Galactic plane

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NGC 4452.jpg
This edge-on view of the galaxy NGC 4452 from Earth shows its galactic plane, with the nucleus at the center.
This edge-on view of the spiral galaxy NGC 891 shows the profile of a dusty 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.

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Galactic astronomy

Galactic astronomy is the study of the Milky Way galaxy and all its contents. This is in contrast to extragalactic astronomy, which is the study of everything outside our galaxy, including all other galaxies.

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

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Proper motion

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Spiral galaxy Class of galaxy having a number of arms of younger stars

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Galactic Center Rotational center of the Milky Way galaxy

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Supergalactic coordinate system

In the 1950s the astronomer Gérard de Vaucouleurs recognized the existence of a flattened “local supercluster” from the Shapley-Ames Catalog in the environment of the Milky Way. He noticed that when one plots nearby galaxies in 3D, they lie more or less on a plane. A flattened distribution of nebulae had earlier been noted by William Herschel over 200 years. Vera Rubin had also identified the supergalactic plane in the 1950s, but her data remained unpublished. The plane delineated by various galaxies defined in 1976 the equator of the supergalactic coordinate system he developed. In years thereafter with more observation data available de Vaucouleurs findings about the existence of the plane proved right.

Sagittarius Dwarf Spheroidal Galaxy Dwarf galaxy

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Sagittarius A* The supermassive black hole at the center of the Milky Way

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Carina–Sagittarius Arm

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Milky Way Barred spiral galaxy containing our Solar System

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Sagittarius A

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