page_title: Andromeda Galaxy
section_title: Mass estimates
hierarchical_section_title: Andromeda Galaxy / General / Mass estimates
caption_attribution_description: English: This diagram shows how scientists determined the size of the halo of the Andromeda galaxy. Because the gas in the halo is dark, the team measured it by using the light from quasars, the very distant bright cores of active galaxies powered by black holes. They observed the quasars’ light as it traveled through the intervening gas. The halo’s gas absorbed some of that light and made the quasar appear darker in a very small wavelength range. By measuring the tiny dip in brightness at that specific range, scientists could tell how much gas is between us and each quasar. Some quasars showed no dip in brightness, and this helped define the size of the halo. Links: NASA press release
context_page_description: The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy approximately 2.5 million light-years from Earth and the nearest major galaxy to the Milky Way. The galaxy's name stems from the area of Earth's sky in which it appears, the constellation of Andromeda, which itself is named after the Ethiopian princess who was the wife of Perseus in Greek mythology.
  The virial mass of the Andromeda Galaxy is of the same order of magnitude as that of the Milky Way, at 1 trillion solar masses. The mass of either galaxy is difficult to estimate with any accuracy, but it was long thought that the Andromeda Galaxy is more massive than the Milky Way by a margin of some 25% to 50%. This has been called into question by a 2018 study that cited a lower estimate on the mass of the Andromeda Galaxy,
  combined with preliminary reports on a 2019 study estimating a higher mass of the Milky Way. The Andromeda Galaxy has a diameter of about 220,000 ly, making it the largest member of the Local Group in terms of extension, if not mass.
context_section_description: Until 2018, mass estimates for the Andromeda Galaxy's halo (including dark matter) gave a value of approximately 1.5×10¹² M☉, compared to 8×10¹¹ M☉ for the Milky Way. This contradicted earlier measurements that seemed to indicate that the Andromeda Galaxy and Milky Way are almost equal in mass. In 2018, the equality of mass was re-established by radio results as approximately 8×10¹¹ M☉
  In 2006, Andromeda Galaxy's spheroid was determined to have a higher stellar density than that of the Milky Way, and its galactic stellar disk was estimated at about twice the diameter of that of the Milky Way. The total mass of Andromeda Galaxy is estimated to be between 8×10¹¹ M☉ and 1.1×10¹² M☉. The stellar mass of M31 is 10-15×10¹⁰ M☉, with 30% of that mass in the central bulge, 56% in the disk, and the remaining 14% in the stellar halo. The radio results (similar mass to Milky Way galaxy) should be taken as likeliest as of 2018, although clearly this matter is still under active investigation by a number of research groups worldwide.
  As of 2019, current calculations based on escape velocity and dynamical mass measurements put the Andromeda Galaxy at 0.8×10¹² M☉, which is only half of the Milky Way's newer mass, calculated in 2019 at 1.5×10¹² M☉.
  In addition to stars, Andromeda Galaxy's interstellar medium contains at least 7.2×10⁹ M☉ in the form of neutral hydrogen, at least 3.4×10⁸ M☉ as molecular hydrogen (within its innermost 10 kiloparsecs), and 5.4×10⁷ M☉ of dust.
  Andromeda Galaxy is surrounded by a massive halo of hot gas that is estimated to contain half the mass of the stars in the galaxy. The nearly invisible halo stretches about a million light-years from its host galaxy, halfway to our Milky Way galaxy. Simulations of galaxies indicate the halo formed at the same time as the Andromeda Galaxy. The halo is enriched in elements heavier than hydrogen and helium, formed from supernovae and its properties are those expected for a galaxy that lies in the "green valley" of the Galaxy color–magnitude diagram (see below). Supernovae erupt in Andromeda Galaxy's star-filled disk and eject these heavier elements into space. Over Andromeda Galaxy's lifetime, nearly half of the heavy elements made by its stars have been ejected far beyond the galaxy's 200,000-light-year-diameter stellar disk.
caption: Giant halo around Andromeda Galaxy.[44]
exif: {"Image ImageDescription": "This diagram shows how scientists determined the size of the halo of the Andromeda galaxy. Because the gas in the halo is dark, the team measured it by using the light from quasars, the very distant bright cores of active galaxies powered by black holes. They observed the quasars\u2019 light as it traveled through the intervening gas. The halo\u2019s gas absorbed some of that light and made the quasar appear darker in a very small wavelength range. By measuring the tiny dip in brightness at that specific range, scientists could tell how much gas is between us and each quasar. Some quasars showed no dip in brightness, and this helped define the size of the halo. Links: NASA press release", "Image Software": "Adobe Photoshop CS4 Macintosh"}