<P> The Milky Way is the second - largest galaxy in the Local Group, with its stellar disk approximately 100,000 ly (30 kpc) in diameter, and, on average, approximately 1,000 ly (0.3 kpc) thick . As a guide to the relative physical scale of the Milky Way, if the Solar System out to Neptune were the size of a US quarter (24.3 mm (0.955 in)), the Milky Way would be approximately the size of the continental United States . A ring - like filament of stars wrapping around the Milky Way may belong to the Milky Way itself, rippling above and below the relatively flat galactic plane . If so, that would mean a diameter of 150,000--180,000 light - years (46--55 kpc). </P> <P> Estimates of 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 × 10 solar masses (M), somewhat less than that of the Andromeda Galaxy . Measurements using the Very Long Baseline Array in 2009 found velocities as large as 254 km / s (570,000 mph) for stars at the outer edge of the Milky Way . Because 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 × 10 M within 160,000 ly (49 kpc) of its center . In 2010, a measurement of the radial velocity of halo stars found that the mass enclosed within 80 kilo parsecs is 7 × 10 M . According to a study published in 2014, the mass of the entire Milky Way is estimated to be 8.5 × 10 M, which is about half the mass of the Andromeda Galaxy . </P> <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 (kpc) 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>

Size of the milky way galaxy in kilometers