--- title: Milky Way summary: The barred spiral galaxy that contains the Sun, some 100 to 400 billion stars in a disk about 30 kiloparsecs across, and the setting of Pax Abyssi. science_status: [observed, model, sim] categories: [Galactic structure, The Galaxy] aliases: [The Milky Way, The Galaxy, Our Galaxy, Galaxy model, Milky Way model] infobox: type: galaxy image: File:Gaia_sky_in_colour_DR2.jpg hubble_type: "Barred spiral, about SBbc (estimated from inside; the Galaxy cannot be seen face-on)" stars: "100 to 400 billion (depends on how many faint red dwarfs are assumed)" stellar_mass_msun: "5 to 6 x 10^10" total_mass_msun: "about 1.3 x 10^12 including dark matter (virial estimate; published values differ by a factor of about two)" stellar_disk_diameter_kpc: "about 30 (about 100,000 light years); the disk thins out with no sharp edge" thin_disk_scale_length_kpc: 2.6 thin_disk_scale_height_pc: 300 thick_disk_scale_length_kpc: 3.6 thick_disk_scale_height_pc: 900 bar_and_bulge_stellar_mass_msun: "1.88 x 10^10" bar_angle_to_sun_centre_line_deg: "27 to 28" sun_distance_from_centre_kpc: "8.2 to 8.3 (GRAVITY: 8.178 in 2019, 8.275 in 2021)" sun_height_above_midplane_pc: 20.8 circular_speed_at_sun_km_s: "about 229" galactic_year_myr: "about 220" oldest_disk_stars_gyr: "about 13" satellite_galaxies: "dozens; the largest are the Large and Small Magellanic Clouds" central_black_hole: "Sagittarius A*, about 4.3 million solar masses" sim: entity: galaxy.milky_way model: "six-component star-count model; 1.11 x 10^11 stars, 2.22 x 10^10 brown dwarfs" refs: - id: bhg2016 type: article-journal author: [{family: Bland-Hawthorn, given: Joss}, {family: Gerhard, given: Ortwin}] title: "The Galaxy in Context: Structural, Kinematic, and Integrated Properties" container-title: Annual Review of Astronomy and Astrophysics issued: 2016 volume: 54 page: 529-596 DOI: 10.1146/annurev-astro-081915-023441 - id: juric2008 type: article-journal author: [{family: Juric, given: Mario}, {literal: "et al."}] title: "The Milky Way Tomography with SDSS. I. Stellar Number Density Distribution" container-title: The Astrophysical Journal issued: 2008 volume: 673 page: 864-914 DOI: 10.1086/523619 - id: licquia2015 type: article-journal author: [{family: Licquia, given: Timothy C.}, {family: Newman, given: Jeffrey A.}] title: "Improved Estimates of the Milky Way's Stellar Mass and Star Formation Rate from Hierarchical Bayesian Meta-Analysis" container-title: The Astrophysical Journal issued: 2015 volume: 806 page: 96 DOI: 10.1088/0004-637X/806/1/96 - id: mcmillan2017 type: article-journal author: [{family: McMillan, given: Paul J.}] title: "The mass distribution and gravitational potential of the Milky Way" container-title: Monthly Notices of the Royal Astronomical Society issued: 2017 volume: 465 page: 76-94 DOI: 10.1093/mnras/stw2759 - id: gravity2019 type: article-journal author: [{literal: "GRAVITY Collaboration"}, {family: Abuter, given: R.}, {literal: "et al."}] title: "A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty" container-title: Astronomy & Astrophysics issued: 2019 volume: 625 page: L10 DOI: 10.1051/0004-6361/201935656 - id: gravity2021 type: article-journal author: [{literal: "GRAVITY Collaboration"}, {family: Abuter, given: R.}, {literal: "et al."}] title: "Improved GRAVITY astrometric accuracy from modeling optical aberrations" container-title: Astronomy & Astrophysics issued: 2021 volume: 647 page: A59 DOI: 10.1051/0004-6361/202040208 - id: gravity2022 type: article-journal author: [{literal: "GRAVITY Collaboration"}, {family: Abuter, given: R.}, {literal: "et al."}] title: "Mass distribution in the Galactic Center based on interferometric astrometry of multiple stellar orbits" container-title: Astronomy & Astrophysics issued: 2022 volume: 657 page: L12 DOI: 10.1051/0004-6361/202142465 - id: bennett2019 type: article-journal author: [{family: Bennett, given: Morgan}, {family: Bovy, given: Jo}] title: "Vertical waves in the solar neighbourhood in Gaia DR2" container-title: Monthly Notices of the Royal Astronomical Society issued: 2019 volume: 482 page: 1417-1425 DOI: 10.1093/mnras/sty2813 - id: eilers2019 type: article-journal author: [{family: Eilers, given: Anna-Christina}, {family: Hogg, given: David W.