Absolute magnitude

Hbolometric magnitudeabsolute magnitude (H)
Placing a radiation source (e.g. star) at the standard distance of 10 parsecs, it follows that the zero point of the apparent bolometric magnitude scale m bol = 0 corresponds to irradiance f 0 = 2.518021002 W/m 2. Using the IAU 2015 scale, the nominal total solar irradiance ("solar constant") measured at 1 astronomical unit (1361 W/m 2) corresponds to an apparent bolometric magnitude of the Sun of m bol,⊙ = −26.832.


Even if the Sun were eternal and stable, 27% of the water in the modern oceans will descend to the mantle in one billion years, due to reduced steam venting from mid-ocean ridges. The Sun will evolve to become a red giant in about 5 Bys. Models predict that the Sun will expand to roughly 1 AU, about 250 times its present radius. Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, Earth will move to an orbit 1.7 AU from the Sun when the star reaches its maximum radius. Most, if not all, remaining life will be destroyed by the Sun's increased luminosity (peaking at about 5,000 times its present level).

Milky Way

galaxyMilky Way Galaxyour galaxy
On the other hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarfs) within 5 pc of the Sun, giving a density of about one star per 8.2 cubic parsecs, or one per 284 cubic light-years (from List of nearest stars). This illustrates the fact that there are far more faint stars than bright stars: in the entire sky, there are about 500 stars brighter than apparent magnitude 4 but 15.5 million stars brighter than apparent magnitude 14. The apex of the Sun's way, or the solar apex, is the direction that the Sun travels through space in the Milky Way.

61 Cygni

61 Cyg61 Cygni A61 Cyg A
This corresponds to a distance of about 600,000 astronomical units, or about 10.4 light-years. This was the first direct and reliable measurement of the distance to a star other than the Sun. His measurement was published only shortly before similar parallax measurements of Vega by Friedrich Georg Wilhelm von Struve and Alpha Centauri by Thomas Henderson that same year. Bessel continued to make additional measurements at Königsberg, publishing a total of four complete observational runs, the last in 1868. The best of these placed the center point at 360.2 ±12.1 mas, made during observations in 1849. This is close to the currently accepted value of 287.18 mas (yielding 11.36 light-years).


α OriBetelg'''euseBetelgeuse mass loss
Betelgeuse, also designated α Orionis (Latinised to Alpha Orionis, abbreviated Alpha Ori, α Ori), is on average the ninth-brightest star in the night sky and second-brightest in the constellation of Orion. It is distinctly reddish, and is a semiregular variable star whose apparent magnitude varies between 0.0 and 1.3, the widest range of any first-magnitude star. Betelgeuse is one of three stars that make up the Winter Triangle asterism, and it marks the center of the Winter Hexagon. If the human eye could view all wavelengths of radiation, Betelgeuse would be the brightest star in the night sky.

Proxima Centauri

Alpha Proximaits host starProxima
Proxima Centauri, or Alpha Centauri C, is a red dwarf, a small low-mass star, about 4.244 ly from the Sun in the constellation of Centaurus. It was discovered in 1915 by Robert Innes and is the nearest-known star to the Sun. With a quiescent apparent magnitude of 11.13, it is too faint to be seen with the naked eye. Proxima Centauri forms a third component of the Alpha Centauri system, currently with a separation of about 12950 AU and an orbital period of 550,000 years. At present Proxima is 2.18° to the southwest of Alpha Centauri. Because of Proxima Centauri's proximity to Earth, its angular diameter can be measured directly. The star is about one-seventh the diameter of the Sun.

Eta Carinae

η Carinaeη CarEta Carinae A
Observations at the Cape of Good Hope indicated it peaked in brightness, surpassing Canopus, over March 11 to 14, 1843 before beginning to fade, then brightened to between the brightness of Alpha Centauri and Canopus between March 24 and 28 before fading once again. For much of 1844 the brightness was midway between Alpha Centauri and Beta Centauri, around magnitude +0.2, before brightening again at the end of the year. At its brightest in 1843 it likely reached an apparent magnitude of −0.8, then −1.0 in 1845. The peaks in 1827, 1838, and 1843 are likely to have occurred at the periastron passage—the point the two stars are closest together—of the binary orbit.


