One of the greatest discoveries in the 20th century was made by one of today’s lesser-known scientists, a man called Vesto Slipher. When we hear the term red shift or the expanding universe many people automatically associate those concepts with Edwin Hubble, the famous pioneer who in the 1920s found a straight-line correlation between galactic distance and velocity of recession, since known as the Hubble Law. Finding the slope of that line, the so-called Hubble Constant, has been an active subject of astronomical inquiry for a century. But fewer people know that Vesto Slipher, a diligent and enterprising astronomer at Lowell Observatory in Arizona, was the actual discoverer of galactic recession: the apparent fleeing away from us of galaxies, or “spiral nebulae” as they were then called, at astonishing speeds.
Slipher began his investigation just over 100 years ago, in 1912, by turning his spectroscope to the Andromeda nebula. What he found was surprising. But a greater thrill came when he later turned his instrument to other galaxies. What he discovered amazed the world. It prepared the way for Edwin Hubble’s systematic studies of larger numbers of galaxies. It opened the door wide for the fruitful and quantitative study of cosmology.
To appreciate what Vesto Slipher did and get a feel for the hard-fought progress in astronomy in its earlier years – and that sense of the ‘wow’ in scientific breakthroughs – we need to see how his discovery came to be.
Measuring the Speeds of Stars
In the late 1800s, a few enterprising scientists had used the new science of spectroscopy to detect radial motion in stars. That is, motion in the line of sight, towards us (velocity approach) or away from us (velocity of recession). Progress was fast. Henry Draper in his private observatory in New York secured the first photographic spectrum of a star, Vega, in August of 1872. Two decades later, Hermann Carl Vogel, director of the Astrophysical Observatory at Potsdam, published a list of the radial motions of 51 bright stars painstakingly obtained through an 11” refractor. The fastest motion he detected was a speedy 30 km/s in recession for Aldebaran, though the average for all stars on his list was about 10.4 miles per second (1.67 km/s). (Later, more precise measurements through larger telescopes put Aldebaran’s motion away from us even faster, at about 50 km/s.)
The idea that you can find the speed of star from its spectrum may sound puzzling. But it’s just another application of the Doppler principle: the change in pitch you hear when a moving train approaches then passes us. The phenomenon was named after Christian Doppler (1803-1853), professor of mathematics at the Polytechnic Institute in Prague. He proposed in 1842 that the color of a luminous body might appear changed depending on whether it was moving towards us or away from us, as sound does. We cannot detect color differences in stars this way. But the idea started others thinking. Fizeau in 1848 thought maybe it could be tested on spectral lines to determine velocities of astronomical objects.
Needless to say, it was a great idea.
In March of 1868, William Huggins, a silk merchant and self-taught amateur astronomer, focused on the so-called Fraunhofer F line of hydrogen in the spectrum of Sirius. Huggins observed Sirius using his new micrometer and a comparison hydrogen spectrum. He was convinced that there was a slight but noticeable shift in the F line towards the red, although not everyone was convinced. There is always uncertainty in visual observations, no matter how scrupulous the observer. There was reluctance as well among the scientific community to accept them, for they challenged the prevailing view of a static universe of stars. But evidence accumulated over the years that stars, clusters, and even gaseous nebula all exhibited radial motion to one degree or another. Vogel saw evidence of the Doppler effect even in solar rotation. The spectral lines of the sun were slightly shifted to a shorter (bluer) wavelength in the advancing, eastern limb and to a longer (redder) wavelength on the receding limb. By the end of the century there was no longer any doubt about the efficacy of using the spectroscope to determine radial velocities of stars.
The First Galactic Spectrum
The discovery that stars had detectable motions led naturally to the question of whether spiral nebulae did – those faint wisps of light that looked in telescopes like faint little pinwheels, needles, or smudges. No one knew then whether those nebulae tucked amongst the stars lay within our own galactic system, the Milky Way, or were remote, distant realms. They looked different from the beautiful little cities of stars and illuminated gas like the famous one in the belt of Orion, or the glowing patches of such “gaseous nebula” in the rich summertime star fields of Sagittarius and Scorpius, in the densest part of the Milky Way. The spiral nebulae were compact, organized and faint.
And that was the problem: Galaxies are not sparkling little points of starlight; they are dim and diffuse. Exposures of many hours over many nights were typically required to spread their tiny trickles of photons into a readable, high dispersion spectrum whose line displacements – shifts toward the red or blue – could be measured. This was true even of the brightest and biggest objects, like the Great Nebula in Andromeda, M31, in the constellation of Andromeda.
The challenge was great, but Vesto Slipher decided to have a go at it. From September through December of 1912, Slipher used the 24-inch Lowell telescope to photograph the spectrum of the Andromeda Nebula. His first exposure was almost 7 hours long through a prism mounted on the telescope. Slipher used a spectro-comparator on the images to measure any shift in spectral lines against a standard. Slipher’s plate measurements showed that the Andromeda galaxy was approaching our solar system at 300 kilometers per second [1].
Slipher’s velocity for M31 of 300 km/sec in approach was sensational: it was the highest velocity ever measured. It was greater by a factor of 15 times the average velocity of stars (about 20 km/s) then known. The result emphatically pointed to the extra-galactic nature of this object [2]. Slipher concluded his paper with modest understatement:
“That the velocity of the first spiral observed should be so high intimates that the spirals as a class have higher velocities than do the stars and that it might not be fruitless to observe some of the more promising spirals for proper motion. Thus extension of the work to other objects promises results of fundamental importance, but the faintness of the spectra makes the work heavy and the accumulation of results slow.”
