Astronomy

Astronomy is the study of outer space and everything in it. This includes stars, planets and galaxies as well as other things. 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.

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 make affect human lives.

History of Astronomy

Main page: History of astronomy

Ancient

Early astronomers used only their eyes to look at the stars. They used maps of the stars for religious reasons and also to work out the time of year. Early civilisations 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.

Arabic astronomers made many advancements during the Middle Ages including improved star maps and ways to estimate the size of the Earth.

Copernicus, Galileo and the Heliocentric Model

Some drawings of the Moon by Galileo. His drawings were more detailed than anyone before him because he used a telescope to look at the Moon.

During the renaissance an astronomer named Nicolaus Copernicus thought of a new idea. He thought, from looking at the way the planets moved, that the Earth was not the center of the Solar System. He said, and he was right, that the Sun was at the centre of the Solar System and all the planets including Earth moved around it. This is known as a Heliocentric model. Another astronomer called Galileo Galilei built his own telescope, and used it to look more closely at the stars and planets for the first time. What he saw backed up Copernicus' idea. Their ideas were also improved by Johannes Kepler and Isaac Newton who came up with the theory of gravity. At this time the Christian church decided that Galileo was wrong. The Pope gave the order to lock Galileo up in his house and they did not let him write any more books until he died.

Renaissance to Modern Era

After Galileo, people used telescopes more often and began to see farther-away objects such as the planets Uranus and Neptune. They also saw how stars were similar to our Sun, but in a range of colours and sizes. They also saw thousands of other faraway objects such as galaxies and nebulae.

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 period after World War 2 saw the rise of dedicated observatories, where large and accurate telescopes are built and operated at good observing sites, normally by governments. For example, radio astronomy started out of leftover military equipment at Jodrell Bank after the war. 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 (such as those that beam satellite television).

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.

By placing a telescope in orbit above the atmosphere, astronomers had access, for the first time in history, to the entire electromagnetic spectrum including those 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.

The period from 1970s onwards can be generalised that 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.

Future Plans

Making new progress in astronomy is becoming more limited by the observatories and facilities being used at this time. To make up for these limitations, astronomers have proposed larger and more ambitious projects. These proposals, some of which are being examined already by governments to see if they will be funded, allow us to glimpse into what the astronomers are planning for the future of astronomy.

Discoveries

This section summarises all the discoveries that Astronomy has made.

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

• Planets
• Asteroids
• Comets

Galactic

Stars

Diffuse Objects:

• Nebulas
• Clusters

Compact Stars:

• White dwarves
• Neutron stars
• Black holes

Cosmic

• Galaxies
• Clusters
• Superclusters.

Phenomena

Phenomena are those discoveries related to events and happenings in the Universe.

Broadly, events and happenings in the Universe can be divided into three types: burst, periodic and noise.

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.

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.

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.

Burst

• Supernovas
• Novas

Periodic

• Pulsars
• Variable stars

Noise

Cosmic background.

Methodology

This section tries to detail how astronomy is done.

Instrumentation

Much of astronomy is done through observation of the heavens using instruments. This section attempts to describe the various instruments used by astronomers, how they work and how they are used.

• Telescopes: Telescopes 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 are not very bright. Telescopes make objects look bigger and closer.
• Spectrometers
• Satellites

Techniques

These are a list of some of the common ways in which astronomers can use their equipment to get better pictures of the heavens.

Integration

Light from a distant source reaches a telescope or satellite 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 identical to longer exposures in photography).

When a normal picture is taken with a camera, the film or CCD plate is exposed to the light for a short amount of time, which is normally useful because a short time means that whatever we are taking a picture of will not have moved much. Astronomical sources do not move much (only the rotation and movement of the Earth causes them to move across the heavens) the film or CCD plate can be left exposed for longer than an instant. As light particles reach the camera over time, they hit the same place making it brighter and more visible then the background, until it can be seen.

Short exposures are easy. However, longer ones are affected by the Earth's rotation, causing a star wheel effect. 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

Smaller telescopes can be combined together to create a larger one which is effectively as large as the distance between the two smaller telescopes. This technique was first pioneered in radio astronomy.

Adaptive Optics

Adaptive optics is when the actual shape of the mirror or lens changes to better view the objects you are looking at with the telescope.

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 applied to it (to analyse it), out of which we may get an answer.

Fourier Analysis

Fourier analysis in astronomy uses the techniques of Fourier analysis in mathematics to see if an observation (over a length of time) is changing periodically (changes like a wave). If so, it can extract any and all frequencies and the type of wave pattern as well.

Fields

Astronomy is a very large subject with many different things to study, and many astronomers specialise in particular areas of astronomy, called fields. Fields are often themselves further sub-divided into sub-fields and so on.

Astronomers assign fields based on what they perceive to be the largest differences between different bodies and phenomena. However, as astronomers learn more about the Universe, they often notice that things which they thought were different at first turn out to be similar and things which were similar turn out to be different. As a result, fields in astronomy are a constantly changing.

A good example of changing 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.

Because of its changing nature, the organisation of astronomy can be confusing, even to astronomers, and there are also many terms left over from previous fields that astronomers use which have both modern and older meanings.

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.

For convenience, the fields are grouped by the major difference that inspired them.

Fields by Body

Solar Astronomy

Main page: Sun

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 so 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 effect 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

See Radio telescope

Theoretical Fields

MagnetoHydroDynamics

MagnetoHydroDynamics, called MHD for short, is Hydrodynamics done in the presence of magnetic fields, where Hydrodynamics is the mathematical modelling of fluid flows. Hydrodynamics is already useful in astronomy for mathematically modelling how gases behave (as gases behave like fluids). 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, making MHD a very 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.

Related pages

• Solar system
• Planet
• Satellite (natural) (word for moons of other planets)
• Comet
• Meteor
• Asteroid
• Star
• Black hole
• Galaxy
• Universe
• List of comets

Other websites

• Astronomy site specifically designed for kids and their parents.
• Astronomy Picture of the Day
• Astronomy Around the World
• Astronomy with sections for beginners and younger people