Apparent magnitude

apparent visual magnitudemagnitudevisual magnitude
Luminosity in astronomy. List of nearest bright stars. List of nearest stars. Surface brightness. Distance modulus.

Stellar classification

spectral typeK-typeG-type
K5V – 61 Cygni A. K5III – Gamma Draconis. Spectral standards:. M0IIIa – Beta Andromedae. M2III – Chi Pegasi. M1-M2Ia-Iab – Betelgeuse. M2Ia – Mu Cephei. WN – spectrum dominated by N III-V and He I-II lines. WNE (WN2 to WN5 with some WN6) – hotter or "early". WNL (WN7 to WN9 with some WN6) – cooler or "late". Extended WN classes WN10 and WN11 sometimes used for the Ofpe/WN9 stars. h tag used (e.g. WN9h) for WR with hydrogen emission and ha (e.g. WN6ha) for both hydrogen emission and absorption. WN/C – WN stars plus strong C IV lines, intermediate between WN and WC stars. WC – spectrum with strong C II-IV lines.

Naked eye

naked-eyeunaided eyenaked human eye
In astronomy, the naked eye may be used to observe celestial events and objects visible without equipment, such as conjunctions, passing comets, meteor showers, and the brightest asteroids, including 4 Vesta. Sky lore and various tests demonstrate an impressive variety of phenomena visible to the unaided eye. The basic accuracies of the human eye are: Visual perception allows a person to gain much information about their surroundings: The visibility of astronomical objects is strongly affected by light pollution.

Main sequence

main-sequencemain sequence dwarfmain-sequence star
K5 || 0.74 || 0.69 || 0.16 || 4,410. style="text-align: left;"|61 Cygni A. M0 || 0.63 || 0.47 || 0.063 || 3,920. style="text-align: left;"|Gliese 185. M5 || 0.32 || 0.21 || 0.0079 || 3,120. style="text-align: left;"|EZ Aquarii A. M8 || 0.13 || 0.10 || 0.0008 || 2,660. style="text-align: left;"|Van Biesbroeck's star.


starsmassive starstellar radius
One of the most massive stars known is Eta Carinae, which, with 100–150 times as much mass as the Sun, will have a lifespan of only several million years. Studies of the most massive open clusters suggests as an upper limit for stars in the current era of the universe. This represents an empirical value for the theoretical limit on the mass of forming stars due to increasing radiation pressure on the accreting gas cloud.

Angular diameter

apparent diameterangular sizeapparent size
Thus the angular diameter of the Sun is about 250,000 times that of Sirius. (Sirius has twice the diameter and its distance is 500,000 times as much; the Sun is 10 10 times as bright, corresponding to an angular diameter ratio of 10 5, so Sirius is roughly 6 times as bright per unit solid angle.) The angular diameter of the Sun is also about 250,000 times that of Alpha Centauri A (it has about the same diameter and the distance is 250,000 times as much; the Sun is 4×10 10 times as bright, corresponding to an angular diameter ratio of 200,000, so Alpha Centauri A is a little brighter per unit solid angle). The angular diameter of the Sun is about the same as that of the Moon.


solarSolThe Sun
The proton–proton chain occurs around 9.2 times each second in the core, converting about 3.7 protons into alpha particles (helium nuclei) every second (out of a total of ~8.9 free protons in the Sun), or about 6.2 kg/s. Fusing four free protons (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of the fused mass as energy, so the Sun releases energy at the mass–energy conversion rate of 4.26 million metric tons per second (which requires 600 metric megatons of hydrogen ), for 384.6 yottawatts (3.846 W), or 9.192 megatons of TNT per second.

Stellar parallax

parallax shiftparallaxparallax method
The first successful measurements of stellar parallax were made by Friedrich Bessel in 1838 for the star 61 Cygni using a heliometer. 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.

