Vesto Slipher – the redshift anomaly pioneer

Vesto Slipher was the first to measure the redshift of an extragalactic object. Slipher (1913) noted that the spectroscopic faintness of extragalactic objects were the main reason for the lack of redshift measurements by that time. The object he chose as the target was the “Andromeda nebula” (today known as “Andromeda galaxy”), M31. He gave a detailed description of his spectral measurements and gave a redshift of cz = -300 km/s. This was the first redshift anomaly he found. He said:

The magnitude of this velocity, which is greatest hitherto observed, raises the question whether the velocity-like displacement might not be due to some other cause, but I believe we have at the present no other interpretation for it. Hence we may conclude that the Andromeda Nebula is approaching the solar system with a velocity of about 300 kilometers per second.

Even if wasn’t very difficult problem, back then such high redshift velocities had not been observed in celestial objects, so to him it was a redshift anomaly. I think the fact that the first ever observed extragalactic redshift immediately raised a redshift anomaly nicely highlights the remarkable history of redhifts as problem raisers and solvers. After noting the high apparent velocity and having made a rather peculiar (or so it seems today at the least) remark about M31 having encountered a “dark “star””, he made a comment that showed great intuition (while at the same time being based on a rather small sample):

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.

Slipher (1914) discovered that galaxies rotate and inferred from that observation that some nebulae are edge-on spirals. However, he’s rather vague on the details on the edge-on conclusion, he just seems to suggest that because there is rotation observed in the “spindle” type nebula, it suggests that they are edge-on spirals. I don’t see why they couldn’t have been other types of nebulae, also rotating.

Slipher (1915) gave a summary of galaxy redshifts known at that point. He noted an anomaly he had faced but which seemed to have been solved:

As far as the data go, the average velocity is 400 km. It is positive by about 325 km. It is 400 km on the north side and less than 200 km on the south side of the Milky Way. Before the observation of N.G.C. 1023, 1068, and 7331, which were among the last to be observed, the signs were all negative on one side and all positive on the other, and it then seemed as if spirals might be drifting across the Milky Way.

And right after that, another redshift anomaly:

N.G.C. 3115, 4565, and 5866 are spindle nebulae – doubtless spirals seen edge-on. Their average velocity is about 800 km, which is much greater than for remaining objects and suggests that the spirals move edge forward.

That sounds rather funny in the context of the today’s knowledge, but back then it was perfectly sound remark.

Slipher (1917a) analyzed the galaxy redshifts known at that point. He first discussed the general features of spectral features of all nebulae. He then pointed out that the faintness of galaxies was still a problem for their spectroscopy, followed by a discussion on the measurements techniques. He then proceeded to analyze the known redshifts. By that time, he noted, it had become clear that (spiral) galaxies were a separate class of objects (remember that it was not yet clear if galaxies were objects of our own galaxy or extragalactic objects):

Referring to the table of velocities again: the average velocity 570 km is about thirty times the average velocity of the 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.

He mentioned that as the redshift velocity of most spiral galaxies seemed to suggest that they are receding from us, it might imply that they are scattering (an early reference to expanding space perhaps?), but that their tendency to cluster seemed to argue against it. He then returned to the spirals move edge forward hypothesis. He showed a table where he had divided the spiral galaxies to three groups; face-on, inclined, and edge-on. Face-on spirals had generally lowest velocities, inclined spirals had generally higher velocities, and edge-on spirals had highest average velocity (but it seems to me that the inclined and edge-on difference is not very clear).

He then discussed the evidence of rotation in spiral galaxies. He specifically adressed the problem of the direction of the rotation. There was a problem of not knowing which side of inclined spiral galaxy is closer to us. He suggested an indirect way to find that out by looking at the dark bands that are seen in edge-on spiral galaxies:

If now we imagine we view such a nebula from a point somewhat outside its plane the dark band would shift to the side and render the nebula unsymmetrical – the deficient edge being of course the one nearer to us.

And when that was checked in practice, it turned out the spirals were rotating all to the same direction:

The central part – which is all of the nebulae the spectrograms record – turns into the spiral arms as a spring turns in winding up.

He mentioned also the possibility he had discussed in his previous work, that spirals in one side of the sky are receding and in one side are approaching (which we know today was just accidental due to his small sample). He suggested that it might be due to our own motion, and even calculated the direction and speed of the motion from the available evidence. However, as the other stars didn’t seem to show this kind of motion. He suggested that this was evidence to the theory that spiral galaxies are extragalactic objects, far away galaxies like our own Milky Way.

In these early days there were some concerns about the accuracy of the redshift measurements, and Slipher (1917b) shows an example where he answers one critic showing that the accuracy is indeed sufficient.

Slipher (1921) announced that a new redshift record had been made; the NGC 584 had been measured to recede from us with a redshift velocity of 1800 km/s.


Slipher, 1913, LowOB, 2, 56, “The radial velocity of the Andromeda nebula”

Slipher, 1914, LowOB, 2, 66, “The detection of nebular rotation”

Slipher, 1915, PA, 23, 21, “Spectrographic observations of nebulae”

Slipher, 1917a, PAPhS, 56, 403, “Nebulae”

Slipher, 1917b, Obs, 40, 304, “Radial velocity observations of spiral nebulae”

Slipher, 1921, PA, 29, 128, “Two nebulae with unparalleled velocities”


Short biography from the Lowell Observatory
Wikipedia – Vesto Slipher
John Peacock’s page on some of Slipher’s work on galaxies

6 Responses

  1. I like the historical component of cosmology, at least up to 1930. Thanks for the summary of Slipher’s work, Ari!

    A couple of quick spelling corrections: missing the R in Andromeda, and wouls should be “would”.

  2. Thanks for the corrections. It was “andomeda” in both cases too. I also have made the “wouls” before. There are also some other words I tend to write wrong every time but in many cases I notice them before publication.

    It is my intention not to leave Slipher as only one presented here like that, I think I’ll go for some completely unknown character next.

  3. spelled practice wrong!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! DX<

  4. Fixed, thanks.

  5. I am fascinated by your website. I offer my CIG Theory to explain red shift anomalies. I can be found in the book “I Have Become Space” by douglas w lipp

  6. Ari,

    Please feel free to add a link to my book:

    I used to have a website but due to cost, have temporarily withdrawn it.

    I gave a copy of my book to Andrice Arp, the daughter of Halton Arp and many physicists and cosmologists.

    It’s a short book and I recommend that cosmologists and physicists read it.

    The explanation of the double slit also explains red shift anomalies, and dark matter and dark energy and the horizon problem and the measurement problem and more, all in one concept theory.

    I am looking for feedback both for and against the rationale I put forward in the theory.

    Thank you
    douglas w lipp

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