Movatterモバイル変換

[0]ホーム

Jump to content

Galactic coordinate system

Edit links

From Wikipedia, the free encyclopedia

(Redirected fromNorth Galactic Pole)

Celestial coordinate system in spherical coordinates, with the Sun as its center

An artistic depiction of the Milky Way Galaxy showing the origin and orientation of galactic longitude. The galactic longitude (l) runs from the Sun upwards in the image through the center of the galaxy. The galactic latitude (b) is perpendicular to the image (i.e. coming out of the image) and also centered on the Sun.

Thegalactic coordinate system (GCS) is acelestial coordinate system inspherical coordinates, with theSun as its center, the primary direction aligned with the approximate center of theMilky Way Galaxy, and thefundamental plane parallel to an approximation of thegalactic plane but offset to its north. It uses theright-handed convention, meaning that coordinates are positive toward the north and toward the east in thefundamental plane.^[1]

Spherical coordinates

[edit]

Galactic longitude

[edit]

Longitude (symboll) measures theangular distance of an object eastward along the galactic equator from the Galactic Center. Analogous to terrestriallongitude, galactic longitude is usually measured in degrees (°).

Galactic latitude

[edit]

Latitude (symbolb) measures theangle of an object northward of the galactic equator (or midplane) as viewed from Earth. Analogous to terrestriallatitude, galactic latitude is usually measured in degrees.

Definition

[edit]

The first galactic coordinate system was used byWilliam Herschel in 1785. A number of different coordinate systems, each differing by a few degrees, were used until 1932, whenLund Observatory assembled a set of conversion tables that defined a standard galactic coordinate system based on a galactic north pole atRA12^h 40^m,dec +28° (in theB1900.0 epoch convention) and a 0° longitude at the point where thegalactic plane andequatorial plane intersected.^[1]

In 1958, theInternational Astronomical Union (IAU) defined the galactic coordinate system in reference to radio observations of galactic neutralhydrogen through thehydrogen line, changing the definition of the Galactic longitude by 32° and the latitude by 1.5°.^[1] In theequatorial coordinate system, forequinox and equator of 1950.0, the north galactic pole is defined atright ascension12^h 49^m,declination +27.4°, in the constellationComa Berenices, with a probable error of ±0.1°.^[2] Longitude 0° is the great semicircle that originates from this point along the line inposition angle 123° with respect to theequatorial pole. The galactic longitude increases in the same direction as right ascension. Galactic latitude is positive towards the north galactic pole, with a plane passing through the Sun and parallel to the galactic equator being 0°, whilst the poles are ±90°.^[3] Based on this definition, the galactic poles and equator can be found fromspherical trigonometry and can beprecessed to otherepochs; see the table.

J2000.0 equatorial coordinates approximating the galactic reference points^[1]

Right ascension

Declination

Constellation

North Pole
+90° latitude

12^h 51.4^m

+27.13°

Coma Berenices
(near31 Com)

South Pole
−90° latitude

0^h 51.4^m

−27.13°

Sculptor
(nearNGC 288)

Center
0° longitude

17^h 45.6^m

−28.94°

Sagittarius
(inSagittarius A)

Anticenter
180° longitude

5^h 45.6^m

+28.94°

Auriga
(nearHIP 27180)

Galactic north	Galactic south	Galactic center

Approx galactic quadrants (NGQ/SGQ, 1–4) indicated, along with differentiating Galactic Plane (containing the Galactic Center) and the Galactic Coordinates Plane (containing the Sun / Solar System)

The IAU recommended that during the transition period from the old, pre-1958 system to the new, the old longitude and latitude should be designatedl^I andb^I while the new should be designatedl^II andb^II.^[3] This convention is occasionally seen.^[4]

Radio source Sagittarius A*, which is the best physical marker of the trueGalactic Center, is located at17^h 45^m 40.0409^s,−29° 00′ 28.118″ (J2000).^[2] Rounded to the same number of digits as the table,17^h 45.7^m, −29.01° (J2000), there is an offset of about 0.07° from the defined coordinate center, well within the 1958 error estimate of ±0.1°. Due to the Sun's position, which currently lies56.75±6.20 ly north of the midplane, and the heliocentric definition adopted by the IAU, the galactic coordinates of Sgr A* are latitude+0° 07′ 12″ south, longitude0° 04′ 06″. Since as defined the galactic coordinate system does not rotate with time, Sgr A* is actually decreasing in longitude at the rate of galactic rotation at the sun,Ω, approximately 5.7milliarcseconds per year (seeOort constants).

