Astronomical terms – Wissenswertes zur Astronomie

The in magnitudes expressed apparent magnitude that a star would have in a unit distance from Earth of 10 pc or 32.6 light years. The apparent magnitude of a star depends on the amount of emitted light and its distance (the brightness decreases with the square of the distance). If all the stars were equidistant, their apparent magnitudes would be precise indicators of their relative brightness. By definition, the absolute magnitude of stars is a measure of their relative luminosity by comparing the apparent magnitude that stars would have at the same distance.

Absolute Zero Point

The temperature at which the thermal energy of all substances is equal to zero, and thus, it is believed, is the lowest possible temperature. Although at absolute zero in many substances is still nonthermal zero point energy present, this can not be eliminated, so the temperature can not be lowered further. Absolute zero, which was originally described as the temperature at which an ideal gas at constant pressure reduces its volume to zero, plays an important role in thermodynamics and is used as the zero point of the absolute temperature scale. In practice absolute zero is unattainable, but in cryogenic temperatures laboratories a temperatures nearly to zero could be reached.

Albedo

The ratio between the amount of light that is reflected from a surface and the amount of light that impinges on it. The albedo is generally indicated for celestial bodies inside our solar system: the moon reflects about 7% of the impigning sunlight and has so an albedo of 0.07.

Apparent Magnitude

Apparent magnitude of a star or other celestial body, expressed in astronomical magnitudes. The system of the apparent magnitudes has its origin in the of Hipparchus in the second century B.C. classification of the brightness of thousands of stars, which was standardized by Pogson in 1850. The average of the twenty brightest stars in the sky is defined as size 1 (or 1^st size). The stars of size 6 are just visible with the naked eye on a clear night sky and stars that lie between these two extremes, have an intermediate value. The brighter a star is, the lower is its the value of its apparent magnitude.

Astronomical Unit

Distance Unit, which corresponds to the semi-major axis of Earth’s orbit (approximately the average distance of the Earth from the Sun) and is used to describe distances within the solar system. The value of an astronomical unit is 149.6 million kilometers.

Azimuth

Means in astronomy and navigation the along the horizon from east from 0° to 360° measured angle between north point from the obeserver and the section line of the horizon with a through the zenith of the observer and a star or planet bounding vertical circle.

Black Hole

A region in space where matter is collapsed in on itself and in the acceleration of gravity is so strong that nothing, not even electromagnetic radiation (including light), they can leave. The matter, which has formed the black hole is compressed in a point of infinite density, the so-called singularity. A black hole can be caused by a gravitational collapse of a star of large mass, when run in Star no more nuclear reactions which could counteract gravity. In today’s universe, the minimum mass of a star can collapse so is about three solar masses. It is believed that have been compressed during the Big Bang also much smaller masses to “primordial” black holes and that the core of active galaxies exist supermassive black holes of millions or even billions of solar masses.

Brightness

One in astronomy common measure of the radiation intensity of a celestial body. Hipparchus (about 120 B.C.) divided the stars into six size classes. The brightest stars were stars of the first magnitude, the most vulnerable, barely visible to the naked eye stars were those of 6^th mag. This system was later expanded and now also includes stars that can only be seen with a telescope, and particularly bright star, to whom a negative brightness has been assigned (eg. Sirius -1.5). A hundred-fold increase in apparent magnitude is lowered by definition the size class around 5. This apparent magnitudes are strongly dependent on the distance of the star to Earth. The absolute magnitude is defined as the apparent magnitude that a star would have if he would be 10 parsecs (32.6 light years) removed from the earth: Sirius would have a brightness of +1.4. The absolute magnitude is more meaningful than the apparent brightness. Stars also have a bolometer and photographic brightness and a brightness measured in certain invisible wave regions of the spectrum (eg. infrared).

Brown Dwarf

A star of such low mass (probably less than 0.08 solar masses) that there are never thermonuclear reactions in its core. It is believed that brown dwarfs are very cool and light up because of the low heat only very weakly in the infrared.

Constellation

A group of stars describing a picture in the sky, but have no real connection to each other. Already in ancient times these patterns were interpreted as images with mythological character.

Cosmic Radiation

Electrically charged particles, mainly atomic nuclei of hydrogen and other atoms that bombard isotropic and almost with the speed of light the Earth’s upper atmosphere. This primary radiation enters into an interaction with molecules in the upper atmosphere, thereby producing the so-called secondary radiation, which is far less energetic. The secondary radiation consists of subatomic particles which decay quickly into other types of particles. Secondary beams, which are harmless and often pass through our bodies, were originally verified by the Geiger counter and the cloud chamber. Today we have a lot of other detectors that can be used in the study of cosmic rays.

