Editing Hoag's Object (section)

==Characteristics==
A nearly perfect ring of young hot blue [[star]]s circles the older yellow nucleus of this ring galaxy c. 600 million [[light-year]]s away in the constellation [[Serpens]]. The ring structure is so perfect and circular that it has been referred to as "The most perfect ring galaxy".<ref name="Broschetal2013" /> The diameter of the 6 [[arcsecond]] inner core of the galaxy is about {{val|17|0.7|u=kly}} ({{val|5.3|0.2|u=kpc}}) while the surrounding ring has an inner 28″ diameter of {{val|75|3|u=kly}} ({{val|24.8|1.1|u=kpc}}) and an outer 45″ diameter of {{val|121|4|u=kly}} ({{val|39.9|1.7|u=kpc}}).<ref name="OConnelletal1974">{{Cite Q|Q56113476}}</ref> The galaxy is estimated to have a mass of 700 billion suns.<ref name="Schweizeretal1987"/> By comparison, the [[Milky Way]] galaxy has an estimated diameter of 150–200 kly and consists of between 100 and 500 billion stars and a mass between 800 billion and 1.54 trillion suns.<ref>{{Cite web |last=Sample |first=Ian |date=7 March 2019 |title=Scientists discover what the Milky Way weighs |url=https://www.theguardian.com/science/2019/mar/07/scientists-milky-way-weighs-galaxy-hubble-nasa |website=[[The Guardian]] |language=en-GB}}</ref><ref>{{Cite web |date=6 March 2019 |title=What Does the Milky Way Weigh? Hubble and Gaia Investigate |url=https://www.nasa.gov/feature/goddard/2019/what-does-the-milky-way-weigh-hubble-and-gaia-investigate |website=NASA.gov}}</ref>

The gap separating the two stellar populations may contain some [[star cluster]]s that are almost too faint to see. Though ring galaxies are rare, another more distant ring galaxy (SDSS J151713.93+213516.8)<ref name="SkyServer_2015">{{cite web|url=http://skyserver.sdss.org/dr7/en/tools/explore/obj.asp?id=587739719767949564|title=SkyServer Object Explorer – SDSS J151713.93+213516.8}}</ref> can be seen through Hoag's Object, between the nucleus and the outer ring of the galaxy, at roughly the one o'clock position in the image shown here.

[[Noah Brosch]] and colleagues showed that the luminous ring lies at the inner edge of a much larger neutral hydrogen ring.<ref name="Broschetal2013">{{Cite Q|Q56113479}}</ref>

A few other galaxies share the primary characteristics of Hoag's Object, including a bright detached ring of stars, but their centers are elongated or barred, and they may exhibit some spiral structure. While none matches Hoag's Object in symmetry, these galaxies are sometimes called Hoag-type galaxies.<ref>{{Cite web |last=Clarke |first=Gavin |date=2017-01-04 |title=Astroboffins glimpse sighting of ultra-rare circular galaxy |url=https://www.theregister.co.uk/2017/01/04/rare_ring_shaped_galaxy_discovered/ |access-date=2019-10-31 |website=www.theregister.co.uk |language=en}}</ref><ref>{{Cite web |last=Berman |first=Bob |date=2015-11-13 |title=Weird Object: Hoags Object |url=https://astronomy.com/magazine/weirdest-objects/2015/11/7-hoags-object |url-status=dead |archive-url=https://web.archive.org/web/20220724093714/https://astronomy.com/magazine/weirdest-objects/2015/11/7-hoags-object |archive-date=2022-07-24 |access-date=2019-10-31 |website=Astronomy.com}}</ref>