Apparent magnitude

apparent visual magnitudemagnitudevisual magnitude
The brightness of Vega is exceeded by four stars in the night sky at visible wavelengths (and more at infrared wavelengths) as well as the bright planets Venus, Mars, and Jupiter, and these must be described by negative magnitudes. For example, Sirius, the brightest star of the celestial sphere, has an apparent magnitude of −1.4 in the visible. Negative magnitudes for other very bright astronomical objects can be found in the table below. Astronomers have developed other photometric zeropoint systems as alternatives to the Vega system.


solarSolThe Sun
Observations of sunspots were recorded during the Han Dynasty (206 BC–AD 220) by Chinese astronomers, who maintained records of these observations for centuries. Averroes also provided a description of sunspots in the 12th century. The invention of the telescope in the early 17th century permitted detailed observations of sunspots by Thomas Harriot, Galileo Galilei and other astronomers. Galileo posited that sunspots were on the surface of the Sun rather than small objects passing between Earth and the Sun.


constellationsEuropean constellationModern constellation
Three schools of classical Chinese astronomy in the Han period are attributed to astronomers of the earlier Warring States period. The constellations of the three schools were conflated into a single system by Chen Zhuo, an astronomer of the 3rd century (Three Kingdoms period). Chen Zhuo's work has been lost, but information on his system of constellations survives in Tang period records, notably by Qutan Xida. The oldest extant Chinese star chart dates to that period and was preserved as part of the Dunhuang Manuscripts.

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.

Minute and second of arc

masarcsecondarc second
These units originated in Babylonian astronomy as sexagesimal subdivisions of the degree; they are used in fields that involve very small angles, such as astronomy, optometry, ophthalmology, optics, navigation, land surveying, and marksmanship. To express even smaller angles, standard SI prefixes can be employed; the milliarcsecond (mas) and microarcsecond (μas), for instance, are commonly used in astronomy. The number of square arcminutes in a complete sphere is 148,510,660 square arcminutes (the surface area of a unit sphere in square units divided by the solid angle area subtended by a square arcminute, also in square units - so that the final result is a dimensionless number).

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.


Earth – Audio (29:28) – Cain/Gay – Astronomy Cast (2007). Earth – Videos – International Space Station:. Video (01:02) – Earth (time-lapse). Video (00:27) – Earth and auroras (time-lapse). United States Geological Survey – USGS. Google Earth 3D, interactive map.

Astronomical unit

AUastronomical unitsAUs
After Greek astronomy was transmitted to the medieval Islamic world, astronomers made some changes to Ptolemy's cosmological model, but did not greatly change his estimate of the Earth–Sun distance. For example, in his introduction to Ptolemaic astronomy, al-Farghānī gave a mean solar distance of 1170 Earth radii, whereas in his zij, al-Battānī used a mean solar distance of 1108 Earth radii. Subsequent astronomers, such as al-Bīrūnī, used similar values. Later in Europe, Copernicus and Tycho Brahe also used comparable figures (1142 and 1150 Earth radii), and so Ptolemy's approximate Earth–Sun distance survived through the 16th century.

Bayer designation

Although the brightest star in Draco is Eltanin (Gamma Draconis), Thuban was assigned alpha by Bayer because, due to precession, Thuban was the north pole star 4,000 years ago. Sometimes there is no apparent order, as exemplified by the stars in Sagittarius, where Bayer's designations appear almost random to the modern eye. Alpha and Beta Sagittarii are perhaps the most anomalously designated stars in the sky.


The parsec (symbol: pc) is a unit of length used to measure large distances to astronomical objects outside the Solar System. A parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond, which corresponds to 1⁄648000 astronomical units. One parsec is equal to about 3.26 light-years (30 trillion km or 19 trillion miles) in length. The nearest star, Proxima Centauri, is about 1.3 pc from the Sun. Most of the stars visible to the unaided eye in the night sky are within 500 parsecs of the Sun. The parsec unit was probably first suggested in 1913 by the British astronomer Herbert Hall Turner.


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.


luminousbolometric luminosityluminosities
In astronomy, luminosity is the total amount of energy emitted per unit of time by a star, galaxy, or other astronomical object. As a term for energy emitted per unit time, luminosity is synonymous with power. In SI units luminosity is measured in joules per second or watts. Values for luminosity are often given in the terms of the luminosity of the Sun, L ⊙ . Luminosity can also be given in terms of the astronomical magnitude system: the absolute bolometric magnitude (M bol ) of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a logarithmic measure of the luminosity within some specific wavelength range or filter band.

Asterism (astronomy)

Sirius (α CMa), Procyon (α CMi), and Betelgeuse (α Ori) form one to the North (Winter Triangle) while Sirius, Naos (ζ Pup), and Phakt (α Col) form another to the South. Unlike the Diamond, however, these triangles meet, not base-to-base, but vertex-to-vertex, forming the Egyptian X. The name derives from both the shape and, because the stars straddle the Celestial Equator, it is more easily seen from south of the Mediterranean than in Europe. The Lozenge is a small diamond formed from three stars – Eltanin, Grumium, and Rastaban (Gamma, Xi, and Beta Draconis) – in the head of Draco and one – Iota Herculis – in the foot of Hercules.

Stellar parallax

parallax shiftparallaxparallax method
The first successful measurements of stellar parallax were made by Friedrich Bessel in 1838 for the star 61 Cygni using a heliometer. 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.

