Milky Way - Size and Composition

Size and Composition

The stellar disk of the Milky Way Galaxy is approximately 100,000 ly (31 kpc) in diameter, and is, on average, about 1,000 ly (0.3 kpc) thick. As a guide to the relative physical scale of the Milky Way, if it were reduced to 100 m (110 yd) in diameter, the Solar System, including the hypothesized Oort cloud, would be no more than 1 mm (0.039 in) in width. The nearest star, Proxima Centauri, would be 4.2 mm (0.17 in) distant. Alternatively visualized, if the Solar System out to Pluto were the size of a US quarter (25 mm (0.98 in)) in diameter, the Milky way would be a disk approximately 2,000 km (1,200 mi) in diameter, having roughly one-third the area of the United States.

The Milky Way contains at least 100 billion stars and may have up to 400 billion stars. The exact figure depends on the number of very low-mass, or dwarf stars, which are hard to detect, especially at distances of more than 300 ly (90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars. Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars, while the thickness of the gas layer ranges from hundreds of light years for the colder gas to thousands of light years for warmer gas. Both gravitational microlensing and planetary transit observations indicate that there may be at least as many planets bound to stars as there are stars in the Milky Way, while microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars. Earth-sized planets may be more numerous than gas giants.

The disk of stars in the Milky Way does not have a sharp edge beyond which there are no stars. Rather, the concentration of stars drops smoothly with distance from the center of the Galaxy. Beyond a radius of roughly 40,000 ly (12 kpc), the number of stars per cubic parsec drops much faster with radius, for reasons that are not understood. Surrounding the Galactic disk is a spherical Galactic Halo of stars and globular clusters that extends further outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose closest approach to the Galactic center is about 180,000 ly (55 kpc). At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would probably be ejected from the vicinity of the Milky Way. The integrated absolute visual magnitude of the Milky Way is estimated to be −20.9.

Estimates for the mass of the Milky Way vary, depending upon the method and data used. At the low end of the estimate range, the mass of the Milky Way is 5.8×1011 solar masses (M_☉), somewhat smaller than the Andromeda Galaxy. Measurements using the Very Long Baseline Array in 2009 found velocities as large as 254 km/s for stars at the outer edge of the Milky Way, higher than the previously accepted value of 220 km/s. As the orbital velocity depends on the total mass inside the orbital radius, this suggests that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7×1011 M_☉ within 160,000 ly (49 kpc) of its center. A 2010 measurement of the radial velocity of halo stars finds the mass enclosed within 80 kiloparsecs is 7×1011 M_☉. Most of the mass of the Galaxy appears to be matter of unknown form which interacts with other matter through gravitational but not electromagnetic forces; this is dubbed dark matter. A dark matter halo is spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic Center. Mathematical models of the Milky Way suggest that the total mass of the entire Galaxy lies in the range 1-1.5×1012 M_☉.