A giant mosaic of the, a Astronomy (from: ἀστρονομία) is a that studies and phenomena. It applies,, and, in an effort to explain the origin of those objects and phenomena and their. Objects of interest include,,,, and; the phenomena include,, and. More generally, all phenomena that originate outside are within the purview of astronomy.

A related but distinct subject,, is concerned with the study of the as a whole. Astronomy is one of the oldest of the natural sciences. The early civilizations in, such as the,,,,,,,, and many ancient performed methodical observations of the. Historically, astronomy has included disciplines as diverse as,, and the making of, but professional astronomy is now often considered to be synonymous with. During the 20th century, the field of professional astronomy split into and branches.

Observational astronomy is focused on acquiring data from observations of astronomical objects, which is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain observational results and observations being used to confirm theoretical results. Astronomy is one of the few sciences where amateurs still play an active role, especially in the discovery and observation of transient. Have made and contributed to many important astronomical discoveries, such as finding new comets. 19th century is located 12 minutes south of the in,. Astronomy (from the from astron, 'star' and -νομία from nomos, 'law' or 'culture') means 'law of the stars' (or 'culture of the stars' depending on the translation).

Astronomy should not be confused with, the belief system which claims that human affairs are correlated with the positions of celestial objects. Although the share a common origin, they are now entirely distinct. Use of terms 'astronomy' and 'astrophysics' [ ] Generally, either the term 'astronomy' or 'astrophysics' may be used to refer to this subject.

Center for Particle Physics, Institute of Physics, The Academy of Sciences of the Czech Republic, Na Slovance 2. ISM: cosmic rays – magnetic fields – Galaxy: general. The origin of high-energy cosmic rays and ultra-high energy cosmic rays (UHECRs1) is one of the major unresolved ques.

Based on strict dictionary definitions, 'astronomy' refers to 'the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties' and 'astrophysics' refers to the branch of astronomy dealing with 'the behavior, physical properties, and dynamic processes of celestial objects and phenomena.' In some cases, as in the introduction of the introductory textbook The Physical Universe by, 'astronomy' may be used to describe the qualitative study of the subject, whereas 'astrophysics' is used to describe the physics-oriented version of the subject.

However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics. Few fields, such as astrometry, are purely astronomy rather than also astrophysics. Various departments in which scientists carry out research on this subject may use 'astronomy' and 'astrophysics,' partly depending on whether the department is historically affiliated with a physics department, and many professional have physics rather than astronomy degrees. Some titles of the leading scientific journals in this field include,, and. A celestial map from the 17th century, by the Dutch cartographer In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye. In some locations, early cultures assembled massive artifacts that possibly had some astronomical purpose.

In addition to their ceremonial uses, these could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as in understanding the length of the year. Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye. As civilizations developed, most notably in,,,,,, and, astronomical observatories were assembled, and ideas on the nature of the Universe began to be explored. Most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the Universe were explored philosophically. The Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the of the Universe, or the, named after.

A particularly important early development was the beginning of mathematical and scientific astronomy, which began among, who laid the foundations for the later astronomical traditions that developed in many other civilizations. The discovered that recurred in a repeating cycle known as a. Greek equatorial,, present-day 3rd–2nd century BCE Following the Babylonians, significant advances in astronomy were made in and the world. Is characterized from the start by seeking a rational, physical explanation for celestial phenomena. In the 3rd century BC, estimated the, and was the first to propose a model of the. In the 2nd century BC, discovered, calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the. Hipparchus also created a comprehensive catalog of 1020 stars, and most of the of the northern hemisphere derive from Greek astronomy.

150–80 BC) was an early designed to calculate the location of the,, and for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical appeared in. During the Middle Ages, astronomy was mostly stagnant in Europe, at least until the 13th century. However, and other parts of the world.

This led to the emergence of the first astronomical in the by the early 9th century. In 964, the, the largest in the, was discovered by the Persian astronomer and first described in his.

The, the brightest stellar event in recorded history, was observed by the Egyptian Arabic astronomer and the in 1006. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include,,,,,,, and the astronomers of the and observatories. Astronomers during that time introduced many. It is also believed that the ruins at and may have housed an astronomical observatory. Europeans had previously believed that there had been no astronomical observation in pre-colonial Middle Ages but modern discoveries show otherwise. The Roman Catholic Church gave more financial and social support to the study of astronomy for over six centuries, from the recovery of ancient learning during the late Middle Ages into the Enlightenment, than any other, and, probably, all other, institutions. Among the Church's motives was finding the date for Easter.

Scientific revolution [ ]. An astronomical chart from an early scientific manuscript, c.

