Astrophysics

**Astrophysics** is the study of how stars, planets, and other things in the Universe work, and how we can learn about them. Astrophysicists use physics and chemistry to explain what astronomers find and see.

Astrophysics is also the study of how the Universe started and how it is changing with time. This part of astrophysics is called cosmology.

## History 
For a long time, bodies in the sky seemed to be unchanging spheres moving in a circle. But on Earth, growth and decay happened, and natural motion was in a straight line. Therefore, people thought the celestial region was made of a fundamentally different kind of matter from that found on Earth.

During the 16th and 17th centuries, natural philosophers such as Nicolaus Copernicus, Johannes Kepler, Galileo, Descartes, and Newton, began to maintain that the celestial and terrestrial regions were made of similar material and were subject to the same natural laws.

When they were able to work out how the planets moved, the science of astrophysics was born. Sir Isaac Newton realized that the same rules of mechanics that he had found on the surface of the Earth also could be used to predict how the planets moved. He said, "As above, so below". By this, he meant that we can study how things work on this planet to find out how things work in space.

Later scientists looked at the light from stars using spectroscopy.  They were able to work out what they were made of, and how they changed with time.

## Types of astrophysics 
 remnant LMC N 63A imaged in x-ray (blue), optical (green) and radio (red) wavelengths. The X-ray glow is from material heated to about ten million degrees Celsius by a shock wave generated by the supernova explosion.]]

There are two main types of astrophysics:

***Observational astrophysics**. Observational astrophysicists use telescopes to study the Universe, but astrophysicists study the physics of what they see to explain the Universe.
**Radio astronomy
**Optical astronomy
**Infrared astronomy
**Ultraviolet, X-ray, and gamma-ray astronomy using space telescopes
***Theoretical astrophysics** uses information from astronomers, theories, and mathematics to explain how the Universe works. Cosmology is a type of theoretical astrophysics. Problems investigated include: 
**Solar System: its formation and evolution
**Star formation and evolution
**Galaxy formation and evolution
**large-scale structure of matter in the universe 
**Origin of cosmic rays 
**General relativity 
**Evolution of the universe
**Hydrodynamics is used for mathematically modelling how gases behave. Strong magnetic fields found around many bodies can drastically change how these gases behave, affecting things from star formation to the flows of gases around compact stars. 

### An example 
Cecilia Helena Payne (later Cecilia Payne-Gaposchkin) published her doctoral dissertation at Radcliffe College in 1925. She had discovered that hydrogen and helium were the main content of stars. More than that, the spectral classes relate to the temperature of stars. Despite Eddington's suggestion, her discovery was so unexpected that her dissertation readers told her to modify the conclusion before publication. Later research confirmed her discovery.

## Related pages
* Astronomy
* Celestial mechanics
* Quasar

## References