Parsec

MpcpckpcparsecsmegaparseckiloparsecmegaparsecskiloparsecsGpcμpc
The parsec (symbol: pc) is a unit of length used to measure large distances to astronomical objects outside the Solar System.wikipedia
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Astronomical unit

AUastronomical unitsAUs
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. In August 2015, the IAU passed Resolution B2, which as part of the definition of a standardized absolute and apparent bolometric magnitude scale, mentioned an existing explicit definition of the parsec as exactly 1⁄648000 astronomical units, or approximately 3.08567758149137 metres (based on the IAU 2012 exact SI definition of the astronomical unit).
It is also a fundamental component in the definition of another unit of astronomical length, the parsec.

Light-year

light yearlight yearsly
One parsec is equal to about 3.26 light-years (30 trillion km or 19 trillion miles) in length.
The unit most commonly used in professional astrometry is the parsec (symbol: pc, about 3.26 light-years; the distance at which one astronomical unit subtends an angle of one second of arc).

Absolute magnitude

Hbolometric magnitudeabsolute magnitude (H)
In August 2015, the IAU passed Resolution B2, which as part of the definition of a standardized absolute and apparent bolometric magnitude scale, mentioned an existing explicit definition of the parsec as exactly 1⁄648000 astronomical units, or approximately 3.08567758149137 metres (based on the IAU 2012 exact SI definition of the astronomical unit).
An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it were viewed from a distance of exactly 10 parsecs (32.6 light-years), with no extinction (or dimming) of its light due to absorption by interstellar dust particles.

Parallax

trigonometric parallaxsolar parallaxmotion parallax
Named as a portmanteau of the parallax of one arcsecond, it was defined so as to make calculations of astronomical distances quick and easy for astronomers from only their raw observational data.
The parsec (3.26 light-years) is defined as the distance for which the annual parallax is 1 arcsecond.

Minute and second of arc

masarcsecondarc second
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.
an object of diameter one astronomical unit (149,597,871 km) at a distance of one parsec, by definition.

Cosmic distance ladder

standard candlestandard candlesdistance
The parallax method is the fundamental calibration step for distance determination in astrophysics; however, the accuracy of ground-based telescope measurements of parallax angle is limited to about 0.01 arcseconds, and thus to stars no more than 100 pc distant.
A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" (within about a thousand parsecs) to Earth.

Milky Way

galaxyMilky Way Galaxyour galaxy
Although parsecs are used for the shorter distances within the Milky Way, multiples of parsecs are required for the larger scales in the universe, including kilo parsecs (kpc) for the more distant objects within and around the Milky Way, mega parsecs (Mpc) for mid-distance galaxies, and giga parsecs (Gpc) for many quasars and the most distant galaxies. The galaxy RXJ1242-11, observed to have a supermassive black hole core similar to the Milky Way's, is about 200 Mpc (650 million light-years) from the Earth.
In 2010, a measurement of the radial velocity of halo stars found that the mass enclosed within 80 kiloparsecs is 7 . According to a study published in 2014, the mass of the entire Milky Way is estimated to be 8.5, but this is actually only half the mass of the Andromeda Galaxy.

Andromeda Galaxy

AndromedaM31Andromeda Nebula
The Andromeda Galaxy (M31) is about 780 kpc (2.5 million light-years) away from the Earth.
The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224, is a spiral galaxy approximately 780 kiloparsecs (2.5 million light-years) from Earth, and the nearest major galaxy to the Milky Way.

Galaxy

galaxiesgalacticgalactic nuclei
Astronomers typically use kiloparsecs to express distances between parts of a galaxy, or within groups of galaxies.
Most of the galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3000 to 300,000 light years) and separated by distances on the order of millions of parsecs (or megaparsecs).

Herbert Hall Turner

H. H. TurnerH.H. TurnerHerbert Turner
The parsec unit was probably first suggested in 1913 by the British astronomer Herbert Hall Turner.
He is also credited with coining the word parsec.

Virgo Cluster

VirgoVirgo Cluster of galaxiesVirgo Cl.
The nearest large galaxy cluster, the Virgo Cluster, is about 16.5 Mpc (54 million light-years) from the Earth.
The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly (16.5 ± 0.1 Mpc)

Galaxy filament

filamentsfilamentgalactic filament
The galaxy filament Hercules–Corona Borealis Great Wall, currently the largest known structure in the universe, is about 3 Gpc (10 billion light-years) across.
They are massive, thread-like formations, with a typical length of 50 to 80 megaparsecs h −1 (163 to 261 million light-years) that form the boundaries between large voids in the universe.

