Galactic coordinate system

The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way galaxy, and the fundamental plane parallel to an approximation of the galactic plane but offset to its north. It uses the right-handed convention, meaning that coordinates are positive toward the north and toward the east in the fundamental plane.[1]

Artist's impression of the Milky Way (updated - annotated)
Artist's depiction of the Milky Way galaxy, showing the galactic longitude relative to the Galactic Center

Galactic longitude

Galactic coordinates
The galactic coordinates use the Sun as the origin. Galactic longitude (l) is measured with primary direction from the Sun to the center of the galaxy in the galactic plane, while the galactic latitude (b) measures the angle of the object above the galactic plane.

Longitude (symbol l) measures the angular distance of an object eastward along the galactic equator from the galactic center. Analogous to terrestrial longitude, galactic longitude is usually measured in degrees (°).

Galactic latitude

Latitude (symbol b) measures the angle of an object north or south of the galactic equator (or midplane) as viewed from Earth; positive to the north, negative to the south. For example, the north galactic pole has a latitude of +90°. Analogous to terrestrial latitude, galactic latitude is usually measured in degrees (°).


The first galactic coordinate system was used by William Herschel in 1785. A number of different coordinate systems, each differing by a few degrees, were used until 1932, when Lund Observatory assembled a set of conversion tables that defined a standard galactic coordinate system based on a galactic north pole at RA 12h 40m, dec +28° (in the B1900.0 epoch convention) and a 0° longitude at the point where the galactic plane and equatorial plane intersected.[1]

Equatorial coordinates J2000.0 of galactic reference points[1]
  Right ascension Declination Constellation
North Pole
+90° latitude
12h 51.4m +27.13° Coma Berenices
(near 31 Com)
South Pole
−90° latitude
0h 51.4m −27.13° Sculptor
(near NGC 288)
0° longitude
17h 45.6m −28.94° Sagittarius
(in Sagittarius A)
180° longitude
5h 45.6m +28.94° Auriga
(near HIP 27088)
Galactic north pole

Galactic north
Galactic south pole

Galactic south
Galactic zero longitude

Galactic center

In 1958, the International Astronomical Union (IAU) defined the galactic coordinate system in reference to radio observations of galactic neutral hydrogen through the hydrogen line, changing the definition of the Galactic longitude by 32° and the latitude by 1.5°.[1] In the equatorial coordinate system, for equinox and equator of 1950.0, the north galactic pole is defined at right ascension 12h 49m, declination +27.4°, in the constellation Coma Berenices, with a probable error of ±0.1°.[2] Longitude 0° is the great semicircle that originates from this point along the line in position angle 123° with respect to the equatorial 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 from spherical trigonometry and can be precessed to other epochs; see the table.

The IAU recommended that during the transition period from the old, pre-1958 system to the new, the old longitude and latitude should be designated lI and bI while the new should be designated lII and bII.[3] This convention is occasionally seen.[4]

Radio source Sagittarius A*, which is the best physical marker of the true galactic center, is located at 17h 45m 40.0409s, −29° 00′ 28.118″ (J2000).[2] Rounded to the same number of digits as the table, 17h 45.7m, −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 lies 56.75±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, longitude 0° 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 5.7 milliarcseconds per year (see Oort constants).

Conversion between Equatorial and Galactic Coordinates

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

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

Rectangular coordinates

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 the x-axis always goes to the centre of the galaxy.[6]

There are two major rectangular variations of galactic coordinates, commonly used for computing space velocities of galactic objects. In these systems the xyz-axes are designated UVW, but the definitions vary by author. In one system, the U axis is directed toward the galactic center (l = 0°), and it is a right-handed system (positive towards the east and towards the north galactic pole); in the other, the U 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]

In the constellations

Milky Way infrared
The anisotropy 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 the Mollweide projection has been applied, typical in maps using galactic coordinates.

