Sun

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.

Earth

Earth's surfaceterrestrialworld
Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.0989 seconds of mean solar time (UT1), or 23 56 4.0989. Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86,164.0905 seconds of mean solar time (UT1) (23 56 4.0905). Thus the sidereal day is shorter than the stellar day by about 8.4 ms. The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005 and 1962–2005.

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.

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).

Absolute magnitude

Hbolometric magnitudeabsolute magnitude (H)
The luminosity of the star in watts can be calculated as a function of its absolute bolometric magnitude M bol as: using the variables as defined previously. For planets and asteroids a definition of absolute magnitude that is more meaningful for non-stellar objects is used. The absolute magnitude, commonly called H, is defined as the apparent magnitude that the object would have if it were one Astronomical Unit (AU) from both the Sun and the observer, and in conditions of ideal solar opposition (an arrangement that is impossible in practice).

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.

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.

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 VV Cephei, Antares, and Betelgeuse, which are also class M.

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.

Orders of magnitude (length)

mm100 nmGm
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.

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
It is a high-mass X-ray binary, with the optical counterpart being a variable star. 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.

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.

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.

Astrometry

astrometricastrometristastrometrical
When telescopes became commonplace, setting circles sped measurements James Bradley first tried to measure stellar parallaxes in 1729. The stellar movement proved too insignificant for his telescope, but he instead discovered the aberration of light and the nutation of the Earth's axis. His cataloguing of 3222 stars was refined in 1807 by Friedrich Bessel, the father of modern astrometry. 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.

Proper motion

proper motionsproper-motionhigh proper motion star
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. 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.

Luminosity

luminousbolometric luminosityluminosities
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.

List of nearest stars and brown dwarfs

List of nearest starspassing starsnearest stars
Despite the relative proximity of these 78 objects to Earth, only nine are bright enough in visible light to reach or exceed the dimmest brightness to be visible to the naked eye from Earth, 6.5 apparent magnitude. All of these objects are currently moving 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 parsecs in 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.

International Astronomical Union

IAUWorking Group for Planetary System NomenclatureInternational Astronomical Union (IAU)
List of proper names of stars. Planetary nomenclature. XXVIth General Assembly 2006. XXVIIth General Assembly 2009. XXVIIIth General Assembly 2012. XXIXth General Assembly 2015. XXXth General Assembly 2018.

Parallax

trigonometric parallaxsolar parallaxmotion parallax
The parsec (3.26 light-years) is defined as the distance for which the annual parallax is 1 arcsecond. Annual parallax is normally measured by observing the position of a star at different times of the year as the Earth moves through its orbit. Measurement of annual parallax was the first reliable way to determine the distances to the closest stars. The first successful measurements of stellar parallax were made by Friedrich Bessel in 1838 for the star 61 Cygni using a heliometer. Stellar parallax remains the standard for calibrating other measurement methods.

Arcturus

ArcturiansArcturianAlpha Boo
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).

Canopus

Alpha Carinaeα Carinaea first magnitude star
Canopus is the brightest star in the southern constellation of Carina, and is located near the western edge of the constellation around 310 light-years from the Sun. Its proper name is generally considered to originate from the mythological Canopus, who was a navigator for Menelaus, king of Sparta. Canopus has the Bayer designation α Carinae, which is Latinised to Alpha Carinae and abbreviated Alpha Car or α Car. It is the second-brightest star in the night sky, after Sirius. Canopus' visual apparent magnitude is −0.74, and it has an absolute magnitude of −5.71. Canopus is an aging bright giant of spectral type A9 or F0, so it is essentially white when seen with the naked eye.

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. Angular diameter. Compact star. List of exoplanet extremes. List of extremes in the sky. Peculiar velocity. Proper motion. Radial velocity. Rotational speed. Star. Star system.

Rigel

Beta Orionisβ Ori (Rigel)β Orionis
As such, much of our understanding about their characteristics is based on theoretical stellar evolution models. Although Rigel is often considered the most luminous star within 1,000 light-years of the Sun, its energy output is poorly known. For example, using the Hipparcos distance of 264 pc, the estimated relative luminosity for Rigel is about 120,000 times that of the Sun, but another recently published distance of 360 ± suggests an even higher luminosity of.

Star system

multiple star systemtriple starmultiple systems
Alpha Centauri, APOD, 2002 April 25. Alpha Centauri, APOD, 2002 April 25.