1000 During the, proposed a of the. His work was defended, expanded upon, and corrected by and. Galileo used telescopes to enhance his observations. Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was left to invention of and his to finally explain the motions of the planets. Newton also developed the.

The English astronomer catalogued over 3000 stars. Further discoveries paralleled the improvements in the size and quality of the telescope. More extensive star catalogues were produced.

The astronomer made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet, the first new planet found. The distance to a star was first announced in 1838 when the of was measured. During the 18–19th centuries, the study of the by,, and led to more accurate predictions about the motions of the Moon and planets. This work was further refined by and, allowing the masses of the planets and moons to be estimated from their perturbations.

Significant advances in astronomy came about with the introduction of new technology, including the and. Discovered about 600 bands in the spectrum of the Sun in 1814–15, which, in 1859, ascribed to the presence of different elements. Stars were proven to be similar to the Earth's own Sun, but with a wide range of,, and sizes. The existence of the Earth's galaxy, the, as a separate group of stars, was only proved in the 20th century, along with the existence of 'external' galaxies.

The observed recession of those galaxies led to the discovery of the expansion of the. Theoretical astronomy led to speculations on the existence of objects such as and, which have been used to explain such observed phenomena as,,, and. Made huge advances during the 20th century, with the model of the, which is heavily supported by evidence provided by,, and the. Have enabled measurements in parts of the electromagnetic spectrum normally blocked or blurred by the atmosphere. In February 2016, it was revealed that the project had detected evidence of in the previous September.

Observational astronomy [ ]. Main article: Radio astronomy uses radiation outside the visible range with greater than approximately one millimeter. Radio astronomy is different from most other forms of observational astronomy in that the observed can be treated as rather than as discrete. Hence, it is relatively easier to measure both the and of radio waves, whereas this is not as easily done at shorter wavelengths. Although some are emitted directly by astronomical objects, a product of, most of the radio emission that is observed is the result of, which is produced when orbit.

Additionally, a number of produced by, notably the spectral line at 21 cm, are observable at radio wavelengths. A wide variety of objects are observable at radio wavelengths, including, interstellar gas,, and. Infrared astronomy [ ]. Observatory is one of the highest observatory sites on Earth.

Atacama, Chile. Infrared astronomy is founded on the detection and analysis of radiation, wavelengths longer than red light and outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, or nebulae whose light is blocked by dust. The longer wavelengths of infrared can penetrate clouds of dust that block visible light, allowing the observation of young stars embedded in and the cores of galaxies.

Observations from the (WISE) have been particularly effective at unveiling numerous Galactic and their host. With the exception of infrared close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as the atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space. Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space; more specifically it can detect water in comets. Optical astronomy [ ].

Main article: Gamma ray astronomy observes astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the or by specialized telescopes called. The Cherenkov telescopes do not detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere. Most emitting sources are actually, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources.

These steady gamma-ray emitters include pulsars,, and candidates such as active galactic nuclei. Fields not based on the electromagnetic spectrum [ ] In addition to electromagnetic radiation, a few other events originating from great distances may be observed from the Earth. In, astronomers use heavily shielded such as,, and for the detection of. The vast majority of the neutrinos streaming through the Earth originate from the, but 24 neutrinos were also detected from., which consist of very high energy particles (atomic nuclei) that can decay or be absorbed when they enter the Earth's atmosphere, result in a cascade of secondary particles which can be detected by current observatories. Some future may also be sensitive to the particles produced when cosmic rays hit the Earth's atmosphere. Is an emerging field of astronomy that employs to collect observational data about distant massive objects.

A few observatories have been constructed, such as the Laser Interferometer Gravitational Observatory. LIGO made its on 14 September 2015, observing gravitational waves from a.

A second was detected on 26 December 2015 and additional observations should continue but require extremely sensitive instruments. The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as. Astrometry and celestial mechanics [ ].

Star cluster with a nebula One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in (the use of celestial objects to guide navigation) and in the making of. Careful measurement of the positions of the planets has led to a solid understanding of gravitational, and an ability to determine past and future positions of the planets with great accuracy, a field known as. More recently the tracking of will allow for predictions of close encounters or potential collisions of the Earth with those objects. The measurement of of nearby stars provides a fundamental baseline in the that is used to measure the scale of the Universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, as their properties can be compared.

Measurements of the and of stars allows astronomers to plot the movement of these systems through the Milky Way galaxy. Astrometric results are the basis used to calculate the distribution of speculated in the galaxy.

During the 1990s, the measurement of the of nearby stars was large orbiting those stars. Theoretical astronomy [ ] • • • • Related topics • • • • •. Main article: Theoretical astronomers use several tools including and; each has its particular advantages.