Binary star

spectroscopic binaryeclipsing binarybinary
Other interesting binaries include 61 Cygni (a binary in the constellation Cygnus, composed of two K class (orange) main-sequence stars, 61 Cygni A and 61 Cygni B, which is known for its large proper motion), Procyon (the brightest star in the constellation Canis Minor and the eighth-brightest star in the night time sky, which is a binary consisting of the main star with a faint white dwarf companion), SS Lacertae (an eclipsing binary which stopped eclipsing), V907 Sco (an eclipsing binary which stopped, restarted, then stopped again) and BG Geminorum (an eclipsing binary which is thought to contain a black hole with a K0 star in orbit around it), 2MASS J18082002-5104378 (a binary in the "thin


constellationsEuropean constellationModern constellation
Three schools of classical Chinese astronomy in the Han period are attributed to astronomers of the earlier Warring States period. The constellations of the three schools were conflated into a single system by Chen Zhuo, an astronomer of the 3rd century (Three Kingdoms period). Chen Zhuo's work has been lost, but information on his system of constellations survives in Tang period records, notably by Qutan Xida. The oldest extant Chinese star chart dates to that period and was preserved as part of the Dunhuang Manuscripts.


The Andromeda Galaxy (M31) is about 780 kpc (2.5 million light-years) away from the Earth. The Andromeda Galaxy is about 0.78 Mpc (2.5 million light-years) from the Earth. The nearest large galaxy cluster, the Virgo Cluster, is about 16.5 Mpc (54 million light-years) from the Earth. The galaxy RXJ1242-11, observed to have a supermassive black hole core similar to the Milky Way's, is about 200 Mpc (650 million light-years) from the Earth. The galaxy filament Hercules–Corona Borealis Great Wall, currently the largest known structure in the universe, is about 3 Gpc (10 billion light-years) across.


Earth – Audio (29:28) – Cain/Gay – Astronomy Cast (2007). Earth – Videos – International Space Station:. Video (01:02) – Earth (time-lapse). Video (00:27) – Earth and auroras (time-lapse). United States Geological Survey – USGS. Google Earth 3D, interactive map.

Friedrich Bessel

BesselFriedrich Wilhelm BesselBessel, Friedrich
Astronomers had believed for some time that parallax would provide the first accurate measurement of interstellar distances—in fact, in the 1830s there was a fierce competition between astronomers to be the first to measure a stellar parallax accurately. In 1838 Bessel won the race, announcing that 61 Cygni had a parallax of 0.314 arcseconds; which, given the diameter of the Earth's orbit, indicated that the star is 10.3 ly away. Given the current measurement of 11.4 ly, Bessel's figure had an error of 9.6%. Nearly at the same time Friedrich Georg Wilhelm Struve and Thomas Henderson measured the parallaxes of Vega and Alpha Centauri.

Astronomical unit

AUastronomical unitsAUs
After Greek astronomy was transmitted to the medieval Islamic world, astronomers made some changes to Ptolemy's cosmological model, but did not greatly change his estimate of the Earth–Sun distance. For example, in his introduction to Ptolemaic astronomy, al-Farghānī gave a mean solar distance of 1170 Earth radii, whereas in his zij, al-Battānī used a mean solar distance of 1108 Earth radii. Subsequent astronomers, such as al-Bīrūnī, used similar values. Later in Europe, Copernicus and Tycho Brahe also used comparable figures (1142 and 1150 Earth radii), and so Ptolemy's approximate Earth–Sun distance survived through the 16th century.

Minute and second of arc

masarcsecondarc second
These units originated in Babylonian astronomy as sexagesimal subdivisions of the degree; they are used in fields that involve very small angles, such as astronomy, optometry, ophthalmology, optics, navigation, land surveying, and marksmanship. To express even smaller angles, standard SI prefixes can be employed; the milliarcsecond (mas) and microarcsecond (μas), for instance, are commonly used in astronomy. The number of square arcminutes in a complete sphere is 148,510,660 square arcminutes (the surface area of a unit sphere in square units divided by the solid angle area subtended by a square arcminute, also in square units - so that the final result is a dimensionless number).


light yearlight yearsly
The star was 61 Cygni, and he used a 6.2 in heliometer designed by Joseph von Fraunhofer. The largest unit for expressing distances across space at that time was the astronomical unit, equal to the radius of the Earth's orbit (1.50 km or 9.30 mi). In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be 660000 astronomical units (9.9 km or 6.1 mi). Bessel added that light employs 10.3 years to traverse this distance. He recognized that his readers would enjoy the mental picture of the approximate transit time for light, but he refrained from using the light-year as a unit.