2828Botercadentconstellation of Vega
Vega, also designated α Lyrae (Latinised to Alpha Lyrae, abbreviated Alpha Lyr or α Lyr), is the brightest star in the constellation of Lyra, the fifth-brightest star in the night sky, and the second-brightest star in the northern celestial hemisphere, after Arcturus. It is relatively close at only 25 light-years from the Sun, and, together with Arcturus and Sirius, one of the most luminous stars in the Sun's neighborhood. Vega has been extensively studied by astronomers, leading it to be termed “arguably the next most important star in the sky after the Sun”. Vega was the northern pole star around 12,000 BC and will be so again around the year 13,727, when the declination will be +86°14'.


Jovianplanet JupiterGiove
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a giant planet with a mass one-thousandth that of the Sun, but two-and-a-half times that of all the other planets in the Solar System combined. Jupiter and Saturn are gas giants; the other two giant planets, Uranus and Neptune, are ice giants. Jupiter has been known to astronomers since antiquity. It is named after the Roman god Jupiter. When viewed from Earth, Jupiter can reach an apparent magnitude of −2.94, bright enough for its reflected light to cast shadows, and making it on average the third-brightest natural object in the night sky after the Moon and Venus.

Solar System

outer Solar Systeminner Solar SystemSol system
The Local Interstellar Cloud is an area of denser cloud in an otherwise sparse region known as the Local Bubble, an hourglass-shaped cavity in the interstellar medium roughly 300 light-years (ly) across. The bubble is suffused with high-temperature plasma, that suggests it is the product of several recent supernovae. There are relatively few stars within ten light-years of the Sun. 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.

Angular diameter

apparent diameterangular sizeapparent size
an object of diameter 1 cm at a distance of 2.06 km. an object of diameter 725.27 km at a distance of 1 astronomical unit (AU). an object of diameter 45 866 916 km at 1 light-year. an object of diameter 1 AU (149 597 871 km) at a distance of 1 parsec (pc). 360 degrees in a full circle. 60 arc-minutes in one degree. 60 arc-seconds in one arc-minute. Angular diameter distance. Angular resolution. Solid angle. Visual acuity. Visual angle. Visual Angle Illusion. List of stars with resolved images. Small-Angle Formula. Visual Aid to the Apparent Size of the Planets.

Mercury (planet)

Mercuryplanet MercuryMercurio
Mercury's apparent magnitude is calculated to vary between −2.48 (brighter than Sirius) around superior conjunction and +7.25 (below the limit of naked-eye visibility) around inferior conjunction. The mean apparent magnitude is 0.23 while the standard deviation of 1.78 is the largest of any planet. The mean apparent magnitude at superior conjunction is −1.89 while that at inferior conjunction is +5.93. Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the Sun's glare for much of the time. Mercury can be observed for only a brief period during either morning or evening twilight.


α Carinaea first magnitude starCanopean
Before the launch of the Hipparcos satellite telescope, distance estimates for Canopus varied widely, from 96 light-years to 1200 light-years. Had the latter distance been correct, Canopus would have been one of the most luminous stars in the Milky Way galaxy. Hipparcos established Canopus as being 310 light-years (96 parsecs) from the Solar System; this is based on its 2007 parallax measurement of 10.43 ± 0.53 mas. Canopus has an MK spectral type of A9 II, although it has also been classified as F0Ib (Ib referring to "less luminous supergiant") on account of its high luminosity, or F0II. The effective temperature of Canopus has been measured to be 6,998 K.

Galactic Center

galactic centregalactic corecenter
Star formation does not seem to be occurring currently at the Galactic Center, although the Circumnuclear Disk of molecular gas that orbits the Galactic Center at two parsecs seems a fairly favorable site for star formation. Work presented in 2002 by Antony Stark and Chris Martin mapping the gas density in a 400-light-year region around the Galactic Center has revealed an accumulating ring with a mass several million times that of the Sun and near the critical density for star formation.

Minute and second of arc

masarcsecondarc second
For instance, a toolmaker's optical comparator will often include an option to measure in "minutes and seconds". * MOA / mils By Robert Simeone an object of diameter 725.27 km at a distance of one astronomical unit. an object of diameter 45,866,916 km at one light-year. an object of diameter one astronomical unit (149,597,871 km) at a distance of one parsec, by definition. Hubble Space Telescope has calculational resolution of 0.05 arcseconds and actual resolution of almost 0.1 arcseconds, which is close to the diffraction limit. crescent Venus measures between 60.2 and 66 seconds of arc.