Covering

The obscuring of a celestial body by another celestial body. Typically these will be referred the passage of a planet in front of the stars or the passage of the moon in front of stars or planets.

Declination

The angular distance of a celestial body from the celestial equator along the meridian through the body. The declination of a celestial body north of the equator is positive and negative for a southern celestial bodies. The declination and right ascension together determine the position of a celestial body in the sky.

Double Star

A pair of stars moving around a common center. As double star is designated less frequently two stars which are seen close together but in reality have very different distances from Earth (optical double stars) or two stars whose movements are communicating with each other, but are not mutually orbiting (physical double stars). Approximately 50% of all stars are part of double or multiple systems. The latters consist of more than two components. It is assumed that the components of double or multiple star systems arise simultaneously. As visual double stars we refer to stars which can be seen with a telescope. There are relatively few visual double stars, because the distances between the components are compared to interstellar distances very small. Examples of visual double stars are Capella, Procyon, Sirium and Alpha Centauri.

Doppler Effect

The observed change in the wavelength of a wave (eg. sound wave or electromagnetic wave), if the source of the waves and the observer moving relatively to each other. If the source approaches the observer, each wave is closer to the preceding wave, as it would be the case if the source would not move relative to the observer. This is perceived as a rising frequency, the tone of a sound source is higher and the color of a light source appears more blue. When a sound source reaches supersonic speed, you will hear a sonic boom. If the source of a wave moves away from the observer, will each wave train be farther submitted by him, as it would otherwise be the case. The keynote therefore seems to be deeper and the color to be redder than usual. The by Christian Johann Doppler (1803 – 1853) called Doppler effect was first described in 1842 and is of great importance in astronomy. By observing the stellar spectra the relative speed can be determined with the stars move towards us or away from us.

Ecliptic

The great heavenly circuit, which the sun describes seemingly over a year on the celestial sphere and corresponds to the movement of the earth around the sun. The ecliptic passes through twelve constellations, which are known as the zodiac signs.

Eclipse

The partial or total coverage of a celestial body by another celestial body, or the entry of the Moon in the shadow of the Earth. The components of a double star (see double star) can darken each other when viewed from the Earth and are referred to ecliptic double stars in this case. In the so-called coverage the moon eclipses a star or planet.

Fireball

A particularly bright meteor with a brightness of at least -5. The name Fireball is also used for ball lightning.

Fraunhofer Lines

Dark lines in the solar spectrum, which (thus energies) are caused by atoms in the outer layers of the solar atmosphere by the absorption of radiation of certain frequencies. Analysis of the solar spectrum thus enable the identification of these atoms. The lines were first recorded by Joseph von Fraunhofer. The most prominent of them were marked with letters. The lines A and B are caused by oxygen, C is hydrogen, D by sodium, E by iron etc.

Galactic Heap

Star clusters, which are in or near the galactic plane and comprise several hundred stars. They are also called open clusters because of their irregular shape. The most famous galactic clusters of the northern sky are the Pleiades.

Galactic Clusters

Galaxies are often found in clusters, which include one to several thousand galaxies. Our galaxy is a member of a small cluster, called the local group. But there are also larger clusters, for example, in the Virgo cluster are several thousand galaxies. Several clusters of galaxies sometimes form a so-called supercluster.

Geostationary Orbit

When a satellite 35,880 kilometers above the Earth’s equator orbits the Earth, it moves at the same speed at which the earth rotates. Seen from Earth, the position of the satellite seems to be stationary in the sky. This is also called earth synchronous 24-hour orbit.

Globular Clusters

Seemingly similar elliptical, densely concentrated heap of up to a million stars, orbiting a galaxy. The Milky Way and the Andromeda Galaxy each have about 200 such globular clusters. These contain a large amount of cool red stars and RR Lyrae variables. RR Lyrae variables are periodic variable stars, commonly found in globular clusters. The study of the RR Lyrae variables allows the calculation of the distances of globular clusters.

Halo

One or more bright rings or arcs around the sun or moon, which occur at high floating, fine ice crystals by refraction and reflection their beams. The most frequent ones is the so-called small ring with a half angle of 22° around the sun or moon.

Inclination

The angle of inclination between two planes, for example:

The angle of inclination of a planetary orbit is normally understood as the angle between the plane of the planets orbit and the plane through the orbit of the Earth.
An equatorial inclination angle is the angle between the plane through the equator of a planet and the plane with its orbit.
The angle of inclination of the orbit of an artificial satellite is the angle between the plane of its orbit and the plane through the equator of the earth.

