Size

The Milky Way is one of the two largest galaxies in the Local Group (the other being the Andromeda Galaxy), although the size for its galactic disc and how much it defines the isophotal diameter is not well understood.^[13] It is estimated that the significant bulk of stars in the galaxy lies within the 26 kiloparsecs (80,000 light-years) diameter, and that the number of stars beyond the outermost disc dramatically reduces to a very low number, with respect to an extrapolation of the exponential disk with the scale length of the inner disc.^[120]^[13]

There are several methods being used in astronomy in defining the size of a galaxy, and each of them can yield different results with respect to one another. The most commonly employed method is the D₂₅ standard – the isophote where the photometric brightness of a galaxy in the B-band (445 nm wavelength of light, in the blue part of the visible spectrum) reaches 25 mag/arcsec².^[121] An estimate from 1997 by Goodwin and others compared the distribution of Cepheid variable stars in 17 other spiral galaxies to the ones in the Milky Way, and modelling the relationship to their surface brightnesses. This gave an isophotal diameter for the Milky Way at 26.8 ± 1.1 kiloparsecs (87,400 ± 3,600 light-years), by assuming that the galactic disc is well represented by an exponential disc and adopting a central surface brightness of the galaxy (μ₀) of 22.1±0.3 B-mag/arcsec⁻² and a disk scale length (h) of 5.0 ± 0.5 kpc (16,300 ± 1,600 ly).^[122]^[12]^[123]

This is significantly smaller than the Andromeda Galaxy's isophotal diameter, and slightly below the mean isophotal sizes of the galaxies being at 28.3 kpc (92,000 ly).^[12] The paper concludes that the Milky Way and Andromeda Galaxy were not overly large spiral galaxies, nor were among the largest known (if the former not being the largest) as previously widely believed, but rather average ordinary spiral galaxies.^[124] To compare 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 at least the greatest north–south line of the contiguous United States.^[125] An even older study from 1978 gave a lower diameter for Milky Way about 23 kpc (75,000 ly).^[12]

A 2015 paper discovered that there is a ring-like filament of stars called Triangulum–Andromeda Ring (TriAnd Ring) rippling above and below the relatively flat galactic plane, which alongside Monoceros Ring were both suggested to be primarily the result of disk oscillations and wrapping around the Milky Way, at a diameter of at least 50 kpc (160,000 ly),^[126] which may be part of the Milky Way's outer disk itself, hence making the stellar disk larger by increasing to this size.^[127] A more recent 2018 paper later somewhat ruled out this hypothesis, and supported a conclusion that the Monoceros Ring, A13 and TriAnd Ring were stellar overdensities rather kicked out from the main stellar disk, with the velocity dispersion of the RR Lyrae stars found to be higher and consistent with halo membership.^[128]

Another 2018 study revealed the very probable presence of disk stars at 26–31.5 kpc (84,800–103,000 ly) from the Galactic Center or perhaps even farther, significantly beyond approximately 13–20 kpc (40,000–70,000 ly), in which it was once believed to be the abrupt drop-off of the stellar density of the disk, meaning that few or no stars were expected to be above this limit, save for stars that belong to the old population of the galactic halo.^[13]^[129]^[130]

A 2020 study predicted the edge of the Milky Way's dark matter halo being around 292 ± 61 kpc (952,000 ± 199,000 ly), which translates to a diameter of 584 ± 122 kpc (1.905 ± 0.3979 Mly).^[26]^[27] The Milky Way's stellar disk is also estimated to be approximately up to 1.35 kpc (4,000 ly) thick.^[131]^[132]

Mass

The Milky Way is approximately 890 billion to 1.54 trillion times the mass of the Sun in total (8.9×10¹¹ to 1.54×10¹² solar masses),^[7]^[8]^[9] although stars and planets make up only a small part of this. Estimates of the mass of the Milky Way vary, depending upon the method and data used. The low end of the estimate range is 5.8×10¹¹ solar masses (M_☉), somewhat less than that of the Andromeda Galaxy.^[133]^[134]^[135] 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.^[136]

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 7×10¹¹ M_☉ within 160,000 ly (49 kpc) of its center.^[137] In 2010, a measurement of the radial velocity of halo stars found that the mass enclosed within 80 kiloparsecs is 7×10¹¹ M_☉.^[138] In a 2014 study, the mass of the entire Milky Way is estimated to be 8.5×10¹¹ M_☉,^[139] but this is only half the mass of the Andromeda Galaxy.^[139] A recent 2019 mass estimate for the Milky Way is 1.29×10¹² M_☉.^[140]

Much of the mass of the Milky Way seems to be dark matter, an unknown and invisible form of matter that interacts gravitationally with ordinary matter. A dark matter halo is conjectured to 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 1–1.5×10¹² M_☉.^[141]^[142]^[143] 2013 and 2014 studies indicate a range in mass, as large as 4.5×10¹² M_☉^[144] and as small as 8×10¹¹ M_☉.^[145] By comparison, the total mass of all the stars in the Milky Way is estimated to be between 4.6×10¹⁰ M_☉^[146] and 6.43×10¹⁰ M_☉.^[141]

In addition to the stars, there is also interstellar gas, comprising 90% hydrogen and 10% helium by mass,^[147] ^{[unreliable source?]} with two thirds of the hydrogen found in the atomic form and the remaining one-third as molecular hydrogen.^[148] The mass of the Milky Way's interstellar gas is equal to between 10%^[148] and 15%^[147] of the total mass of its stars. Interstellar dust accounts for an additional 1% of the total mass of the gas.^[147]

In March 2019, astronomers reported that the virial mass of the Milky Way Galaxy is 1.54 trillion solar masses within a radius of about 39.5 kpc (130,000 ly), over twice as much as was determined in earlier studies, suggesting that about 90% of the mass of the galaxy is dark matter.^[7]^[8]

In September 2023, astronomers reported that the virial mass of the Milky Way Galaxy is only 2.06 10¹¹ solar masses, only a 10th of the mass of previous studies. The mass was determined from data of the Gaia spacecraft.^[149]