From the Library: “Radial Velocity of the Andromeda Nebula”

If you were to list the great astronomers of the last century, just based on your own memory, it’s a safe bet that Vesto Slipher would not be a name you come up with.

Vesto Melvin Slipher, “V.M.” to most folks, was the head astronomer at Lowell Observatory for over fifty years. If he is forgotten, it’s because he worked at it. Slipher was a quiet, unassuming and humble man. He’s also forgotten because his boss, Percival Lowell, was none of those things.

Vesto Slipher (1875-1969)

Vesto Slipher (1875-1969)

While Lowell gallivanted, Slipher worked diligently with his 24” Clark refractor and spectrograph and generally let his boss take the spotlight. Even the Lowell Observatory website gives him little attention. He’s mentioned briefly in connection with his telescope, with which he “revolutionized our understanding of space.”

One of the revolutions that Slipher contributed to was the understanding that many of the little blobs of light in the night sky called nebula were actually other galaxies like our own Milky Way. The debate over the nature of these nebula ran from the mid-18th century until the 1920s, and suggestions included stars in the process of forming, clouds of gas and finally other galaxies. This last suggestion, called the “island universe” theory, was finally settled by Edwin Hubble, but a great deal of work went into the theory before that point.

In 1910, prompted by Lowell, Slipher began to take spectrographic readings of the Andromeda nebula. Even though this is the brightest nebula in the northern sky, it was still an arduous process, requiring that a photographic plate be left in place for many hours to capture sufficient light.  Slipher spent the next two years rebuilding his spectrograph to capture more light until he finally got a workable spectrum in September of 1912.

When he looked at the spectrum – so small that it required a microscope to examine – he got a shock. By looking at how much the spectral lines had shifted, an astronomer could tell how fast an object was moving either towards or away from the earth. This variable is called the “radial velocity.” The radial velocity of Andromeda showed it to be headed towards us at an incredible rate of speed.

Andromeda Nebula, 1899, by Isaac Robert

Andromeda Nebula, 1899, by Isaac Roberts

After taking several more photographs, Slipher calculated that Andromeda was moving at 300 kilometers per second, about ten times faster than the average star in our galaxy. This find changed the debate over the nature of nebula; something moving that fast was unlikely to be part of our galaxy and didn’t fit the understanding of forming stars.

Slipher, true to his nature, did not publish his discovery in a major scientific journal, or even in a journal at all. Instead it appeared in the Lowell’s own newsletter, the Lowell Observatory Bulletin.  In nine sparse paragraphs, covering both of the two pages of the newsletter, Slipher laid out his process and findings. He suggested that “it might not be fruitless” to look at other spiral nebula.

It wasn’t. In 1914, with fourteen spiral nebula spectra in hand, Slipher had found that Andromeda was a relative slowpoke; one nebula was moving away from us at 1,100 kps, the fastest celestial object recorded at the time. That year, Slipher presented before the Astronomical and Astrophysical Society of America (which changed its name to the American Astronomical Society at that meeting). He received the first standing ovation in the institution’s history. In the audience was a recently elected member, a Wisconsin graduate student named Edwin Hubble.

Because he shunned the spotlight, Slipher rarely published or presented papers. Most of his output went out in the tiny Lowell Observatory Bulletin, which was not collected by many institutions. It’s a very fortunate thing that the Dudley Observatory was one that did. There are few revolutions that can compete in scope with the realization that our galaxy is only one of billions. And this short little paper was a vital part of the process.

Radial Velocity of the Andromeda Nebula

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