One Galaxy or Many?

This World Wide Web page written by Dan Styer, Oberlin College Physics Department;;
last updated 29 November 2000.

How did we find out about galaxies? The answer is interesting in its own right, but it also reveals a lot about the nature of controversy and debate in the development of science. This sketchy timeline outlines the history of our understanding.

1781: Charles Messier

In 1781 comet hunter Messier publishes a list of 103 "fuzzy objects", not to be confused with comets. (The list has since been extended to 110 objects.)

In this course we have encountered many of the objects Messier cataloged -- we classify them into a host of categories including:

However, other objects in the catalog don't fit into any of these categories. Because these are shaped somewhat like pinwheels, they are called "spiral nebulae".

1770-1785: William and Caroline Herschel

The Herschel siblings, musicians and astronomers, develop the "grindstone" model of the galaxy, with the sun near its center. This model is generally accepted until the early years of the twentieth century.

1908: 60-inch telescope

The huge reflecting telescope atop Mount Wilson is commissioned. Its mirror is 60 inches (five feet) across, and for good observing conditions it is located in the clear mountain air high above the small agricultural community of Los Angeles, California.

Because the observatory was deliberately positioned at such a remote site, transportation of telescope components to the site posed difficulties:

But finally the assembled instrument was commissioned:

(Note the rivet-dominated construction, similar to that used in the steamship Titanic.)

1912: Henrietta Leavitt

Leavitt (who worked at Harvard and had studied at Oberlin College from 1885 to 1888) discovers the uncalibrated period-luminosity relation for Cepheid variable stars.

1917: Harlow Shapley

Shapley, working with the 60-inch telescope at Mount Wilson, calibrates the period-luminosity relation for Cepheids and measures the distances to globular clusters. He postulates a galaxy much larger than previously thought, and with the sun near its edge.

Questions remain

Does the center of the globular clusters really mark the center of our galaxy, as Shapley assumed? Or were they, say, the result of an explosion that happened to occur in the constellation Sagittarius?

What are the spiral nebulae? Their spectra are like those of stars, so they are not emission nebulae: they could be either reflection nebulae or else collections of stars.

As so often happens in science, there was controversy. But as rarely happens, a formal debate was arranged to air all sides of these questions. On 26 April 1920 two astronomers, Harlow Shapley and Heber Curtis, met in Washington, DC, for a debate on "The Scale of the Universe".

The Shapley-Curtis debate

Harlow Shapley

Heber Curtis

Globular clusters mark the center of our galaxy. The clumped distribution of globular clusters is a fluke.
Our galaxy is about 300,000 light years in diameter. Our galaxy is about 30,000 light years in diameter.
The sun is near the edge of our galaxy. The sun is near the center of our galaxy.
Spiral nebulae are reflection nebulae within our own galaxy: they are condensing into planetary systems ("baby solar systems"). Spiral nebulae are star systems outside our galaxy.
Spiral nebulae can't be outside our galaxy: We see novae in spiral nebulae, and no one star explosion could possibly create so much energy. Those novae are exploding stars within our own galaxy that happen to be in front of a spiral nebula.
The astronomer Van Maanen has detected the rotation of a spiral nebula, using two pictures taken a year apart. If spiral nebulae were entire, distant, star systems, they would rotate once in several thousand years, not once in two or three years. Van Maanen's observation was an artifact. They do rotate once in several thousand years.
The absorption of light by dust outside our galactic disk is unimportant. Agreed.

Using radio telescopes and larger optical telescopes, all of these issues have been resolved. The current consensus (which is probably but not necessarily correct) is that:

A larger telescope was needed to resolve these issues.

1917: 100-inch telescope

You might think that a 100-inch telescope would be better than a 60-inch telescope by a factor of 100/60 = 5/3 = 1 2/3 times. In fact, however, the quality of a telescope is limited by the area of its mirror, not the diameter. So a 100-inch telescope is better by a factor of (5/3)2 = 2.78.

In 1917 Mount Wilson Observatory commissioned the Hooker telescope with a mirror diameter of 100 inches (8 feet, 4 inch). It is still in use, and looks like this:

Here's the exterior of the Hooker telescope's dome:

And an overview of the Mount Wilson site:

(Dome for the 100-inch on left, dome for the 60-inch at center. The towers belong to small telescopes for observing the sun.)

1923: Edwin Hubble

In this year, Hubble uses the 100-inch telescope to photograph stars in the edge of M31, the "Andromeda Nebula". He looks for novae on his photographic plates, and believes he identifies three. Comparison with a similar plate taken a week or two earlier shows that one of the supposed novae is actually a Cepheid variable!

Hubble's plate:

The three stars identified as novae are marked "N". One "N" is crossed out and replaced by "VAR!" (variable star!).

Hubble measures the period of the Cepheid variable and its apparent magnitude. Using the period-luminosity relation he finds its absolute magnitude. Using the magnitude-distance relation he finds its distance. Hubble announces that M31 is 900,000 light years away, far outside of our own galaxy. The "Andromeda Nebula" had become the "Andromeda Galaxy".

(Note: The modern distance estimate is 2,200,000 light years. And Hubble himself continued to use the old-fashioned term "Andromeda Nebula" until the day he died.)

1944: Walter Baade

Stars in the outer edge of M31 constitute important evidence that this "spiral nebula" is in fact a galaxy. So does its immense distance. But final proof awaits confirmation that the densely packed interior also consists of stars.

The German-born astronomer Walter Baade had been working at Mount Wilson since 1931. Although he had applied for US citizenship, either he or the Immigration and Naturalization Service lost some papers, and the application never went through. When World War II broke out, most of the Mount Wilson scientific staff went off to work on war-related research. Such work was forbidden to Baade, who had been classified as an enemy alien. As such, he had the 100-inch telescope all to himself.

Furthermore, the observing conditions at Mount Wilson had been deteriorating due to extraneous light from the no-longer quaint agricultural center of Los Angeles. That city turned off its lights under a wartime blackout order, and Baade made use of the darkest skies Mount Wilson would ever see.

Even with these advantages, Baade could not resolve individual stars within the center of M31. Finally, in desperation, Baade turned from the type of photographic plates traditionally used in astronomy, which are most sensitive to blue light, to a type most sensitive to red light. He exposed them for eight hours are more. He introduced new techniques for keeping the telescope stable and accurately pointed for such long exposure times.

Finally, Baade managed to obtain a plate resolving thousands of individual stars in the heart of M31. In Baade's own words: "After the shooting was over, it was quite clear that all the precautions had actually been necessary; I had just managed to get under the wire, with nothing to spare." Baade's article announcing his achievement in Astrophysical Journal had to be printed with a special process, as conventional printing would render the image smudgy.

1946: It's a Wonderful Life

Beautiful, distant, and mysterious, spiral galaxies entered the public imagination. The famous 1946 film "It's a Wonderful Life" (directed by Frank Capra, starring James Stewart) starts with a sequence visualizing angels as spiral galaxies.