Astronomical Objects of Research

Modern astronomy explores the whole Universe and its different forms of matter and energy. Astronomers study the contents of the Universe from the level of elementary particles and molecules (with masses of 10-30 kg) to the largest superclusters of galaxies (with masses of 1050 kg).

Astronomy can be divided into different branches in several ways. The division can be made according to either the methods or the objects of research.

The Earth (Fig. 1.3) is of interest to astronomy for many reasons. Nearly all observations must be made through the atmosphere, and the phenomena of the upper atmosphere and magnetosphere reflect the state of interplanetary space. The Earth is also the most important object of comparison for planetologists.

The Moon is still studied by astronomical methods, although spacecraft and astronauts have visited its surface and brought samples back to the Earth. To amateur astronomers, the Moon is an interesting and easy object for observations.

In the study of the planets of the solar system, the situation in the 1980's was the same as in lunar exploration 20 years earlier: the surfaces of the planets and their moons have been mapped by fly-bys of spacecraft or by orbiters, and spacecraft have soft-landed on Mars and Venus. This kind of exploration has tremendously added to our knowledge of the conditions on the planets. Continuous monitoring of the planets, however, can still only be made from the Earth, and many bodies in the solar system still await their spacecraft.

The Solar System is governed by the Sun, which produces energy in its centre by nuclear fusion. The Sun is our nearest star, and its study lends insight into conditions on other stars.

Some thousands of stars can be seen with the naked eye, but even a small telescope reveals millions of them. Stars can be classified according to their observed characteristics. A majority are like the Sun; we call them main sequence stars. However, some stars are much larger, giants or supergiants, and some are much smaller, white dwarfs. Different types of stars represent different stages of stellar evolution. Most stars are components of binary or multiple

Fig. 1.3. The Earth as seen from the Moon. The picture was taken on the first Apollo flight around the Moon, Apollo 8 in 1968. (Photo NASA)

systems, many are variable: their brightness is not constant.

Among the newest objects studied by astronomers are the compact stars: neutron stars and black holes. In them, matter has been so greatly compressed and the gravitational field is so strong that Einstein's general

Fig. 1.4. The dimensions of the Universe

Fig. 1.4. The dimensions of the Universe theory of relativity must be used to describe matter and space.

Stars are points of light in an otherwise seemingly empty space. Yet interstellar space is not empty, but contains large clouds of atoms, molecules, elementary particles and dust. New matter is injected into interstellar space by erupting and exploding stars; at other places, new stars are formed from contracting interstellar clouds.

Stars are not evenly distributed in space, but form concentrations, clusters of stars. These consist of stars born near each other, and in some cases, remaining together for billions of years.

The largest concentration of stars in the sky is the Milky Way. It is a massive stellar system, a galaxy, consisting of over 200 billion stars. All the stars visible to the naked eye belong to the Milky Way. Light travels across our galaxy in 100,000 years.

The Milky Way is not the only galaxy, but one of almost innumerable others. Galaxies often form clusters ofgalaxies, and these clusters can be clumped together into superclusters. Galaxies are seen at all distances as

far away as our observations reach. Still further out we see quasars - the light of the most distant quasars we see now was emitted when the Universe was one-tenth of its present age.

The largest object studied by astronomers is the whole Universe. Cosmology, once the domain of theologicians and philosophers, has become the subject of physical theories and concrete astronomical observations.

Among the different branches of research, spherical, or positional, astronomy studies the coordinate systems on the celestial sphere, their changes and the apparent places of celestial bodies in the sky. Celestial mechanics studies the movements of bodies in the solar system, in stellar systems and among the galaxies and clusters of galaxies. Astrophysics is concerned with the physical properties of celestial objects; it employs methods of modern physics. It thus has a central position in almost all branches of astronomy (Table 1.1).

Astronomy can be divided into different areas according to the wavelength used in observations. We can

Fig. 1.5. The globular cluster M13. There are over a million stars in the cluster. (Photo Palomar Observatory)

Table 1.1. The share of different branches of astronomy in 1980, 1998 and 2005. For the first two years, the percantage of the number of publications was estimated from the printed pages of Astronomy and Astrophysics Abstracts, published by the Astronomische Rechen-Institut, Heidelberg. The publication of the series was discontinued in 2000, and for 2005, an estimate was made from the Smithsonian/NASA Astrophysics Data System (ADS) Abstract Service in the net. The difference between 1998 and 2005 may reflect different methods of classification, rather than actual changes in the direction of research.

Branch of

Percentage of publications


in the





Astronomical instruments and techniques




Positional astronomy, celestial mechanics




Space research




Theoretical astrophysics












Planetary system




Interstellar matter, nebulae




Radio sources, X-ray sources, cosmic rays




Stellar systems, Galaxy, extragalactic

objects, cosmology




speak of radio, infrared, optical, ultraviolet, X-ray or gamma astronomy, depending on which wavelengths of the electromagnetic spectrum are used. In the future, neutrinos and gravitational waves may also be observed.

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