DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Mixmaster universe
Mixmaster universe A dynamically
anisotropic but homogeneous theoretical cosmology
(Misner, 1966) which expands from a
Big Bang to a maximum size and recollapses to
a final “big crunch”. The constant-time 3-spaces
are distorted 3-spheres. A nonlinear oscillation
occurs because the spatial curvature generates
a “force term” in the motion of the anisotropy,
bounding the anistropy by an amount which is
finite at any one time, though the bound itself diverges
at the Big Bang and big crunch. The universe
behaves like an expanding, then contracting,
ball that shrinks to a flattened spindle and
then puffs out first in one direction and then in
another, changing into a pancake configuration,
and so on. Any shape can eventually arise. Further,
cosmological horizons can be eliminated
in some situations, allowing mixing of matter
and providing, in principle, a homogenization
(though this has been shown to be of very small
probability); thus the name, which also stems
from the analogy with a particular kitchen appliance.
MJD See Modified Julian Date.
MK system of stellar classification In the
1890s Harvard Observatory began to classify
stars according to hydrogen features, such as the
strength of Balmer lines. In the 1920s this system
of classification based on temperature was
revised and compiled by E.C. Pickering, A.J.
Cannon, and others, leaving us with O, B, A, F,
G, K, M. The types were divided up into 10 subtypes,
from 0 to 9, and special types R, N, and S
were added for carbon-rich stars. However, often
stars of the same temperature type differed
greatly in luminosity, and so several luminosity
classification schemes were used. In the 1930s
W.W. Morgan and P.C. Keenan of Yerkes Observatory
documented a luminosity classification
in combination with a temperature type that is
still used today. The system used Roman numerals
along with the subdivided temperature
classification. Class V stars are on the main sequence,
class Ia, b are the supergiants, class II
are the bright giants, class III are the normal giants,
class IV are the sub-giants, class VI are the
dwarfs, and class VII are the white dwarfs. Each
of these luminosity classes occupy distinct positions
on the Hertzprung–Russell diagram. Stars
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with the same temperature yet different luminosity
must differ in surface area (see effective
temperature). This difference produces measurable
effects in some spectral features, such as
pressure broadening, and can thus be measured.
Later, Keenan developed, along with others, a
dual type of classification for the difficult latetype
carbon-rich stars, the carbon stars. Types
N and R have been replaced by a C classification
followed by two numbers, one indicating
the temperature and the other the strength of the
carbon abundance.
M magnitude Stellar magnitude derived
from observations in the infrared at a wavelength
of 5 µm.
model atmosphere A computationally constructed
atmosphere model, created according to
physics laws and representing the atmospheric
status by discrete values on horizontal grid mesh
and vertical levels. By inputting initial atmospheric
state conditions, numerical atmospheric
models can start time integration to simulate
the real atmospheric processes approximately.
Their accuracy depends on the model’s resolution,
the number and quality of contained
physics processes, and parameterization packages
to describe the sub-grid processes in the
models. By simulating real atmospheric processes,
numerical atmospheric models can provide
products for weather forecasts, research on
different atmospheric science issues, and on climate
status in ancient times.
mode water An ocean water mass characterized
by small vertical gradient in temperature
and density. For this, it is also called thermostad
or pycnostad. Occupying a great depth range,
this water mass stands out as a distinct mode
in a volumetric census taken against temperature
or density. Subtropical mode waters, found
in northwestern parts of the North Pacific and
North Atlantic subtropical gyres, are the most
famous ones, with a characteristic temperature
centered on 18 ◦ C, they are sometimes called
18 ◦ -waters. They form in the deep mixed layer
under intense cooling in winter and are advected
southwestward by the wind-driven subtropical
gyres. Other mode waters with different tem-