Absolute magnitude

Hbolometric magnitudeabsolute magnitude (H)
Examples include Rigel (−7.0), Deneb (−7.2), Naos (−6.0), and Betelgeuse (−5.6). For comparison, Sirius has an absolute magnitude of 1.4, which is brighter than the Sun, whose absolute visual magnitude is 4.83 (it actually serves as a reference point). The Sun's absolute bolometric magnitude is set arbitrarily, usually at 4.75. Absolute magnitudes of stars generally range from −10 to +17. The absolute magnitudes of galaxies can be much lower (brighter). For example, the giant elliptical galaxy M87 has an absolute magnitude of −22 (i.e. as bright as about 60,000 stars of magnitude −10).

Proper motion

proper motionsproper-motionhigh proper motion star
The motion of the Andromeda Galaxy was measured in 2012, and an Andromeda–Milky Way collision is predicted in about 4 billion years. Proper motion of the NGC 4258 (M106) galaxy in the M106 group of galaxies was used in 1999 to find an accurate distance to this object. Measurements were made of the radial motion of objects in that galaxy moving directly toward and away from us, and assuming this same motion to apply to objects with only a proper motion, the observed proper motion predicts a distance to the galaxy of 7.2 Mpc.

Bayer designation

Of the 88 modern constellations, there are at least 30 in which "Alpha" is not the brightest star, and four of those lack an alpha star altogether. (Constellations with no alpha include Vela and Puppis – both formerly part of Argo Navis, whose alpha is Canopus in Carina.) Orion provides a good example of Bayer's method. Bayer first designated Betelgeuse and Rigel, the two 1st-magnitude stars (those of magnitude 1.5 or less), as Alpha and Beta from north to south, with Betelgeuse (the shoulder) coming ahead of Rigel (the foot), even though the latter is usually the brighter. (Betelgeuse is a variable star and can at its maximum occasionally outshine Rigel.)


luminousbolometric luminosityluminosities
In astronomy, luminosity is the total amount of energy emitted per unit of time by a star, galaxy, or other astronomical object. As a term for energy emitted per unit time, luminosity is synonymous with power. In SI units luminosity is measured in joules per second or watts. Values for luminosity are often given in the terms of the luminosity of the Sun, L ⊙ . Luminosity can also be given in terms of the astronomical magnitude system: the absolute bolometric magnitude (M bol ) of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a logarithmic measure of the luminosity within some specific wavelength range or filter band.

Solar System

outer Solar Systeminner Solar SystemSol system
The closest is the triple star system Alpha Centauri, which is about 4.4 light-years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the small red dwarf, Proxima Centauri, orbits the pair at a distance of 0.2 light-year. In 2016, a potentially habitable exoplanet was confirmed to be orbiting Proxima Centauri, called Proxima Centauri b, the closest confirmed exoplanet to the Sun. The stars next closest to the Sun are the red dwarfs Barnard's Star (at 5.9 ly), Wolf 359 (7.8 ly), and Lalande 21185 (8.3 ly).

Astronomical naming conventions

formally designatednaming conventionalso written
For example, Sirius, the brightest star in the sky, is actually a double star, consisting of the naked-eye Sirius A and its dim white-dwarf companion Sirius B. The first exoplanet tentatively identified around the second brightest star in the triple star system Alpha Centauri is accordingly called Alpha Centauri Bb. If an exoplanet orbits both of the stars in a binary system, its name can be, for example, Kepler-34(AB) b. Earth's natural satellite is simply known as the Moon, or the equivalent in the language being spoken (for instance, two astronomers speaking French would call it la Lune). It is sometimes called Luna (which is simply Latin for "moon"), in science fiction.

Milky Way

galaxyMilky Way Galaxyour galaxy
With the discovery that the disk of the Andromeda Galaxy (M31) extends much further than previously thought, the possibility of the disk of the Milky Way extending further is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the Cygnus Arm and of a similar extension of the Scutum-Centaurus Arm. With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart.

Hubble Space Telescope

The Space Telescope European Coordinating Facility (ST-ECF), established at Garching bei München near Munich in 1984, provided similar support for European astronomers until 2011, when these activities were moved to the European Space Astronomy Centre. One rather complex task that falls to STScI is scheduling observations for the telescope. Hubble is in a low-Earth orbit to enable servicing missions, but this means that most astronomical targets are occulted by the Earth for slightly less than half of each orbit.


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.