2828Botercadentconstellation of Vega
The faintest stars visible to the unaided eye are sixth magnitude, while the brightest in the night sky, Sirius, is of magnitude −1.46. To standardize the magnitude scale, astronomers chose Vega to represent magnitude zero at all wavelengths. Thus, for many years, Vega was used as a baseline for the calibration of absolute photometric brightness scales. However, this is no longer the case, as the apparent magnitude zero point is now commonly defined in terms of a particular numerically specified flux. This approach is more convenient for astronomers, since Vega is not always available for calibration and varies in brightness.


Major astronomers who practised as court astrologers included Tycho Brahe in the royal court of Denmark, Johannes Kepler to the Habsburgs, Galileo Galilei to the Medici, and Giordano Bruno who was burnt at the stake for heresy in Rome in 1600. The distinction between astrology and astronomy was not entirely clear. Advances in astronomy were often motivated by the desire to improve the accuracy of astrology. Ephemerides with complex astrological calculations, and almanacs interpreting celestial events for use in medicine and for choosing times to plant crops, were popular in Elizabethan England.

Milky Way

galaxyMilky Way Galaxyour galaxy
The Milky Way is visible from Earth as a hazy band of white light, some 30° wide, arching across the night sky. In night sky observing, although all the individual naked-eye stars in the entire sky are part of the Milky Way, the term “Milky Way” is limited to this band of light. The light originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where interstellar dust blocks light from distant stars. The area of sky that the Milky Way obscures is called the Zone of Avoidance. The Milky Way has a relatively low surface brightness.

Mercury (planet)

Mercuryplanet MercuryMercurio
In medieval Islamic astronomy, the Andalusian astronomer Abū Ishāq Ibrāhīm al-Zarqālī in the 11th century described the deferent of Mercury's geocentric orbit as being oval, like an egg or a pignon, although this insight did not influence his astronomical theory or his astronomical calculations. In the 12th century, Ibn Bajjah observed "two planets as black spots on the face of the Sun", which was later suggested as the transit of Mercury and/or Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.


HH 2 hydrogen gas
They are the source of the important 21 cm hydrogen line in astronomy at 1420 MHz. Hydrogen has three naturally occurring isotopes, denoted, and. Other, highly unstable nuclei ( to ) have been synthesized in the laboratory but not observed in nature. Hydrogen is the only element that has different names for its isotopes in common use today. During the early study of radioactivity, various heavy radioactive isotopes were given their own names, but such names are no longer used, except for deuterium and tritium.


In astronomy, the stellar classification of stars and their place on the Hertzsprung–Russell diagram are based, in part, upon their surface temperature, known as effective temperature. The photosphere of the Sun, for instance, has an effective temperature of 5778 K. Digital cameras and photographic software often use colour temperature in K in edit and setup menus. The simple guide is that the higher the colour temperature, the more white or blue the image will be. The reduction in colour temperature will give an image more dominated by reddish, "warmer" colours.


Martianplanet MarsRed Planet
The existence of Mars as a wandering object in the night sky was recorded by the ancient Egyptian astronomers and, by 1534 BCE, they were familiar with the retrograde motion of the planet. By the period of the Neo-Babylonian Empire, the Babylonian astronomers were making regular records of the positions of the planets and systematic observations of their behavior. For Mars, they knew that the planet made 37 synodic periods, or 42 circuits of the zodiac, every 79 years. They invented arithmetic methods for making minor corrections to the predicted positions of the planets. In Ancient Greek, the planet was known as.


β Ori (Rigel)B2 IaeBeta Orionis
Rigel, also designated β Orionis (Latinized to Beta Orionis, abbreviated Beta Ori, β Ori), is on average the seventh-brightest star in the night sky and the brightest in the constellation of Orion—though occasionally it is outshone within the constellation by the variable star Betelgeuse. It is a luminous object some 863 light-years distant from Earth, varying irregularly between apparent magnitude +0.05 and +0.18. Rigel as seen from Earth is actually a multiple star system of three to five stars. A companion, Rigel B, is 500 times fainter than the supergiant Rigel A and visible only with a telescope.

Orion (constellation)

Orionconstellation of OrionOrion constellation
A line from Rigel through Betelgeuse points to Castor and Pollux (α Gem and β Gem). Additionally, Rigel is part of the Winter Circle asterism. Sirius and Procyon, which may be located from Orion by following imaginary lines (see map), also are points in both the Winter Triangle and the Circle. Orion's seven brightest stars form a distinctive hourglass-shaped asterism, or pattern, in the night sky. Four stars—Rigel, Betelgeuse, Bellatrix and Saiph—form a large roughly rectangular shape, in the centre of which lie the three stars of Orion's Belt—Alnitak, Alnilam and Mintaka.


He made the first measurement of stellar parallax: 0.3 arcsec for the binary star 61 Cygni. Being very difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century, mostly by use of the filar micrometer. Astrographs using astronomical photographic plates sped the process in the early 20th century. Automated plate-measuring machines and more sophisticated computer technology of the 1960s allowed more efficient compilation of star catalogues. In the 1980s, charge-coupled devices (CCDs) replaced photographic plates and reduced optical uncertainties to one milliarcsecond.

John Herschel

Sir John HerschelHerschelJohn
Herschel collaborated with Thomas Maclear, the Astronomer Royal at the Cape of Good Hope and the members of the two families became close friends. During this time, he also witnessed the Great Eruption of Eta Carinae (December, 1837). In addition to his astronomical work, however, this voyage to a far corner of the British empire also gave Herschel an escape from the pressures under which he found himself in London, where he was one of the most sought-after of all British men of science. While in southern Africa, he engaged in a broad variety of scientific pursuits free from a sense of strong obligations to a larger scientific community.

