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| A giant Hubble
mosaic of the Crab Nebula, a supernova remnant. |
Astronomy
Astronomy is a natural science. It is the study of
everything outside the atmosphere of Earth.
It studies celestial objects (such as stars, galaxies,
planets, moons, asteroids, comets and nebulae) and
processes (such as supernovae explosions, gamma ray
bursts, and cosmic microwave background radiation). This
includes the physics, chemistry of those objects and
processes.
A related subject, physical cosmology, is concerned with
studying the Universe as a whole, and the way the
universe changed over time.
The word astronomy comes from the Greek words astron
which means star and nomos which means law. A person who
studies astronomy is called an astronomer. |
 |
| A celestial map from
the 17th century, by the Dutch cartographer
Frederik de Wit. |
Astronomy is one of the oldest sciences. Ancient people
used the positions of the stars to navigate, and to find
when was the best time to plant crops. Astronomy is very
similar to astrophysics. Since the 20th century there
have been two main types of astronomy, observational and
theoretical astronomy. Observational astronomy uses
telescopes and cameras to observe or look at stars,
galaxies and other astronomical objects. Theoretical
astronomy uses maths and computer models to predict what
should happen. The two often work together, the
theoretical predicts what should happen and the
observational shows whether the prediction works.
Astronomy is not the same as astrology, the belief that
the patterns the stars and the planets may affect human
lives. |
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 |
| The
Suryaprajnaptisūtra, a 6th-century BC astronomy
text of Jains at The Schoyen Collection, London.
Above: its manuscript from c.1500 AD. |
History of astronomy
Ancient
Early astronomers used only their eyes to look at the
stars. They used maps of the constellations and stars
for religious reasons and also to work out the time of
year. Early civilizations such as the Maya people and
the Ancient Egyptians built simple observatories and
drew maps of the stars positions. They also began to
think about the place of Earth in the universe. For a
long time people thought Earth was the center of the
universe, and that the planets, the stars and the sun
went around it. This is known as the geocentric model of
the Universe.
Ancient Greeks tried to explain the motions of the sun
and stars by taking measurements. A mathematician named
Eratosthenes was the first who measured the size of the
Earth and proved that the Earth is a sphere. A theory by
another mathematician named Aristarchus was, that the
sun is in the center and the Earth is moving around it.
This is known as the Heliocentric model. Only a small
group of people thought it was right. The rest continued
to believe in the geocentric model.
Most of the names of constellations and stars that we
have, come from Greeks of that time.
Arabic astronomers made many advancements during the
Middle Ages including improved star maps and ways to
estimate the size of the Earth. |
 |
| Portrait of Newton
at 46 by Godfrey Kneller, 1689. |
Renaissance to modern era
During the renaissance a priest named Nicolaus
Copernicus thought, from looking at the way the planets
moved, that the Earth was not the center of everything.
Based on previous works, he said that the Earth was a
planet and all the planets moved around the sun. This
heliocentrism was an old idea. A physicist called
Galileo Galilei built his own telescopes, and used them
to look more closely at the stars and planets for the
first time. He agreed with Copernicus. Their ideas were
also improved by Johannes Kepler and Isaac Newton who
invented the theory of gravity. At this time the
Catholic Church decided that Galileo was wrong. He had
to spend the rest of his life under house arrest.
After Galileo, people made better telescopes and used
them to see farther objects such as the planets Uranus
and Neptune. They also saw how stars were similar to our
Sun, but in a range of colors and sizes. They also saw
thousands of other faraway objects such as galaxies and
nebulae. |
 |
| The Very Large Array
in New Mexico, an example of a radio telescope. |
Modern era
The 20th century saw important changes in astronomy.
In 1931, Karl Jansky discovered radio emission from
outside the Earth when trying to isolate a source of
noise in radio communications, marking the birth of
radio astronomy and the first attempts at using another
part of the electromagnetic spectrum to observe the sky.
Those parts of the electromagnetic spectrum that the
atmosphere did not block were now opened up to
astronomy, allowing more discoveries to be made.
