Professori f m. f d. I. Sattorov ttymi qoshidagi al o‘qituvchisi L

A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas dust, and an important but poorly understood component tentatively dubbed dark matter.The name is from the Greek word galaxias, literally meaning "milky", a reference to the Milky Way galaxy. Typical galaxies range from dwarfs with as few as ten million (107) stars, up to giants with a hundred trillion (1014) stars,[4] all orbiting the galaxy's center of mass. Galaxies may contain many star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.

A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Spiral galaxies are named for the spiral structures that extend from the center into the disk. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disk because of the young, hot OB stars that inhabit them.

The galactic disk is surrounded by a spheroid halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc),[50] suggesting a stellar halo diameter of 200,000 light-years. However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years away from the galactic center. About 40% of these clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation. The globular clusters can follow rosette orbits about the galaxy, in contrast to the elliptical orbit of a planet.

While the disk contains gas and dust which obscure the view in some wavelengths, the spheroid component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but not in the halo. Open clusters also occur primarily in the disk.

Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much further than

Main article: Galactic Center

Observed structure of the Milky Way's spiral arms. The Sun is in the Local Spur.

The galactic disc, which bulges outward at the galactic center, has a diameter of 70,000-100,000 light-years (20-30 kpc). The exact distance from the Sun to the galactic center is actively debated. The latest estimates from geometric-based methods and standard candles yield distances to the Galactic center of 7.6-8.7 kpc (25,000-28,000 ly). The fact that the estimates span over 1 kpc only underscores the true uncertainty associated with the distance to the Galactic center.

The galactic center harbors a compact object of very large mass as determined by the motion of material around the center. The intense radio source named Sagittarius A*, thought to mark the center of the Milky Way, is newly confirmed to be a supermassive black hole. Most galaxies are believed to have supermassive black holes at their centers.

Artist's conception of the spiral structure of the Milky Way with two major stellar arms and a bar.

Maps of the Milky Way's spiral structure are notoriously uncertain and exhibit striking differences. Some 150 years after Alexander (1852) first suggested that the Milky Way was a spiral, there is currently no consensus on the number or nature of the Galaxy's spiral arms. Perfect grand design logarithmic spiral patterns ineptly describe features near the Sun, namely since galaxies commonly exhibit arms that branch, merge, twist unexpectedly, and feature a degree of irregularity. The possible scenario of the Sun within a spur / Local arm emphasizes that point and indicates that such features are likely not unique, and exist elsewhere in the galaxy.