxy very quickly.

Globular Clusters – Globular clusters are gravitationally bound groups of many thousands (sometimes as many as a million) of stars. They consist primarily of very old stars. Globular clusters are not concentrated in the plane of the galaxy but rather are randomly distributed throughout the halo. There are several hundred globular clusters associated with our galaxy. A typical globular cluster is a few hundred light-years across.

Open Clusters – Open clusters are loose aggregations of dozens or hundreds of young stars. They are generally not gravitationally bound and will disperse in a relatively short period of time, astronomically speaking. Also called "galactic clusters" because they are usually found in the plane of the galaxy. A typical open cluster is less than 50 light- years across.

Lenticular - These are, in short, "spiral galaxies without spiral structure", that is, they are smooth disk galaxies, where stellar formation has stopped long ago, because the interstellar matter was used up. Therefore, they consist of old stars only, or at least chiefly. From their appearance and stellar contents, they can often hardly be distinguished from elliptical galaxies observationally.

Elliptical - Elliptical galaxies are actually of ellipsoidal shape. They are different from lenticular galaxies because they are not shaped like a flat disk but are 3 dimensional and are shaped much more like a football. They have little or no global angular momentum, that is to say they do not rotate as a whole (although individual stars rotate in random directions at random speeds). Normally, elliptical galaxies contain very little or no interstellar matter, and consist of old stars only: They appear like luminous bulges without a disk component.

Irregular - Often due to distortion by the gravitation of neighboring galaxies, these galaxies do not fit well into the scheme of disks and ellipsoids, but exhibit peculiar shapes.

This is an illustration of the Hubble galaxy classification.