The opening of this new window on the Universe saw the
discovery of entirely new things, for example pulsars,
which sent regular pulses of radio waves out into space.
The waves were first thought to be alien in origin
because the pulses were so regular that it implied an
artificial source. |
 |
| The Subaru Telescope
(left) and Keck Observatory (center) on Mauna
Kea, both examples of an observatory that
operates at near-infrared and visible
wavelengths. The NASA Infrared Telescope
Facility (right) is an example of a telescope
that operates only at near-infrared wavelengths. |
The period after World War 2 saw more observatories
where large and accurate telescopes are built and
operated at good observing sites, normally by
governments. For example, Bernard Lovell began radio
astronomy at Jodrell Bank using leftover military radar
equipment. By 1957, the site had the largest steerable
radio telescope in the world. Similarly, the end of the
1960s saw the start of the building of dedicated
observatories at Mauna Kea in Hawaii, a good site for
visible and infra-red telescopes thanks to its high
altitude and clear skies. Mauna Kea would eventually
come to host very large and very accurate telescopes
like the Keck Observatory with its 10-meter mirror.
The next great revolution in astronomy was thanks to the
birth of rocketry. This allowed telescopes to be placed
in space on satellites.
Satellite-based telescopes opened up the Universe to
human eyes. Turbulence in the Earth's atmosphere blurs
images taken by ground-based telescopes, an effect known
as seeing. It is this effect that makes stars "twinkle"
in the sky. As a result, the pictures taken by satellite
telescopes in visible light (for example, by the Hubble
Space Telescope) are much clearer than Earth-based
telescopes, even though Earth-based telescopes are very
large.
Space telescopes gave access, for the first time in
history, to the entire electromagnetic spectrum
including rays that had been blocked by the atmosphere.
The X-rays, gamma rays, ultraviolet light and parts of
the infra-red spectrum were all opened to astronomy as
observing telescopes were launched. As with other parts
of the spectrum, new discoveries were made.
From 1970s satellites were launched to be replaced with
more accurate and better satellites, causing the sky to
be mapped in nearly all parts of the electromagnetic
spectrum. |
|
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| The Milky Way as
viewed from La Silla Observatory. |
Discoveries
Discoveries broadly come in two types: bodies and
phenomena. Bodies are things in the Universe, whether it
is a planet like our Earth or a galaxy like our Milky
Way. Phenomena are events and happenings in the
Universe.
Bodies
For convenience, this section has been divided by where
these astronomical bodies may be found: those found
around stars are solar bodies, those inside galaxies are
galactic bodies and everything else larger are cosmic
bodies.
Solar System |
- Sun
- Planets
- Asteroids
- Comets
|
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Galactic |
- Stars
- Diffuse Objects:
- Nebulas
- Clusters
- Compact Stars:
- White dwarves
- Neutron stars
- Black holes
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Cosmic |
- Galaxies
- Galaxy clusters
- Superclusters
|
Phenomena
Burst events are those where there is a sudden change in
the heavens that disappears quickly. These are called
bursts because they are normally associated with large
explosions producing a "burst" of energy. They include: |
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Periodic events are those that happen regularly in a
repetitive way. The name periodic comes from period,
which is the length of time required for a wave to
complete one cycle. Periodic phenomena include: |
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Noise phenomena tend to relate to things that happened a
long time ago. The signal from these events bounce
around the Universe until it seems to come from
everywhere and varies little in intensity. In this way,
it resembles "noise", the background signal that
pervades every instrument used for astronomy. The most
common example of noise is static seen on analogue
televisions. The principal astronomical example is:
Cosmic background radiation.
Methods
Instruments |
- Telescopes are the main tool of
observing. They take all the light in a big area and put
in into a small area. This is like making your eyes very
big and powerful. Astronomers use telescopes to look at
things that are far away and dim. Telescopes make
objects look bigger, closer, brighter.
- Spectrometers study the different
wavelengths of light. This shows what something is made
of.
- Many telescopes are in satellites.
