Astronomers Essay

Part One
Brief Descriptions of the Following Astronomers:
Walter Baade : Baade was a German-born American, whose work gave new estimates for the age and size
of the universe. During the wartime, blackouts aided his observatons and allowed him to indentify and
classify stars in a new and useful way, and led him to increase and improve Hubble’s values for the size and
age of the universe (to the great relief of geologists.) He also worked on supernovae and radiostars.

Milton Humason : Humason was a colleague of Edwin Hubble’s at Mt. Wilson and Palomar Mtn. who was
instrumental in measuring faint galaxy spectra providing evidence for the expansion of the universe.

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Jan Oort : In 1927, this Dutch astronomer proved by observation (in the Leiden observatory) that our
galaxy is rotating, and calculated the sirance of the sun from the centre of the galaxy and the period of its
orbit. In 1950 he sugested the exsistence of a sphere of incipent cometary material surrounding the solar
system, which is now called the ‘Oort cloud.’ He proposed that comets detached themsleves from this ‘Oort-
cloud’ and went into orbit around the sun. From 1940 onwards he carried out notable work in radio

Harlow Shapley : Shapley deduced that the Sun lies near the central plane of the Galaxy some 30,000 light-
years away from the centre.

In 1911 Shapley, working with results given by Henry N. Russell, began finding the dimensions of
stars in a number of binary systems from measurements of their light variation when they eclipse one
another. These methods remained the standard procedure for more than 30 years. Shapley also showed that
Cepheid variables cannot be star pairs that eclipse each other. He was the first to propose that they are
pulsating stars.

In the Mount Wilson Observatory, Pasadena Calif., in 1914, he made a study of the distribution of
the globular clusters in the Galaxy; these clusters are immense, densely packed groups of stars, some
containing as many as 1,000,000 members. He found that of the 100 clusters known at the time, one-third
lay within the boundary of the constellation Sagittarius. Utilizing the newly developed concept that variable
stars accurately reveal their distance by their period of variation and apparent brightness, he found that the
clusters were distributed roughly in a sphere whose centre lay in Sagittarius. Since the clusters assumed a
spherical arrangement, it was logical to conclude that they would cluster around the centre of the Galaxy;
from this conclusion and his other distance data Shapley deduced that the Sun lies at a distance of 50,000
light-years from the centre of the Galaxy; the number was later corrected to 30,000 light-years. Before
Shapley, the Sun was believed to lie ne!
ar the centre of the Galaxy. His work, which led to the first realistic estimate for the actual size of the
Galaxy, thus was a milestone in galactic astronomy.

Allan Sandage : Sandage (U.S) discovered the first quasi-stellar radio source (quasar), a starlike object that
is a strong emitter of radio waves. He made the discovery in collaboration with the U.S. radio astronomer
Thomas A. Matthews. Sandage became a member of the staff of the Hale Observatories (now the Mount
Wilson and Palomar Observatories), in California, in 1952 and carried out most of his investigations there.

Pursuing the theoretical work of several astronomers on the evolution of stars, Sandage, with Harold L.

Johnson, demonstrated in the early 1950s that the observed characteristics of the light and colour of the
brightest stars in various globular clusters indicate that the clusters can be arranged in order according to
their age. This information provided insight into stellar evolution and galactic structure.

Later, Sandage became a leader in the study of quasi-stellar radio sources, comparing accurate
positions of radio sources with photographic sky maps and then using a large optical telescope to find a
visual starlike source at the point where the strong radio waves are being emitted. Sandage and Matthews
identified the first of many such objects Sandage later discovered that some of the remote, starlike objects
with similar characteristics are not radio sources. He also found that the light from a number of the sources
varies rapidly and irregularly in intensity.

Part Two
Cerro Tololo Interamerican Observatory
Cerro Tololo is a mountatin peak in the nothern range of the Andes in South America. At the
summit, 7,200 ft. above sea level, the US has built on of the world’s foremost astronomical observatories.

This is called the Cerro Tololo Interamerican Observatory or CTIO. It was founded in 1965 in Chile as the
southern branch of the Kitt Peak National Observatory. It is located about 285 miles north of Santiago and
50 miles inland from the coastal city of La Serena. The European Southern Observatory and the Carnegie
Institution of Washington also operate major astronomical observatories nearby. It’s coordinates are : W
70d48m52.7s S 30d09m55.5s
CTIO’s facilities are available for use for approved projects by all qualified astronomers in the
western hemisphere. CTIO is operated by the Association of Universities for Research in Astronomy Inc.

(AURA), under a cooperative agreement with the National Science Foundation as part of the National
Optical Astronomy Observatories, which also operates Kitt Peak National Observatory in Tucson Arizona
and is the operating agency for the US portion of the International Gemini Project.

The CTIO houses several telescopes and auxiliary instruments, the most significant of which is a
reflecting telescope with a 4-metre mirror. On site are six optical telescopes, and one radio telescope:
4.0 Meter Blanco Telescope.

