solarSolThe Sun
The heavy elements could most plausibly have been produced by endothermic nuclear reactions during a supernova, or by transmutation through neutron absorption within a massive second-generation star. The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, Sirius, which has an apparent magnitude of −1.46. The mean distance of the Sun's center to Earth's center is approximately 1 AU, though the distance varies as Earth moves from perihelion in January to aphelion in July.

Absolute magnitude

Hbolometric magnitudeabsolute magnitude (H)
The absolute magnitude can also be approximated using apparent magnitude and stellar parallax : or using apparent magnitude and distance modulus : Rigel has a visual magnitude m V of 0.12 and distance about 860 light-years Vega has a parallax of 0.129″, and an apparent magnitude m V of 0.03 Alpha Centauri A has a parallax of 0.742″ and an apparent magnitude m V of −0.01 The Black Eye Galaxy has a visual magnitude m V of 9.36 and a distance modulus of 31.06 The bolometric magnitude M bol, takes into account electromagnetic radiation at all wavelengths. It includes those unobserved due to instrumental passband, the Earth's atmospheric absorption, and extinction by interstellar dust.


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). A 2008 simulation indicates that Earth's orbit will eventually decay due to tidal effects and drag, causing it to enter the Sun's atmosphere and be vaporized. The shape of Earth is approximately oblate spheroidal.

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.

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.

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.

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).

International Astronomical Union

IAUInternational Astronomical Union (IAU)I.A.U.
List of proper names of stars. XXVIth General Assembly 2006. XXVIIth General Assembly 2009. XXVIIIth General Assembly 2012. XXIXth General Assembly 2015. XXXth General Assembly 2018.


Jovianplanet JupiterGiove
Depending on Jupiter's position with respect to the Earth, it can vary in visual magnitude from as bright as −2.94 at opposition down to −1.66 during conjunction with the Sun. The mean apparent magnitude is -2.20 with a standard deviation of 0.33. The angular diameter of Jupiter likewise varies from 50.1 to 29.8 arc seconds. Favorable oppositions occur when Jupiter is passing through perihelion, an event that occurs once per orbit. Earth overtakes Jupiter every 398.9 days as it orbits the Sun, a duration called the synodic period. As it does so, Jupiter appears to undergo retrograde motion with respect to the background stars.

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.

Magnitude (astronomy)

Two of the main types of magnitudes distinguished by astronomers are: The difference between these concepts can be seen by comparing two stars. Betelgeuse (apparent magnitude 0.5, absolute magnitude −5.8) appears slightly dimmer in the sky than Alpha Centauri (apparent magnitude 0.0, absolute magnitude 4.4) even though it emits thousands of times more light, because Betelgeuse is much farther away.


α Carinaea first magnitude starCanopean
Canopus, also designated α Carinae (Latinised to Alpha Carinae, abbreviated Alpha Car, α Car), is the brightest star in the southern constellation of Carina, and the second-brightest star in the night sky, after Sirius. Canopus' visual magnitude is −0.74, and it has an absolute magnitude of −5.71. Canopus is a bright giant of spectral type A9, so it is essentially white when seen with the naked eye. It is located in the far southern sky, at a year 2000 declination of −52°42′ and a right ascension of 06:24.0:0. Its name is generally considered to originate from the mythological Canopus, who was a navigator for Menelaus, king of Sparta (see below).

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.

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.

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.

Friedrich Bessel

BesselFriedrich Wilhelm BesselBessel, Friedrich
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. As well as helping determine the parallax of 61 Cygni, Bessel's precise measurements using a new meridian circle from Adolf Repsold allowed him to notice deviations in the motions of Sirius and Procyon, which he deduced must be caused by the gravitational attraction of unseen companions.


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).


luminousbolometric luminosityluminosities
For stars on the main sequence, luminosity is also related to mass: The magnitude of a star, a unitless measure, is a logarithmic scale of observed visible brightness. The apparent magnitude is the observed visible brightness from Earth which depends on the distance of the object. The absolute magnitude is the apparent magnitude at a distance of 10 parsecs, therefore the bolometric absolute magnitude is a logarithmic measure of the bolometric luminosity.

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.


2828Botercadentconstellation of Vega
This apparent magnitude is a numerical value that decreases in value with increasing brightness of the star. 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.

Extraterrestrial skies

Mercurian daylunar skynight sky
Due to the proximity of the Alpha Centauri system, the constellations would, for the most part, appear similar. However, there are some notable differences with the position of other nearby stars; for example, Sirius would appear about one degree from the star Betelgeuse in the constellation Orion. Also, Procyon would appear in the constellation Gemini, about 13 degrees below Pollux. A planet around either α Centauri A or B would see the other star as a very bright secondary.


β Ori (Rigel)B2 IaeBeta Orionis
The apparent visual magnitude of Rigel is 0.13, making it on average the seventh-brightest star in the celestial sphere excluding the Sun—just fainter than Capella. It is an irregular pulsating variable with a visual range of magnitude 0.05–0.18. Although Rigel has the Bayer designation "beta", it is almost always brighter than Alpha Orionis (Betelgeuse). Since 1943, the spectrum of this star has served as the spectral reference, for class B8Ia, by which other stars are classified. Rigel has a color index (B–V) of −0.03, meaning it appears white or slightly blue-white.

Night sky

night skiessky''' of the Earthsky of Earth
The stars of the night sky cannot be counted unaided because they are so numerous and there is no way to track which have been counted and which have not. Further complicating the count, fainter stars may appear and disappear depending on exactly where the observer is looking. The result is an impression of an extraordinarily vast star field. Because stargazing is best done from a dark place away from city lights, dark adaptation is important to achieve and maintain. It takes several minutes for eyes to adjust to the darkness necessary for seeing the most stars, and surroundings on the ground are hard to discern.

Open cluster

open star clusterstar clusteropen clusters
The core is typically about 3–4 light years across, with the corona extending to about 20 light years from the cluster centre. Typical star densities in the centre of a cluster are about 1.5 stars per cubic light year; the stellar density near the Sun is about 0.003 stars per cubic light year. Open clusters are often classified according to a scheme developed by Robert Trumpler in 1930.

Stellar classification

spectral typeK-typeG-type
However, class M main-sequence stars (red dwarfs) have such low luminosities that none are bright enough to be seen with the unaided eye, unless under exceptional conditions. The brightest known M-class main-sequence star is M0V Lacaille 8760, with magnitude 6.6 (the limiting magnitude for typical naked-eye visibility under good conditions is typically quoted as 6.5), and it is extremely unlikely that any brighter examples will be found. Although most class M stars are red dwarfs, most of the largest ever supergiant stars in the Milky Way are M stars, such as VY Canis Majoris, Antares and Betelgeuse, which are also class M.