Brown dwarf

brown dwarfsbrown dwarvesPlanetar
Since then, over 1,800 brown dwarfs have been identified, even some very close to Earth like Epsilon Indi Ba and Bb, a pair of brown dwarfs gravitationally bound to a Sun-like star 12 light-years from the Sun, and Luhman 16, a binary system of brown dwarfs at 6.5 light-years from the Sun. The standard mechanism for star birth is through the gravitational collapse of a cold interstellar cloud of gas and dust. As the cloud contracts it heats due to the Kelvin–Helmholtz mechanism. Early in the process the contracting gas quickly radiates away much of the energy, allowing the collapse to continue. Eventually, the central region becomes sufficiently dense to trap radiation.


National Aeronautics and Space AdministrationNational Aeronautics and Space Administration (NASA)space program
The National Aeronautics and Space Administration (NASA, ) is an independent agency of the United States Federal Government responsible for the civilian space program, as well as aeronautics and aerospace research.

Bayer designation

Bayer first designated Betelgeuse and Rigel, the two 1st-magnitude stars (those of magnitude 1.5 or less), as Alpha and Beta from north to south, with Betelgeuse (the shoulder) coming ahead of Rigel (the foot), even though the latter is usually the brighter. (Betelgeuse is a variable star and can at its maximum occasionally outshine Rigel.) Bayer then repeated the procedure for the stars of the 2nd magnitude (those between magnitudes 1.51 and 2.5), labeling them from gamma through zeta in "top-down" (north-to-south) order. The "First to Rise in the East" order is used in a number of instances.


AchenarAchernar Three
Alpha Eridani was not visible from Alexandria until about 1600 CE. Until about March 2000, Achernar and Fomalhaut were the two first-magnitude stars furthest in angular distance from any other first-magnitude star in the celestial sphere. Antares, in the constellation of Scorpius, is now the most isolated first-magnitude star, although Antares is located in a constellation with many bright second-magnitude stars, whereas the stars surrounding Alpha Eridani and Fomalhaut are considerably fainter. The first star catalogue to contain Achernar in the chart of Eridanus is Johann Bayer's Uranometria. Bayer did not observe it himself, and it is attributed to Keyser and the voyages of the Dutch.

Eridanus (constellation)

Theta Eridani, called Acamar, is a binary star with blue-white components, distinguishable in small amateur telescopes and 161 light-years from Earth. The primary is of magnitude 3.2 and the secondary is of magnitude 4.3. 32 Eridani is a binary star 290 light-years from Earth. The primary is a yellow-hued star of magnitude 4.8 and the secondary is a blue-green star of magnitude 6.1. 32 Eridani is divisible in small amateur telescopes. 39 Eridani is a binary star also divisible in small amateur telescopes, 206 light-years from Earth.

Main sequence

main-sequencemain sequence dwarfmain-sequence star
K5 || 0.74 || 0.69 || 0.16 || 4,410. style="text-align: left;"|61 Cygni A. M0 || 0.63 || 0.47 || 0.063 || 3,920. style="text-align: left;"|Gliese 185. M5 || 0.32 || 0.21 || 0.0079 || 3,120. style="text-align: left;"|EZ Aquarii A. M8 || 0.13 || 0.10 || 0.0008 || 2,660. style="text-align: left;"|Van Biesbroeck's star.

Red clump

clump giantred-clumpclump giants
These stars are an order of magnitude less common than sun-like stars, even rarer compared to the sub-solar stars that can form red clump giants, and the duration of the blue loop is far less than the time spent by a red clump giant on the horizontal branch. This means that these imposters are much less common in the H-R diagram, but still detectable. Stars with will also pass through the red clump as they evolve along the subgiant branch. This is again a very rapid phase of evolution, but stars such as OU Andromedae are found in the red clump region (5,500 K and ) even though it is thought to be a subgiant crossing the Hertzsprung gap.

Sombrero Galaxy

M1044594M104, Sombrero Galaxy
The galaxy has an apparent magnitude of +8.0, making it easily visible with amateur telescopes, and it is considered by some authors to be the galaxy with the highest absolute magnitude within a radius of 10 megaparsecs of the Milky Way. Its large bulge, its central supermassive black hole, and its dust lane all attract the attention of professional astronomers. The Sombrero Galaxy was discovered on May 11, 1781 by Pierre Méchain, who described the object in a May 1783 letter to J. Bernoulli that was later published in the Berliner Astronomisches Jahrbuch.


