# Astrophysics

Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties (luminosity, density, temperature and chemical composition) of astronomical objects such as stars, galaxies, and the interstellar medium, as well as their interactions. The study of cosmology is theoretical astrophysics at the largest scales.

Because it is a very broad subject, astrophysicists typically apply many disciplines of physics including, but not limited to, mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics. In practice, modern astronomical research involves a substantial amount of physics. The name of a university's department ("astrophysics" or "astronomy") often has to do more with the department's history than with the contents of the programs.

## History

Although astronomy is as old as recorded history, it was long separated from the study of physics. In the Aristotelian worldview, the celestial pertained to perfection—bodies in the sky being perfect spheres moving in perfectly circular orbits—while the earthly pertained to imperfection; these two realms were seen as unrelated.

For centuries, the apparently common-sense view that the Sun and other planets went round the Earth went basically unquestioned, until Nicolaus Copernicus suggested in the 16th century that the Earth and all the other planets in the Solar System orbited the Sun. This idea had been around, though, for nearly 2000 years when Aristarchus first suggested it, but not in such a nice mathematical model. Galileo Galilei made quantitative measurements central to physics, but in astronomy his observation did not have astrophysical significance.

The availability of accurate observational data led to research into theoretical explanations for the observed behavior. At first, only ad-hoc rules were discovered, such as Kepler's laws of planetary motion, discovered at the start of the 17th century. 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.

After Isaac Newton published his Principia, maritime navigation was transformed. 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.

At the end of the 19th century it was discovered that, when decomposing the light from the Sun, a multitude of spectral lines were observed (regions where there was less or no light). Experiments with hot gases showed that the same lines could be observed in the spectra of gases, specific lines corresponding 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. Indeed, the element helium was first discovered in the spectrum of the sun and only later on earth, hence its name. During the 20th century, spectrometry (the study of these spectral lines) advanced, particularly as a result of the advent of quantum physics that was necessary to understand the astronomical and experimental observations.

## Observational astrophysics

Most astrophysical processes cannot be reproduced in laboratories on Earth. However, there is a huge variety of astronomical objects visible all over the electromagnetic spectrum. The study of these objects through passive collection of data is the goal of observational astrophysics.

The equipment and techniques required to study an astrophysical phenomenon can vary widely. Many astrophysical phenomena that are of current interest can only be studied by using very advanced technology and were simply not known until very recently.

The majority of astrophysical observations are made using the electromagnetic spectrum.

Other than electromagnetic radiation, few things may be observed from the Earth that originate from great distances. A few gravitational wave observatories have been constructed, but gravitational waves are extremely difficult to detect. Neutrino observatories have also been built, primarily to study our Sun. Cosmic rays consisting of very high energy particles can be observed hitting the Earth's atmosphere.

Observations can also vary in their time scale. Most optical observations take minutes to hours, so phenomena that change faster than this cannot readily be observed. 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) or combine years of data (pulsar deceleration studies). The information obtained from these different timescales is very different.

The study of our own Sun has a special place in observational astrophysics. Due to the tremendous distance of all other stars, the Sun can be observed in a kind of detail unparalleled by any other star. Our understanding of our own sun serves as a guide to our understanding of other stars.

The topic of how stars change, or stellar evolution, is often modelled by placing the varieties of star types in their respective positions on the Hertzsprung-Russell diagram, which can be viewed as representing the state of a stellar object, from birth to destruction. The material composition of the astronomical objects can often be examined using:

## Theoretical astrophysics

Main article: Theoretical astrophysics

Theoretical astrophysicists create and evaluate models to reproduce and properly predict observations. They use a wide variety of tools which include analytical models (for example, polytropes to approximate the behaviors of a star) and computational numerical simulations.

A few examples of this process:

 Physical process Experimental tool Theoretical model Explains/predicts Gravitation Radio telescopes Self-gravitating system Emergence of a star system Nuclear fusion Spectroscopy Stellar evolution How the stars shine The Big Bang Hubble Space Telescope, COBE Expanding universe Age of the Universe Quantum fluctuations Cosmic inflation Flatness problem Gravitational collapse X-ray astronomy General relativity Black holes at the center of Andromeda galaxy CNO cycle in stars

Dark matter and dark energy are the current leading topics in astrophysics, as their discovery and controversy originated during the study of the galaxies.

## Astrodynamics

Main article: Astrodynamics

Astrodynamics is the branch of celestial mechanics concerned with the motion of rockets, satellites and missiles. It is based upon Newton's laws of motion, and law of universal gravitation. The formula for escape velocity is defined in astrodynamics as:

${\displaystyle v\geq {\sqrt {2GM/r}}}$

Astrodynamics is also used to compute the position of a satellite at a given time, a problem first solved by Johannes Kepler, who computed the formula:

${\displaystyle MT=E-e\sin E\;}$

This formula is commonly referred to as Kepler's equation, and can compute the time required for a satellite to travel from periapsis P to a given point S.

Modern techniques for computing time-of-flight include the patched conic approximation, where one must choose the one dominant gravitating body in each region of space through which the trajectory will pass, and to model only that body's effects in that region, or the universal variable formulation.

## Astrophysicists

Main article: List of astrophysicists

## References

Herman Roth, "A Slowly Contracting or Expanding Fluid Sphere and its Stability" Phys. Rev. 39, 525–529 (1932) [Issue 3 – 1 February 1932 ]