Before I recount the rest of my Sunday, there’s a little material remaining in your lesson in astronomy and cosmology and the important role of the Lowell Observatory so let’s get started on that ending.
Red shift, blue shift (more than) a few words about Vesto Slipher.
If those of you reading are unfamiliar with the name Edwin Hubble, it’s likely that you are at least familiar with his namesake telescope that NASA launched in 1990 and that has, over the subsequent quarter century, provided us with unimaginably startling and breathtaking views of the solar system, the galaxy, and the cosmos beyond. Had he been a different sort of person, although, we might not be talking about the Slipher Telescope, we would likely be much more familiar with the modest Indianan.
[Photo from alchetron – Public Doman].
Edwin Hubble was a true giant of 20th century astronomy. Unlike Slipher, who also did important and groundbreaking work, Hubble was, as one biographer describes him, “a tireless self-promoter and master networker, who married into wealth and befriended Hollywood celebrities.”
Hubble’s discoveries included Cepheid variables, stars also known as standard candles, that play an important role in measuring cosmic distances. Observations he made in the early 1920’s particularly of the galaxies Triangulum and Andromeda proved conclusively that these rather mysterious things long called spiral nebulae were much too distant to be part of the Milky Way as was the prevailing belief at the time and were, in fact, entire galaxies far outside our home galaxy. It was an earthshaking discovery.
In 1929, Hubble, working with his assistant Milton Humason, quantified the relationship between the distances of the galaxies and their redshifts. This became known as Hubble’s Law. So why, you might be wondering, am I spending all these pixels writing about Edwin Hubble when the section says it’s going to be about Vesto Slipher? The answer is because of the connection between some of the work that made Hubble famous and observations Slipher made and reported that predate Hubble.
When Slipher came to Arizona in 1909 or 1910 to work at the Lowell Observatory, it had acquired a relatively new instrument called a spectrograph and Percival Lowell assigned Slipher to work with it. While spectroscopy was a long-established science (credited to Sir Isaac Newton though a number of scientists experimented with it before Newton published his theoretical explanations in Optiks), it’s become an important part of modern astronomy. In the middle of the 18th century, work by a pair of Germans – physicist Gustav Kirchhoff and chemist Robert Bunsen – established the link between each element and its unique electromagnetic signature that manifests itself across the spectrum from infrared to ultraviolet. A spectrograph takes a picture of this signature. (Our guide demonstrated spectroscopy for us during our tour of the Clark Dome.)
(This “break up the text video” has nothing to do with spectroscopy but it does have a cool look at Neptune from the Hubble telescope.)
After his initial struggles using a machine to which he had never been previously exposed, Slipher eventually mastered its intricacies and began taking excellent spectra of planetary atmospheres. He also used it to measure the rotation periods of the outer planets, detect matter in the space between stars, and to discover a thin layer of sodium atoms in the Earth’s upper atmosphere.
When Lowell saw Slipher’s success with the spectrograph, he directed him to turn it toward the mysterious spiral nebulae. Although some astronomers believed that these bodies were, indeed, outside the Milky Way, Lowell sided with the prevailing belief they were spirals of gas within our own galaxy that were coalescing into new solar systems. He thought that if Slipher could measure the spectra of the light from the outer edges of the spiral nebulae, the chemical makeup would show they resembled the makeup of the outer planets of our solar system thus proving this hypothesis.
As often happens in science, reality undermined the hypothesis. By the end of 1912, Slipher had made two astonishing discoveries. The first was that the light from what was called the Sombrero nebula was shifted toward the red end of the spectrum indicating that it was flying away from the Milky Way at a speed of 621 miles per second. The second was that Andromeda was headed in the opposite direction. It showed a blue shift indicating that Andromeda was heading toward the Milky Way at 186 miles per second. That measurement has since been adjusted down to a mere 68 MPS based in part on more accurate measurements of distance based on Hubble’s Law.
(Yes, that means that it’s likely that Andromeda and the Milky Way will someday be involved in a massive intergalactic collision. While even 68 MPS seems terribly fast on a terrestrial scale – {at that speed you’d circle the Earth in a tad more than six minutes}, it’s not very fast on a cosmic scale. There’s no need to worry about finding a big enough airbag to survive this galactic pileup. Here’s why. At 68 MPS, you need about 2,000 years to travel one light year. Andromeda is about 2,500,000 light years distant. This means it will be more than four billion years before the collision happens and that’s about half a billion years after the sun will have made the planet too hot to hold any liquid water. So, maybe you should worry about surviving that first. 😎 )
The concept of “red shift” and “blue shift” is called the Doppler Effect. As an electromagnetic wave approaches an observer it increases in frequency and, as it recedes, the frequency decreases. In terms of the visual spectrum, higher frequencies occur at the blue end (hence approaching objects are “blue shifted”) and lower frequencies are at the red end. Thus, receding objects are “red shifted.”
The most familiar example of the Doppler Effect occurs with sound waves. (Other than requiring a medium to move through, sound waves behave no differently than waves along the E M spectrum.) High pitched sounds have higher frequencies than low pitched sounds. Thus, as you stand on a corner listening to the approach of a car blaring its horn it appears to have a higher pitch as it moves toward you and a lower pitch as it moves away – something like this:
The lesson ends with the next post.