Chapter 15: Galaxies

The Two Micron All Sky Survey provides our first global look at galaxies in infrared light. This image presents the 30 largest galaxies seen in the infrared sky.

Our Sun orbits roughly one-third of the way out from the center of the Milky Way galaxy. This collection of billions of stars, dust, gas, and dark matter is just one of billions and billions of galaxies that make up our Universe. Galaxies were referred to as Island Universes by Kant, who saw our own disk-like system as the heart of the entire cosmos. With the construction of modern telescopes with photographic detectors at the beginning of the twentieth century we discovered that our galaxy is not alone. Galaxies come in a myriad of shapes in sizes, ranging from star cluster-sized dwarf galaxies to giant ellipticals that would dwarf our own Milky Way. These galaxy morphologies are often a reflection of a galaxy's history and its related environment.

The position of our Sun in our Galaxy

Our Milky Way is adrift in a vast volume of space that is loosely sprinkled with other galaxies. Gravitationally, we are bound within a small "Local Group" of galaxies that includes our system, Andromeda (M31), the Triangulum galaxy (M33), and swarms of smaller systems that orbit each of these large galaxies. NGC 3109 and its companions may also be part of the Local Group, but the evidence is ambiguous. Our Local Group is gravitationally being pulled into the nearby Virgo cluster. In general, galaxies are rarely found alone, and instead tend to be found in pairs, groups, clusters, and even vast collections called superclusters.

Looking at them statistically, the vast majority of galaxies are dwarf elliptical and dwarf irregular systems typically found orbiting around larger systems. Most often, the elliptical systems are devoid of gas and dust and populated with old red stars. The irregular systems are quite the opposite and are often rich in gas and dust that is ablaze with star formation. Giant galaxies have similar shapes — coming in both elliptical and irregular varieties, as well as the well-known spiral shape. Typically the largest elliptical galaxies contain little gas or dust and have red populations (very rare exceptions do exist, but are not the norm), while spiral and irregular systems typically do have gas and dust, as well as star formation.

The differences between having dust and not having dust, having mostly red stars, and having star formation are reflections of evolutionary histories. A given system will start with a certain amount of gas and dust with which it can form stars. In most cases, this initial supply of stellar building materials is augmented by gas and dust that is cannibalized from smaller systems. Eventually, however, galaxies do run out of material to build stars, and when this happens all the existing stars simply age, with the blue stars winking out as the red stars undergo a slow burn of their fuel. This early death of blue stars leads to consistently red stellar populations in galaxies that aren't undergoing star formation. The consumption of gas and dust can be accelerated during galaxy collisions, which may compress the material and also drive it into the hearts of the colliding galaxies. The compressed gas will burst with star formation, while the material driven toward the center will in part fall into the galaxies' central black holes, and also in part become new stars. Once the collision is over, little or no gas and dust may remain to form future generations of stars.