Milky Way
The Milky Way's Galactic Center in the night sky above the Paranal Observatory (the laser creates a guide-star for the telescope).
| |
| Observation data | |
|---|---|
| Type | Sb, Sbc, or SB(rs)bc[1][2] (barred spiral galaxy) |
| Diameter | 100–180 kly (31–55 kpc)[3] |
| Thickness of thin stellar disk | ≈2 kly (0.6 kpc)[4][5] |
| Number of stars | 100–400 billion (2.5 × 1011 ± 1.5 × 1011)[6][7][8] |
| Oldest known star | ≥13.7 Gyr[9] |
| Mass | 0.8–1.5×1012 M☉[10][11][12][13] |
| Angular momentum | ≈1×1067 J s[14] |
| Sun's distance to Galactic Center | 26.4 ± 1.0 kly (8.09 ± 0.31 kpc)[15][16][17] |
| Sun's Galactic rotation period | 240 Myr[18] |
| Spiral pattern rotation period | 220–360 Myr[19] |
| Bar patternrotation period | 100–120 Myr[19] |
| Speed relative to CMB rest frame | 631 ± 20 km/s[20] |
| Escape velocity at Sun's position | 550 km/s[13] |
| Dark matter density at Sun's position | 0.0088+0.0024 −0.0018 M☉pc-3 or 0.35+0.08 −0.07 GeV cm-3[13] |
| See also: Galaxy, List of galaxies | |
The Milky Way is the galaxy[nb 1] that contains our Solar System. The descriptive "milky" is derived from the appearance from Earth of the galaxy – a band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος (galaxías kýklos, "milky circle").[21][22][23] From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe.[24] Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis,[25] observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.
The Milky Way is a barred spiral galaxy with a diameter between 100,000[26] and 180,000 light-years (ly).[27] It is estimated to contain 100–400 billion stars.[28][29] There are probably at least 100 billion planets in the Milky Way.[30][31] The Solar System is located within the disk, about 26,000 light-years from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The galactic center is an intense radio source known as Sagittarius A*, likely a supermassive black hole.
Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been termed "dark matter".[32] The rotational period is about 240 million years at the position of the Sun.[18] The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.[9]
The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.[33][34]
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