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Astronomy & Cosmology III
 The Hertzsprung-Russell diagram shown on
the Mexican stamp at left is a graphic relation of the absolute magnitude
of stars to their spectral class or temperature. Such classification aims
at grouping like stars together in a meaningful way so that each star
does not have to be described anew individually. Attempts at classification
were already made by Secchi,
who grouped stellar spectra into four classes, depending on the absorption
lines seen, and were continued by the Danish astronomer Ejnar Hertzsprung (1873-1967) and independently by the American astronomer Henry Norris
Russell (1877-1957) after whom the resulting composite graph is named.
The main sequence of stars runs from the upper left corner where the hottest,
brightest stars are placed (blue giants) to the lower right, where the
smallest, dimmest stars reside (red dwarfs). Our sun, a yellow star, falls
about into the middle of this sequence, which can also be viewed in time
as the evolution of a star from its hot, bright, early stages to cooler,
dim, late stages. In the upper right corner are very bright giants or
supergiants who are however rather cool, as evidenced by their red color
(Betelgeuse). |
 Hello! It is cold outside, 3 degrees! Three degrees
Kelvin, that is, in intergalactic space. This surprising discovery was
made by researchers Arno Penzias (1933-) and Robert Wilson (1936-) at
Bell Labs in New Jersey, with a receiver originally built for satellite
communication. They found that cosmic microwave radiation of uniform strength
was received from all directions, and it was supposed that this radiation
was the remains of the Big Bang. The shape of the spectrum is indeed like
that of a black body with a temperature of 3 degrees Kelvin, supporting
the Big Bang theory of creation rather than the steady state hypothesis.
Penzias and Wilson shared one half of the 1978 Nobel physics prize for
this discovery; the other half went to Piotr Leontevitch Kapitsa (1894-1984),
for his discoveries in the area of low temperature physics. A Swedish
stamp has not yet been issued in his honor. |
 Subramanian Chandrasekhar (1910-1995) shared the
1983 Nobel prize for physics for his theoretical studies of the physical
processes relating to the structure and evolution of the stars, particularly
white dwarfs, stars at the evolutionary end of stellar development. Chandrasekhar
calculated that white dwarfs cannot have a mass greater than 1.4 solar
masses without collapsing into an even denser state, a neutron star. The
expression for this critical mass is shown on the stamp at left. |
 William A Fowler (1911-1995) shared the 1993 Nobel
physics prize with Chandrasekhar for important astrophysical discoveries
in his theoretical and experimental studies of nuclear reactions in the
formation of the chemical elements in the universe, from the mostly hydrogen
and helium created in the Big Bang.The chart of nuclides on the stamp
shows stable heavy nuclei between hafnium and lead. |
 Georges Edouard Lemaitre (1894-1966), a Belgian
cosmologist, astrophysicist and priest, was interested in the problem
of the creation of the universe and proposed a theory of an expanding
universe based on his solutions to Einsteins equations of general relativity.
He envisioned a primal atom containing all the matter in the universe,
which exploded at some point 10-20 billion years ago, hurtling elementary
particles and photons outward, where they cooled and condensed eventually
to form the light elements, heavy elements, molecules, and finally galaxies.
The Big Bang theory, as it was derisively called by Fred Hoyle, a proponent
of a steady state theory of the universe, is supported by many independent
astronomical observations, starting with Hubble's observations of receding
galaxies and the discoveries of Penzias, Wilson, and Fowler, above. |
  Radioastronomer Antony Hewish (1924- ) and his graduate
student Jocelyn Bell discovered radio sources in space that were called
pulsars, because of their regular emission of pulsed energy. This confirmed
the existence of neutron stars, extremely dense collapsed stars at the
center of the pulsars which are highly magnetized. He shared the 1974
Nobel prize in physics with Martin Ryle (1918-1984), who was recognized
for his achievements in radiotelescope construction. Known as the aperture
synthesis technique, this method employs several small telescopes deployed
over a distance of about three miles whose positions are mutually adjustable
and who act as one enormous composite telescope of corresponding size.
The Ryle stamp shows two symbolic radiotelescopes receiving radiation
from a radiogalaxy. The Hewish stamp shows a stylized pulse signal superimposed
on the Crab Nebula, a supernova remnant which was first observed by Chinese
astronomers in the 11th century, and which has a pulsar emitting energy
bursts across the whole spectrum at its center. The five stamps of this
stunning 1987 Swedish set vary in color from a deep blue to black, like
the night sky. |
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