Astronomy

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NCG 602 in the Small Magellanic Cloud

Astronomy[1] is the study of all aspects of the objects which populate the universe: their formation, structure, motion, and evolution. This includes the universe as a whole, and astronomy tries to answer questions including those about the origin of the universe, and the existence of life outside Earth.

Astronomy is one of the oldest of the sciences, as the study of the sky has fascinated all peoples throughout history, both because of its practical value for activities such as agriculture and navigation, and because of its inherent beauty, which was often associated with religious beliefs (e.g., heaven). The coexistence of practical and mystical aspects led to the belief that celestial events might mysteriously affect human life, and astrology was an integral part of ancient astronomy.

The link with astrology and mysticism in general was severed only in the 17th century, after the adoption of the scientific method transformed astronomy into a formal science.[2] As such, modern astronomy incorporates parts of physics, mathematics, chemistry, and even biology. Among these, physics is now so fundamental that astrophysics is often used as a synonym for astronomy.

History

For more information, see: History of Astronomy and archaeoastronomy.

Until the invention of the telescope (around 1608), astronomy only comprised the observation and predictions of the motions of the objects which could be observed with the naked eye. In some locations, such as Stonehenge, early cultures assembled massive artifacts that likely had astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as the length of the year.[3]

As civilizations developed, most notably the Chaldeans, Egypt, ancient Greece, India, and China, astronomical observatories were assembled and ideas on the nature of the universe began to be explored. Early ideas on the motions of the planets were developed, and the nature of the sun, moon and the earth in the universe were explored philosophically. These included speculations on the spherical nature of the earth and moon, and the rotation and movement of the earth through the heavens.

A few notable astronomical discoveries were made prior to the application of the telescope. For example, the obliquity of the ecliptic was estimated as early as 1,000 B.C by the Chinese. The Chaldeans discovered that eclipses recurred in a repeating cycle known as a saros. In the 2nd century B.C., the size and distance of the moon were estimated by Hipparchus, who also authored the first stellar catalog.

During the Middle Ages, observational astronomy was mostly stagnant in Europe until the 15th century. However, observational astronomy flourished in the regions conquered by the Arabs (from Persia to Spain). Arab astronomers introduced many names that are now used for individual stars.[4][5]

Scientific revolution

During the Renaissance, Copernicus proposed a heliocentric model of the Solar System. His work was defended, expanded upon, and corrected by Galileo and Kepler: the former started using the telescopes he built to enhance his observations, the latter was the first to describe correctly the motion of the planets in a system with the sun at the center[6].

However, Kepler did not succeed in formulating a theory behind the empirical laws he wrote down. It was left to Newton's Laws of Motion and his law of gravitation to predict the laws discovered by Kepler. Newton also developed the reflecting telescope.

During the 18th century, attention to the theoretical three-body problem by Euler, Clairaut and D'Alembert led to more accurate predictions about the motions of the moon and planets. This work was further refined by Lagrange and Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.

Further observational discoveries paralleled the improvements in size and quality of the telescope. More extensive star calatogues were produced by Lacaille. The astronomer William Herschel made an extensive catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found; he also found out that the Milky Way is a system of stars with the shape of a disk. The distance to a star was first measured in 1838, when the parallax of 61 Cygni was measured by Friedrich Bessel.

In the 19th century the introduction of new technologies (including spectroscopy and photography) brought even more advancements. For example, in 1814-15 Fraunhofer discovered about 600 bands in the spectrum of the sun, which were later (1859) ascribed to the presence of different chemical elements by Kirchhoff. Stars were proven to be similar to Earth's own sun, but with a wide range of temperatures, masses, and sizes.[4]

The existence of galaxies external to the Milky Way was only proved in the early 20th century; soon after, the expansion of the universe was discovered through measurements of the recession velocities of galaxies.

Thanks to the new instrumentation (e.g. radiotelescopes or artificial satellites) allowing the observation of the whole electromagnetic spectrum, modern astronomy has also discovered many exotic objects such as active galaxies (radio galaxies, quasars, blazars) and pulsars. Observations have been used to develop physical theories which describe some of these objects in terms of objects such as black holes and neutron stars.

Physical cosmology made huge advances during the 20th century, with the model of the Big Bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation,[7] Hubble's law, and cosmological abundances of elements.

Astronomical observations

For more information, see: Observational astronomy.


Astronomy is different from most other sciences because it is impossible to manipulate and conduct experiments upon the objects which are being investigated. Astronomers can only observe the objects they are studying, gathering as much information as possible. This is mainly accomplished by detecting and analysing the various form of electromagnetic radiation which reaches the observer, although methods of data collection which do not involve radiation are currently becoming available.