Variable star

variablevariable starsvariability
Society for Popular Astronomy – Variable Star Section.

Star, Russia

Star is the name of several inhabited localities in Russia: * Urban localities * Star, Bryansk Oblast, a work settlement in Dyatkovsky District of Bryansk Oblast; 53.63333°N, 34.15°W

List of Arabic star names

traditional star namesArabic and Persian namesArabic names now used for individual stars
Islamic astronomy. List of proper names of stars. Islamic Crescents' Observation Project (ICOP) website, which in turn cites. Paul Kunitzsch, Tim Smart, Short Guide to Modern Star Names and Their Derivations (1986). Guy Ottewell, The Astronomical Companion. Abdul-Rahim Bader, Rasd al-Sama ("Observing the Sky"). Hamid Al-Naimy Elm al-Falak ("Astronomy"). ICOP: Arabic Star Names. Kaler, James, "Star Names: Proper Names", University of Illinois. The Astronomy Corner: Reference – Star Names v1.1.2 (2006-05-28) from Lebling, Robert W., "Arabic in the Sky", Saudi Aramco World magazine, September/October 2010, v.61, n.5.


In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Arcturus for this star. It is now so entered in the IAU Catalog of Star Names. With an apparent visual magnitude of −0.05, Arcturus is the brightest star in the northern celestial hemisphere and the fourth-brightest star in the night sky, after Sirius (−1.46 apparent magnitude), Canopus (−0.72) and α Centauri (combined magnitude of −0.27).


Canis minorProcyon Aα CMi
Procyon is usually the eighth-brightest star in the night sky, culminating at midnight on January 14. It forms one of the three vertices of the Winter Triangle asterism, in combination with Sirius and Betelgeuse. The prime period for evening viewing of Procyon is in late winter in the northern hemisphere. It has a color index of 0.42, and its hue has been described as having a faint yellow tinge to it. Procyon is a binary star system with a bright primary component, Procyon A, having an apparent magnitude of 0.34, and a faint companion, Procyon B, at magnitude 10.7.

Radial velocity

radial velocitiesradial-velocitydoppler velocity tests
Space velocity (astronomy).

Stellar evolution

Astronomy 606 (Stellar Structure and Evolution) lecture notes, Cole Miller, Department of Astronomy, University of Maryland. Astronomy 162, Unit 2 (The Structure & Evolution of Stars) lecture notes, Richard W. Pogge, Department of Astronomy, Ohio State University. Stellar evolution simulator. Pisa Stellar Models. MESA stellar evolution codes (Modules for Experiments in Stellar Astrophysics). "The Life of Stars", BBC Radio 4 discussion with Paul Murdin, Janna Levin and Phil Charles (In Our Time, Mar. 27, 2003).

Spectral line

emission lineabsorption lineabsorption lines
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to identify atoms and molecules. These "fingerprints" can be compared to the previously collected "fingerprints" of atoms and molecules, and are thus used to identify the atomic and molecular components of stars and planets, which would otherwise be impossible.

Chinese astronomy

Chinese astronomersastronomyastronomer
Boundaries Crossing: Western Astronomy in Confucian China, 1600-1800 by Pingyi Chu. Homepage of the National Astronomical Observatories, Chinese Academy of Sciences. Chinese astronomy at the University of Maine.

CNO cycle

CNOcarbon-nitrogen-oxygen (CNO) cyclecarbon-nitrogen-oxygen cycle
The CNO cycle (for carbon–nitrogen–oxygen) is one of the two known sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain reaction (pp-chain reaction). Unlike the latter, the CNO cycle is a catalytic cycle. It is dominant in stars that are more than 1.3 times as massive as the Sun.

International Astronomical Union

IAUInternational Astronomical Union (IAU)I.A.U.
The International Astronomical Union (IAU; Union astronomique internationale, UAI) is an international association of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy. Among other activities, it acts as the internationally recognized authority for assigning designations and names to celestial bodies (stars, planets, asteroids, etc.) and any surface features on them. The IAU is a member of the International Council for Science (ICSU). Its main objective is to promote and safeguard the science of astronomy in all its aspects through international cooperation.

Star system

multiple star systemmultiple systemstriple star
It has an apparent magnitude of around −0.47, making Capella one of the brightest stars in the night sky. 4 Centauri. Mizar is often said to have been the first binary star discovered when it was observed in 1650 by Giovanni Battista Riccioli, p. 1 but it was probably observed earlier, by Benedetto Castelli and Galileo. Later, spectroscopy of its components Mizar A and B revealed that they are both binary stars themselves. HD 98800. The Kepler-64 system has the planet PH1 (discovered in 2012 by the Planet Hunters group, a part of the Zooniverse) orbiting two of the four stars, making it to be the first known planet to be in a quadruple star system.

Magnitude (astronomy)

A more complex definition of absolute magnitude is used for planets and small Solar System bodies, based on its brightness at one astronomical unit from the observer and the Sun. The Sun has an apparent magnitude of −27 and Sirius, the brightest visible star in the night sky, −1.46. Apparent magnitudes can also be assigned to artificial objects in Earth orbit with the International Space Station (ISS) sometimes reaching a magnitude of −6. The magnitude system dates back roughly 2000 years to the Greek astronomer Hipparchus (or the Alexandrian astronomer Ptolemy—references vary) who classified stars by their apparent brightness, which they saw as size (magnitude means "bigness, size" ).


NeptunianNeptune-masseighth planet
Neptune Astronomy Cast episode No. 63, includes full transcript. Neptune Profile at NASA's Solar System Exploration site. Planets – Neptune A children's guide to Neptune. Neptune by amateur (The Planetary Society). Interactive 3D visualisation of Neptune and its inner moons.