<P> Much of the mass of the Milky Way appears to be dark matter, an unknown and invisible form of matter that interacts gravitationally with ordinary 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 mass of dark matter is 1--1.5 × 10 M. Recent studies indicate a range in mass, as large as 4.5 × 10 M and as small as 8 × 10 M . </P> <P> The total mass of all the stars in the Milky Way is estimated to be between 4.6 × 10 M and 6.43 × 10 M. In addition to the stars, there is also interstellar gas, comprising 90% hydrogen and 10% helium by mass, with two thirds of the hydrogen found in the atomic form and the remaining one - third as molecular hydrogen . The mass of this gas is equal to between 10% and 15% of the total mass of the galaxy's stars . Interstellar dust accounts for an additional 1% of the total mass of the gas . </P> <P> The Milky Way contains between 200 and 400 billion stars and at least 100 billion planets . The exact figure depends on the number of very - low - mass 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 (10) 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, whereas the thickness of the gas layer ranges from hundreds of light years for the colder gas to thousands of light years for warmer gas . </P> <P> 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 decreases with distance from the center of the Milky Way . For reasons that are not understood, beyond a radius of roughly 40,000 ly (13 kpc) from the center, the number of stars per cubic parsec drops much faster with radius . 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 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 around − 20.9 . </P>

Names of other planets in the milky way
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