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.

Milky Way

galaxyMilky Way Galaxyour galaxy
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). 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.

Proxima Centauri

Alpha Proximaits host starProxima
Proxima Centauri, or Alpha Centauri C, is a red dwarf, a small low-mass star, about 4.244 ly from the Sun in the constellation of Centaurus. It 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 component of the Alpha Centauri system, currently with a separation of about 12950 AU and an orbital period of 550,000 years. At present Proxima is 2.18° to the southwest of Alpha Centauri. Because of Proxima Centauri's proximity to Earth, its angular diameter can be measured directly. The star is about one-seventh the diameter of the Sun.

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.


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. Most, if not all, remaining life will be destroyed by the Sun's increased luminosity (peaking at about 5,000 times its present level). A 2008 simulation indicates that Earth's orbit will eventually decay due to tidal effects and drag, causing it to enter the Sun's atmosphere and be vaporized. The shape of Earth is approximately oblate spheroidal.

Eta Carinae

η Carinaeη CarEta Carinae A
Observations at the Cape of Good Hope indicated it peaked in brightness, surpassing Canopus, over March 11 to 14, 1843 before beginning to fade, then brightened to between the brightness of Alpha Centauri and Canopus between March 24 and 28 before fading once again. For much of 1844 the brightness was midway between Alpha Centauri and Beta Centauri, around magnitude +0.2, before brightening again at the end of the year. At its brightest in 1843 it likely reached an apparent magnitude of −0.8, then −1.0 in 1845. The peaks in 1827, 1838, and 1843 are likely to have occurred at the periastron passage—the point the two stars are closest together—of the binary orbit.

Minute and second of arc

masarcsecondarc second
For instance, a toolmaker's optical comparator will often include an option to measure in "minutes and seconds". * MOA / mils By Robert Simeone 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 (149,597,871 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.

Mercury (planet)

Mercuryplanet MercuryMercurio
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.


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).


Jovianplanet JupiterGiove
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. As it does so, Jupiter appears to undergo retrograde motion with respect to the background stars.


Saturn's atmosphereExploration of Saturnhome planet
The mean apparent magnitude of Saturn is 0.46 with a standard deviation of 0.34. Most of the magnitude variation is due to the inclination of the ring system relative to the Sun and Earth. The brightest magnitude, -0.55, occurs near in time to when the plane of the rings is inclined most highly, and the faintest magnitude, 1.17, occurs around the time when they are least inclined. It takes approximately 29.5 years for the planet to complete an entire circuit of the ecliptic against the background constellations of the zodiac.


NeptunianNeptune-masseighth planet
The average distance between Neptune and the Sun is 4.5 billion km (about 30.1 astronomical units (AU)), and it completes an orbit on average every 164.79 years, subject to a variability of around ±0.1 years. The perihelion distance is 29.81 AU; the aphelion distance is 30.33 AU. On 11 July 2011, Neptune completed its first full barycentric orbit since its discovery in 1846, although it did not appear at its exact discovery position in the sky, because Earth was in a different location in its 365.26-day orbit.

Open cluster

open star clusterstar clusteropen clusters
The core is typically about 3–4 light years across, with the corona extending to about 20 light years from the cluster centre. Typical star densities in the centre of a cluster are about 1.5 stars per cubic light year; the stellar density near the Sun is about 0.003 stars per cubic light year. Open clusters are often classified according to a scheme developed by Robert Trumpler in 1930.

International Astronomical Union

IAUInternational Astronomical Union (IAU)I.A.U.
List of proper names of stars. XXVIth General Assembly 2006. XXVIIth General Assembly 2009. XXVIIIth General Assembly 2012. XXIXth General Assembly 2015. XXXth General Assembly 2018.


Many sorts of mounts have been developed over the years, with the majority of effort being put into systems that can track the motion of the stars as the Earth rotates. The two main types of tracking mount are: Since the atmosphere is opaque for most of the electromagnetic spectrum, only a few bands can be observed from the Earth's surface. These bands are visible – near-infrared and a portion of the radio-wave part of the spectrum. For this reason there are no X-ray or far-infrared ground-based telescopes as these have to be observed from orbit. Even if a wavelength is observable from the ground, it might still be advantageous to place a telescope on a satellite due to astronomical seeing.

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.

Red dwarf

redred dwarf starsred dwarfs
A red dwarf (or M dwarf) is a small and cool star on the main sequence, of M spectral type. Red dwarfs range in mass from about 0.075 to about 0.50 solar mass and have a surface temperature of less than 4,000 K. Sometimes K-type main-sequence stars, with masses between 0.50-0.8 solar mass, are also included. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs cannot be easily observed. From Earth, not one is visible to the naked eye. Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are fifty of the sixty nearest stars.


Its brightness at a distance of 10 parsecs, the absolute visual magnitude, is −7.6, three magnitudes brighter than Venus ever appears from Earth. Its brightness at the distance of the nearest star to Earth, Proxima Centauri (just over a parsec), would be about the same as the full Moon. R136a1 supplies c. 7% of the ionizing flux of the entire 30 Doradus region, as much as 70 O7 dwarf stars. Along with R136a2, a3, and c, it produces 43–46% of the Lyman continuum radiation of the whole R136 cluster. Massive stars lie close to the Eddington limit, the luminosity at which the radiation pressure acting outwards at the surface of the star equals the force of the star's gravity pulling it inward.