They are space observatories.
|
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| Amateur astronomers
can build their own equipment, and hold star
parties and gatherings, such as Stellafane. |
Techniques
There are ways astronomers can get better pictures of the
heavens. Light from a distant source reaches a sensor and
gets measured, normally by a human eye or a camera. For very
dim sources, there may not be enough light particles coming
from the source for it to be seen. One technique that
astronomers have for making it visible is using integration,
(which is like longer exposures in photography).
Integration
Astronomical sources do not move much: only the rotation and
movement of the Earth causes them to move across the
heavens. As light particles reach the camera over time, they
hit the same place making it brighter and more visible than
the background, until it can be seen.
Telescopes at most observatories (and satellite instruments)
can normally track a source as it moves across the heavens,
making the star appear still to the telescope and allowing
longer exposures. Also, images can be taken on different
nights so exposures span hours, days or even months. In the
digital era, digitised pictures of the sky can be added
together by computer, which overlays the images after
correcting for movement.
Aperture synthesis
With radio telescopes smaller telescopes can be combined
together to create a big one, which works like one as big as
the distance between the two smaller telescopes.
Adaptive optics
Adaptive optics means changing the shape of the mirror or
lens while looking at something, to see it better.
Data analysis
Data analysis is the process of getting more information out
of an astronomical observation than by simply looking at it.
The observation is first stored as data. This data will then
have various techniques used to analyse it.
Fourier analysis
Fourier analysis in mathematics can show if an observation
(over a length of time) is changing periodically (changes
like a wave). If so, it can extract the frequencies and the
type of wave pattern, and find many things including new
planets.
Fields
A good example of a fields comes from pulsars which
pulse regularly in radio waves. These turned out to be
similar to some (but not all) of a type of bright source
in X-rays called a Low-mass X-ray binary. It turned out
that all pulsars and some LMXBs are neutron stars and
that the differences were due to the environment in
which the neutron star was found. Those LMXBs that were
not neutron stars turned out to be black holes. |
|
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| Star cluster Pismis
24 with a nebula. |
Fields in astronomy
This section attempts to provide an overview of the
important fields of astronomy, their period of
importance and the terms used to describe them. It
should be noted that astronomy in the Modern Era has
been divided mainly by electromagnetic spectrum,
although there is some evidence this is changing.
Fields by body
Solar astronomy
Solar astronomy is the study of the Sun. The Sun is the
closest star to Earth at around 92 million (92,000,000)
miles away. It is the easiest to observe in detail.
Observing the Sun can help us understand how other stars
work and are formed. Changes in the Sun can affect the
weather and climate on Earth. A stream of charged
particles called the Solar wind is constantly sent off
from the Sun. The Solar Wind hitting the Earth's
magnetic field causes the northern lights. Studying the
Sun helped people understand how nuclear fusion works.
Planetary astronomy
Planetary Astronomy is the study of planets, moons,
dwarf planets, comets and asteroids as well as other
small objects that orbit stars. The planets of our own
Solar System have been studied in depth by many visiting
spacecraft such as Cassini-Huygens (Saturn) and the
Voyager 1 and 2.
Galactic astronomy
Galactic Astronomy is the study of distant galaxies.
Studying distant galaxies is the best way of learning
about our own galaxy, as the gases and stars in our own
galaxy make it difficult to observe. Galactic
Astronomers attempt to understand the structure of
galaxies and how they are formed through the use of
different types of telescopes and computer simulations.
Fields by electromagnetic
spectrum
Radio astronomy
Radio telescope is used.
Magnetohydrodynamics (MHD)
Hydrodynamics is used in astronomy 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. This makes MHD
an important and useful tool in astronomy.
Other fields
Gravitational wave astronomy
Gravitational wave astronomy is the study of the
Universe in the gravitational wave spectrum. So far, all
astronomy that has been done has used the
electromagnetic spectrum. Gravitational Waves are
ripples in spacetime emitted by very dense objects
changing shape, which include white dwarves, neutron
stars and black holes. Because no one has been able to
detect gravitational waves directly, the impact of
Gravitational Wave Astronomy has been very limited. |
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