Analytical models of a process are generally better for giving broader insight into the heart of what is going on. Numerical models reveal the existence of phenomena and effects otherwise unobserved. Theorists in astronomy endeavor to create theoretical models and from the results predict observational consequences of those models. The observation of a phenomenon predicted by a model allows astronomers to select between several alternate or conflicting models as the one best able to describe the phenomena. Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency between the data and model's results, the general tendency is to try to make minimal modifications to the model so that it produces results that fit the data.

In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model. Phenomena modeled by theoretical astronomers include: and;; of in the; origin of; and, including and. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for ( astro) and the study of. Some widely accepted and studied theories and models in astronomy, now included in the are the,,, and fundamental theories of.

A few examples of this process: Physical process Experimental tool Theoretical model Explains/predicts Emergence of a How the stars shine and how, at the center of in stars The dominant source of energy for massive star. And are the current leading topics in astronomy, as their discovery and controversy originated during the study of the galaxies.

Specific subfields [ ] Solar astronomy [ ]. See also: At a distance of about eight light-minutes, the most frequently studied star is the, a typical main-sequence of G2 V, and about 4.6 billion years (Gyr) old. The Sun is not considered a, but it does undergo periodic changes in activity known as the. This is an 11-year oscillation in. Sunspots are regions of lower-than- average temperatures that are associated with intense magnetic activity.

The Sun has steadily increased in luminosity 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, for example, is believed to have caused the phenomenon during the. The visible outer surface of the Sun is called the. Above this layer is a thin region known as the. This is surrounded by a transition region of rapidly increasing temperatures, and finally by the super-heated.

At the center of the Sun is the core region, a volume of sufficient temperature and pressure for to occur. Above the core is the, where the plasma conveys the energy flux by means of radiation. Above that is the where the gas material transports energy primarily through physical displacement of the gas known as convection. It is believed that the movement of mass within the convection zone creates the magnetic activity that generates sunspots. A solar wind of plasma particles constantly streams outward from the Sun until, at the outermost limit of the Solar System, it reaches the.

As the solar wind passes the Earth, it interacts with the () and deflects the solar wind, but traps some creating the that envelop the Earth. The are created when solar wind particles are guided by the magnetic flux lines into the Earth's polar regions where the lines the descend into the. Planetary science [ ]. The black spot at the top is a climbing a crater wall on. This moving, swirling column of (comparable to a terrestrial ) created the long, dark streak. The Solar System is subdivided into the inner planets, the, and the outer planets.

The inner consist of,,, and. The outer planets are,,, and.

Beyond Neptune lies the, and finally the, which may extend as far as a light-year. The planets were formed 4.6 billion years ago in the 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 of the 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 on the Moon. During this period, some of the protoplanets may have collided and one such collision may have.

Once a planet reaches sufficient mass, the materials of different densities segregate within, during. This process can form a stony or metallic core, surrounded by a mantle and an outer crust. The core may include solid and liquid regions, and some planetary cores generate their own, which can protect their atmospheres from solar wind stripping. A planet or moon's interior heat is produced from the collisions that created the body, by the decay of radioactive materials ( e.g.,, and ), or caused by interactions with other bodies. Some planets and moons accumulate enough heat to drive geologic processes such as and tectonics. Those that accumulate or retain an can also undergo surface from wind or water.

Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering. Stellar astronomy [ ]. Main article: The study of stars and is fundamental to our understanding of the Universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.

Occurs in dense regions of dust and gas, known as. When destabilized, cloud fragments can collapse under the influence of gravity, to form a. A sufficiently dense, and hot, core region will trigger, thus creating a. Almost all elements heavier than and were inside the cores of stars.

The characteristics of the resulting star depend primarily upon its starting mass. Nuclear Reactor Dynamics Pdf Printer. The more massive the star, the greater its luminosity, and the more rapidly it fuses its hydrogen fuel into helium in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to.

The fusion of helium requires a higher core temperature. A star with a high enough core temperature will push its outer layers outward while increasing its core density. The resulting formed by the expanding outer layers enjoys a brief life span, before the helium fuel in the core is in turn consumed.

Very massive stars can also undergo a series of evolutionary phases, as they fuse increasingly heavier elements. The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse; while smaller stars blow off their outer layers and leave behind the inert core in the form of a. The ejection of the outer layers forms a. The remnant of a supernova is a dense, or, if the stellar mass was at least three times that of the Sun, a. Closely orbiting binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova. Planetary nebulae and supernovae distribute the ' produced in the star by fusion to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.

Main article: Our orbits within the, a that is a prominent member of the 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, a bar-shaped bulge with what is believed to be a at its center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, stars. The disk is surrounded by a of older, stars, as well as relatively dense concentrations of stars known as. Between the stars lies the, a region of sparse matter. In the densest regions, of and other elements create star-forming regions.

