Milky Way

Milky Way Galaxygalaxyour galaxy
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. The general direction of the Sun's Galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center.

Night sky

night skiessky''' of the Earthsky of Earth
Several sources can be identified as the source of the intrinsic brightness of the sky, namely airglow, indirect scattering of sunlight, scattering of starlight, and artificial light pollution. Depending on local sky cloud cover, pollution, humidity, and light pollution levels, the stars visible to the unaided naked eye appear as hundreds, thousands or tens of thousands of white pinpoints of light in an otherwise near black sky together with some faint nebulae or clouds of light. In ancient times the stars were often assumed to be equidistant on a dome above the earth because they are much too far away for stereopsis to offer any depth cues.

Astronomical object

celestial bodiescelestial bodycelestial object
Depending on the initial mass of the star and the presence or absence of a companion, a star may spend the last part of its life as a compact object; either a white dwarf, neutron star, or black hole. The table below lists the general categories of bodies and objects by their location or structure. * Monthly skymaps for every location on Earth List of light sources. List of Solar System objects. List of Solar System objects by size. Lists of astronomical objects. Outer space. Monthly skymaps for every location on Earth.

Orion Nebula

1976M42Messier 42
The nebula has revealed much about the process of how stars and planetary systems are formed from collapsing clouds of gas and dust. Astronomers have directly observed protoplanetary disks, brown dwarfs, intense and turbulent motions of the gas, and the photo-ionizing effects of massive nearby stars in the nebula. The Nebula is visible with the naked eye even from areas affected by some light pollution. It is seen as the middle "star" in the "sword" of Orion, which are the three stars located south of Orion's Belt. The star appears fuzzy to sharp-eyed observers, and the nebulosity is obvious through binoculars or a small telescope.


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.


Alpha OrionisBételgeuseα Ori
Betelgeuse is generally the ninth-brightest star in the night sky and second-brightest in the constellation of Orion (after Rigel). It is a distinctly reddish, semiregular variable star whose apparent magnitude varies between +0.0 and +1.3, the widest range of any first-magnitude star. At near-infrared wavelengths, Betelgeuse is the brightest star in the night sky. It has the Bayer designation α Orionis, which is Latinised to Alpha Orionis and abbreviated Alpha Ori or α Ori. Classified as a red supergiant of spectral type M1-2, Betelgeuse is one of the largest stars visible to the naked eye.


Alpha Draconisα DraconisAdib
Thuban, designation Alpha Draconis (α Draconis, abbreviated Alpha Dra, α Dra), is a star (or star system) in the constellation of Draco. A relatively inconspicuous star in the night sky of the Northern Hemisphere, it is historically significant as having been the north pole star from the 4th to 2nd millennium BCE. Even though Johann Bayer gave Thuban the designation Alpha, its apparent magnitude of 3.65 means it is 3.7 times fainter than the brightest star in the constellation, Gamma Draconis (Eltanin), whose apparent magnitude is 2.24. α Draconis (Latinised to Alpha Draconis) is the star's Bayer designation.


Sirius BSirius superclusterDog Star
An interaction with a third star, to date undiscovered, has also been proposed as a possibility for a red appearance. Alternative explanations are either that the description as red is a poetic metaphor for ill fortune, or that the dramatic scintillations of the star when rising left the viewer with the impression that it was red. To the naked eye, it often appears to be flashing with red, white, and blue hues when near the horizon. With an apparent magnitude of −1.46, Sirius is the brightest star in the night sky, almost twice as bright as the second-brightest star, Canopus. From Earth, Sirius always appears dimmer than Jupiter and Venus, as well as Mercury and Mars at certain times.

Epsilon Draconis

ε Draε Dra63/ε Dra
The star is circumpolar throughout all of Europe, China, most of India and as far south as the tip of the Baja peninsula in North America as well as other locations around the globe having a latitude greater than ± 20° North. Since Epsilon Draconis has an apparent magnitude of almost 4.0, the star is easily observable to the naked eye as long as one's stargazing is not hampered by the light pollution common to most cities.


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.

