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Astrophysics
Although astronomy is as old as recorded history, it was long separated from the study of physics. In the 17th century Galileo Galilei made quantitative measurements central to physics and very important observation in astronomy. Kepler discovered the laws of planetary motion, and later that century Isaac Newton bridged the gap between Kepler's laws and Galileo's dynamics, discovering that the same laws that rule the dynamics of objects on earth rules the motion of planets and the moon. Celestial mechanics, the application of Newtonian gravity and Newton's laws to explain Kepler's laws of planetary motion, was the first unification of astronomy and physics.
These discoveries transformed maritime navigation: starting around 1670, the entire world was measured using essentially modern latitude instruments and the best available clocks. The needs of navigation provided a drive for progressively more accurate astronomical observations and instruments, providing a background for ever more available data for scientists.
Spectroscopy
Spectroscopy is a branch of physics (optics) that is central to astrophysics. The word 'spectrum' is used today to mean a display of electromagnetic radiation (plural is 'spectra'). In 1671 Newton reported his experiment of decomposing the sunlight into colors using a prism. When you see a rainbow, you are seeing a spectrum.
At the end of the 19th century it was discovered that, when decomposing the light from the Sun, a multitude of dark lines were observed inside the spectrum (in these areas where there was less or no light). Experiments with hot gases showed that the same lines could be observed in physics and chemistry laboratories on Earth, in the spectra of gases. Spectral lines from specific patterns that correspond to unique chemical elements. In this way it was proved that the chemical elements found in the Sun (chiefly hydrogen) were also found on Earth. The element helium was first discovered in the spectrum of the Sun and later on Earth.
During the 19th century several scientists thought that it would be impossible to determine the chemical composition of the stars. Since then, astrophysicists have proved that this is possible, using spectroscopy.
During the 20th century spectrometry as a result of the advent of quantum physics that was necessary to understand the spectral lines produced by the atoms of the specific chemical elements.
Stars can be classified on their spectral characteristics, that are related to their colors and temperatures. The main spectral classes are: O, B, A, F, G, K, M.
A popular mnemonic for remembering the order is "Oh Be A Fine Girl, Kiss Me".
O and B stars are bluish. A and F are white. G are yellow (like the Sun). K are orange. M are reddish.
The topic of how stars change, or stellar evolution, is often modelled by placing the varieties of star types on a special diagram, called Hertzsprung-Russell diagram.
Read more on Star classification.
Observational astrophysics
Many astrophysical processes cannot be reproduced in laboratories on Earth. However, there is a huge variety of astronomical objects that are visible. The study of these objects through collection of data is the goal of observational astrophysics.
The majority of astrophysical observations are made using light and the electromagnetic spectrum.
- Optical astronomy is the oldest kind of astronomy. Today Telescopes and spectroscopes are the most common instruments used to study light coming from the stars and the other astronomical objects. Since the Earth's atmosphere interferes somewhat with optical observations, adaptive optics and space telescopes are used to obtain the better image quality. Many chemical spectra can be observed to study the chemical composition of stars, galaxies and nebulae. However, light is only a form of electromagnetic radiation. Its wavelenght is less than one micron.
- Radio astronomy studies radiation with a wavelength greater than a few millimeters. Radio waves are usually emitted by cold objects, including interstellar gas and dust clouds. The cosmic microwave background radiation is the "redshifted" radiation that is believed to be due to the Big Bang. Pulsars were first detected at microwave frequencies.
- Infrared astronomy studies radiation with a wavelength that is longer than visible light but shorter than radio waves. Infrared observations are usually made with telescopes similar to the usual optical telescopes. Objects colder than stars, such as planets, are usually studied at infrared frequencies.
- Ultraviolet, X-ray and gamma ray astronomy study very energetic processes such as binary pulsars and supposed black holes. These kinds of radiation do not penetrate the Earth's atmosphere well, so they are usually studied with space-based telescopes.
Other than electromagnetic radiation, few things may be observed. A few gravitational wave observatories have been constructed, but these waves are extremely difficult to detect. Neutrino observatories have also been built, primarily to study our Sun. Cosmic rays consisting of very high energy (very fast) subatomic particles can be observed hitting the Earth's atmosphere.
Observations can also vary in their time scale. Most optical observations take minutes to hours. However, historical data on some objects is available spanning centuries or millennia. On the other hand, radio observations may look at events on a millisecond timescale (millisecond pulsars).
The study of the Sun has a special place in astrophysics. Due to the distance of other stars, the Sun can be observed in a kind of detail that is unparalleled. Of course, understanding of the Sun is useful as a guide to understand other stars.
Theoretical astrophysics
Theoretical astrophysicists create and evaluate models to reproduce and predict real observations. A wide variety of tools are used, which include analytical models and computational numerical simulations.
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