Star

starsstellarmassive star
Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years. As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.

Earth

Earth's surfaceterrestrialworld
Earth, along with the Solar System, is situated in the Milky Way and orbits about 28,000 light-years from its center. It is about 20 light-years above the galactic plane in the Orion Arm. The axial tilt of the Earth is approximately 23.439281° with the axis of its orbit plane, always pointing towards the Celestial Poles. Due to Earth's axial tilt, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes the seasonal change in climate, with summer in the Northern Hemisphere occurring when the Tropic of Cancer is facing the Sun, and winter taking place when the Tropic of Capricorn in the Southern Hemisphere faces the Sun.

Stellar classification

spectral typeK-typeG-type
A1V – Sirius A. A2Ia – Deneb. A3Va – Fomalhaut. F0IIIa – Zeta Leonis. F0Ib – Alpha Leporis. F2V – 78 Ursae Majoris. G0V – Beta Canum Venaticorum. G0IV – Eta Boötis. G0Ib – Beta Aquarii. G2V – Sun. G5V – Kappa Ceti. G5IV – Mu Herculis. G5Ib – 9 Pegasi. G8V – 61 Ursae Majoris. G8IV – Beta Aquilae. G8IIIa – Kappa Geminorum. G8IIIab – Epsilon Virginis. G8Ib – Epsilon Geminorum. K0V – Sigma Draconis. K0III – Pollux. K0III – Epsilon Cygni. K2V – Epsilon Eridani. K2III – Kappa Ophiuchi. K3III – Rho Boötis. K5V – 61 Cygni A.

Sun

solarSolThe Sun
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. At this average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 19 seconds, while light from the closest points of the Sun and Earth takes about two seconds less.

Angular diameter

apparent diameterangular sizeapparent size
(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.

Parsec

Mpcpckpc
One parsec is approximately 3.26 light-years. Proxima Centauri, the nearest known star to earth other than the sun, is about 1.3 parsecs (4.24 light-years) away, by direct parallax measurement. The distance to the open cluster Pleiades is 130 pc (420 ly) from us, per Hipparcos parallax measurement. The centre of the Milky Way is more than 8 kiloparsecs (26000 ly) from the Earth, and the Milky Way is roughly 34 kpc (110000 ly) across. 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.

Naked eye

naked-eyeunaided eyenaked human eye
Under such "typical" dark sky conditions, the naked eye can see stars with an apparent magnitude up to +6 m. Under perfect dark sky conditions where all light pollution is absent, stars as faint as +8 m might be visible. The angular resolution of the naked eye is about 1′; however, some people have sharper vision than that. There is anecdotal evidence that people had seen the Galilean moons of Jupiter before telescopes were invented.

Proper motion

proper motionsproper-motionhigh proper motion star
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. Of the stars visible to the naked eye (by convention, limiting visual magnitude of 6.0), 61 Cygni A (magnitude V=5.20) has the highest proper motion at 5.281 arcsec/a, although Groombridge 1830 (magnitude V=6.42), proper motion 7.058 arcsec/a, might be visible for an observer with exceptionally keen vision.

Astronomical unit

AUastronomical unitsAUs
Examples are listed by increasing distance. * rowspan=5 valign=top|1 astronomical unit. = 149597870700 metres (exactly). ≈ 92.955807 million miles. ≈ 499.00478384 light-seconds. ≈ 4.8481368 millionths (4.8481368E-6) of a parsec. ≈ 15.812507 millionths (15.812507E-6) of a light-year. }. ≈ 499.00478384 light-seconds. ≈ 4.8481368 millionths (4.8481368E-6) of a parsec. ≈ 15.812507 millionths (15.812507E-6) of a light-year. }. ≈ 15.812507 millionths (15.812507E-6) of a light-year. }. Orders of magnitude (length). Gigametre. The IAU and astronomical units. Recommendations concerning Units (HTML version of the IAU Style Manual). Chasing Venus, Observing the Transits of Venus. Transit of Venus.

Magnitude (astronomy)

magnitudemagnitudesmag
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.

Friedrich Bessel

Friedrich Wilhelm BesselBesselBessel, Friedrich
In 1838 Bessel announced 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.

Jupiter

JovianGioveplanet Jupiter
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.

Binary star

spectroscopic binaryeclipsing binarybinary
Sirius is another binary and the brightest star in the night time sky, with a visual apparent magnitude of −1.46. It is located in the constellation Canis Major. In 1844 Friedrich Bessel deduced that Sirius was a binary. In 1862 Alvan Graham Clark discovered the companion (Sirius B; the visible star is Sirius A). In 1915 astronomers at the Mount Wilson Observatory determined that Sirius B was a white dwarf, the first to be discovered. In 2005, using the Hubble Space Telescope, astronomers determined Sirius B to be 12000 km in diameter, with a mass that is 98% of the Sun. An example of an eclipsing binary is Epsilon Aurigae in the constellation Auriga.