The electromagnetic spectrum

A traditional division of astronomy derives from the region of the electromagnetic spectrum which is observed (and, in practice, by the type of instrument which is used for the observation).

Radio and microwave astronomy

For more information, see: Radio astronomy.


At the low frequency end of the spectrum, radio astronomy detects radiation of wavelength larger than about 0.3 millimetres (corresponding to a frequency of 1000 Gigahertz)[8]. Except for their dimensions and sensitivity, the radio telescope receivers are similar to those used in radio broadcast transmission. Single radio telescopes typically have a high spectral resolution and a low spatial resolution; however, when combined through interferometry, radio telescopes can reach spatial resolutions of about 0.001 arc seconds (much better than any other existing telescope).

Since the radio emission from stars is usually weak, the most intense radio sources are objects with strong magnetic fields (such as pulsars and radio galaxies) emitting synchrotron radiation; radio observations can also detect diffuse interstellar gas such as neutral hydrogen (HI), which is important also for the study of the dynamics of galaxies. Numerous molecules have been detected through radio astronomical techniques (e.g. H2, OH, CO, ammonia, and water).

Microwaves (at the high frequency end of the radio spectrum) are particularly important for the study of cosmology, as this is the band where the cosmic microwave background radiation is more intense. In the radio band atmospheric absorption is usually not a problem, but microwave wavelengths are an exception, and sensitive microwave observatories are often placed on artificial satellites, or taken to high altitudes by research balloons.

Infrared astronomy

For more information, see: Infrared astronomy.


Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths between those of red light and of microwaves, i.e. between about 0.8 and 300 microns).

The most common observational tool for infrared radiation is a "normal" (optical) telescope, but a detector which is specifically sensitive to the infrared is a strong requirement. Furthermore, measures must be taken in order to prevent the thermal radiation produced by the telescope itself from swamping the astronomical signal.

Infrared radiation is heavily absorbed by atmospheric water vapor, so infrared observatories have to be located in high, dry places[9], on planes, balloons, or in outer space. Space telescopes also avoid the effects of atmospheric thermal emission, atmospheric opacity, and the negative effects of astronomical seeing; however, the life span of infrared space missions is relatively short (2-5 years).

The infrared radiation can cross regions rich of gas and dust more easily than the optical one; furthermore, dust grains re-radiate in the infrared the energy they absorb in the optical and ultraviolet bands. So, infrared observations are particularly useful for studying heavily obscured patches such as the galactic centre, regions of intense star formation, or dust disks surrounding protostars, where planets are believed to be forming. Observations in the infrared band are also crucial for cosmology, because the bulk of the radiation emitted at early cosmological times has been "redshifted" to infrared wavelengths.


Optical astronomy

For more information, see: Optical astronomy.
The Pleiades, also known as The Seven Sisters

Historically, most astronomical data has been collected through optical astronomy, which observes visible light (i.e. light which can be seen with human eyes, at wavelengths between 360 and 800 nm). Optical telescopes, built out of combinations of lenses and/or mirrors are usually equipped with CCD detectors, or photografic films.

The optical band is particularly remarkable because the atmosphere is quite transparent to this kind of light, and because normal stars (e.g. the sun itself) emit the bulk of their luminosity at such wavelengths, although most other sources can usually be observed also in visible light.

Ultraviolet astronomy

For more information, see: Ultraviolet astronomy.


Ultraviolet astronomy deals with photons with wavelengths between 10 and 360 nm, which can still be focused with optical components similar to those found in normal telescopes, and can be collected through CCD detectors; like infrared telescopes, ultraviolet telescopes are quite similar to optical ones.

With the exception of those at near ultraviolet wavelengths (the part of the spectrum which is closest to visible light, where the atmospheric absorption is strong but not complete), the ultraviolet photons are totally absorbed by the Earth's atmosphere; for this reason, almost all ultraviolet observations are carried out by space telescopes.

The ultraviolet band is particularly indicated in order to observe young massive stars (whose high surface temperatures imply that the peak of their emission is in this band), and to look for spectral lines which give precious information about the physical conditions (chemical composition, density, temperature) of the interstellar medium.

High energy astronomy

For more information, see: X-ray astronomy and gamma ray astronomy.


Photons with wavelengths shorter than about 10 nm (or, almost equivalently, with an energy larger than 100 eV) are considered of high energy; they are further sub-divided inro X-rays (energies below about 100 keV[10]) and gamma rays.