Alpha Centauri

α CentauriAlphaα Centauri A
The Winter Triangle would have a very different appearance, with Procyon outshining Pollux in the middle of Gemini, and Sirius lying less than a degree from Betelgeuse in Orion. With a magnitude of −1.2, Sirius would be a little fainter than from Earth but still the brightest star in the night sky. Both Vega and Altair would be shifted northwestward relative to Deneb, giving the Summer Triangle a more equilateral appearance. Alpha Centauri is inside the G-cloud, and its nearest known system is the binary brown dwarf system Luhman 16 at 3.6 ly.


Morning Starevening starCytherocentric
In the second century, in his astronomical treatise Almagest, Ptolemy theorized that both Mercury and Venus are located between the Sun and the Earth. The 11th century Persian astronomer Avicenna claimed to have observed the transit of Venus, which later astronomers took as confirmation of Ptolemy's theory. In the 12th century, the Andalusian astronomer Ibn Bajjah observed "two planets as black spots on the face of the Sun"; these were later identified as the transits of Venus and Mercury by the Maragha astronomer Qotb al-Din Shirazi in the 13th century, though this identification cannot be true as there were no Venus transits in Ibn Bajjah's lifetime.


Jovianplanet JupiterGiove
Jupiter has been known to astronomers since antiquity. It is named after the Roman god Jupiter. When viewed from Earth, Jupiter can reach an apparent magnitude of −2.94, bright enough for its reflected light to cast shadows, and making it on average the third-brightest natural object in the night sky after the Moon and Venus. Jupiter is primarily composed of hydrogen with a quarter of its mass being helium, though helium comprises only about a tenth of the number of molecules. It may also have a rocky core of heavier elements, but like the other giant planets, Jupiter lacks a well-defined solid surface.

Light pollution

The International Dark-Sky Association works to designate areas that have high quality night skies. These areas are supported by communities and organizations that are dedicated to reducing light pollution (e.g.Dark-sky preserve). The National Park Service Natural Sounds and Night Skies Division has measured night sky quality in national park units across the U.S. Sky quality in the U.S. ranges from pristine (Capitol Reef National Park and Big Bend National Park) to severely degraded (Santa Monica Mountains National Recreation Area and Biscayne National Park). The National Park Service Night Sky Program monitoring database is available online (2015).

Astronomical spectroscopy

Astronomical spectroscopy is the study of astronomy using the techniques of spectroscopy to measure the spectrum of electromagnetic radiation, including visible light and radio, which radiates from stars and other celestial objects. A stellar spectrum can reveal many properties of stars, such as their chemical composition, temperature, density, mass, distance, luminosity, and relative motion using Doppler shift measurements. Spectroscopy is also used to study the physical properties of many other types of celestial objects such as planets, nebulae, galaxies, and active galactic nuclei.

Johann Bayer

Bayer Johann BayerBayer, Johann
Bayer's atlas included twelve new constellations invented a few years earlier to fill in the far south of the night sky, which was unknown to ancient Greece and Rome. The crater Bayer on the Moon is named after him. * Full digital facsimile, Linda Hall Library Julius Schiller. Bayer designation.

Angular diameter

apparent diameterangular sizeapparent size
However, much finer units are needed to measure the angular sizes of galaxies, nebulae, or other objects of the night sky. Degrees, therefore, are subdivided as follows: To put this in perspective, the full Moon as viewed from Earth is about 1⁄2°, or 30′ (or 1800″). The Moon's motion across the sky can be measured in angular size: approximately 15° every hour, or 15″ per second. A one-mile-long line painted on the face of the Moon would appear from Earth to be about 1″ in length.

Naked eye

naked-eyeunaided eyenaked human eye
Naked Eye Observing in Astronomy. Naked-Eye Stargazing: Learning the Sky and its constellations. Naked Eye Navigation, Polynesia Voyages. Detection of weak optical signals by the human visual system: Perspectives in Neuroscience and in Quantum Physics. The Naked-eye Planets in the Night Sky (and how to identify them).

Binary star

spectroscopic binaryeclipsing binarybinary
Other interesting binaries include 61 Cygni (a binary in the constellation Cygnus, composed of two K class (orange) main-sequence stars, 61 Cygni A and 61 Cygni B, which is known for its large proper motion), Procyon (the brightest star in the constellation Canis Minor and the eighth-brightest star in the night time sky, which is a binary consisting of the main star with a faint white dwarf companion), SS Lacertae (an eclipsing binary which stopped eclipsing), V907 Sco (an eclipsing binary which stopped, restarted, then stopped again) and BG Geminorum (an eclipsing binary which is thought to contain a black hole with a K0 star in orbit around it), 2MASS J18082002-5104378 (a binary in the "thin

Friedrich Bessel

BesselFriedrich Wilhelm BesselBessel, Friedrich
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. His announcement of Sirius's "dark companion" in 1844 was the first correct claim of a previously unobserved companion by positional measurement, and eventually led to the discovery of Sirius B.


light yearlight yearsly
The star was 61 Cygni, and he used a 6.2 in heliometer designed by Joseph von Fraunhofer. The largest unit for expressing distances across space at that time was the astronomical unit, equal to the radius of the Earth's orbit (1.50 km or 9.30 mi). In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be 660000 astronomical units (9.9 km or 6.1 mi). Bessel added that light employs 10.3 years to traverse this distance. He recognized that his readers would enjoy the mental picture of the approximate transit time for light, but he refrained from using the light-year as a unit.