The galactic equator runs through the following constellations:[8]

See also


  1. ^ a b c d Blaauw, 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.
  2. ^ a b Reid, M.J.; Brunthaler, A. (2004). "The Proper Motion of Sagittarius A*". The Astrophysical Journal. The American Astronomical Society. 616 (2): 874, 883. arXiv:astro-ph/0408107. Bibcode:2004ApJ...616..872R. doi:10.1086/424960.
  3. ^ a b James Binney, Michael Merrifield (1998). Galactic Astronomy. Princeton University Press. pp. 30–31. ISBN 0-691-02565-7.
  4. ^ For example in Kogut, 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.
  5. ^ Carroll, Bradley; Ostlie, Dale. An Introduction to Modern Astrophysics (2nd ed.). Pearson Addison-Wesley. p. 1054-1056. ISBN 978-0805304022.
  6. ^ For example Bobylev, 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.
  7. ^ 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.
  8. ^ "SEDS Milky Way Constellations".

External links

Celestial sphere

In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemispherical screen over the observing location.

The celestial sphere is a practical tool for spherical astronomy, allowing astronomers to specify the apparent positions of objects in the sky if their distances are unknown or irrelevant. In the equatorial coordinate system, the celestial equator divides the celestial sphere into two halves: the northern and southern celestial hemispheres.

Coma Filament

Coma Filament is a galaxy filament. The filament contains the Coma Supercluster of galaxies and forms a part of the CfA2 Great Wall.

Delta Scuti

Delta Scuti (δ Sct, δ Scuti) is a giant star in the southern constellation Scutum. With an apparent visual magnitude of 4.72, it is the fifth-brightest star in this small and otherwise undistinguished constellation. Analysis of the parallax measurements made during the Hipparcos mission place this star at a distance of about 202 light-years (62 parsecs) from Earth. Delta Scuti is the prototype of the Delta Scuti type variable stars. It is a high-amplitude δ Scuti type pulsator with light variations of about 0.15 minutes. The peculiar chemical abundances of this star are similar to those of Am stars.In 1900, William W. Campbell and William H. Wright used the Mills spectrograph at the Lick Observatory to determine that this star has a variable radial velocity. The 0.19377-day period of this variability as well as 0.2 magnitude changes in luminosity demonstrated in 1935 that the variability was intrinsic, rather than being the result of a spectroscopic binary. In 1938, a secondary period was discovered and a pulsation theory was proposed to model the variation. Since then, observation of Delta Scuti has shown that it pulsates in multiple discrete radial and non-radial modes. The strongest mode has a frequency of 59.731 μHz, the next strongest has a frequency of 61.936 μHz, and so forth, with a total of eight different frequency modes now modeled.The space velocity components of this star in the galactic coordinate system are [U, V, W] = [–42, –17, –1] km·s−1. It is following an orbit through the Milky Way galaxy that has an eccentricity of 0.11, carrying it as close as 22.31 kly (6.84 kpc) to, and as far as 27.59 kly (8.46 kpc) from the galactic center. If Delta Scuti maintains its current movement and brightness, it will pass within 10 light-years of the solar system, becoming the brightest star in the sky between 1150000 and 1330000 CE. It will reach an apparent magnitude of -1.84, brighter than the current -1.46 of Sirius.This star has two optical companions. The first is a +12.2 magnitude star that is 15.2 arcseconds from Delta Scuti. The second is a +9.2 magnitude star that is 53 arcseconds away.Flamsteed did not recognise the constellation Scutum and included several of its stars in Aquila. δ Scuti was catalogued as 2 Aquilae. The Bayer designation δ was assigned by Gould rather than Bayer.

Digital Universe Atlas

Digital Universe Atlas is a free open source software planetarium application, available under the terms of the Illinois Open Source License, and running on Linux, Windows, macOS (10.5 and above), AmigaOS 4, and IRIX.

It is a standalone 4-dimensional space visualization application built on the programmable Partiview data visualization engine designed by Stuart Levy of the National Center for Supercomputing Applications (NCSA) as an adjunct of the NCSA's Virtual Director virtual choreography project. The Virtual Universe Atlas project was launched by the American Museum of Natural History's Hayden Planetarium with significant programming support from the National Aeronautics and Space Administration as well as Stuart Levy. The database draws on the National Virtual Observatory.