Infrared Astronomy

Part of the astronomy, which studies the infrared radiation emitted by celestial bodies. By positioning an infrared telescope in space the infrared background radiation can be reduced a million times. The infrared sources include cold clouds of dust, warm dust around young stars in our galaxy, nearby galaxies, active galaxies and quasars.

Kepler’s Laws

Three laws formulated by Johannes Kepler that describe the motions of the planets in the solar system:

Planets move around the Sun in ellipses, with the Sun at one focus.
The line connecting the Sun to a planet sweeps equal areas in equal times.
The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

Light Year

In astronomy, a unit of length defining the route, which the light travels in a vacuum during a sidereal year and corresponds 9.4605 trillion km. The light year was largely replaced by the Parsec (1 Ly = 0.3069 pc).

Magellanic Cloud

Two irregular galaxies near the south celestial that are orbiting the Milky Way. The Large Magellanic Cloud (Nubecula Major) has a diameter of about 29,000 light years and a clearly recognizable axis, why it is assumed that it could be an embryonic spiral nebulae. The Small Magellanic Cloud (Nubecula Minor) has a diameter of about 16,000 light years. Both clouds have a variety of Cepheids. Their distance from the solar system is about 163,000 light-years (Nubecula Major) or 195,000 light years (Nubecula Minor). In the Large Magellanic Cloud is the giant Tarantula Nebula, near which in 1987 a supernova was observed.

Meridian

In astronomy, the name of the great circle passing through the celestial poles and the zenith of the observer. It intersects the horizon to the north and south. The term meridian is also used for longitudes on the earth.

Neutron Star

A star that has been compressed so much that a large part of its positively charged protons and negatively charged electrons have formed neutral neutrons. Such a star has a radius of about 10 kilometers and a density from 1013 to 1014 g/cm³.

Pleiades

A star cluster in the constellation Taurus. Seven of its stars, named after the daughters of Atlas, are visible without optical help. The Pleiades are 500 light years away from the Sun and sourrounded by slightly reflecting nebulae.

Polaris

Alpha Ursae Majoris, a Cephei star in the constellation of the Ursa Minor. Because of its proximity to the north celestial pole star it is also known as the North Star, and was used for orientation in shipping for centuries. Due to the precession of the Earth’s axis, the Polar star moves away from the north celestial pole.

Quasar

A short form for “Quasi Stellar Object”, a star in the optical range of similar celestial bodies, whose spectrum has an unusually large redshift. The largest redshift of a quasar that has ever been measured, is 4.01. Quasars are generally considered distant objects that erase from us with high speed. Some scientists hold against the fact that the redshift of quasars is not only due to the expansion of the universe. In this case, the quasars were not so far away as they seem.

Radio Astronomy

Part of astronomy that concerns with the study of the by celestial bodies emitted or reflected electromagnetic radiation in the wavelength range between 1 mm 30 m. This is usually measured with a radio telescope. Radio astronomy was invented at random in 1932 by Karl Jansky, who found out during testing the interference of a telephone network, that its source was the Milky Way. 1937 the Americans Grote Reber built in his backyard a radio telescope with a diameter of 9.5 m and patted the sky at wavelengths of about 2 m. After the 2nd World War, the research in this area has been strengthened. Studies of the sky revealed that clouds of hydrogen gas emit radio emissions in the Milky Way. The creation of a map of clouds confirmed the spiral shape of our galaxy.

Radio Telescope

The main instrument of radio astronomy. The receiver of the radio telescope consists of a large antenna that has as the most telescopes the shape of a parabolic dish. This large dish that works the like a parabolic mirror of a reflecting telescope, reflects radio signals to a central antenna. The received signals are then amplified and investigated. In practice, radio telescopes can be built, which are much larger than any single dish by connecting several bowls electronically. Such a radio telescope is called Array (More mirror telescope). The largest more mirror telescope in the world, the Very Large Array radio telescope, is located in the United States near Socorro, New Mexico and consists of 27 controllable bowls.

Star

A large, luminous ball of gas, which is held together by its own gravity. The sun is in its chemical composition, the color, and also regarding other parameters a normal star. The lifetime of a star depends on its mass and brightness. A very bright star can life only one million year, the sun ten billion years and the faintest stars of the main sequence 10 trillion years. Stars are classified into two categories: Population I and II. The stars of Population I move more slowly and are generally found in the spiral arms of galaxies. It is also supposed that these stars are relatively young. The stars of Population II move faster and are mainly found in the spherical halo of stars around a galaxy and in globular clusters. They have a smaller amount of metals than the stars of population I. Many stars are double stars.