Conversion between equatorial and galactic coordinates

[edit]

An object's location expressed in theequatorial coordinate system can be transformed into the galactic coordinate system. In these equations,α isright ascension,δ isdeclination. NGP refers to the coordinate values of the north galactic pole and NCP to those of the north celestial pole.^[5]

{\begin{aligned}\sin(b)&=\sin(\delta _{\text{NGP}})\sin(\delta )+\cos(\delta _{\text{NGP}})\cos(\delta )\cos(\alpha -\alpha _{\text{NGP}})\\\cos(b)\sin(l_{\text{NCP}}-l)&=\cos(\delta )\sin(\alpha -\alpha _{\text{NGP}})\\\cos(b)\cos(l_{\text{NCP}}-l)&=\cos(\delta _{\text{NGP}})\sin(\delta )-\sin(\delta _{\text{NGP}})\cos(\delta )\cos(\alpha -\alpha _{\text{NGP}})\end{aligned}}

The reverse (galactic to equatorial) can also be accomplished with the following conversion formulas.

{\begin{aligned}\sin(\delta )&=\sin(\delta _{\text{NGP}})\sin(b)+\cos(\delta _{\text{NGP}})\cos(b)\cos(l_{\text{NCP}}-l)\\\cos(\delta )\sin(\alpha -\alpha _{\text{NGP}})&=\cos(b)\sin(l_{\text{NCP}}-l)\\\cos(\delta )\cos(\alpha -\alpha _{\text{NGP}})&=\cos(\delta _{\text{NGP}})\sin(b)-\sin(\delta _{\text{NGP}})\cos(b)\cos(l_{\text{NCP}}-l)\end{aligned}}

Where:

\alpha _{\text{NGP}}=12^{h}51.4^{m}\qquad \delta _{\text{NGP}}=27.13^{\circ }\qquad l_{\text{NCP}}=122.93314^{\circ }

Rectangular coordinates

[edit]

In some applications use is made of rectangular coordinates based on galactic longitude and latitude and distance. In some work regarding the distant past or future the galactic coordinate system is taken as rotating so that thex-axis always goes to the centre of the galaxy.^[6]

There are two majorrectangular variations of galactic coordinates, commonly used for computing space velocities of galactic objects. In these systems thexyz-axes are designatedUVW, but the definitions vary by author. In one system, theU axis is directed toward the Galactic Center (l = 0°), and it is aright-handed system (positive towards the east and towards the north galactic pole); in the other, theU axis is directed toward the galactic anticenter (l = 180°), and it is a left-handed system (positive towards the east and towards the north galactic pole).^[7]

Theanisotropy of the star density in the night sky makes the galactic coordinate system very useful for coordinating surveys, both those that require high densities of stars at low galactic latitudes, and those that require a low density of stars at high galactic latitudes. For this image theMollweide projection has been applied, typical in maps using galactic coordinates.

In the constellations

[edit]

The galactic equator runs through the followingconstellations:^[8]

References

[edit]

^^a ^b ^c ^dBlaauw, A.; Gum, C.S.; Pawsey, J.L.; Westerhout, G. (1960)."The new IAU system of galactic coordinates (1958 revision)".Monthly Notices of the Royal Astronomical Society.121 (2): 123.Bibcode:1960MNRAS.121..123B.doi:10.1093/mnras/121.2.123.
^^a ^bReid, M.J.; Brunthaler, A. (2004). "The Proper Motion of Sagittarius A*".The Astrophysical Journal.616 (2): 874, 883.arXiv:astro-ph/0408107.Bibcode:2004ApJ...616..872R.doi:10.1086/424960.S2CID 16568545.
^^a ^bJames Binney, Michael Merrifield (1998).Galactic Astronomy.Princeton University Press. pp. 30–31.ISBN 0-691-02565-7.
^For example inKogut, A.; et al. (1993). "Dipole Anisotropy in the COBE Differential Microwave Radiometers First-Year Sky Maps".Astrophysical Journal.419: 1.arXiv:astro-ph/9312056.Bibcode:1993ApJ...419....1K.doi:10.1086/173453.
^Carroll, Bradley; Ostlie, Dale (2007).An Introduction to Modern Astrophysics (2nd ed.). Pearson Addison-Wesley. pp. 900–901.ISBN 978-0805304022.
^For exampleBobylev, Vadim V. (March 2010). "Searching for Stars Closely Encountering with the Solar System".Astronomy Letters.36 (3):220–226.arXiv:1003.2160.Bibcode:2010AstL...36..220B.doi:10.1134/S1063773710030060.S2CID 118374161.
^Johnson, Dean R.H.; Soderblom, David R. (1987). "Calculating galactic space velocities and their uncertainties, with an application to the Ursa Major group".Astronomical Journal.93: 864.Bibcode:1987AJ.....93..864J.doi:10.1086/114370.
^"SEDS Milky Way Constellations".