Unit of length

lengthunits of lengthL
The parsec (symbol: pc) is a unit of length used to measure large distances to astronomical objects outside the Solar System.
parsec pc ≈ 30856775814671.9 km or about 3.26156 ly

Hubble's law

Hubble constantcosmological redshiftHubble parameter
h is a parameter in the range 0.5 < h < 0.75 reflecting the uncertainty in the value of the Hubble constant H for the rate of expansion of the universe: h = H⁄100 km/s/Mpc.
1) Objects observed in deep space—extragalactic space, 10 megaparsecs (Mpc) or more—are found to have a redshift, interpreted as a relative velocity away from Earth;

Sun

solarSolThe Sun
The nearest star, Proxima Centauri, is about 1.3 pc from the Sun.
The Sun lies close to the inner rim of the Milky Way's Orion Arm, in the Local Interstellar Cloud or the Gould Belt, at a distance of 7.5–8.5 kpc (25,000–28,000 light-years) from the Galactic Center.

RX J1242-11

Komossa's objectRX J124236.9-111935
The galaxy RXJ1242-11, observed to have a supermassive black hole core similar to the Milky Way's, is about 200 Mpc (650 million light-years) from the Earth.
RX J1242.6-1119A (often abbreviated RX J1242-11) is an elliptical galaxy located approximately 200 megaparsecs (about 650 million light-years) from Earth.

Observable universe

large-scale structure of the universelarge-scale structurelarge-scale structure of the cosmos
One gigaparsec is about 3.26 billion light-years, or roughly 1⁄14 of the distance to the horizon of the observable universe (dictated by the cosmic background radiation).
According to calculations, the current comoving distance—proper distance, which takes into account that the universe has expanded since the light was emitted—to particles from which the cosmic microwave background radiation (CMBR) was emitted, which represent the radius of the visible universe, is about 14.0 billion parsecs (about 45.7 billion light-years), while the comoving distance to the edge of the observable universe is about 14.3 billion parsecs (about 46.6 billion light-years), about 2% larger.

Galaxy group

group of galaxiesgroups of galaxiesgroup
Astronomers typically use kiloparsecs to express distances between parts of a galaxy, or within groups of galaxies.
They typically contain no more than 50 galaxies in a diameter of 1 to 2 megaparsecs (Mpc).

List of humorous units of measurement

attoparsechumorous system of unitshumorous unit of measurement
List of humorous units of measurement#Attoparsec
Parsecs are used in astronomy to measure enormous interstellar distances.

Orders of magnitude (length)

mmnmgigameter
One gigaparsec (Gpc) is one billion parsecs — one of the largest units of length commonly used.
3.1 cm – 1 attoparsec (10 −18 parsecs)

Skinny triangle

skinny right trianglessmall angles
No trigonometric functions are required in this relationship because the very small angles involved mean that the approximate solution of the skinny triangle can be applied.
The distance to a star with a parallax angle of only one arcsecond measured on a baseline of one AU is a unit known as the parsec (pc) in astronomy and is equal to about 3.26 light years.

Globular cluster

globular clustersglobulartidal radius
Distances expressed in parsecs (pc) include distances between nearby stars, such as those in the same spiral arm or globular cluster.
The type of stars found in a globular cluster are similar to those in the bulge of a spiral galaxy but confined to a volume of only a few million cubic parsecs.

Angular distance

angular separationangular separationsapparent distance
The parallax of a star is defined as half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun.
In order to calculate the angular distance \theta in arcseconds for binary star systems, extrasolar planets, solar system objects and other astronomical objects, we use orbital distance (semi-major axis), a, in AU divided by stellar distance D in parsecs, per the small-angle approximation for :

Pleiades

Pleiades star clusterM45the Pleiades
The distance to the open cluster Pleiades is 130 pc (420 ly) from us, per Hipparcos parallax measurement.
Results prior to the launch of the Hipparcos satellite generally found that the Pleiades were about 135 parsecs away from Earth.

Supermassive black hole

supermassive black holesblack holesuper massive black hole
The galaxy RXJ1242-11, observed to have a supermassive black hole core similar to the Milky Way's, is about 200 Mpc (650 million light-years) from the Earth.
Using the Very Long Baseline Array to observe Messier 106, Miyoshi et al. (1995) were able to demonstrate that the emission from an H 2 O maser in this galaxy came from a gaseous disk in the nucleus that orbited a concentrated mass of 3.6 solar mass, which was constrained to a radius of 0.13 parsecs.