High energy photons can be focused only if their energy is lower than a few keV (soft X-rays), through grazing incidence optics; X-ray telescopes built for soft X-rays can then achieve high sensitivities and good spatial resolutions. The situation rapidly deteriorates at energies larger than 5 keV, and above 10 keV no focusing is possible with current technologies. As a result, hard X-ray and gamma ray instruments have bad spatial resolution (some arc minutes), despite recent enhancements due to the introduction of aperture masks. All these kind of telescopes must be placed outside Earth's atmosphere, as the atmosphere is completely opaque to high energy photons. However, photons of extremely high energies (0.1-10 TeV) can be detected from the ground by looking at the large air showers they produce when they hit the atmosphere[11].

In general, high energy observations are important for studying the so-called "compact objects", such as white dwarfs, neutron stars and black holes including the supermassive ones which are believed to be the engines of active galactic nuclei. But there are a number of other sources which can emit high energy photons, from relatively normal stars to explosive phenomena (supernovae, gamma ray bursts), to clusters of galaxies.

Data acquisition through non-electromagnetic channels

There exist several ways of gathering astronomical information without collecting photons; some of them are being vigorously developed, as they promise to give totally new insight on astronomical objects.

Here is a brief list of all "non-electromagnetic" data collection techniques which have provided astronomical data[12][13]:

  • The oldest of all these methods is the collection and examination of meteorites.
  • The detection of charged particles (protons, alpha particles, electrons etc.) started in the early 20th century with the first observations of cosmic rays.
  • In the 1960s neutrinos coming from the sun were detected; a second astronomical source whose neutrino emission was observed was supernova 1987A. Prospects for the observation of neutrinos from several kinds of astronomical sources are rapidly improving, as several experiments are coming online in the next decade.
  • Since the late 1960s, our knowledge of the solar system has benefited from space missions which directly explored celestial bodies. These include fly-by missions with remote sensors, landing vehicles that can perform experiments on the surface materials, impactors that allow remote sensing of buried materials, and sample return missions that allow direct laboratory examination.
  • Several experiments are expected to detect gravitational waves in the near future.

Astrometry and celestial mechanics

For more information, see: Astrometry and Celestial mechanics.


One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects in the sky. Historically, accurate knowledge of the positions of the sun, moon, planets and stars has been essential in celestial navigation.

Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. More recently the tracking of near-Earth objects will allow for predictions of close encounters, and potential collisions, with the Earth.[14]

The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the universe. Parallax measurements of nearby stars provides an absolute baseline for the properties of more distant stars, because their properties can be compared. Measurements of radial velocity and proper motion show the kinematics of these systems through the Milky Way galaxy. Astrometric results are also used to measure the distribution of dark matter in the galaxy.[15]

During the 1990s, the astrometric technique of measuring the stellar wobble led to the discovery of large extrasolar planets orbiting nearby stars.[16]

Interdisciplinary studies

Astronomy has developed significant interdisciplinary links with other major scientific fields. These include:

Astronomical objects

Solar astronomy

For more information, see: Sun.

The most frequently studied star is the sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 Gyr in age. The sun is not considered a variable star, but it does undergo periodic changes in activity known as the sunspot cycle. This is an 11-year fluctuation in sunspot numbers. Sunspots are regions of lower than average temperature that are associated with intense magnetic activity.[17]

The sun has steadily increased in luminosity over the course of its life, increasing by 40% since it first became a main-sequence star. The sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth. The Maunder minimum, for example, is believed to have caused the Little Ice Age phenomenon during the Middle Ages.[18]

The visible outer surface of the sun is called the photosphere. Above this layer is a thin region known as the chromosphere. This is surrounded by a transition region of rapidly increasing temperatures, then by the super-heated corona.

At the center of the sun is the core region, a volume of sufficient temperature and pressure for nuclear fusion to occur. Above the core is the radiation zone, where the plasma conveys the energy flux by means of radiation. The outer layers form a convection zone where the gas material transports energy primarily through physical displacement of the gas. It is believed that this convection zone creates the magnetic activity that generates sun spots.[17]

A solar wind of plasma particles constantly streams outward from the sun until it reaches the heliopause. This solar wind interacts with the magnetosphere of the earth to create the Van Allen radiation belts, as well as the aurora where the lines of the Earth's magnetic field descend into the atmosphere.[19]

Planetary science

For more information, see: planetary science and planetary geology.