These begin as a compact or, which concentrate and collapse (in volumes determined by the ) to form compact protostars. As the more massive stars appear, they transform the cloud into an (ionized atomic hydrogen) of glowing gas and plasma.

The and supernova explosions from these stars eventually cause the cloud to disperse, often leaving behind one or more young of stars. These clusters gradually disperse, and the stars join the population of 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 appears to dominate the mass, although the nature of this dark matter remains undetermined. Extragalactic astronomy [ ]. Main article: The study of objects outside our galaxy is a branch of astronomy concerned with the, their morphology (description) and, the observation of, and at a larger scale, the. Finally, the latter is important for the understanding of the. Most are organized into distinct shapes that allow for classification schemes. They are commonly divided into, and galaxies.

As the name suggests, an elliptical galaxy has the cross-sectional shape of an. The stars move along 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 have been 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 within which massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the and one of our nearest galaxy neighbors, the, are spiral galaxies. Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.

An active galaxy is a formation that emits a significant amount of its energy from a source other than its stars, dust and gas. It is powered by a compact region at the core, thought to be a super-massive black hole that is emitting radiation from in-falling material. A is an active galaxy that is very luminous in the portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit shorter frequency, high-energy radiation include,, and.

Quasars are believed to be the most consistently luminous objects in the known universe. The is represented by groups and clusters of galaxies. This structure is organized into a hierarchy of groupings, with the largest being the. The collective matter is formed into and walls, leaving large between. Physical cosmology [ ].

Observations of the, a branch known as, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the, wherein our Universe began at a single point in time, and thereafter over the course of 13.8 billion years to its present condition. The concept of the big bang can be traced back to the discovery of the 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 experienced a very rapid, which homogenized the starting conditions. Thereafter, produced the elemental abundance of the early Universe.

(See also.) When the first neutral formed from a sea of primordial ions, 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. A hierarchical structure of matter began to form from minute variations in the mass density of space.

Matter accumulated in the densest regions, forming clouds of gas and the earliest stars, the. These massive stars triggered the process and are believed to have created many of the heavy elements in the early Universe, which, through nuclear decay, create lighter elements, allowing the cycle of nucleosynthesis to continue longer. 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 of galaxies, then into larger-scale superclusters. Fundamental to the structure of the Universe is the existence of and.

These are now thought to be its dominant components, forming 96% of the mass of the Universe. For this reason, much effort is expended in trying to understand the physics of these components. Interdisciplinary studies [ ] Astronomy and astrophysics have developed significant interdisciplinary links with other major scientific fields. Is the study of ancient or traditional astronomies in their cultural context, utilizing and evidence. Is the study of the advent and evolution of biological systems in the Universe, with particular emphasis on the possibility of non-terrestrial life. Is the application of statistics to astrophysics to the analysis of vast amount of observational astrophysical data.

The study of found in space, including their formation, interaction and destruction, is called. These substances are usually found in, although they may also appear in low temperature stars, brown dwarfs and planets. Is the study of the chemicals found within the Solar System, including the origins of the elements and variations in the ratios. Both of these fields represent an overlap of the disciplines of astronomy and chemistry. As ', finally, methods from astronomy have been used to solve problems of law and history. Amateur astronomy [ ]. Amateur astronomers can build their own equipment, and hold star parties and gatherings, such as.

Astronomy is one of the sciences to which amateurs can contribute the most. Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with. Common targets of amateur astronomers include the Sun, the Moon, planets, stars, comets,, and a variety of such as star clusters, galaxies, and nebulae. Astronomy clubs are located throughout the world and many have programs to help their members set up and complete observational programs including those to observe all the objects in the Messier (110 objects) or Herschel 400 catalogues of points of interest in the night sky. One branch of amateur astronomy, amateur, involves the taking of photos of the night sky. Many amateurs like to specialize in the observation of particular objects, types of objects, or types of events which interest them. 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, 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. Amateur astronomers continue to make scientific contributions to the field of astronomy and 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. Unsolved problems in astronomy [ ]. See also: Although the scientific discipline of 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. • What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses – the – apparently regardless of the initial conditions? A deeper understanding of the formation of stars and planets is needed. • Is there other? Especially, is there other intelligent life?

If so, what is the explanation for the? The existence of life elsewhere has important scientific and philosophical implications.

Is the Solar System normal or atypical? • What is the nature of and? These dominate the evolution and fate of the cosmos, yet their true nature remains unknown. What will be the? • How did the first galaxies form? How did supermassive black holes form? • What is creating the?

• Why is the abundance of lithium in the cosmos four times lower than predicted by the standard model? • What really happens beyond the?

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