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.

Large Magellanic Cloud

LargeLMCGreater Magellanic Cloud
At a distance of about 50 kiloparsecs (≈163,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal (~16 kpc) and the possible dwarf irregular galaxy known as the Canis Major Overdensity. Based on readily visible stars and a mass of approximately 10 billion solar masses, the diameter of the LMC is about 14000 ly, making it roughly one one-hundredth as massive as the Milky Way. This makes the LMC the fourth-largest galaxy in the Local Group, after the Andromeda Galaxy (M31), the Milky Way, and the Triangulum Galaxy (M33). The LMC is classified as a Magellanic spiral.


HIPTYCHipparcos satellite
It merged two components: first, a survey of around 58,000 objects as complete as possible to the following limiting magnitudes: V<7.9 + 1.1sin|b| for spectral types earlier than G5, and V<7.3 + 1.1sin|b| for spectral types later than G5 (b is the Galactic latitude). Stars constituting this survey are flagged in the Hipparcos Catalogue. The second component comprised additional stars selected according to their scientific interest, with none fainter than about magnitude V=13 mag.


Canis minorProcyon Aα CMi
It forms one of the three vertices of the Winter Triangle asterism, in combination with Sirius and Betelgeuse. The prime period for evening viewing of Procyon is in late winter in the northern hemisphere. It has a color index of 0.42, and its hue has been described as having a faint yellow tinge to it. Procyon is a binary star system with a bright primary component, Procyon A, having an apparent magnitude of 0.34, and a faint companion, Procyon B, at magnitude 10.7. The pair orbit each other with a period of 40.82 years along an elliptical orbit with an eccentricity of 0.407, more eccentric than Mercury's.

Epsilon Cassiopeiae

ε Casε Cassiopeiae&epsilon; Cas
With an apparent visual magnitude of 3.4, this is one of the brightest stars in the constellation. The distance to this star has been determined directly using parallax measurements, yielding a value of around 390 - 430 ly from the Sun. ε Cassiopeiae (Latinised to Epsilon Cassiopeiae) is the star's Bayer designation. The star bore the traditional name Segin, which probably originates from an erroneous transcription of Seginus, the traditional name for Gamma Boötis, which itself is of uncertain origin. Different sources report varying pronunciations, with SEG-in the most common but the variants SAY-gin and seg-EEN also appearing.


Alpha Cygniα CygAlpha Cygni (Deneb)
Deneb, also designated α Cygni (Latinised to Alpha Cygni, abbreviated Alpha Cyg, α Cyg), is the brightest star in the constellation of Cygnus, the swan. It is one of the vertices of the asterism known as the Summer Triangle and forms the 'head' of the Northern Cross. It is generally the 19th brightest star in the night sky, with an average apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous stars.


αα AqlAlpha
Altair (from Arabic "al-ṭā'ir" ), also designated α Aquilae (Latinised to Alpha Aquilae, abbreviated Alpha Aql, α Aql), is the brightest star in the constellation of Aquila and the twelfth brightest star in the night sky. It is currently in the G-cloud—a nearby interstellar cloud, which is an accumulation of gas and dust. Altair is an A-type main sequence star with an apparent visual magnitude of 0.77 and is one of the vertices of the Summer Triangle asterism (the other two vertices are marked by Deneb and Vega). It is 16.7 light-years (5.13 parsecs) from the Sun and is one of the closest stars visible to the naked eye.


2MASXTwo Micron All-Sky SurveyTwo Micron All-Sky Survey (2MASS)
The goals of this survey included: Numerical descriptions of point sources (stars, planets, asteroids) and extended sources (galaxies, nebulae) were cataloged by automated computer programs to an average limiting magnitude of about 14. More than 300 million point sources and 1 million extended sources were cataloged. In November 2003, a team of scientists announced the discovery of the Canis Major Dwarf Galaxy, at that time the closest known satellite galaxy to the Milky Way, based on analysis of 2MASS stellar data. The resulting data and images from the survey are currently in the public domain, and may be accessed online for free by anyone.

