Astrophysics

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Astrophysics

Astrophysics is a branch of astronomy that describes the processes (from birth, evolution, and ending stages of celestial bodies and larger celestial systems) that give rise to the observable features of the universe in terms of previously developed physical theories and laws. It ties together physics and astronomy by describing astronomical phenomena in terms of the physics and chemistry familiar in everyday life. Astrophysicists deal primarily with radiations emitted over the entire length of the electromagnetic spectrum. For instance, they study radiation in order to estimate energy states of atoms, which help to evaluate temperatures and pressures of celestial objects.

Background

Why do the stars shine? How did the Milky Way galaxy form? Will the universe expand forever? These questions are the types of questions asked by astrophysicists in an attempt to understand the processes that cause the universe, and everything in it, to behave in certain ways. From the low-energy gravitational interactions between planets and stars to the violent, high energy processes occurring in the centers of galaxies, astrophysical theories are used to explain what is seen and to understand how phenomena are related.

For thousands of years, astronomy was simply an observational science—humans could observe phenomena in the sky, but had no physical explanation for what they saw. Early humans could offer only supernatural explanations for what they observed, which seemed drastically different from what they experienced in everyday life. Only in the twentieth century have scientists been able to explain many astronomical phenomena in terms of detailed physical theories, relating them to the same chemistry and physics at work in everyday lives.

Astrophysical experiments, unlike experiments in many other sciences, cannot be done under controlled conditions or repeated in laboratories; the energies, distances, and time scales involved are simply too great. As a result, astrophysicists are forced into roles as observers, watching events as they happen without being able to control the parameters of the experiment.

For centuries, humans have made such observations and attempted to understand the forces at work. However, how did scientists develop the picture of the universe if they could not reproduce what they saw in the laboratory? Instead of controlling the experiments, they used what data they were able to obtain in order to develop theories based on extensions of the physical laws that govern day-to-day experiences on Earth.

Astrophysics often involves the creation of mathematical models as a means of interpreting observations. This theoretical modeling is important not just for explaining what has already been seen, but also for predicting other observable effects. These models are often based on well-established physics, but often must be simplified, because real astronomical phenomena can be enormously complex.

Processes in the universe

Even looking close to the Earth, within the solar system astrophysics see widely varying conditions. The properties of the rocky planet Mercury, very close to the Sun, differ dramatically from those of the gas giant Saturn, with its complex ring structure, and from the cold, icy dwarf planet Pluto. However, the range of variations found in the solar system is minuscule when compared to that of the stars, galaxies, and more exotic objects such as quasars. The properties of all of these objects, however, can be measured by observation, and an understanding of how they work can be reached by the extension and application of the same physical laws with which scientists are familiar.

The first astrophysical concept or law to be recognized was the law of gravity. Most people are familiar with the force of gravity. Although it is a very weak force compared to the other fundamental forces of nature, it is the dominant factor determining the structure and the fate of the universe. Large structures, such as galaxies, and smaller ones, such as stars and planets, coalesced due to the force of gravity, which acts over vast distances of space.

Much of the evolution of the universe is due to gravity’s effects. However, scientists generally hold the view that the understanding of atomic processes marks the true beginning of astrophysics. Indeed, even such enormous objects as stars are governed by the interaction and behavior of atoms. Thus, it is often said that astrophysics began in the early decades of the twentieth century, when quantum mechanics and atomic physics were born.

Importance of instrumentation

Scientists learn about distant objects by measuring the properties that they can observe directly—for example, by detecting emissions from the objects. The most common measurements are of electromagnetic radiation, extending from radio waves through visible wavelengths to high-energy gamma rays. Each time a class of objects has been studied in a new wavelength region, astrophysicists gain insights into composition, structure, and properties. Emissions from each wavelength region are generated by and affected by different processes and so provide fresh understanding of the object. For this reason, the development of new instrumentation has been crucial to the development of astrophysics.

The development of space instrumentation that can detect photons before they are obscured by the Earth’s atmosphere has been critical to the understanding of the universe. Large space-based observatories, such as the Hubble Space Telescope, continually spawn major advances in astrophysics due to their ability to study the universe over specific regions of the electromagnetic spectrum with unprecedented sensitivity. In addition, probes such as the Voyagers, which visited most of the outer planets of the solar system, have provided detailed measurements of the physical environment throughout the solar system. The use of spectroscopy, which can determine the chemical composition of distant objects from their wavelength distribution, is a particularly important tool of the astrophysicist.

In addition to the photons of electromagnetic radiation, emitted particles can be detected. These can be protons and electrons, the constituents of ordinary matter on Earth (though often with extremely high energies), or ghostly neutrinos, which only weakly interact with matter on Earth (and are thus extremely difficult to detect) but help scientists learn about the nuclear reactions that power stars.

Astrophysics proceeds through hypothesis, prediction, and testing (via observation); its common belief is that laws of physics are consistent throughout the universe. These laws of physics have served us well, and scientists are most skeptical of proposed explanations that violate them.