This astronomical field examines the assemblage of planets, moons, dwarf planets, comets, asteroids, and other bodies orbiting the sun, as well as extrasolar planets. The solar system has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of this planetary system, although many new discoveries are still being made.[20]

The solar system is subdivided into the inner planets, the asteroid belt, and the outer planets. The inner terrestrial planets consist of Mercury, Venus, Earth, and Mars. The outer gas giant planets are Jupiter, Saturn, Uranus and Neptune.[21]

The planets formed from a protoplanetary disk that surrounded the early sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that with time became protoplanets. The radiation pressure of the solar wind then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up or eject the remaining matter during a period of intense bombardment evidenced by the many impact craters on the moon. During this period some protoplanets may have collided, the leading hypothesis for how the moon was formed.[22]

Once a planet reaches sufficient mass, the materials with different densities segregate within its interior during planetary differentiation. This process can form a stony or metallic core surrounded by a mantle and outer surface. The core may include solid and liquid regions, and some planetary cores generate their own magnetic field, which can protect its atmosphere from solar wind stripping.[23]

A planet or moon's interior heat is produced from the collisions that created the body, radioactive materials (e.g. uranium, thorium, and 26Al), or tidal heating. Some planets and moons accumulate enough heat to drive geologic processes such as volcanism and tectonics. Those that accumulate or retain an atmosphere can also undergo surface erosion due to wind or water. Smaller bodies without tidal heating cool more quickly and their geological activity ceases with the exception of impact cratering.[24]

Comets and asteroids that travel in an orbit which takes them into the Earth's neighborhood are called Near Earth Objects.

Stellar astronomy

The Helix Nebula, NGC 7293
For more information, see: star and stellar astronomy.

The study of stars and stellar evolution is fundamental to our understanding of the universe. The astrophysics of stars has been determined through observation, theoretical understanding and from computer simulations of the interior.

Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. When destabilized, cloud fragments can collapse under the influence of gravity to form a protostar. A sufficiently dense and hot core region will trigger nuclear fusion and it becomes a main-sequence star.[25]

The characteristics of the resulting star depend primarily on its starting mass. The more massive the star, the greater its luminosity and the more rapidly it expends the hydrogen fuel in its core. Over time this hydrogen fuel is completely converted into helium and the star begins to evolve. Fusion of helium requires a higher core temperature, so the star both expands in size and increases in density at the core. The resulting red giant enjoys a brief life span before the helium fuel is in turn consumed. Very massive stars can also undergo a series of shorter and shorter evolutionary phases as they fuse increasingly heavier elements.

The final fate of the star depends on its mass, with stars of mass greater than 1.4 times the Sun becoming supernovae, while smaller stars will form planetary nebulae and evolve into white dwarfs. The remnant of a supernova is a dense neutron star, or, if the stellar mass was at least three times that of the sun, a black hole.[26]

Galactic astronomy

For more information, see: Galactic astronomy.


Our solar system orbits within the Milky Way, a barred spiral galaxy that is a prominent member of the Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.

In the center of the Milky Way is the core region, a bar-shaped bulge with what is believed to be a supermassive black hole at the center. This is surrounded by four primary arms that spiral out from the core. This is a region of active star formation that contains many younger, population II stars. The disk is surrounded by a spheroid halo of older, population I stars, as well as relatively dense concentrations of stars known as globular clusters.[27][28]

Between the stars lies the interstellar medium, a region of sparse matter. In the densest regions, molecular clouds of molecular hydrogen and other elements create star-forming regions. These begin as irregular dark nebulae, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars.[29]

As the more massive stars appear, they transform the cloud into an H II region of glowing gas and plasma. The stellar wind and supernova explosions from these stars eventually serve to disperse the cloud, often leaving behind one or more young open clusters of stars. These gradually disperse to join the population of stars in the Milky Way.

Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A dark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined.[30]

Galaxies and clusters

For more information, see: Extragalactic astronomy.

The study of objects outside our galaxy is a branch of astronomy concerned with the formation and evolution of galaxies, their morphology and classification, the examination of active galaxies and the groups and clusters of galaxies. The later is important for the understanding of the large-scale structure of the cosmos.

Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral, elliptical and irregular galaxies.[31]

As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The stars move along random orbits with no preferred direction. These galaxies contain little or no interstellar dust, few star-forming regions and generally older stars. Elliptical galaxies are more commonly found at the core of galactic clusters and may be formed through mergers of large galaxies.

A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation where massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the Milky Way and the Andromeda Galaxy are spiral galaxies.

Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical in form. About a quarter of all galaxies are irregular, and their peculiar shape may be the result of gravitational interaction.

An active galaxy is a formation that is emitting a significant amount of its energy from a source other than stars, dust and gas. They are powered by a compact region at the core, usually thought to be a supermassive black hole that is emitting radiation due to infalling material.

