Image 2 Caption: Vesto Slipher (1875--1969) of Lowell Observatory in Flagstaff, Arizona.

Features:

1. Vesto Slipher (1875--1969) working at Lowell Observatory began starting in 1912 an observational program of measuring the redshifts of galaxies (see No-522--523).

   Slipher's measurements were all in the visible band (fiducial range 0.4--0.7 μm =400--700 nm = 4000--7000 Å) since he had NO ultraviolet NOR infrared measurement capability. He probably mostly measured the hydrogen Balmer lines (i.e., the visible band atomic hydrogen lines). Since galaxies are made of stars, their overall spectra are a kind of star average in which the hydrogen Balmer lines would be seen as absorption lines as in stellar spectra.

2. Slipher, of course, did NOT know the nebulae (historical usage) (i.e., spiral nebulae and elliptical nebulae) were galaxies for sure until Edwin Hubble's (1889--1953) convincing proof in 1924 of the galactic nature of the spiral nebulae and, the elliptical nebulae (by immediate implication since they were in galaxy clusters with spiral nebulae) (see Wikipedia: Edwin Hubble: Universe goes beyond the Milky Way galaxy). Note, spiral galaxies and elliptical galaxies were then called nebulae, a term which we now use for galaxies only when speaking historically.

3. Most galaxies showed redshifts. A very few showed blueshifts which we regard as negative redshifts.

4. By 1925, Slipher had redshifts for 45 galaxies (see No-522--523). This is very slow progress by modern standards, but "sufficient unto the day is the technique thereof".

   Here is a partial list of the Vesto Slipher's (1875--1969) redshift velocities (with modern values in parentheses):

   1. Andromeda Galaxy (M31, NGC 224, oblique barred spiral, r = 0.765 Mpc = 2.50 Mly, z = -0.001004, heliocentric v_redshift = -301(1)): -300 km/s
      which is a blueshift since negative, and so the Andromeda Galaxy is approaching the Solar System.
   2. M81 (NGC 3031, Bode's Galaxy, oblique grand design spiral, r = 3.675(49) Mpc = 11.99(16) Mly, z = 0.000130(900), Galactrocentric v_redshift = 73(6) km/s, heliocentric v_redshift = -39(3) km/s): -30 km/s
      which is a blueshift since negative, and so M81 is approaching the Solar System, but receding from the Milky Way.
   3. NGC 404 (a dwarf lenticular galaxy, r = 3.07(21) Mpc = 10.0(7) Mly, heliocentric? v_redshift = -48(9) km/s): -25 km/s
      which is a blueshift since negative, and so NGC 404 is approaching the Solar System.
   4. M65 (NGC 3623, oblique intermediate spiral, r = 12.73(16) Mpc = 41.5(1) Mly, z = 0.002692(10) heliocentric v_redshift = 807(3) km/s): 800 km/s
   5. Sombrero Galaxy (M104, NGC 4594, a nearly edge-on peculiar spiral, r = 9.55(31) Mpc = 31.1 (10) Mly, z = 0.003416(17), heliocentric v_redshift = 1024(5) km/s): 1,100 km/s.
   6. M77 (NGC 1068, face-on barred spiral, r = 10.72(52) Mpc = 35.0(17) Mly, z = 0.003793, heliocentric v_redshift = 1137(3) km/s): 1,100 km/s.
   7. NGC_4565 (edge-on spiral, r = 11.700 (697) = 38.16(227) Mly, z = 0.004276, heliocentric v_redshift = 1282(1) km/s): 1,100 km/s.
   8. NGC 584 (elliptical, r = 19.094(3323) Mpc = 62.28(1084) Mly, z = 0.006011, heliocentric v_redshift = 1802 km/s): 1800 km/s
      the fastest in Slipher's 1921 dataset.

   The data is a response to Google AI question: Give a list of Vesto Slipher's redshift velocities.

   Note, nearby galaxies can have blueshifts since their heliocentric velocities (i.e., velocities relative to the Solar System) are dominated by some combination of their own peculiar velocity, the Milky Way's peculiar velocity 630 km/s in some direction relative to the Milky Way comoving frame), and the Solar System orbital velocity 220 km/s around the Milk Way center of mass. These peculiar velocities dominate over the recession velocities that express the mean expansion of the universe. At larger distances, recession velocities dominate the peculiar velocities which become negligible when heliocentric velocities are ⪆ 1000 km/s.

5. Interpreted as Doppler shifts (which, in fact, is NOT correct: see below) though that is what was assumed originally by Slipher and, perhaps for awhile, by Edwin Hubble (1889--1953)---these results showed that most galaxies (the redshifted ones) were moving away from us. The blueshifted galaxies are moving toward us.

   In fact, the redshifts are a combination of Doppler shifts for nearby galaxies and cosmological redshifts (as discussed above in language of velocities rather than redshifts). The two kinds of redshifts do have different formulae in general. However, conflating the two kinds of redshifts for the very local observable universe did NOT cause much of problem in the early 1920s since their formulae to agree to 1st order in small z, and so the the results of Vesto Slipher (1875--1969) and Edwin Hubble (1889--1953) led to the correct observational result of the expansion of the universe.

   By the end of the 1920s, all people do research in cosmology (maybe 10 people) were probably aware of the distinction between Doppler shift and cosmological redshifts.

6. The upshot of Slipher's work (even before Hubble's work) was that most galaxies were in relative motion away from Milky Way.

   Alternatively, one could hypothesize that the Milky Way was at the center of an outward flow of galaxies.

   But that hypothesis violates the Copernican principle that we occupy NO special place in the universe.

7. By the by, in yours truly's days in Flag, yours truly learnt that old Vesto was also a real estate developer---West Saturn Way, Meteor Drive, etc.---he made a killing.

   

8. Image 3 Caption: The Slipher Rotunda Museum at the Lowell Observatory in Flagstaff, Arizona.

9. This may be the dome where Vesto Slipher (1875--1969) discovered the systematic cosmological redshift of the galaxies---but no one's telling.

10. Despite spending a year Flag (2010 Aug--2011 May) and visiting the Lowell Observatory several times, yours truly never noticed the Slipher Rotunda Museum---yours truly was probably just being obtuse.

Images:

1. Credit/Permission: Anonymous photographer?, 1905 (uploaded to Wikimedia Commons by User:Victuallers, 2016) / Public domain.
   Image link: Wikimedia Commons: File:Group at Clark Telescope in 1905.jpg.
   
2. Credit/Permission: Anonymous photographer, 1909 / Public domain.
   Image link: Linda Hall Library, News, Scientist of the Day, Vesto Slipher.
   
3. Credit/Permission: Leslie Connell, before or circa 2006 (uploaded to Wikipedia by User:Howcheng, 2006) / Public domain.
   Image link: Wikipedia: File:LowellObservatory.jpg.