DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

M
M51 Object 51 in the Messier list, a bright
Sc spiral galaxy notable for the grand design of
its spiral arms (and sometimes referred to as the
“Whirlpool” galaxy). M 51 is perturbed by a
smaller companion galaxy which appears to be
in touch with and distorts one of its spiral arms.
Optical spectra of the M 51 nucleus show emission
lines suggestive of non-thermal nuclear activity,
and whose intensity ratios are intermediate
between those of LINERs and Seyfert 2
nuclei.
M87 (NGC 4486) Giant elliptical galaxy,
type E1, in Virgo at RA 12 h 30 m .8, dec +12 ◦ 24 ′ ,
mV = 8.6, angular diameter 7 ′ . Distance
18 Mpc, giving a linear diameter of about 40 kpc
at the center of the Virgo cluster. Also a radio
source, Virgo A; also a strong source of X-rays.
Totalmassabout3 ×10 12 M⊙. M87hastwovery
notable features. One is a strong jet (∼ 2 kpc in
length), which is also a source in the radio and
connects the central core of M87 with the more
diffuse emission of its northwestern lobe. The
other remarkable feature is the large number (up
to 15,000) of globular clusters associated with
M87.
M87 has also been identified as containing
a probable supermassive black hole, of mass
about 3 × 10 9 M⊙. See cD galaxies.
M101 (NGC 5457). The “Pinwheel” galaxy,
type Sc, in Ursa Major at RA 14 h 03 m .2, Dec
+54 ◦ 21 ′ , mV = 7.9, angular diameter 22 ′ .0.
Distance, by recent measurements using the
period-magnitude relation of Cepheid variable
stars detected by the Hubble Space Telescope,
is 8.3 Mpc, giving a physical diameter of about
50 kpc.
Mach, Ernst (1838–1916) An outstanding
experimental physicist whose seminal works
(essentially, his criticisms of the Newtonian
world picture) paved the way for the major revision
of physics in the beginning of the 20th
© 2001 by CRC Press LLC
Mach number
century. He is sometimes considered more as
a philosopher of science, though he persistently
denied this title.
Born on February 18, 1838, in 1855 he enrolled
in Vienna University, and received from
it his Ph.D. degree in 1860. He continued as a
Privat–Dozent delivering lectures on mathematics,
psychophysics, and Helmholtz’s studies in
the perception of sound. From 1864, he held
the chair of mathematics in Graz, then (from
1867) for 28 years he taught at the experimental
physics department of the Charles University
(its German branch). In 1895, Mach moved to
Vienna as the philosophy professor of the university
(the theory and history of inductive sciences)
where he worked up to his retirement in
1901. He died on February 19, 1916.
Mach’s scientific interests were primarily focused
on acoustic phenomena and processes in
continuous media (for example, the unit of velocity
in terms of the velocity of sound in a
medium, now generally used in aerospace techniques,
bears his name — mach), but he was in
general a deep thinker, and his achievements in
experimental studies were intimately connected
with his fundamental methodological quest as
well as with his research in the organs of perception
physiology and, finally, the epistemology
(he influenced these areas profoundly). His
sharp critical analysis of the basic concepts of
Newtonian mechanics is widely known, and it
influenced the development of both the relativities
by Einstein. Especially popular among
gravitational physicists is Mach’s principle (the
term introduced by Einstein) which relates local
inertial properties of matter with its global distribution
(and, finally, motion). Mach was the first
who clearly formulated the idea of Gedankenexperiment
(thought experiment) and pointed out
its great importance for physics.
Mach’s ideas were greatly appreciated by
Einstein and L. Boltzmann, and disdained by
Planck (who was shocked by Mach’s negative attitude
toward religion) and Lenin (who accused
Mach of fideism, i.e., a point-blank acceptance
of religion).
Mach number Ratio of the speed of an object
to the sound (or by extension, the light) speed in