Normal Galaxies

Galaxies are huge collections of stars, dust and gas. They usually contain several million to over a trillion stars and can range in size from a few thousand to several hundred thousand light years across. There are hundreds of billions of galaxies in the Universe. Galaxies vary in size, structure, and luminosity, and, like stars, are found alone, in pairs, or in clusters. Galaxies are divided into three basic types: spirals, ellipticals and irregulars.

Examples of the three main types of galaxies: spiral (left), elliptical (middle), and irregular (right)

Spiral Galaxies

Spiral galaxies get their name from the shape of their disks, in which stars, gas and dust are concentrated in spiral arms that extend outward from the central nucleus of the galaxies. They are divided into three main types according to how tightly wound the spiral arms are: Sa, Sb and Sc. Sa galaxies have very tightly wound arms around a larger central nucleus. Sc galaxies have very loosely wound arms around a smaller nucleus. Sb's are between, having moderately wound arms around an average sized nucleus. Our Milky Way galaxy is an example of a spiral galaxy. Spiral galaxies are rich in gas and dust and have a high rate of star formation. Since spirals contain a high fraction of hot, young stars, they are often among the brightest galaxies in the universe.

Elliptical Galaxies

Elliptical galaxies are elliptical in shape and are divided into eight subgroups: E0-E7 depending on their elongation. E0 ellipticals are nearly circular, while E7s are highly elongated. Elliptical galaxies contain primarily old stars, and do not have much gas and dust. There is very little new star formation in these galaxies.

Irregular Galaxies

Irregular galaxies have no particular shape. They are among the smallest galaxies and they contain a vast amount of gas and dust. As a result they have a very high rate of star formation. The Large and Small Magellanic Clouds are examples of irregular galaxies.

An infrared image of the spiral galaxy M83 (European Southern Observatory) showing the infrared glow of stars in the spiral arms.

The infrared emission from galaxies comes primarily from three sources: stars, interstellar gas, and dust. The emission from stars peaks in the near-infrared (1-3 micrometers). Emission from atoms and molecules in interstellar gas makes up only a few percent of the infrared output of galaxies. The primary source of infrared radiation beyond 3 micrometers is thermal emission from dust grains heated by starlight. The brightest infrared galaxies are usually the ones which have a lot of dust (from star-forming regions for example). Spiral galaxies which are rich in gas and dust are strong infrared sources and are still forming new stars. About half of the luminosity of an average spiral galaxy is radiated at far-infrared wavelengths. Elliptical galaxies are faint in the infrared because they do not have much gas and dust.

The unprecedented sensitivity and spatial resolution of the Spitzer Space Telescope will allow astronomers to conduct extensive studies of normal galaxies, from nearby galaxies to those in the distant Universe. By studying the details of dust, gas and stars in galaxies within a few few tens of millions of light years, Spitzer will gather valuable data for understanding its own observations of more distant galaxies.