Along with Celestia and Orbiter, and unlike most other planetarium applications, Digital Universe shares the capacity to visualize space from points outside Earth. Building on work by Japan's RIKEN, its planet renderings and zoom visualizations can match or exceed Celestia and Orbiter. Unlike Celestia and Orbiter, highly accurate visualization from distances beyond the Milky Way galaxy is integral to the software and the datasets. This allows for unrivaled flexibility in plotting itineraries that reveal true distances and configurations of objects in the observable sky. It therefore improves understanding of the surroundings of the solar system in terms of observer-neutral celestial coordinate systems—systems that are neither geocentric nor heliocentric—such as the galactic coordinate system and supergalactic coordinate system.

The Digital Universe Atlas has spun off a commercial-grade planetarium platform from SCISS called Uniview that was featured in the White House star party on October 7, 2009. The Atlas database and Partiview interface is compatible with professional planetarium software such as Evans & Sutherland's Digistar and Sky-Skan's DigitalSky 2.

Fundamental plane (spherical coordinates)

The fundamental plane in a spherical coordinate system is a plane of reference that divides the sphere into two hemispheres. The latitude of a point is then the angle between the fundamental plane and the line joining the point to the centre of the sphere.For a geographic coordinate system of the Earth, the fundamental plane is the Equator. Celestial coordinate systems have varying fundamental planes:

The horizontal coordinate system uses the observer's horizon.

The Besselian coordinate system uses Earth's terminator (day/night boundary). This is a Cartesian coordinate system (x, y, z).

The equatorial coordinate system uses the celestial equator.

The ecliptic coordinate system uses the ecliptic.

The galactic coordinate system uses the Milky Way's galactic equator.

Galactic anticenter

The galactic anticenter is a direction in space directly opposite to the Galactic Center, as viewed from Earth. This direction corresponds to a point on the celestial sphere. From the perspective of an observer on Earth, the galactic anticenter is located in the constellation Auriga, and Beta Tauri is the bright star that appears nearest this point.

In terms of the galactic coordinate system, the Galactic Center (in Sagittarius) corresponds to a longitude of 0°, while the anticenter is located exactly at 180°. In the equatorial coordinate system, the anticenter is found at roughly RA 05h 46m, dec +28° 56'.

Galactic corona

The terms galactic corona and gaseous corona have been used in the first decade of the 21st century to describe a hot, ionised, gaseous component in the Galactic halo of the Milky Way. A similar body of very hot and tenuous gas in the halo of any spiral galaxy may also be described by these terms.

This coronal gas may be sustained by the galactic fountain, in which superbubbles of ionised gas from supernova remnants expand vertically through galactic chimneys into the halo. As the gas cools, it is pulled back into the galactic disc of the galaxy by gravitational forces.

Galactic coronas have been and are currently being studied extensively, in the hope of gaining a further understanding of galaxy formation. Although, considering how galaxies differ in shaping and sizing, no particular theory has been able to adequately illustrate how the galaxies in the Universe originally formed.

Galactic plane

The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms galactic plane and galactic poles usually refer specifically to the plane and poles of the Milky Way, in which Planet Earth is located.

Some galaxies are irregular and do not have any well-defined disk. Even in the case of a barred spiral galaxy like the Milky Way, defining the galactic plane is slightly imprecise and arbitrary since the stars are not perfectly coplanar. In 1959, the IAU defined the position of the Milky Way's north galactic pole as exactly RA = 12h 49m, Dec = 27° 24′ in the then-used B1950 epoch; in the currently-used J2000 epoch, after precession is taken into account, its position is RA 12h 51m 26.282s, Dec 27° 07′ 42.01″. This position is in Coma Berenices, near the bright star Arcturus; likewise, the south galactic pole lies in the constellation Sculptor.

The "zero of longitude" of galactic coordinates was also defined in 1959 to be at position angle 123° from the north celestial pole. Thus the zero longitude point on the galactic equator was at 17h 42m 26.603s, −28° 55′ 00.445″ (B1950) or 17h 45m 37.224s, −28° 56′ 10.23″ (J2000), and its J2000 position angle is 122.932°. The galactic center is located at position angle 31.72° (B1950) or 31.40° (J2000) east of north.

Galactic quadrant

A galactic quadrant, or quadrant of the Galaxy, is one of four circular sectors in the division of the Milky Way Galaxy.