A radio galaxy is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit high-energy radiation include Seyfert galaxies, quasars, and blazars. Quasars are believed to be the most consistently luminous objects in the known universe.[32]

The large-scale structure of the cosmos is represented by groups and clusters of galaxies. This structure is organized in a hierarchy of groupings, with the largest being the superclusters. The collective matter is formed into filaments and walls, leaving large voids in between.[33]

Cosmology

For more information, see: physical cosmology and timeline of the Big Bang.

Observations of the large-scale structure of the universe, a branch known as physical cosmology, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the big bang, wherein our universe began at a single point in time and thereafter expanded over the course of 13.7 Gyr to its present condition. The concept of the big bang can be traced back to the discovery of the microwave background radiation in 1965.

In the course of this expansion, the universe underwent several evolutionary stages. In the very early moments, it is theorized that the universe underwent a very rapid cosmic inflation, which homogenized the starting conditions. Thereafter nucleosynthesis produced the elemental abundance of the early universe.

When the first atoms formed space became transparent to radiation; releasing the energy viewed today as the microwave background radiation. The expanding universe then underwent a dark age due to the lack of stellar energy sources.[34]

A hierarchical structure of matter began to form from minute variations in the mass density. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars. These massive stars triggered the reionization process and are believed to have created many of the heavy elements in the early universe.

Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually organizations of gas and dust merged to form the first primitive galaxies. Over time these pulled in more matter, and were often organized into groups and clusters of galaxies, then into larger-scale superclusters.[35]

Fundamental to the structure of the universe is the existence of dark matter and dark energy. These are now thought to be the dominant components, forming 96% of the density of the universe. So much effort is being spent to try and understand the physics of these components.[36]

Amateur astronomy

For more information, see: Amateur astronomy.


Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with equipment they build themselves. Common targets of amateur astronomers include the Moon, planets, stars, comets, meteor showers, and a variety of deep sky objects such as star clusters, galaxies, and nebulae. One branch of amateur astronomy, amateur astrophotography, involves the taking of photos of the night sky. Many amateurs like to specialise in observing particular objects, types of objects, or types of events which interest them.[37][38]

Most amateurs work at visible wavelengths, but a small minority experiment with wavelengths outside the visible spectrum. This includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. The pioneer of amateur radio astronomy was Karl Jansky who started observing the sky at radio wavelengths in the 1930s. A number of amateur astronomers use either homemade telescopes or use radio telescopes which were originally built for astronomy research but which are now available to amateurs (e.g. the One-Mile Telescope).[39][40]

Amateur astronomers continue to make scientific contributions to the field of astronomy. Indeed it is one of the few scientific disciplines where amateurs can still make significant contributions. Amateurs can make occultation measurements that are used to refine the orbits of minor planets. They can also discover comets and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make impressive advances in the field of astrophotography. [41][42][43]

Major questions in astronomy

Although the scientific discipline of astronomy has made tremendous strides in understanding the nature of the universe and its contents, there remain some important unanswered questions. Answers to these may require the construction of new ground and space-based instruments, and possibly new developments in theoretical and experimental physics.

  • Are there Earth-like planets around other stars? Astronomers have found massive stars and disks of debris around other stars. So the existence of smaller, terrestrial planets seems likely.[44]
  • Is there other life in the universe? Especially, is there other intelligent life? If so, what is the explanation for the Fermi paradox? The existence of life elsewhere has important scientific and philosophical implications.[45][46]
  • What is the nature of dark matter and dark energy? These dominate the evolution and fate of the cosmos, yet we are still uncertain about their true nature.[47]
  • Why did the universe come to be? Why, for example, are the physical constants so finely tuned that they permit the existence of life?
  • What caused the cosmic inflation that produced our homogeneous universe?[48]

See also

References

  1. Etymology: The word Astronomy comes from the Greek αστρονομία (astronomia), which is the combination of the words άστρον (astron), meaning star, and νόμος (nomos), meaning law.
  2. However, amateur astronomers have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena.
  3. George Forbes (1909). History of Astronomy (Free e-book from Project Gutenberg). London: Watts & Co.. 
  4. 4.0 4.1 Arthur Berry (1961). A Short History of Astronomy From Earliest Times Through the Nineteenth Century. New York: Dover Publications, Inc.. 
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  6. Kepler's discoveries were based on empirical data provided by the naked-eye observations of Tycho Brahe
  7. Tests of the Big Bang: The CMB Wilkinson Microwave Anistropy Probe, NASA
  8. The wavelength limits of the various bands which are quoted here and in the following only have an indicative value, as there exists no hard border between the various bands, and different people will quote different values.
  9. . However, even in the best ground locations, large parts of the infrared spectrum are not accessible, and observations can be carried out only in narrow "windows"
  10. There is no agreement about this value, which could be chosen to be as low as 5 keV, or as high as 511 keV.
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