1.5 Meter Ritchey-Chretien Telescope.

Yale 1.0 Meter Ritchey-Chretien Telescope f/10 (19.5 arcsec/mm).

0.9 Meter Telescope f/13.5 (16.5 arcsec/mm).

Curtis/Schmidt Telescope (0.6/0.9 Meter) f/3.5 (96.6 arcsec/mm).

Lowell (0.6m) Telescope f/13.5, f/75 (25.0, 4.5 arcsec/mm)
1.2m Radio Telescope (SCMT, Universidad de Chile).

The observatory is best noted for its research on the central region of the Milky Way Galaxy, the
Magellanic Clouds, and high-energy cosmic radio and X-ray sources.

Part Three
How Galaxies Evolve :
The study of the origin and evolution of galaxies has only just begun. In the past
there has not been much data to work with, and many models of galaxy formation and evolution have been
constructed on the basis of presumptions about conditions in the early universe, which are in turn based on
models of the expansion of the Cosmos after the “big bang”–the explosion from which the Universe is
thought to have originated. Prevailing theory has it that at crucial points in time there condensed from the
expanding matter smaller clouds (protogalaxies) that could collapse under their own gravitational field and
eventually form galaxies. At the time when the mass of such a stable perturbation in the cloud was
approximately 10 solar masses, the galaxies formed. It is still not known whether the clusters of galaxies
emerged first or whether they resulted as accumulations of already formed galaxies. Following the
separation of mass into individual galaxies, the next step probably depe!
nded on the characteristics of the particular clump of matter involved, especially on its mass and angular
momentum. The latter quantity was the most likely determinant of the form of the galaxy that eventually
evolved. It is thought that a protogalaxy with a large amount of angular momentum tended to form a flat,
rapidly rotating system (a spiral galaxy), whereas one with very little angular momentum developed into a
more nearly spherical system (an elliptical galaxy.)
Calculations show that a galaxy very gradually becomes dimmer and redder as time progresses
and its constituent stars evolve. There is some evidence from very distant galaxies–those whose light was
emitted billions of years ago when they were younger–that the effects of this kind of slow evolution can
actually be seen.

Part Four
Three Great Scientists Of The Past (In My Opinion)
Based on my readings I believe that the following scientist have all made valuable contributions
to astronomy :
Here are some brief descriptions of their contributon to the understanding of astronomy:
Copernicus (1473-1543) : Nicolas Copernicus is often considered the founder of modern astronomy. His
study led to his theory that the Earth rotates on its axis and that the Earth and the planets revolve around the

The Copernican theory was contrary to the Ptolemaic theory then generally accepted. In 1530 he
finished his great book, ‘Concerning the Revolutions of the Celestial Spheres’.

His theory was in opposition to the teachings of the Roman Catholic church, and the book was not
published for 13 years. Copernicus apparently received the first copy as he was dying, on May 24, 1543.

The book opened the way to a truly scientific approach to astronomy. Such men as Galileo and Kepler were
profoundly influenced by it.

Galilieo (1564-1642) : Modern physics owes its beginning to Galileo, who was the first astronomer to use a
telescope. By discovering four satellites of the planet Jupiter, he gave visual evidence that supported the
Copernican theory. Galileo thus helped disprove much of the medieval thinking in science.

In 1583 Galileo discovered the law of the pendulum by watching a chandelier swing in the
cathedral at Pisa. He timed it with his pulse and found that, whether it swung in a wide or a narrow arc, it
always took the same time to complete an oscillation. He thus gave society the first reliable means of
keeping time.

In about 1609, after word from Holland of Hans Lippershey’s newly invented telescope reached
him, he built his own version of the instrument. He developed magnifying power until on Jan. 7, 1610, he
saw four satellites of Jupiter. He also saw the mountains and craters on the moon and found the Milky Way
to be a dense collection of stars.

Kepler (1571-1630) : This Renaissance astronomer and astrologer is best known for his discovery that the
orbits in which the Earth and the other planets of the solar system travel around the sun are elliptical, or
oval, in shape. He was also the first to explain correctly how human beings see and to demonstrate what
happens to light when it enters a telescope. In addition, he designed an instrument that serves as the basis of
the modern refractive telescope.

Kepler’s great work on planetary motion is summed up in three principles, which have become
known as “Kepler’s laws”: (1) The path of every planet in its motion about the sun forms an ellipse, with
the sun at one focus. (2) The speed of a planet in its orbit varies so that a line joining it with the sun sweeps
over equal areas in equal times. (3) The squares of the planets’ periods of revolution are proportional to the
cubes of the planets’ mean distances from the sun. These laws removed all doubt that the Earth and planets
go around the sun. Later Sir Isaac Newton used Kepler’s laws to establish his law of universal gravitation.


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