List of nearest stars and brown dwarfs

List of nearest starspassing starsnearest stars
Many brown dwarfs are not listed by visual magnitude but are listed by near-infrared J band apparent magnitude due to how dim (and often invisible) they are in visible color bands (U, B or V). Absolute magnitude (with electromagnetic wave, 'light' band denoted in subscript) is a measurement at a 10-parsec distance across imaginary empty space devoid of all its sparse dust and gas. Some of the parallaxes and resultant distances are rough measurements. Over long periods of time, the slow independent motion of stars change in both relative position and in their distance from the observer.

Astrometry

astrometricastrometristastrometrical
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.

Absolute magnitude

Hbolometric magnitudeabsolute magnitude (H)
To compare the magnitudes of very distant objects with those of local objects, a K correction might have to be applied to the magnitudes of the distant objects. The absolute magnitude M can also be approximated using apparent magnitude m and stellar parallax p: or using apparent magnitude m and distance modulus μ: Rigel has a visual magnitude m V of 0.12 and distance about 860 light-years : Vega has a parallax p of 0.129″, and an apparent magnitude m V of 0.03 : The Black Eye Galaxy has a visual magnitude m V of 9.36 and a distance modulus μ of 31.06 : The bolometric magnitude M bol, takes into account electromagnetic radiation at all wavelengths.

Milky Way

Milky Way Galaxygalaxyour galaxy
There are about 208 stars brighter than absolute magnitude 8.5 within a sphere with a radius of 15 pc from the Sun, giving a density of one star per 69 cubic parsecs, or one star per 2,360 cubic light-years (from List of nearest bright stars). 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).

Solar System

outer Solar Systeminner Solar Systemouter planets
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.

Arcturus

ArcturiansArcturianAlpha Boo
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). However, α Centauri AB is a binary star, whose components are both fainter than Arcturus. This makes Arcturus the third-brightest individual star, just ahead of α Centauri A (officially named Rigil Kentaurus), whose apparent magnitude is −0.01.

Luminosity

luminousbolometric luminosityluminosities
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. The difference in bolometric magnitude between two objects is related to their luminosity ratio according to: where: The zero point of the absolute magnitude scale is actually defined as a fixed luminosity of 3.0128 W.

Chinese star names

Chinese nameChinese uranographyTraditional Chinese star names
Added Stars The Rooftop mansion corresponds to constellations Andromeda, Aquarius, Cepheus, Cygnus, Draco, Lacerta, Pegasus, Piscis Austrinus and Vulpecula. Added Stars The Encampment mansion corresponds to constellations Andromeda, Aquarius, Capricornus, Cassiopeia, Cepheus, Cygnus, Lacerta, Pegasus, Pisces and Piscis Austrinus. Added Stars The Wall mansion corresponds to constellations Andromeda, Cetus, Pegasus and Pisces. Added Stars The Legs mansion represents the tail of White Tiger. It corresponds to constellations Andromeda, Cassiopeia, Cetus, Pisces and Triangulum. Added Stars The Bond mansion represents the body of White Tiger.

Main sequence

main-sequencemain-sequence starmain sequence dwarf
In April 2018, astronomers reported the detection of the most distant "ordinary" (i.e., main sequence) star, named Icarus (formally, MACS J1149 Lensed Star 1), at 9 billion light-years away from Earth. When a protostar is formed from the collapse of a giant molecular cloud of gas and dust in the local interstellar medium, the initial composition is homogeneous throughout, consisting of about 70% hydrogen, 28% helium and trace amounts of other elements, by mass. The initial mass of the star depends on the local conditions within the cloud. (The mass distribution of newly formed stars is described empirically by the initial mass function.)

List of star extremes

most extreme stars
A star is a sphere that is mainly composed of hydrogen and plasma, held together by gravity and is able to produce light through nuclear fusion. Stars exhibit many diverse properties, resulting from different masses, volumes, velocities, stage in stellar evolution and even proximity to earth. Some of these properties are considered extreme and sometimes disproportionate by astronomers.

Minute and second of arc

masarcsecondarc second
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 (149,597,870.7 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. To adjust a 1⁄2 MOA scope 3 MOA down and 1.5 MOA right, the scope needs to be adjusted 3 × 2 = 6 clicks down and 1.5 x 2 = 3 clicks right.

List of brightest stars

brightest starsbrightest starone of the brightest stars
However, other kinds of magnitude systems do exist based on different wavelengths, some well away from the distribution of the visible wavelengths of light, and these apparent magnitudes vary dramatically in the different systems. For example, Betelgeuse has the K-band (infrared) apparent magnitude of −4.05. Some stars, like Betelgeuse and Antares, are variable stars, changing their magnitude over days, months or years. In the table, the range of variation is indicated with var . Single magnitude values quoted for variable stars come from a variety of sources.