Earth

Earth's surfaceterrestrialworld
Anthropogenic emissions are "probably insufficient" to cause a runaway greenhouse at current solar luminosity. 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 billion years. 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.

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).

Jupiter

JovianGioveplanet Jupiter
Jupiter is usually the fourth brightest object in the sky (after the Sun, the Moon and Venus); at times Mars is brighter than 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.

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.

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.

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.

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
There are also giant K-type stars, which range from hypergiants like RW Cephei, to giants and supergiants, such as Arcturus, whereas orange dwarfs, like Alpha Centauri B, are main-sequence stars. They have extremely weak hydrogen lines, if those are present at all, and mostly neutral metals (Mn I, Fe I, Si I). By late K, molecular bands of titanium oxide become present. There is a suggestion that K-spectrum stars may potentially increase the chances of life developing on orbiting planets that are within the habitable zone. Spectral standards: Class M stars are by far the most common. About 76% of the main-sequence stars in the solar neighborhood are class M stars.

Naked eye

naked-eyeunaided eyenaked human eye
Under typical dark sky conditions Uranus (magnitude +5.8) can be seen as well with averted vision, as can the asteroid Vesta at its brighter oppositions. The Sun and the Moon—the remaining noticeable naked-eye objects of the solar system—are sometimes added to make seven "planets." During daylight only the Moon and Sun are obvious naked eye objects, but in many cases Venus can be spotted in daylight and in rarer cases Jupiter. Close to sunset and sunrise bright stars like Sirius or even Canopus can be spotted with the naked eye as long as one knows the exact position in which to look. Historically, the zenith of naked-eye astronomy was the work of Tycho Brahe (1546–1601).

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).

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.

Proxima Centauri

Alpha Centauri Cits host starProxima
Proxima Centauri is a small, low-mass star located 4.244 ly away from the Sun in the southern constellation of Centaurus. Its Latin name means the "nearest [star] of Centaurus". This object 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 member of the Alpha Centauri system, being identified as component Alpha Centauri C, and is 2.18° to the southwest of the Alpha Centauri AB pair. Currently it has a physical separation of about 12950 AU from AB and an orbital period of 550,000 years.

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.

Vega

Alpha Lyrae2828Botercadent
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.

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.

Mercury (planet)

MercuryMercurioplanet Mercury
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.

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.

Main sequence

main-sequencemain-sequence starmain sequence dwarf
(A popular mnemonic for memorizing this sequence of stellar classes is "Oh Be A Fine Girl/Guy, Kiss Me".) The luminosity class ranged from I to V, in order of decreasing luminosity. Stars of luminosity class V belonged to the main sequence. 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.

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.

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.

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.

Eta Carinae

η Carinaeη CarEta Carinae A
Eta Carinae (η Carinae, abbreviated to η Car), formerly known as Eta Argus, is a stellar system containing at least two stars with a combined luminosity greater than five million times that of the Sun, located around 7,500 light-years (2,300 parsecs) distant in the constellation Carina. Previously a 4th-magnitude star, it brightened in 1837 to become brighter than Rigel marking the start of the Great Eruption. Eta Carinae became the second-brightest star in the sky between 11 and 14 March 1843 before fading well below naked eye visibility after 1856. In a smaller eruption, it reached 6th magnitude in 1892 before fading again.

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.

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.