Alpha Centauri

Alpha Centauri BAlpha Centauri Aα Centauri
At −0.27 apparent magnitude (combined for A and B magnitudes), Alpha Centauri is fainter only than Sirius and Canopus. It forms the outer star of The Pointers or The Southern Pointers, so called because the line through Beta Centauri (Hadar/Agena), some 4.5° west, points to the constellation Crux—the Southern Cross. The Pointers easily distinguish the true Southern Cross from the fainter asterism known as the False Cross. South of about 29° S latitude, Alpha Centauri is circumpolar and never sets below the horizon. North of about 29° N latitude, Alpha Centauri never rises.

Eta Carinae

η Carinaeη CarEta Carinae A
Eta Carinae is too far south to be part of the mansion-based traditional Chinese astronomy, but it was mapped when the Southern Asterisms were created at the start of the 17th century. Together with s Carinae, λ Centauri, and λ Muscae, Eta Carinae forms the asterism 海山 (Sea and Mountain). Eta Carinae has the names Tseen She (from the Chinese 天社 [Mandarin: tiānshè] "Heaven's altar") and Foramen. It is also known as 海山二 (Hǎi Shān èr, the Second Star of Sea and Mountain). Halley gave an approximate apparent magnitude of "4" at the time of discovery, which has been calculated as magnitude 3.3 on the modern scale.


Alpha CygniAridedα Cyg
Deneb is a first-magnitude star in the constellation of Cygnus, the swan. Deneb is one of the vertices of the asterism known as the Summer Triangle and the "head" of the Northern Cross. It is the brightest star in Cygnus and the 19th brightest star in the night sky, with an average apparent magnitude of +1.25. A blue-white supergiant, Deneb rivals Rigel as the most luminous first magnitude star. However its distance, and hence luminosity, is poorly known; its luminosity is somewhere between 55,000 and 196,000 times that of the 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.

Star catalogue

star catalogNLTTLHS
Completeness and accuracy is described by the weakest apparent magnitude V (largest number) and the accuracy of the positions. From their existing records, it is known that the ancient Egyptians recorded the names of only a few identifiable constellations and a list of thirty-six decans that were used as a star clock. The Egyptians called the circumpolar star "the star that cannot perish" and, although they made no known formal star catalogues, they nonetheless created extensive star charts of the night sky which adorn the coffins and ceilings of tomb chambers.

Omega Carinae

ω Carω CarinaeOmega
Omega Carinae, Latinized from ω Carinae, is a star in the constellation Carina. With a declination greater than 70 degrees south of the celestial equator, it is the most southerly of the bright stars of Carina (third-magnitude or brighter), and it is part of a southern asterism known as the Diamond Cross. This star has an apparent visual magnitude of 3.3 and is located at a distance of about 342 ly from Earth. Omega Carinae has a stellar classification of B8 IIIe, which places it in the category of Be stars, that display emission lines of hydrogen their spectrum. Omega Carinae is a shell star, having a circumstellar disk of gas surrounding its equator.

Zeta Ophiuchi

ζ Ophiuchiζ Oph*zet Oph
Zeta Ophiuchi (ζ Oph, ζ Ophiuchi) is a star located in the constellation of Ophiuchus. It has an apparent visual magnitude of 2.57, making it the third-brightest star in the constellation. Parallax measurements give an estimated distance of roughly 366 ly from the Earth. ζ Ophiuchi is an enormous star with more than 19 times the Sun's mass and eight times its radius. The stellar classification of this star is O9.5 V, with the luminosity class of V indicating that it is generating energy in its core by the nuclear fusion of hydrogen. This energy is being emitted from the outer envelope at an effective temperature of 34,000K, giving the star the blue hue of an O-type star.

Ceres (dwarf planet)

Ceres1 CeresAtmosphere of Ceres
When in opposition near its perihelion, Ceres can reach an apparent magnitude of +6.7. This is generally regarded as too dim to be visible to the naked eye, but under ideal viewing conditions, keen eyes with 20/20 vision may be able to see it. The only other asteroids that can reach a similarly bright magnitude are 4 Vesta and, when in rare oppositions near their perihelions, 2 Pallas and 7 Iris. When in conjunction, Ceres has a magnitude of around +9.3, which corresponds to the faintest objects visible with 10×50 binoculars; thus it can be seen with such binoculars in a naturally dark and clear night sky around new moon.