List of nearest stars and brown dwarfs

passing starsnearest starsclosest stars
Despite the relative proximity of these objects to Earth, only nine (not including the Sun) are brighter than 6.5 apparent magnitude, the dimmest magnitude visible to the naked eye from Earth. All of these objects are located in the Local Bubble, a region within the Orion–Cygnus Arm of the Milky Way. Based on results from the Gaia telescope's second data release from April 2018, an estimated 694 stars will possibly approach the Solar system to less than 5.0 pc over the next 15 million years. Of these, 26 have a good probability to come within 1.0 pc and another 7 within 0.5 pc.

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" ).


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).


Saturn's atmosphereExploration of Saturnhome planet
The mean apparent magnitude of Saturn is 0.46 with a standard deviation of 0.34. Most of the magnitude variation is due to the inclination of the ring system relative to the Sun and Earth. The brightest magnitude, -0.55, occurs near in time to when the plane of the rings is inclined most highly, and the faintest magnitude, 1.17, occurs around the time when they are least inclined. It takes approximately 29.5 years for the planet to complete an entire circuit of the ecliptic against the background constellations of the zodiac.

Orders of magnitude (length)

Comet of 1910. 9.5 Pm – 63,241.1 AU – One light year, the distance travelled by light in one year. 15 Pm – 1.59 light years – Possible outer radius of Oort cloud. 20 Pm – 2.11 light years – maximum extent of influence of the Sun's gravitational field. 30.9 Pm – 3.26 light years – 1 parsec. 39.9 Pm – 4.22 light years – Distance to Proxima Centauri (nearest star to Sun). 81.3 Pm – 8.59 light years – Distance to Sirius. 110 Pm – 12 light years – Distance to Tau Ceti. 230 Pm – 24 light years – Diameter of the Orion Nebula. 240 Pm – 25 light years – Distance to Vega. 260 Pm – 27 light years – Distance to Chara, a star approximately as bright as our Sun.

Red dwarf

redred dwarf starsred dwarfs
A red dwarf (or M dwarf) is a small and cool star on the main sequence, of M spectral type. Red dwarfs range in mass from about 0.075 to about 0.50 solar mass and have a surface temperature of less than 4,000 K. Sometimes K-type main-sequence stars, with masses between 0.50-0.8 solar mass, are also included. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs cannot be easily observed. From Earth, not one is visible to the naked eye. Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are fifty of the sixty nearest stars.

Binary star

spectroscopic binaryeclipsing binarybinary
Monotonic period increases have been attributed to mass transfer, usually (but not always) from the less massive to the more massive star The components of binary stars are denoted by the suffixes A and B appended to the system's designation, A denoting the primary and B the secondary. The suffix AB may be used to denote the pair (for example, the binary star α Centauri AB consists of the stars α Centauri A and α Centauri B.) Additional letters, such as C, D, etc., may be used for systems with more than two stars.

Extraterrestrial skies

Mercurian daylunar skynight sky
The angular separation between the two stars would not be exactly the same after one orbit of the planet however, because during that time the parent star will have completed part of its orbit around the other star in the system. From 40 Eridani, 16 light years away, the Sun would be an average looking star of about apparent magnitude 3.3 in the constellation Serpens Caput. At this distance most of the stars nearest to us would be in different locations to those in our sky, including Alpha Centauri, Sirius, and Procyon.

Proper motion

proper motionsproper-motionhigh proper motion star
Barnard's star has the largest proper motion of all stars, moving at 10.3 seconds of arc per year. Large proper motion is usually a strong indication that a star is relatively close to the Sun. This is indeed the case for Barnard's Star, located at a distance of about 6 light-years. After the Sun and the Alpha Centauri system, it is the nearest known star to Earth. Because it is a red dwarf with an apparent magnitude of 9.54, it is too faint to see without a telescope or powerful binoculars. A proper motion of 1 arcsec per year at a distance of 1 light-year corresponds to a relative transverse speed of 1.45 km/s.

International Astronomical Union

IAUInternational Astronomical Union (IAU)I.A.U.
Working groups include the Working Group for Planetary System Nomenclature (WGPSN), which maintains the astronomical naming conventions and planetary nomenclature for planetary bodies, and the Working Group on Star Names (WGSN), which catalogs and standardizes proper names for stars. The IAU is also responsible for the system of astronomical telegrams which are produced and distributed on its behalf by the Central Bureau for Astronomical Telegrams. The Minor Planet Center also operates under the IAU, and is a "clearinghouse" for all non-planetary or non-moon bodies in the Solar System.