Milky Way

galaxyMilky Way Galaxyour 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).

Stellar classification

spectral typeK-typeG-type
K5V – 61 Cygni A. K5III – Gamma Draconis. Spectral standards:. M0IIIa – Beta Andromedae. M2III – Chi Pegasi. M1-M2Ia-Iab – Betelgeuse. M2Ia – Mu Cephei. WN – spectrum dominated by N III-V and He I-II lines. WNE (WN2 to WN5 with some WN6) – hotter or "early". WNL (WN7 to WN9 with some WN6) – cooler or "late". Extended WN classes WN10 and WN11 sometimes used for the Ofpe/WN9 stars. h tag used (e.g. WN9h) for WR with hydrogen emission and ha (e.g. WN6ha) for both hydrogen emission and absorption. WN/C – WN stars plus strong C IV lines, intermediate between WN and WC stars. WC – spectrum with strong C II-IV lines.

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.

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.


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.

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. 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. Barnard's star's transverse speed is 90 km/s and its radial velocity is 111 km/s (which is at right angles to the transverse velocity), which gives a true motion of 142 km/s.

Hubble Space Telescope

The images reveal galaxies billions of light years away, and have generated a wealth of scientific papers, providing a new window on the early Universe. The Wide Field Camera 3 improved the view of these fields in the infrared and ultraviolet, supporting the discovery of some of the most distant objects yet discovered, such as MACS0647-JD. The non-standard object SCP 06F6 was discovered by the Hubble Space Telescope in February 2006. During June and July 2012, US astronomers using Hubble discovered a tiny fifth moon moving around icy Pluto. In March 2015, researchers announced that measurements of aurorae around Ganymede revealed that the moon has a subsurface ocean.


starsmassive starstellar radius
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.

Light curve

lightcurvelight-curvelight curves
For example, PA-99-N2 is a microlensing event that may have been due to a star in the Andromeda galaxy that has an exoplanet. U = 0 → Result later proven incorrect. U = 1 → Result based on fragmentary light curve(s), may be completely wrong. U = 2 → Result based on less than full coverage. Period may be wrong by 30 percent or ambiguous. U = 3 → Secure result within the precision given. No ambiguity. U = n.a. → Not available.

List of most luminous stars

most luminousmost luminous starsmost luminous stars known
For example, 3C 273 has an average apparent magnitude of 12.8 (when observing with a telescope), but an absolute magnitude of −26.7. If this object were 10 parsecs away from Earth it would appear nearly as bright in the sky as the Sun (apparent magnitude −26.74). This quasar's luminosity is, therefore, about 2 trillion (10 12 ) times that of the Sun, or about 100 times that of the total light of average large galaxies like our Milky Way. (Note that quasars often vary somewhat in luminosity.) In terms of gamma rays, a magnetar (type of neutron star) called SGR 1806-20, had an extreme burst reach Earth on 27 December 2004.

Star catalogue

star catalogNLTTLTT
Examples include 61 Cygni and 47 Ursae Majoris. Bayer and Flamsteed covered only a few thousand stars between them. In theory, full-sky catalogues try to list every star in the sky. There are, however, billions of stars resolvable by telescopes, so this is an impossible goal; with this kind of catalog, an attempt is generally made to get every star brighter than a given magnitude. Jérôme Lalande published the Histoire Céleste Française in 1801, which contained an extensive star catalog, among other things. The observations made were made from the Paris Observatory and so it describes mostly northern stars.

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 (infra-red) 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.

First magnitude star

brightestfirst magnitudefirst magnitude stars
First magnitude stars are the brightest stars in the night sky, with apparent magnitudes lower than +1.50. Hipparchus, in the 1st century B.C., introduced the magnitude scale. He allocated first magnitude to the 20 brightest stars and the sixth magnitude to the faintest stars visible to the naked eye. In the 19th Century, this ancient scale of apparent magnitude was logarithmically defined—so that a star of 1.00 mag is exactly 100 times brighter than a star of 6.00 magnitude. The scale also was extended to even brighter celestial bodies like Sirius (-1.5 mag), Venus (-4 mag), full Moon (-12.7 mag) and Sun (-26.7 mag). Hipparchus ranked his stars in a very simple way.

Proxima Centauri

Alpha Proximaits host starProxima
Even from Alpha Centauri A or B, Proxima would only be seen as a fifth magnitude star. It has an apparent visual magnitude of 11, so a telescope with an aperture of at least 8 cm is needed to observe it, even under ideal viewing conditions—under clear, dark skies with Proxima Centauri well above the horizon. In 2018, a superflare was observed from Proxima Centauri, the strongest flare ever seen. The optical brightness increased by a factor of 68 to approximately magnitude 6.8. It is estimated that similar flares occur around five times every year but are of such short duration, just a few minutes, that they have never been observed before.


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).

Triangulum Galaxy

M33TriangulumM33 Galaxy
The Triangulum Galaxy is a spiral galaxy approximately 3 million light-years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598. The Triangulum Galaxy is the third-largest member of the Local Group of galaxies, behind the Milky Way and the Andromeda Galaxy. It is one of the most distant permanent objects that can be viewed with the naked eye. The galaxy is the smallest spiral galaxy in the Local Group and it is believed to be a satellite of the Andromeda Galaxy due to their interactions, velocities, and proximity to one another in the night sky. It also has an H II nucleus.


cataloghis book on astronomyMagna Syntaxis
A series of stars in Centaurus are off by a couple degrees, including the star we call Alpha Centauri. These were probably measured by a different person or persons from the others, and in an inaccurate way. Ptolemy assigned the following order to the planetary spheres, beginning with the innermost: Other classical writers suggested different sequences. Plato (c. 427 – c. 347 BC) placed the Sun second in order after the Moon. Martianus Capella (5th century AD) put Mercury and Venus in motion around the Sun. Ptolemy's authority was preferred by most medieval Islamic and late medieval European astronomers.


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. The French mathematician and astronomer Jean-Baptiste Morin observed Arcturus in the daytime with a telescope in 1635, a first for any star other than the Sun and supernovae.

Double star

visual companiondouble starsoptical double
Alpha Centauri. Capella. p Eridani. Polaris. Procyon. Sirius. Alpha 1 and Alpha 2 Capricorni. Theta Muscae and Theta Muscae B. Eta 1 and Eta 2 Coronae Australis. Kappa 1 and Kappa 2 Coronae Australis. Winnecke 4 (Messier 40). Alpha Centauri system (AB) and Proxima Centauri (thus α Cen C): association is generally considered a physically connected system. Castor system (Aa/Ab/Ba/Bb) and YY Geminorum (thus Castor Ca/Cb) is generally considered a physical system. Mizar system (Aa/Ab/Ba/Bb) and Alcor (itself a binary, thus Mizar Ca/Cb, though generally not considered physical until 2009).

List of nearest stars and brown dwarfs

passing starsnearest starsclosest stars
Its approach has not been fully described due to it being a distant binary star with a red dwarf, but almost certainly passed less than 1 light year from the Solar System roughly 1.16 million years ago. * "The 100 nearest star systems", Research Consortium on Nearby Stars * The dynamics of the closest stars Interstellar travel. List of brightest stars. List of star systems within 20–25 light-years. List of star systems within 25–30 light-years. List of nearest bright stars. List of nearest exoplanets. List of nearest galaxies. Lists of stars. Nearby Stars Database. Stars and planetary systems in fiction. The Magnificent Seven. List of Solar System objects by greatest aphelion.