The motion of M31 Slipher found was with respect to our Solar System, a heliocentric frame of reference, not its motion with respect to the Milky Way's galactic center, around which the solar system revolves like a giant pinwheel. But this motion was not then known. If he had taken galactic rotation into account, it would have become apparent to him that the Andromeda Galaxy is approaching our Milky Way at approximately 100 km/sec. [3].
An Historic Discovery
Buoyed by this success, Slipher expanded his program at Lowell. By 1914 he had obtained even more surprising results that he published in a classic 1915 paper. Out of 15 galaxies he observed, the spectra of 11 were (about ¾ of his sample) significantly shifted toward the red, indicating recession at absolutely extraordinary speeds. It was a remarkable finding, rich with implication, though the causes of the large shifts were unsuspected at the time. Slipher received a standing ovation at the August 1914 meeting of the American Astronomical Society. His findings gave the first indication of galactic “redshift” in our universe, one of the most fundamental discoveries in the history of science. This was also the year Einstein published his General Theory of Relativity, which conceived of gravity in a new way as curvature of a 4-dimensional continuum of space and time.
By 1917 Slipher had measured and re-measured the spectra of 25 galaxies. The table below summarizes his results from his 1917 paper [5]. “NGC” stands for the New General Catalog.
Out of the 25 galaxies surveyed, 4 were approaching (indicated by the negative sign) and 21 were receding, more than 4/5th of this set. The evidence was piling up that galactic recession was a very real phenomenon.
Our Island Universe
Slipher did not conclude from these findings that the universe was expanding. What was the most logical conclusion of this data at the time? Evidence was already accumulating for a systematic “streaming motion” of stars in our neighborhood (later deduced to be the result of our motion around the galaxy’s center). Slipher found the average velocity of the group to be far higher than stellar velocities:
“[T]he average velocity 570 km, is about thirty times the average velocity of stars. And it is so much greater than that known of any other class of celestial bodies as to set the spiral nebulae aside in a class to themselves. Their distribution over the sky likewise shows them to be unique -- they shun the Milky Way and cluster about its poles”[6].
Slipher then took this information and noted that it “calls to mind the radial velocities of the stars which, in the sky about Orion, are receding and in the opposite part of the sky are approaching.” So Slipher concluded, quite logically, that the data shows we must be moving relative to the spirals:
"We may in like manner determine our motion relative to the spiral nebulae, when sufficient material becomes available. A preliminary solution of the material at present available indicates that we are moving in the direction of right-ascension 22 hours and declination with a velocity of about 700 km. While the number of nebulae is small and their distribution poor this result may still be considered as indicating that we have some such drift through space. For us to have such motion and the stars not to show it means that our whole stellar system moves and carries us with it. It has for a long time been suggested that the spiral nebulae are stellar systems seen at great distances. This is the so-called “island universe” theory, which regards our stellar system and the Milky Way as a great spiral nebula which we see from within. This theory, it seems to me, gains favor in the present observations."
Why didn’t Slipher conjecture that the universe is expanding? For that leap to have been sensibly made, Slipher would have had to know the distances to his spirals, which he did not. This information was necessary for him to make any correlation between recession velocity and distance that would suggest that the universe is expanding. Distances to close stars were determinable by the parallax method. But these spirals showed no parallax whatever. Yet the discovery at Harvard of the correlation between period and luminosity in the brilliant Cepheid variable stars would enable the measurement of far greater distances than the parallax method. All that remained was to find cepheids in spiral nebula. With the construction of a huge new reflecting telescope on Mt. Wilson in California, and the sharp eyes of Edwin Hubble and Milton Humason, that possibility was ripe for investigation [7].
Math Notes
The radial velocity of a galaxy is proportional to the shift in wavelength of the spectral line in question to the rest wavelength of that line. To find the velocity of the object relative to the Earth, use this equation:
where c is the speed of light in km/sec, and the fraction is the ratio of the shift in wavelength of the line in nanometers to the rest wavelength of the line in the same units. The velocity is in units of km/s. For example, if the rest wavelength of the Calcium II (Ca II) line is 396.8 nm, the speed of light is 2.99 x 10E5 km/s, and the shift in that line detected from the plates was - .488 nm, the resulting velocity is - 300 km/s. That is its velocity of approach relative to the earth.
NOTES [1] For Slipher’s paper on the velocity of the Andromeda nebula, see Slipher, V.M., “The Radial Velocity of the Andromeda Nebula,” at https://articles.adsabs.harvard.edu/pdf/1913LowOB...2...56S. Slipher’s 1913 paper does not reveal which lines he used. He does specifically mention the F and H lines of Fraunhofer. [2] This value has held up over the years, the accepted modern value being about 300 km/s. [3] For further information on M31, see https://www.messier.seds.org/m/m031.html. [4] Visible are the Fraunhofer F line at 486.1 nm, and the two sharp Fraunhofer K and H lines of singly ionized calcium, at 393.4 nm and 396.8 nm. This photograph also shows the “G band” of absorptions from the CH molecule, titanium and iron, and the weaker b neutral magnesium bands at and near 517.3 nm. [5] Slipher, V.M., “Nebulae,” Proc. Amer. Phil. Soc. 56, 403-09 (1917). You can find it online at https://articles.adsabs.harvard.edu/pdf/1917PAPhS..56..403S.
[6] This and the following quotes come from Slipher’s 1917 paper referred to in note 5. [7] Slipher’s papers can be found online. See the links to his papers and related resources at http://www.phys-astro.sonoma.edu/brucemedalists/Slipher/SlipherRefs.html.
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