External links

[edit]

Universal coordinate converter Archived 2004-10-17 at theWayback Machine.
Galactic Coordinate System - Wolfram Demonstration
Galactic coordinates, TheInternet Encyclopedia of Science
Fiona Vincent,Positional Astronomy: Galactic coordinates Archived 2012-09-10 at theWayback Machine,University of St Andrews
An Atlas of the Universe

Milky Way

Location

Milky Way →Milky Way subgroup →Local Group→Local Sheet→Virgo Supercluster→Laniakea Supercluster →Local Hole →Observable universe →Universe
Each arrow (→) may be read as "within" or "part of".

Structure

Galactic Center	Sagittarius A Sagittarius A* Fermi bubbles Supermassive black hole Galactic Center filaments Galactic Center GeV excess Galactic Center Radio Arc
Disk	Carina–Sagittarius Norma–Cygnus Orion–Cygnus Perseus Scutum–Centaurus Near 3 kpc Arm Far 3 kpc Arm
Halo	Aquarius Stream Fimbulthul stream Gaia Sausage Helmi stream Magellanic Stream Monoceros Ring Palomar 5 stream Pisces–Eridanus stream Sagittarius Stream Virgo Stream

Satellite
galaxies

Magellanic Clouds	Large Magellanic Cloud Small Magellanic Cloud Magellanic Bridge
Dwarfs	Antlia II Boötes I Boötes II Boötes III Canes Venatici I Canes Venatici II Canis Major Carina Coma Berenices Crater 2 Draco Eridanus II Fornax Hercules Leo I Leo II Leo IV Leo V Leo T Phoenix Pisces Overdensity Pisces II Reticulum II Sagittarius Spheroidal Segue 1 Segue 2 Sculptor Sextans Triangulum II Ursa Major I Ursa Major II Ursa Major III Ursa Minor Virgo I Willman 1

v t e Galaxies
Morphology	Disc Thick disk Thin disk Lenticular barred unbarred Spiral anemic barred flocculent grand design intermediate Magellanic unbarred Dwarf galaxy elliptical irregular spheroidal spiral Elliptical galaxy cD Irregular barred Peculiar Ring Polar
Structure	Active galactic nucleus Bar Bulge Central massive object Galactic Center Dark matter halo Disc Disc galaxy Halo corona Galactic plane Galactic ridge Interstellar medium Protogalaxy Galaxy filament Spiral arm Supermassive black hole Ultramassive
Active nuclei	Blazar BL Lacertae object LINER Little Red Dots Markarian Quasar Radio X-shaped DRAGN Relativistic jet Seyfert
Energetic galaxies	Lyman-alpha emitter Lyman-break Luminous infrared ULIRG HLIRG ELIRG Starburst blue compact dwarf blueberry pea faint blue Luminous infrared galaxy Hot dust-obscured Green bean galaxy Hanny's Voorwerp Wolf-Rayet galaxy
Low activity	Low surface brightness Ultra diffuse Dark galaxy Red nugget Blue Nugget Dead disk
Interaction	Binary galaxy Field Galactic tide Groups and clusters group cluster Brightest cluster galaxy fossil group Interacting merger collision cannibalism Jellyfish Satellite Stellar stream Superclusters Walls Voids and supervoids void galaxy
Lists	Galaxies Galaxies named after people Largest Nearest Polar-ring Ring Spiral Groups and clusters Large quasar groups Quasars List of the most distant astronomical objects Starburst galaxies Superclusters Voids
Related	Extragalactic astronomy Galactic astronomy Galactic coordinate system Galactic habitable zone Galactic magnetic fields Galactic orientation Galactic quadrant Galaxy color–magnitude diagram Galaxy formation and evolution Galaxy rotation curve Gravitational lens Gravitational microlensing Illustris project Intergalactic dust Intergalactic stars Intergalactic travel Population III stars Galaxy X (galaxy)
Category Portal