}, {family: Rix, given: Hans-Walter}, {family: Ness, given: Melissa K.}] title: "The Circular Velocity Curve of the Milky Way from 5 to 25 kpc" container-title: The Astrophysical Journal issued: 2019 volume: 871 page: 120 DOI: 10.3847/1538-4357/aaf648 - id: reid2019 type: article-journal author: [{family: Reid, given: M. J.}, {literal: "et al."}] title: "Trigonometric Parallaxes of High-mass Star-forming Regions: Our View of the Milky Way" container-title: The Astrophysical Journal issued: 2019 volume: 885 page: 131 DOI: 10.3847/1538-4357/ab4a11 - id: wegg2013 type: article-journal author: [{family: Wegg, given: Christopher}, {family: Gerhard, given: Ortwin}] title: "Mapping the three-dimensional density of the Galactic bulge with VVV red clump stars" container-title: Monthly Notices of the Royal Astronomical Society issued: 2013 volume: 435 page: 1874-1887 DOI: 10.1093/mnras/stt1376 - id: portail2017 type: article-journal author: [{family: Portail, given: Matthieu}, {family: Gerhard, given: Ortwin}, {family: Wegg, given: Christopher}, {family: Ness, given: Melissa}] title: "Dynamical modelling of the galactic bulge and bar: the Milky Way's pattern speed, stellar and dark matter mass distribution" container-title: Monthly Notices of the Royal Astronomical Society issued: 2017 volume: 465 page: 1621-1644 DOI: 10.1093/mnras/stw2819 - id: sormani2022 type: article-journal author: [{family: Sormani, given: Mattia C.}, {family: Gerhard, given: Ortwin}, {family: Portail, given: Matthieu}, {family: Vasiliev, given: Eugene}, {family: Clarke, given: Jonathan}] title: "The stellar mass distribution of the Milky Way's bar: an analytical model" container-title: Monthly Notices of the Royal Astronomical Society Letters issued: 2022 volume: 514 page: L1-L5 DOI: 10.1093/mnrasl/slac046 - id: hunter2024 type: article-journal author: [{family: Hunter, given: G. H.}, {literal: "et al."}] title: "Testing kinematic distances under a realistic Galactic potential" container-title: Astronomy & Astrophysics issued: 2024 volume: 692 page: A216 DOI: 10.1051/0004-6361/202450000 - id: sormani2022nsd type: article-journal author: [{family: Sormani, given: Mattia C.}, {family: Sanders, given: Jason L.}, {family: Fritz, given: Tobias K.}, {literal: "et al."}] title: "Self-consistent modelling of the Milky Way's nuclear stellar disc" container-title: Monthly Notices of the Royal Astronomical Society issued: 2022 volume: 512 page: 1857-1884 DOI: 10.1093/mnras/stac639 - id: eilers2020 type: article-journal author: [{family: Eilers, given: Anna-Christina}, {family: Hogg, given: David W.}, {family: Rix, given: Hans-Walter}, {literal: "et al."}] title: "The Strength of the Dynamical Spiral Perturbation in the Galactic Disk" container-title: The Astrophysical Journal issued: 2020 volume: 900 page: 186 DOI: 10.3847/1538-4357/abac0b - id: deason2019 type: article-journal author: [{family: Deason, given: Alis J.}, {family: Belokurov, given: Vasily}, {family: Sanders, given: Jason L.}] title: "The total stellar halo mass of the Milky Way" container-title: Monthly Notices of the Royal Astronomical Society issued: 2019 volume: 490 page: 3426-3439 DOI: 10.1093/mnras/stz2793 - id: xiang2022 type: article-journal author: [{family: Xiang, given: Maosheng}, {family: Rix, given: Hans-Walter}] title: "A time-resolved picture of our Milky Way's early formation history" container-title: Nature issued: 2022 volume: 603 page: 599-603 DOI: 10.1038/s41586-022-04496-5 - id: vallenari2023 type: article-journal author: [{literal: "Gaia Collaboration"}, {family: Vallenari, given: A.}, {literal: "et al."