Supernova

Tremendous Starbursts, during this a large part of mass of a star is thrust into the space or the star was completely destroyed. During this the brightness of the star increases millions to billions of times, so that it seems to be briefly as bright as billions of suns. The differences between supernovae are classified between type I and type II. Supernovae Type I are ten times brighter than supernovae of type II generally. In type II, in contrast to Type I, hydrogen lines in the spectrum can be seen. As a precursor of a supernova of type I, a white dwarf star is asumed, who receives matter from a binary companion until it is so strongly compressed and heated by gravity so that it explodes due to thermonuclear reactions. Supernovae Type II occur in the population I and can be attributed to a massive star that has used up nuclear energy in its interior. The core collapses and eventually forms a neutron star, while the outer layers of the star are flung into space, forming an expanding supernova remnant such as the Crab nebula. Supernovae are very rare. On average there occur in a typical galaxy about three supernovae per century. The most recent visible supernova in the Milky Way occurred in 1604. The last supernova that could be seen from Earth with the naked eye, was detected in a neighboring galaxy in 1987, the Large Magellanic Cloud.

Ultraviolet Radiation

Electromagnetic radiation with wavelengths from 0.1 to 380 nm, which is generated for example by gas discharge tubes. Five percent of the solar radiation is composed of ultraviolet radiation, but most of it is prevented by the oxygen and ozone in the atmosphere from reaching the Earth and destroy life there. This means that an optical device for the measurement of ultraviolet radiation must be evacuated. For a similar reason lenses and prisms must be made of quartz or fluorite because of the strong absorption of glass. The detection of ultraviolet radiation is done photographically or using fluorescent screens. Ultraviolet radiation is mainly used in fluorescent tubes, but they are also found in medical field applications, such as in germicidal lamps, in the treatment of rickets and some skin diseases, and in the enrichment of milk and eggs with vitamin D.

Van Allen Belt

A the earth surrounding belt of high-energy particles, mostly protons and electrons, named after Van Allen who discovered the belt in 1958. The particles are trapped by the magnetic field of the earth in these areas which are located from a few hundred to 50 000 km above the Earth’s surface. The radiation is so strong that astronauts have to be specially protected against it.

Variable Star

A star showing variations in its brightness. There are two main categories: eclipsing binary and physical variables. Eclipsing binary are stars whose change in apparent brightness is caused by external causes, such as in photometric binary stars. In physical variables, the absolute brightness changes within themselves due to physical changes. They can change either regularly or irregularly. Most physical variables are pulsating variables or eruptive variables. Pulsating variables whose size changes, are the most common variable stars.They include the RR Lyrae stars, whose periods last from 1.5 hours to a little more than one day, W Virginis stars and RV Tauri stars (all 3 types mainly occur in clusters), long-period and semi-regular variables, the so-called red giants, and Cephei variables. Variables whose periods are related to their absolute magnitude, are of particular concern because they can be used to determine large astronomical distances. The eruptive variables include flare star (UV Ceti star), weak red stars that light up once or twice a day one or two sizes brighter. Nova and Supernova expose much more extreme eruptions.

X-ray Astronomy

Part of the astronomy which is about the emitted x-rays of celestial bodies. Since most of the X-rays is absorbed by the Earth’s atmosphere before they reach the Earth’s surface, the observations are made of altitude balloons, satellites and missiles. X-rays are usually emitted by very hot clouds of matter. The X-ray sources include solar corona, solar flares, compact stars in binary stars, quasars and intergalactic gas.

X-rays

High-energy, invisible electromagnetic radiation with a wavelength between 0.1 pm and 1 nm. X-rays are generally produced with the aid of a vacuum electron tube in which electrons are accelerated by applying a potential difference of between 1 to a few hundred kV from a heated cathode to a large molybdenum or tungsten anode. The electrons release their energy to the anode, which then emits X-rays. X-rays are verified by using phosphor screens (eg. like in the medical X-ray fluoroscopy), Geiger and scintillation counter as well as on photographic plates. They were discovered in 1895 by W. C. Roentgen, but their wave nature was confirmed in 1911 because of their extremely short wavelength. This was varified by Laude, who showed that X-rays were diffracted by crystal lattices.X-rays are often used in medicine for diagnostic and treatment purposes and in the technique for detecting minimal material damage in components. X-ray tubes have to be screened, as X-rays damage living tissue.