Astronomy and astrophysics

Astronomy, which etymologically means laws of the stars, is the science whose subject is the observation and explanation of events outside Earth. Astrophysics is the part of astronomy (and physics) that deals with the application of physics to the phenomena observed by astronomy.

Nearly all astronomers now have a strong background in physics, and the results of observations are usually put in an astrophysical context, so astronomy and astrophysics are used with very close meaning. Astronomy is one of the few sciences where amateurs still play an active role, especially in the discovery and monitoring of transient phenomena.

Astronomy is not to be confused with astrology, a pseudoscience which attempts to predict a person’s destiny by tracking the paths of astronomical objects. Although the two fields share a common origin, they are quite different; astronomy embraces the scientific method, while astrology does not.

Divisions of Astronomy

Given its huge scope, astronomy is divided into different branches. The
divisions are not unique, however, and the intersections, as well as
astronomers who work in several areas, are the rule more than the exception.

A first distinction is between theoretical and observational astronomy and
astrophysics. Observers use a variety of means to obtain data about
different phenomena, data that is then used by theorists to create and
constrain theories and models, to explain observations and to predict new
ones. Fields of study are also categorized in another two main ways: by
subject, usually according to the region of space (e.g. Galactic astronomy)
or problems addressed (such as star formation or cosmology); and according
to the means of obtaining the data (e.g. optical astronomy or
radioastronomy)

By way of obtaining information

In astronomy, the main way of obtaining information is through the detection
and analysis of electromagnetic radiation, photons, but we also receive
information from outside the earth carried by cosmic rays, neutrinos, and,
in the near future, gravitational waves (see LIGO and LISA).

A traditional division of astronomy is given by the region of the electromagnetic spectrum observed:

• Optical astronomy refers to the techniques used to detect and analyze
  light in and slightly around the wavelengths than can be detected with
  the eyes (about 400 – 800 nm). The most common tool is the telescope,
  with electronic imagers and spectrographs.
• Infrared astronomy deals with using infrared radiation (wavelengths
  longer than the red light). Again, the most common tool is the
  telescope, but with instruments sensitive to longer wavelengths; the
  telescope itself can be optimized for infrared. Space telescopes are
  also used to eliminate noise ( electromagnetic interference) from the
  atmosphere.
• Radio astronomy uses completely different instruments to detect
  radiation of wavelengths of mm to cm. The receivers are similar to
  those used in radio broadcast transmission (which uses those
  wavelengths of radiation). See also Radio telescopes.
• High-energy astronomy

Optical and radio astronomy can be done using ground-based observatories,
because the atmosphere is transparent at those wavelengths. Infrared light
is heavily absorbed by water vapor, so infrared observatories have to be
located in high, dry places or in space.

The atmosphere is opaque at the wavelengths used by X-ray astronomy,
gamma-ray astronomy, UV astronomy and, except for a few wavelength
“windows”, Far infrared astronomy , and so observations can be carried out
only from balloons or space observatories.

Short History

The Bible has a number of statements on the position of the earth in the
universe, the designation of the stars, planets, fixed stars. A separate
article exists on Astronomy in the Bible.

In the early part of its history, astronomy involved only the observation
and predictions of the motions of the objects in the sky that could be seen
with the naked eye. The Rigveda refers to the 27 constellations associated
with the motions of the sun and also the 12 zodiacal divisions of the sky.

The ancient Greeks made many important contributions to astronomy, among
them the definition of the magnitude system. In 500 AD, Aryabhata presented
a mathematical system that took the earth to spin on its axis and considered
the motions of the planets with respect to the sun.

The study of astronomy almost stopped during the middle ages, except for the
work of some Arabic astronomers. The renaissance came to astronomy with the
work of Copernicus, who proposed a heliocentric model of the Solar System.
His work was defended, expanded upon, and corrected by the likes of Galileo
Galilei and Johannes Kepler. The latter of these was the first to provide a
system which described correctly the details of the motion of the planets
with the Sun at the center.

However, Kepler did not understand the reasons behind the laws he wrote down. It was left to Newton’s invention of celestial dynamics and his law of gravitation, the final explanation of the motions of the planets. Astrophysics was a later development, which only became possible once it was understood that the elements that made up the “celestial objects” were the same that made up the Earth, and that the same laws of physics applied.

Stars were found to be far away objects, and with the advent of spectroscopy
it was proved that they were similar to our own sun, but with a wide range
of temperatures, masses and sizes. The existence of our galaxy, the Milky
Way, as a separate group of stars was only proven in the 20th century, along
with the existence of “external” galaxies, and soon after, the expansion of
the universe seen in the recession of most galaxies from us.

Cosmology, a discipline that has a large intersection with astronomy, made huge advances during the 20th century, with the model of the hot big bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation, Hubble’s Law and cosmological
abundances of elements.