}] title: "Gaia Data Release 3: Summary of the content and survey properties" container-title: Astronomy & Astrophysics issued: 2023 volume: 674 page: A1 DOI: 10.1051/0004-6361/202243940 images_wanted: - file: File:Gaia_sky_in_colour_DR2.jpg subject: "Gaia's all-sky map of the Milky Way in colour, from about 1.7 billion stars (Gaia DR2)" source: other source_page: https://www.esa.int/ESA_Multimedia/Images/2018/04/Gaia_s_sky_in_colour2 credit: "ESA/Gaia/DPAC" licence: "CC BY-SA 3.0 IGO (marked on the page; share-alike applies to any derivative)" note: "esa.int image, allowed only because the page marks it CC BY-SA 3.0 IGO" - file: File:Milky_Way_barred_spiral_concept_PIA10748.jpg subject: "Artist's concept of the Milky Way seen face-on: a central bar with two major arms (Scutum-Centaurus, Perseus) and two minor ones" source: nasa source_page: https://www.jpl.nasa.gov/images/pia10748-our-milky-way-gets-a-makeover-artist-concept/ credit: "NASA/JPL-Caltech" licence: "NASA/JPL, public domain with credit; no endorsement implied" - file: File:Galaxy_map_Milky_Way_model_sim.png subject: "The Pax Abyssi galaxy map showing catalogue stars and known compact objects over the Milky Way model" source: sim note: "shot list: no approved galaxy-map capture exists yet" --- The **Milky Way** is the galaxy that contains the Sun: a flattened, rotating system of 100 to 400 billion stars, gas, dust and dark matter, shaped as a barred spiral whose bright disk is about 30 kiloparsecs (roughly 100,000 light years) across. The Sun sits in that disk about 8.2 kiloparsecs from the centre, just above the midplane, so we see the Galaxy edge-on from inside it, as the pale band across the night sky that gave it its name. Every star in [[Pax Abyssi]] belongs to it, and the game's generated sky beyond the [[Star catalogue|real catalogue]] is built from a published model of its structure. ::figure{src="File:Gaia_sky_in_colour_DR2.jpg" size=wide alt="An oval all-sky map with a bright horizontal band of stars and dark dust lanes, brightest at the centre, with two small bright patches below right." caption="Observation. The Milky Way as Gaia measured it: brightness and colour of about 1.7 billion stars, in Galactic coordinates with the Galactic centre in the middle. The Magellanic Clouds are the two patches at lower right."} ## Structure A galaxy seen from inside cannot be photographed whole, so its shape is assembled from counts, motions and distances of stars in every direction. The modern picture has six stellar components plus a dark matter halo. :cite[bhg2016] | Component | Size and shape | Stellar mass (solar masses) | Main source | |---|---|---|---| | Thin disk | exponential, scale length about 2.6 kpc, scale height about 300 pc | about 3.5 x 10^10 | :cite[juric2008] :cite[bhg2016] | | Thick disk | scale length about 3.6 kpc, scale height about 900 pc, older and more metal-poor | about 6 x 10^9 | :cite[juric2008] :cite[bhg2016] | | Bar and boxy bulge | a peanut-shaped bar about 27 to 28 degrees from the Sun-centre line | 1.88 x 10^10 | :cite[wegg2013] :cite[portail2017] | | Nuclear stellar disk | a dense, flattened disk in the central few hundred parsecs, around a nuclear star cluster a few parsecs across | 1.05 x 10^9 (disk) | :cite[sormani2022nsd] | | Stellar halo | a roughly spheroidal cloud of old stars reaching past 100 kpc | 1.4 x 10^9 | :cite[deason2019] | | Central black hole | [[Sagittarius A*]] | about 4.3 x 10^6 | :cite[gravity2022] | Most of the Galaxy's stars live in the **thin disk**. Its density falls off exponentially with distance from the centre and with height above the midplane, which star counts describe well with a double exponential: $$ n(R, z) = n_\odot \, \exp\!\left(-\frac{R - R_\odot}{L}\right) \exp\!