1 The Milky Way

The Milky Way[a] is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy 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 γαλακτικός κύκλος (galaktikos kýklos, "milky circle").[19][20][21] 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.[22] Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis,[23] observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.

Milky Way

Observation data

Constellation

Sagittarius

Right ascension

17h 45m 40.0409s

Declination

−29° 0′ 28.118″

Distance

25.6–27.1 kly (7.86–8.32 kpc)[1][2]

Characteristics

Type

Sb, Sbc, or SB(rs)bc[3][4]

(barred spiral galaxy)

Mass

(0.8–1.5)×1012[5][6][7][8] M☉

Number of stars

100–400 billion

Size

Stellar disk: 185 ± 15 kly [9][10]

Dark matter halo: 1.9 ± 0.4 Mly (580 ± 120 kpc)[11][12]

Thickness of thin stellar disk

≈2 kly (0.6 kpc)[13][14]

Angular momentum

≈1×1067 J s[15]

Sun's Galactic rotation period

240 Myr[16]

Spiral pattern rotation period

220–360 Myr[17]

Bar pattern rotation period

100–120 Myr[17]

Speed relative to CMB rest frame

552.2±5.5 km/s[18]

Escape velocity at Sun's position

550 km/s[8]

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[8]

See also: Galaxy, List of galaxies

The Milky Way is a barred spiral galaxy with an estimated visible diameter of 100,000–200,000 light-years. Recent simulations suggest that a dark matter disk, also containing some visible stars, may extend up to a diameter of almost 2 million light-years.[11][12] 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.[24][25]

It is estimated to contain 100–400 billion stars[26][27] and at least that number of planets.[28][29] The Solar System is located at a radius of about 27,000 light-years from the Galactic Center,[2] 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*, a supermassive black hole of 4.100 (± 0.034) million solar masses. 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 (about 90%)[30][31] of the mass of the Milky Way is invisible to telescopes, neither emitting nor absorbing electromagnetic radiation. This conjectural mass has been termed "dark matter".[32] The rotational period is about 240 million years at the radius of the Sun.[16] 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.