Axial precession

precession of the equinoxesprecessionprecession of equinoxes
The term "precession" typically refers only to this largest part of the motion; other changes in the alignment of Earth's axis—nutation and polar motion—are much smaller in magnitude. Earth's precession was historically called the precession of the equinoxes, because the equinoxes moved westward along the ecliptic relative to the fixed stars, opposite to the yearly motion of the Sun along the ecliptic. This term is still used in non-technical discussions, that is, when detailed mathematics are absent.

Messier 81

M81Bode's Galaxy3031
Messier 81 (also known as NGC 3031 or Bode's Galaxy) is a spiral galaxy about 12 million light-years away, with at diameter of 90,000 light years, about half the size of the Milky Way, in the constellation Ursa Major. Due to its proximity to Earth, large size, and active galactic nucleus (which harbors a 70 million supermassive black hole), Messier 81 has been studied extensively by professional astronomers. The galaxy's large size and relatively high brightness also makes it a popular target for amateur astronomers. Messier 81 was first discovered by Johann Elert Bode on December 31, 1774. Consequently, the galaxy is sometimes referred to as "Bode's Galaxy".

Pollux (star)

Polluxβ Gemβ Geminorum
At an apparent visual magnitude of 1.14, Pollux is the brightest star in its constellation, even brighter than its neighbor Castor (α Geminorum). Pollux is 6.69 degrees north of the ecliptic, so on rare occasions viewers in Earth's southern hemisphere see it occulted by the Moon. Parallax measurements made with the Hipparcos astrometry satellite place Pollux at a distance of about 33.78 ly from the Sun. The star is larger than the Sun, with about two times its mass and almost nine times its radius. Once an A-type main sequence star, Pollux has exhausted the hydrogen at its core and evolved into a giant star with a stellar classification of K0 III.

Bortle scale

dark suburban skiessuitably dark skiesbright enough
It gives several criteria for each level beyond naked-eye limiting magnitude (NELM). The accuracy and utility of the scale have been questioned in recent research. The table below summarizes Bortle's descriptions of the classes. 4673 Bortle. Amateur astronomy. Dark-sky movement. The End of Night (book). International Dark-Sky Association (IDA). Light pollution. Night sky. Sky brightness. Sky & Telescope (S&T). by Sky & Telescope. Interactive demo of the Bortle Scale. International Dark-Sky Association. (North American sites only).

Winter Triangle

The Winter Triangle surrounds most of the faint constellation Monoceros, although its brightest stars are of fourth magnitude and hardly noticeable to the naked eye. The triangle includes two first magnitude stars, while Sirius is even brighter. The other bright stars of the winter sky lie around the triangle: Orion including Rigel; Aldebaran in Taurus; Castor and Pollux in Gemini; and Capella in Auriga. Spring Great Diamond. Summer Triangle. Winter Hexagon. Winter Triangle Night Sky. Deep sky objects inside the Winter Triangle. Winter Triangle at constellation guide. Winter Triangle Jim Kaler's stars.

Uppsala General Catalogue

The catalogue includes essentially all galaxies north of declination -02°30' and to a limiting diameter of 1.0 arcminute or to a limiting apparent magnitude of 14.5. The primary source of data is the blue prints of the Palomar Observatory Sky Survey (POSS). It also includes galaxies smaller than 1.0 arcminute in diameter but brighter than 14.5 magnitude from the Catalogue of Galaxies and of Clusters of Galaxies (CGCG). The catalogue contains descriptions of the galaxies and their surrounding areas, plus conventional system classifications and position angles for flattened galaxies.

Very Large Telescope

When fringe tracking is introduced, the limiting magnitude of the VLTI is expected to improve by a factor of almost 1000, reaching a magnitude of about 14. This is similar to what is expected for other fringe tracking interferometers. In spectroscopic mode, the VLTI can currently reach a magnitude of 1.5. The VLTI can work in a fully integrated way, so that interferometric observations are actually quite simple to prepare and execute. The VLTI has become worldwide the first general user optical/infrared interferometric facility offered with this kind of service to the astronomical community.