- 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.
- 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.
- Most galaxies showed
redshifts.
A very few showed
blueshifts which we regard as
negative
redshifts.
- 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".
-
Interpreted as
Doppler shifts---which, in fact, is
NOT exactly right---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.
Originally, Slipher and, perhaps for awhile,
Hubble
assumed the shifts were entirely Doppler shifts.
This is a wrong assumption.
Most of the shifting is
cosmological redshift.
The somewhat wrong assumption did NOT hurt much since the
1st order Doppler effect formula
coincidentally
gives the right answer anyway for
recession velocities
for relatively nearby galaxies which were all
Slipher measured
redshifts for.
- Using the
1st order Doppler effect formula,
the
recession velocities
of the observed galaxies were known.
- The upshot of Slipher'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.
- 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.
- Image 3 Caption:
The Slipher Rotunda Museum at the
Lowell Observatory in
Flagstaff, Arizona.
- This may be the dome where
Vesto Slipher (1875--1969) discovered the
systematic cosmological redshift of the
galaxies---but no one's telling.
- 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.