What is the galactic bar and bulge?

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⚪ Credits

ObserveTheSky is made with love for anyone who looks up.

• Planet facts & orbits based on public NASA/JPL data.
• Space agencies: NASA & ESA — public images and data.
• Fonts: Fredoka & Nunito (Google Fonts).
• Inspiration: NASA, ESA, and everyone who looks up at night.
• Earth map: NASA Earth Observatory, Blue Marble: Next Generation (Reto Stöckli, NASA GSFC), lightened for the screen so the sea reads as sea; nothing added, nothing removed.
• Earth at night: NASA Earth Observatory, Earth at Night (Black Marble) 2016, from Suomi NPP VIIRS Day-Night Band data.
• Moon map: NASA’s Scientific Visualization Studio, CGI Moon Kit, from Lunar Reconnaissance Orbiter data.
• Mars map: USGS Astrogeology Science Center, Mars Viking Global Color Mosaic, from NASA Viking Orbiter data.
• Mercury map: Applied Coherent Technology Corporation and the MESSENGER team (NASA, Johns Hopkins Applied Physics Laboratory, Arizona State University, Carnegie Institution of Washington), published by the USGS Astrogeology Science Center.
• Venus map: USGS Astrogeology Science Center, Venus Magellan Global C3-MDIR Synthetic Color Mosaic, from NASA Magellan radar data (PDS Geosciences Node). Radar, not a photograph: it shows the rock under the clouds, which no eye can see.
• Io map: USGS Astrogeology Science Center, Io Galileo SSI Global Color Merge Mosaic, from NASA Galileo data (Planetary Data System). False colour: the tones are stretched to bring out the volcanoes, and the real surface is paler and more muted than this.
• Europa map: USGS Astrogeology Science Center, Europa Voyager – Galileo SSI Global Mosaic, from NASA Voyager and Galileo data (Planetary Data System). The mosaic is greyscale — there is no colour version of it — so Europa looks grey here; the real surface is a pale, ochre-streaked white.
• Ganymede map: USGS Astrogeology Science Center, Ganymede Voyager – Galileo SSI Global Mosaic, from NASA Voyager and Galileo data (Planetary Data System). The mosaic is greyscale — there is no colour version of it — so Ganymede looks grey here; the real surface is a dull brownish grey with paler grooved bands.
• Callisto map: USGS Astrogeology Science Center, Callisto Voyager – Galileo SSI Global Mosaic, from NASA Voyager and Galileo data (Planetary Data System). The mosaic is greyscale — there is no colour version of it — so Callisto looks grey here; the real surface is a dark brownish grey, covered in craters, with brighter ice where impacts have dug it up.
• Enceladus map: USGS Astrogeology Science Center, Enceladus Cassini Global Mosaic 110m, from NASA Cassini data — credit: the Cassini Team, the Jet Propulsion Laboratory and the Space Science Institute. The mosaic is greyscale, and stretched to bring out the cracks and craters, so Enceladus looks mid-grey here; the real moon is brilliant white — almost pure water ice, and the most reflective thing in the solar system.
• Ceres map: USGS Astrogeology Science Center, Ceres Dawn FC Global Mosaic 140m, from NASA Dawn Framing Camera data — credit NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. The mosaic is greyscale and its tones are evened out to show detail, so Ceres looks lighter here than it is: the real surface is dark, about as dark as fresh asphalt. The bright spots in Occator crater are real, though — they are salt left behind by water.
• Pluto map: USGS Astrogeology Science Center, Pluto New Horizons LORRI/MVIC Global Mosaic 300m (July 2017), from NASA New Horizons data. Public domain. The mosaic is greyscale — the map was made in black and white, and Pluto’s real colour is a butterscotch tan. New Horizons flew past rather than going into orbit, so the side it passed is sharp and the far side coarse — and the far south, in the dark of winter, was never photographed at all: it is shown as a plain grey.
• Triton map: USGS Astrogeology Science Center, Triton Voyager 2 Global Color Mosaic 600m, by the Lunar and Planetary Institute from NASA/JPL Voyager 2 data — please cite the Lunar and Planetary Institute, NASA, the Jet Propulsion Laboratory and Paul Schenk. Public domain. Voyager 2 saw only the south and the middle of Triton; the north was dark in winter and has never been photographed, so it is shown as a plain surface in the average colour of the rest.
• Io’s volcanoes: positions from the USGS Gazetteer of Planetary Nomenclature (IAU); which vents are persistently active, and that they glow in eclipse, from Davies, Perry, Williams, Veeder & Nelson, The Planetary Science Journal (2024), CC BY 4.0.
• Sun map: courtesy of NASA/SDO and the AIA, EVE, and HMI science teams. HMI white-light images from one turn of the Sun, April and May 2024, laid into a map of its whole surface. Public domain. Shown in grey, because sunlight is white.