Galactic ridge

The galactic ridge is a region of the inner galaxy that is coincident with the galactic plane of the Milky Way. It can be seen from Earth as a band of stars which is interrupted by 'dust lanes'. In these 'dust lanes' the dust in the gaseous galactic disk (or plane) blocks the visible light of the background stars. Due to this, many of the most interesting features of the Milky Way can only be viewed in X-rays. Along with the point X-ray sources which populate the Milky Way, an apparently diffuse X-ray emission concentrated in the galactic plane is also observed. This is known as the galactic ridge X-ray emission (GRXE). These emissions were originally discovered by Diana Worrall and collaborators in 1982, and since then the origins of these emissions have puzzled astrophysicists around the globe.

It was initially believed, due to the difficulty of resolving the GRXE into point sources, that the x-ray emissions were truly diffuse in nature and that their origin might be a Galactic plasma rather than distant stellar sources. It was thought to be caused by low energy cosmic rays interacting with cold gas in the region, which heated up the gas and caused it to emit X-rays. However it was discovered that the temperature of a gas producing such an emission would have to be around tens of millions of degrees. This temperature is far too high for a gas to be bound gravitationally to the galaxy. Therefore, it was proposed that the GRXE might be caused by a large number of extremely remote and outlying stars. In 2009, after decades of attempting to resolve the GRXE, Mikhail Revnivtsev, his partner Sazonov and their colleges managed to resolve approximately 80% of the emissions over the course of 12 days using the Chandra X-ray observatory. During this time period a total of 473 sources of x-ray emission were detected in an area that is significantly smaller than the size of a Full Moon. This is one of the highest densities of x-ray sources ever seen in our Galaxy.

Due to this amazing discovery it is now thought that about 80% of the emission comes from discrete sources such as white dwarfs and stars with active coronae.However, recent work by researchers at the Max Planck Institute for Astrophysics suggests that the GRXE may indeed consist of an additional, diffuse component after all. This diffuse component could arise not from the thermal emission of a very hot plasma but from the reprocessing by the interstellar gas of the X-ray radiation produced by luminous X-ray binary sources located in the Galaxy. X-ray binaries are the most luminous sources of X-rays in galaxies such as the Milky Way. These binary systems emit X-ray radiation when material or substance from a so-called donor star falls into the strong gravitational field of a compact object, such as a neutron star or a black hole. This X-ray radiation illuminates the atoms and molecules in the Galactic interstellar gas, which then scatter the incoming photons in different directions and at different energies. The resulting emission appears truly diffuse to the viewer.The Galactic Ridge has a width of 5° latitude (b) and ±40° longitude (l) in the Galactic coordinate system.The first instrument that was able to measure diffuse X-ray emission was the HEAO A2 (High Energy Astrophysical Observatory). However it was created to study the large-scale structure of the galaxy and the universe, and to yield high-quality spatial and spectral data in the X-ray region. Still, the HEAO A2 produced valuable information on discrete X-ray sources such as binary star systems, hot white dwarfs, cataclysmic variables and supernova remnants. The HEAO A2 also allowed for the study of extragalactic objects, for example radio galaxies, Seyfert galaxies, and quasars. Elihu Boldt was the principal investigator of the HEAO A2 instrument, however he worked alongside G. Gamire on the project. The HEAO A2 was launched into space in 1977, where its job was to scan the sky for approximately 17 months. It (the HEAO A2) produced the first low-background, all-sky maps in the 2-60 keV band, and for its time the HEAO A2 produced the best spectra ever obtained over 2-60 keV energy range.

Gamma Microscopii

Gamma Microscopii (γ Microscopii, γ Mic) is the brightest star in the faint southern constellation of Microscopium. It has an apparent visual magnitude of 4.68, which is too dim to be viewed from city skies. The distance to this star has been determined using parallax measurements made with the Gaia telescope, which place it at 223 ± 8 light-years (68.4 ± 2.5 parsecs).