\left(-\frac{|z|}{h}\right) $$ Here $n_\odot$ is the number of stars per cubic parsec at the Sun, $R$ is the distance from the Galactic centre measured in the plane, $z$ is the height above it, $L \approx 2.6$ kpc is the scale length and $h \approx 300$ pc the scale height for old stars. :cite[juric2008] The formula has a direct reading. Walk 2.6 kpc toward the centre and the stars crowd together by a factor of $e \approx 2.7$; climb 300 pc out of the plane and they thin out by the same factor. Near the Sun there are about 0.1 stars in every cubic parsec, one star per box ten light years on a side. Young, hot stars hug the plane much more tightly than old ones: O and B stars have scale heights of only a few tens of parsecs, because they have not lived long enough to be scattered upward. The **bar** is the Galaxy's dominant inner structure. Red clump giants, stars of nearly fixed brightness that serve as distance markers, show a boxy, X-shaped bulge that is really the thickened inner part of a long bar tilted about 27 degrees to our line of sight. :cite[wegg2013] Dynamical models fitted to the stars' counts and motions give the whole bar and bulge a stellar mass of $(1.88 \pm 0.12) \times 10^{10}$ solar masses, about a third of all the Galaxy's stars by mass. :cite[portail2017] Its density has since been captured in a closed-form analytic fit. :cite[sormani2022] The **spiral arms** are best traced by the youngest objects. Radio astronomers have measured trigonometric parallaxes to about 200 massive star-forming regions using masers, natural microwave lasers in dense gas, and fitted arms through them: Norma, Scutum-Centaurus, Sagittarius-Carina, Perseus, the Outer arm and the short Local arm near the Sun. :cite[reid2019] Old stars barely follow the arms: the surface density of mass differs between arm and inter-arm regions by only about ten percent either way at the Sun's radius. :cite[eilers2020] The arms are patterns that young stars light up, and most of the Galaxy's mass passes through them. The **stellar halo** holds only about 1.4 billion solar masses of old, metal-poor stars. Its mass and average metallicity point to one massive dwarf galaxy that merged with the Milky Way about 10 billion years ago and now dominates the inner halo. :cite[deason2019] Around it lies a far more massive halo of dark matter, which makes up most of the Galaxy's total mass of roughly $1.3 \times 10^{12}$ solar masses. :cite[bhg2016] ## How many stars The number of stars is less certain than the mass in stars, because the faintest stars are the most common. Estimates of the total stellar mass cluster around $5$ to $6 \times 10^{10}$ solar masses. :cite[bhg2016] :cite[licquia2015] :cite[mcmillan2017] Dividing by the mass of an average star gives a count: $$ N_\star \approx \frac{M_\star}{\langle m \rangle} $$ With $M_\star = 5 \times 10^{10}$ solar masses and an average star of 0.4 solar masses, $N_\star \approx 1.3 \times 10^{11}$. If red dwarfs below a fifth of a solar mass are more numerous than assumed, the average falls toward 0.15 solar masses and the count rises past 300 billion. That is why the literature quotes a range of 100 to 400 billion rather than a single figure. ## The Sun's place The distance from the Sun to the Galactic centre, $R_0$, sets the scale of every Galactic model. The most direct measurement comes from the star S2, which orbits the central black hole every 16 years: comparing its motion across the sky with its line-of-sight velocity gives a geometric distance. The GRAVITY instrument at ESO's Very Large Telescope Interferometer measured $R_0 = 8{,}178 \pm 13 \pm 22$ pc in 2019 and, after correcting an optical aberration, $8{,}275 \pm 9 \pm 33$ pc in 2021. :cite[gravity2019] :cite[gravity2021] A value of 8.2 to 8.3 kpc, about 27,000 light years, covers both. The Sun also sits $20.8 \pm 0.3$ pc above the midplane. :cite[bennett2019] Stars and gas near the Sun circle the centre at about 229 km/s, measured from the motions of red giant stars; maser parallaxes give a similar 236 ± 7 km/s. :cite[eilers2019] :cite[reid2019] One lap of about 52 kpc at that speed takes $$ T = \frac{2 \pi R_0}{v_c} \approx \frac{2\pi \times 8.2\ \text{kpc}}{229\ \text{km/s}} \approx 220\ \text{million years}, $$ so the Sun has made about 20 orbits since it formed. The rotation curve stays nearly flat out to 25 kpc, declining only slowly. :cite[eilers2019] If the visible stars and gas were all the mass there is, orbital speeds would fall off steeply beyond the disk. They do not, and that is one of the main lines of evidence for dark matter. ## How we know - **Star counts.