Outline of astronomy

AstronomyList of astronomical topicsoutline
Stellar magnetic field. Magnitude. Absolute magnitude. Solar mass. Metallicity. Stellar rotation. UBV photometric system. Variable star. Star systems. Binary star. Contact binary. Common envelope. Multiple star. Accretion disc. Planetary system. Earth's Solar System. Earth-centric observation of stars. Pole star. Circumpolar star. Magnitude. Apparent magnitude. Photographic magnitude. color-color diagram. Radial velocity. Proper motion. Parallax#Stellar parallax. Photometric-standard star. Lists of stars. List of proper names of stars. List of Arabic star names. Traditional Chinese star names. List of most massive stars. List of least massive stars. List of largest known stars.


Beta Orionisβ Ori (Rigel)β Orionis
It is one of the closest known potential supernova progenitors to Earth, and would be expected to have a maximum apparent magnitude of around -11 (about the same brightness as a quarter Moon or around 300 times brighter than Venus ever gets.) Rigel's complex variability at visual wavelengths is caused by stellar pulsations similar to those of Deneb. Additional observations of radial velocity variations indicate that it simultaneously oscillates in at least 19 non-radial modes with periods ranging from about 1.2 to 74 days. Recent stellar evolution models suggest the pulsations are powered by nuclear reactions in a hydrogen-burning shell that is at least partially non-convective.

61 Cygni

61 Cygni A61 Cyg61 Cyg A
61 Cygni is a binary star system in the constellation Cygnus, consisting of a pair of K-type dwarf stars that orbit each other in a period of about 659 years. Of apparent magnitude 5.20 and 6.05, respectively, they can be seen with binoculars in city skies or with the naked eye in rural areas without photopollution. 61 Cygni first attracted the attention of astronomers when its large proper motion was first demonstrated by Giuseppe Piazzi in 1804.

Zeta Puppis

ζ PuppisNaosζ Pup
Zeta Puppis (ζ Puppis, abbreviated Zeta Pup, ζ Pup), formally named Naos, is a star in the constellation of Puppis. The spectral class of O4 means this is one of the hottest, and most luminous, stars visible to the naked eye. It is one of the sky's few naked-eye class O-type stars as well as one of the closest to Earth. It is a blue supergiant, one of the most luminous stars in the Milky Way. Visually it is over 10,000 times brighter than the Sun, but its high temperature means that most of its radiation is in the ultraviolet and its bolometric luminosity is over 500,000 times that of the Sun. It is also the 72nd brightest star in terms of apparent magnitude from Earth.


HipparchosHipparchus of NicaeaHipparchus of Nicea
Pogson in 1856, who placed the magnitudes on a logarithmic scale, making magnitude 1 stars 100 times brighter than magnitude 6 stars, thus each magnitude is 5√100 or 2.512 times brighter than the next faintest magnitude. Hipparchus is generally recognized as discoverer of the precession of the equinoxes in 127. His two books on precession, On the Displacement of the Solsticial and Equinoctial Points and On the Length of the Year, are both mentioned in the Almagest of Claudius Ptolemy. According to Ptolemy, Hipparchus measured the longitude of Spica and Regulus and other bright stars.

Epsilon Ophiuchi

ε Ophε OphiuchiYed Posterior
Epsilon Ophiuchi (ε Ophiuchi, abbreviated Epsilon Oph, ε Oph), formally named Yed Posterior, is a red giant star in the constellation of Ophiuchus. Located less than five degrees south of the celestial equator in the eastern part of the constellation, it forms a naked eye optical double with Delta Ophiuchi (named Yed Prior ). With an apparent visual magnitude of 3.220, the star can be seen with the naked eye from most of the Earth under suitably dark skies. Parallax measurements yield an estimated distance of 106.4 ly from the Sun. ε Ophiuchi (Latinised to Epsilon Ophiuchi) is the star's Bayer designation. It bore the traditional name Yed Posterior.

Gamma Serpentis

γ Serγ SerpentisGamma
Gamma Serpentis (γ Serpentis, γ Ser) is a star in the equatorial constellation Serpens, in the part of the constellation that represents the serpent's head (Serpens Caput). It has an apparent visual magnitude +3.85, which means it is visible to the naked eye. Based upon parallax measurements by the Hipparcos spacecraft, this star is approximately 36.7 light years from Earth. Gamma Serpentis is an ordinary F-type main sequence star with a stellar classification of F6 V. It is larger and more massive than the Sun, with three times the solar luminosity. Based upon its mass, it may have a convection zone in its core region. The projected rotational velocity along the equator is 10.2 km/s.