Based upon a stellar classification of G6 III, this is a G-type giant star. It is a core helium fusing star that is classified as a member of the red clump evolutionary branch, although the metallicity of this star—meaning the abundance of elements other than hydrogen and helium—is anomalously low for a member of this group. The effective temperature of the star's outer envelope is 5,050 K, giving it the yellow-hued glow typical of G-type stars.In the galactic coordinate system, this star has space velocity components of [U, V, W] = [+13.75, +3.47, –10.50] km s−1. The peculiar velocity of this star, relative to its neighbors, is 1.2 km s−1. It has been listed as likely member of the Ursa Major Moving Group of stars that share a similar location and a common trajectory through space. Backwards extrapolation of the motion of γ Microscopii has shown that approximately 3.8 million years ago, it was only around 6 light-years from the Sun. It would then have had an apparent magnitude of −3 and have been brighter than Sirius is now. Shortly before that, around 3.9 million years ago, it likely passed within 1.14 to 3.45 light-years of the Sun, possibly massive enough and close enough to disturb the Oort cloud.Gamma Microscopii has a visual companion, CCDM J21013-3215B at an angular separation of 26 arcseconds along a position angle of 94°, with an apparent visual magnitude of approximately 13.7. Most likely this star is not gravitationally bound to γ Microscopii, but is merely a line of sight companion.The Bayer designation γ Microscopii was not assigned by Bayer himself. It was given the Flamsteed designation of 1 Piscis Austrini before Lacaille created the constellation of Microscopium in 1756.

HD 218566

HD 218566 is a star in the equatorial zodiac constellation of Pisces. With an apparent visual magnitude of 8.6, this ninth magnitude star can not be viewed with the naked eye. However, it can be readily seen even with a small telescope.HD 218566 is a smaller star than the Sun, with about 81% of the Sun's mass and 86% of the radius of the Sun. It is a K-type main sequence star with a stellar classification of K3 V that is generating energy by the nuclear fusion of hydrogen at its core. HD 218556 is radiating around 35% of the luminosity of the Sun from its outer envelope at an effective temperature of 4,849 K. This heat gives the star the characteristic orange-hued glow of a K-type star.Compared to the Sun, this star has an unusually high abundance of elements other than hydrogen and helium, what astronomers term the metallicity. Based upon the abundance of iron, the metallicity is 2.4 times as high as in the Sun. It is much older than the Sun, with estimates of its age ranging from 8.5 to 11.5 billion years. It appears to have a negligible rate of spin as its projected rotational velocity is too small to measure.This star belongs to the thick disk population of the Milky Way. In the galactic coordinate system, it has space velocity components of [U, V, W] = [77, –61, –8] km s−1. HD 218556 is following an orbit through the galaxy with an eccentricity of 0.36 ± 0.01 that carries it as close as 14.3 kly (4.4 kpc) and as far as 30.3 kly (9.3 kpc) from the Galactic Center. The orbital tilt carries this star as much as 0.6 kly (0.18 kpc) from the galactic plane.Based upon high resolution measurements performed at the W. M. Keck Observatory and analysis performed upon these measurements by amateur astronomer Peter Jalowiczor, HD 218566 shows cyclical variations in radial velocity that suggest gravitational perturbation by orbiting companion. This candidate object is estimated to be orbiting the parent star with a period of 225.7 ± 0.4 days at an eccentricity of 0.3 ± 0.1. The semi-major axis for this Keplerian orbit is an estimated 0.6873 Astronomical Units. Because the inclination of the orbit remains unknown, the mass of this companion has not been determined. However, it can be constrained to have a mass of at least 21% the mass of Jupiter. There is no evidence of additional companions in the system.

Jones-Emberson 1

Jones-Emberson 1 (PK 164+31.1) is a 14th magnitude planetary nebula in the constellation Lynx at a distance of 1600 light years. It is a larger planetary with low surface brightness. The 16.8-magnitude central star is a very blue white dwarf.

Poles of astronomical bodies

The poles of astronomical bodies are determined based on their axis of rotation in relation to the celestial poles of the celestial sphere. Astronomical bodies include stars, planets, dwarf planets and small Solar System bodies such as comets and minor planets (i.e. asteroids), as well as natural satellites and minor-planet moons.

Space velocity

Space velocity may be about,

Space velocity (astronomy), the velocity of a star in the galactic coordinate system

Space velocity (chemistry), the relation between volumetric flow and reactor volume in a chemical reactor

Ursa Major Filament

Ursa Major Filament is a galaxy filament. The filament is connected to the CfA Homunculus, a portion of the filament forms a portion of the "leg" of the Homunculus.

Galactic core
Spiral arms
Satellite galaxies
Active nuclei
Energetic galaxies
Low activity
See also

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