** The Sloan Digital Sky Survey counted 48 million stars at known colours and magnitudes, enough to fit the thin disk, thick disk and halo together. :cite[juric2008] - **Infrared surveys.** Dust hides the inner Galaxy at visible wavelengths but lets near-infrared light through, which is how the bar was mapped with red clump giants. :cite[wegg2013] - **Maser parallaxes.** Radio interferometers spanning continents measure parallaxes to about ten microarcseconds, accurate enough to place star-forming regions on the far side of the Galactic centre. :cite[reid2019] - **Gaia.** ESA's astrometry mission measured positions for about 1.8 billion sources and parallaxes for about 1.5 billion in its third data release (2022), giving distances and motions across a large part of the disk. :cite[vallenari2023] - **Stellar ages.** Combining Gaia with spectroscopic surveys dates subgiant stars individually. They show that the old, thick disk began forming about 13 billion years ago, only 0.8 billion years after the Big Bang. :cite[xiang2022] - **Stars around the central black hole.** Decades of orbits around Sagittarius A* give both its mass, about 4.3 million solar masses, and the Sun's distance from it. :cite[gravity2022] :::callout{type=sim title="In Pax Abyssi"} **The catalogue sky and the galaxy map are BUILT.** The galaxy map places 119,624 catalogue stars and 4,881 known compact objects (every pulsar in the ATNF catalogue, 113 known black holes and 375 other neutron stars) in their measured positions. **The generated galaxy is IN PROGRESS.** Beyond the catalogue, which thins out past about 80 parsecs, the game fills space from a published model of the Galaxy. The model sums six components: star-count disks after Juric et al. (2008), calibrated to 0.10 stars per cubic parsec at the Sun; the bar and bulge of Sormani et al. (2022); a nuclear stellar disk and star cluster; an Einasto stellar halo of 1.4 billion solar masses; and spiral arms from Reid et al. (2019) where they are measured, with Hunter et al. (2024) beyond. :cite[juric2008] :cite[sormani2022] :cite[reid2019] :cite[hunter2024] Its frame puts the Sun 8,275 pc from Sagittarius A* and 20.8 pc above the plane. The model holds $1.11 \times 10^{11}$ stars plus $2.2 \times 10^{10}$ brown dwarfs, $8.4 \times 10^{8}$ neutron stars and $8.7 \times 10^{7}$ black holes. The code runs; the generated view is not yet in front of players. **Two departures from the literature.** The model's stellar mass is $4.2 \times 10^{10}$ solar masses, 15 to 30 percent below the published $5$ to $6 \times 10^{10}$, because it is pinned to the star count at the Sun rather than to dynamical mass models; the gap is an open question in the literature as well. And the procedural fill the game inherited had been built with the celestial equator standing in for the Galactic plane, which tilted the generated disk 62.9 degrees away from the real one and put the model's bulge far below the plane. An audit found it in September 2026 and the model was rebuilt in the correct Galactic frame. ::: :::callout{type=lore title="Human space, to scale"} In the fiction of Pax Abyssi, human space in 2538 spans about 800 light years, roughly 250 parsecs. That is under one percent of the Galaxy's width: a small patch of the disk around the Sun, inside a disk more than 100 times wider. ::: ::figure{src="File:Milky_Way_barred_spiral_concept_PIA10748.jpg" size=wide alt="A face-on spiral galaxy with a bright elongated central bar and two prominent arms, with fainter arms between them." caption="Artist's concept. The Milky Way drawn face-on from infrared star counts made with NASA's Spitzer Space Telescope (2008): a central bar with two major arms, Scutum-Centaurus and Perseus. No image of our Galaxy from outside exists."} ## See also - [[Sagittarius A*]], the black hole at the centre - [[Star catalogue]], the 119,626 real stars in the game - [[Sol]], the Solar System - [[Stellar classification]] - [[Red dwarf]], the most common kind of star - [[Neutron star]] and [[Black hole]] - [[Science in Pax Abyssi]]