March 25, 2008

In Today's One Minute Astronomer...

Galaxy Shapes: Another Legacy of Edwin Hubble

Spring is the time to peer up and out of the plane of our Milky Way at fields of distant galaxies in Leo, Virgo, and Coma Berenices. Every galaxy you see, and nearly every one of the billions of galaxies in the universe conforms to one of four basic shapes first outlined by Edwin Hubble in the 1930's.

The Basics

• Hubble devised a “tuning fork” diagram to illustrate each of the four known galaxy shapes: elliptical, spiral, and lenticular (lens-shaped). Irregular galaxies, with no discernable shape, make up a fourth class

• Spiral galaxies have a classic “pinwheel” shape with a flat disk, central bulge, and graceful arcing arms containing regions where young stars form. Some spirals have a bar through the center. Spirals range from tightly wound (Sa, SBa) to loosely wound (Sc, SBc). Examples: M31, M81, and our own Milky Way

• Elliptical galaxies have no dust lane and are fairly featureless, consisting of mostly older stars. Ellipticals are classified from E0 (circular) to E7 (highly elliptical). Examples: M49, M87

• Lenticulars lie at the center of Hubble's tuning fork. They often have a bulge surrounded by a disk-like structure with dust lanes, but have no spiral shape at all. They also lack star-forming regions. Examples: M85, NGC 5866

• Irregulars have an indeterminate shape. They are usually small galaxies without the gravitational capacity to assume a regular form, like the Magellanic Clouds, or a large galaxy that's undergone a major gravitational disturbance, like M82.

A Deeper Look

• Hubble believed his diagram represented the evolution of a galaxy through time, from elliptical to lenticular and finally to spirals and irregulars.

• Astronomers now hold the opposite view. The early universe was dominated by spirals and irregulars, which in time were pulled together by gravity and merged into elliptical galaxies. Observations of large clusters of galaxies bear this out.

A Bit of History

The "definitive" version of the Hubble classification was set out in an article by Hubble's famed student Alan Sandage in 1975.

Personal View

As a graduate student in astronomy, I once observed through the 100” DuPont telescope at Las Campanas in Chile. On the observing schedule after me was Hubble's student, Alan Sandage, at that time the world's best known observational cosmologist. A modest brush with astronomical fame for your humble editor.