• Sun’s corona and prominences: NASA/Keegan Barber, a photograph of the total solar eclipse of 8 April 2024, from Dallas, Texas.
• Picture of Sagittarius A*: EHT Collaboration, the Event Horizon Telescope’s 2022 image, via ESO (CC BY 4.0).
• Picture of M87*, the black hole at the heart of M87: EHT Collaboration, the Event Horizon Telescope’s 2019 image, via ESO (CC BY 4.0).
• Infrared picture of the stars round Sagittarius A*: ESO, VLT/NACO, 2002 (CC BY 4.0).
• Shapes of Phobos, Deimos, Vesta, Bennu, Didymos, Dimorphos, comet Wild 2 and Halley’s Comet: the missions’ shape models, all public domain — Vesta’s from NASA’s 3D models, the rest from NASA’s Planetary Data System: R. W. Gaskell (Phobos); P. C. Thomas (Deimos); NASA VTAD (Vesta); NASA/OSIRIS-REx, M. G. Daly, O. S. Barnouin, R. C. Espiritu, D. S. Lauretta (Bennu); NASA/Johns Hopkins APL (Didymos, Dimorphos); T. Farnham, M. Duxbury, J.-Y. Li (Wild 2); P. Stooke (Halley). Simplified to about 5,000 triangles each.
• Shape of Ryugu: ISAS/JAXA, from Watanabe et al. 2019. Simplified to about 5,000 triangles.
• Shape of comet 67P/Churyumov-Gerasimenko: ESA/Rosetta/NAVCAM (CC BY-SA 3.0 IGO). The simplified copy on this site is shared under the same licence.
• Spacecraft shapes: NASA 3D Resources’ models, public domain — the International Space Station and the Voyagers by NASA/Michael D. Carbajal, Hubble by NASA VTAD, Parker Solar Probe by NASA/Matthew J. Garcia, Cassini by NASA/JPL (Solar System Simulator, M. Oberle), the James Webb Space Telescope by NASA/Christopher R. Meaney. Simplified to a few thousand triangles each.
• Voyager’s Golden Record: photograph of the record’s cover, courtesy of NASA/JPL-Caltech.
• Galaxy shapes in Space: drawn from published measurements — the S4G survey (Salo et al. 2015, ApJS 219, 4), THINGS (de Blok et al. 2008), Courteau et al. 2011 (Andromeda), Kam et al. 2015 and Regan & Vogel 1994 (Triangulum), Davis et al. 2012 (spiral arms).
• The Large Magellanic Cloud’s shape in Space: from a photograph by Zdeněk Bardon/ESO (CC BY 4.0), laid into its measured tilted disc.
• The Andromeda Galaxy’s shape in Space: from the Local Group Survey’s mosaic, Local Group Survey Team and T.A. Rector (University of Alaska Anchorage), via NOIRLab (CC BY 4.0), laid into its measured tilted disc.
• The Triangulum Galaxy’s shape in Space: from the Local Group Survey’s mosaic, Local Group Survey Team and T.A. Rector (University of Alaska Anchorage), via NOIRLab (CC BY 4.0), laid into its measured tilted disc.
• Bode’s Galaxy’s shape in Space: from T.A. Rector (University of Alaska Anchorage) and NOIRLab/NSF/AURA’s image of M81 and M82 (CC BY 4.0), laid into its measured tilted disc.
• The Whirlpool Galaxy’s shape in Space: from K. Rhode, M. Young and WIYN/NOIRLab/NSF/AURA’s image of M51 (CC BY 4.0), laid into its measured tilted disc.
• The Pinwheel Galaxy’s shape in Space: from George Jacoby, Bruce Bohannan, Mark Hanna/NOIRLab/NSF/AURA’s image of M101 (CC BY 4.0), laid into its measured tilted disc.
• The Fireworks Galaxy’s shape in Space: from KPNO/NOIRLab/NSF/AURA/T.A. Rector (University of Alaska Anchorage)/H. Schweiker (WIYN)’s image of NGC 6946 (CC BY 4.0), laid into its measured tilted disc.
• The Sombrero Galaxy’s photograph in Space, seen from the Earth’s direction: ESO/IDA/Danish 1.5 m/R. Gendler and J.-E. Ovaldsen (CC BY 4.0).
• The Cigar Galaxy’s photograph in Space, seen from the Earth’s direction: from T.A. Rector (University of Alaska Anchorage) and NOIRLab/NSF/AURA’s image of M81 and M82 (CC BY 4.0).
• Star clusters in Space: where the Pleiades’ and the Beehive’s stars are on the sky, from UCAC4 (US Naval Observatory), public domain. How far each one lies in front of the middle or behind it is a model.
• Pillars of Creation in Space: NASA and STScI’s 3D model (Leah Hustak, Ralf Crawford, STScI), public domain under STScI’s content policy, simplified to about 7,500 triangles.
• Orbits of the stars S2, S29, S38 and S55 round Sagittarius A*: GRAVITY Collaboration 2022, A&A 657, L12, Table B.1 (CC BY 4.0).
• Young stars in the Pillars of Creation: positions from the SIMBAD database, operated at CDS, Strasbourg, France.
• Exoplanet systems in Space: This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. Each planet’s numbers are credited to its own paper in the database.
• The Sagittarius Dwarf’s stream in Space: drawn from the simulation of Vasiliev, Belokurov & Erkal 2021 (MNRAS 501, 2279), its present-day snapshot on Zenodo (record 4038141), CC BY 4.0 — 8,000 of its 200,000 star particles.
• The Antennae Galaxies in Space: the shape is traced from a Legacy Surveys image (Legacy Surveys / D. Lang (Perimeter Institute), CC BY 4.0); the depth, which nobody can measure, is a model after Karl et al. 2010 (ApJL 715, L88).