Editing Galaxy (section)

== Observation history ==

=== Milky Way ===
{{Main|Milky Way}}

[[Greek philosophy|Greek]] philosopher [[Democritus]] (450–370 BCE) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.<ref name="Plutarch"/>
[[Aristotle]] (384–322 BCE), however, believed the Milky Way was caused by "the ignition of the fiery exhalation of some stars that were large, numerous and close together" and that the "ignition takes place in the upper part of the [[atmosphere]], in the [[Sublunary sphere|region of the World that is continuous with the heavenly motions]]."<ref name=Montada/> [[Neoplatonism|Neoplatonist]] philosopher [[Olympiodorus the Younger]] ({{circa|495}}–570 CE) was critical of this view, arguing that if the Milky Way was [[sublunary]] (situated between Earth and the Moon) it should appear different at different times and places on Earth, and that it should have [[parallax]], which it did not. In his view, the Milky Way was celestial.<ref name=heidarzadeh23 />

According to Mohani Mohamed, [[Astronomy in the medieval Islamic world|Arabian]] astronomer [[Ibn al-Haytham]] (965–1037) made the first attempt at observing and measuring the Milky Way's parallax,<ref name=Mohamed /> and he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere."<ref name=Bouali_et_al_2008/> [[Persian people|Persian]] astronomer [[al-Biruni]] (973–1048) proposed the Milky Way galaxy was "a collection of countless fragments of the nature of nebulous stars."<ref name=al-Biruni/> [[Al-Andalus|Andalusian]] astronomer [[Avempace]] ({{abbr|d.|died}} 1138) proposed that it was composed of many stars that almost touched one another, and appeared to be a continuous image due to the effect of [[refraction]] from sublunary material,<ref name=Montada /><ref name="heidarzadeh25" /> citing his observation of the [[Conjunction (astronomy and astrology)|conjunction]] of Jupiter and Mars as evidence of this occurring when two objects were near.<ref name=Montada /> In the 14th century, Syrian-born [[Ibn Qayyim al-Jawziyya]] proposed the Milky Way galaxy was "a myriad of tiny stars packed together in the sphere of the fixed stars."<ref name=Livingston/>

Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer [[Galileo Galilei]] used a [[optical telescope|telescope]] to study it and discovered it was composed of a huge number of faint stars.<ref name=Galilei/><ref name="O'Connor_Robertson_2002"/> In 1750, English astronomer [[Thomas Wright (astronomer)|Thomas Wright]], in his ''An Original Theory or New Hypothesis of the Universe'', correctly speculated that it might be a rotating body of a huge number of stars held together by [[gravitation]]al forces, akin to the [[Solar System]] but on a much larger scale, and that the resulting disk of stars could be seen as a band on the sky from a perspective inside it.{{efn|Wright called the Milky Way the ''Vortex Magnus'' (Great Whirlpool) and estimated its diameter to be 8.64×10<sup>12</sup> miles (13.9×10<sup>12</sup> km).{{sfn|Wright|1750|p=73}}}}{{sfn|Wright|1750|pp=48–}}<ref name="our_galaxy" /> In his 1755 treatise, [[Immanuel Kant]] elaborated on Wright's idea about the Milky Way's structure.<ref name=Kant_1755/> [[File:Herschel-Galaxy.png|thumb|The shape of the Milky Way as estimated from star counts by [[William Herschel]] in 1785; the Solar System was assumed to be near the center.]]

The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by [[William Herschel]] in 1785 by counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with [[Galactocentrism|the Solar System close to the center]].<ref name=Herschel_1785/><ref name=paul1993 /> Using a refined approach, [[Jacobus Kapteyn|Kapteyn]] in 1920 arrived at the picture of a small (diameter about 15&nbsp;kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by [[Harlow Shapley]] based on the cataloguing of [[globular cluster]]s led to a radically different picture: a flat disk with diameter approximately 70&nbsp;kiloparsecs and the Sun far from the centre.<ref name="our_galaxy" /> Both analyses failed to take into account the [[extinction (astronomy)|absorption of light]] by [[cosmic dust|interstellar dust]] present in the [[galactic plane]]; but after [[Robert Julius Trumpler]] quantified this effect in 1930 by studying [[open cluster]]s, the present picture of the Milky Way galaxy emerged.<ref name=Trimble_1999/>

=== Distinction from other nebulae ===
A few galaxies outside the Milky Way are visible on a dark night to the [[naked eye|unaided eye]], including the [[Andromeda Galaxy]], [[Large Magellanic Cloud]], [[Small Magellanic Cloud]], and the [[Triangulum Galaxy]]. In the 10th century, Persian astronomer [[Abd al-Rahman al-Sufi]] made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud".<ref name="NSOG" /> In 964, he probably mentioned the Large Magellanic Cloud in his ''[[Book of Fixed Stars]]'', referring to "Al Bakr of the southern Arabs",<ref name="obspm2"/> since at a [[declination]] of about 70° south it was not visible where he lived. It was not well known to Europeans until [[Ferdinand Magellan|Magellan]]'s voyage in the 16th century.<ref name="obspm"/><ref name="obspm2"/> The Andromeda Galaxy was later independently noted by [[Simon Marius]] in 1612.<ref name="NSOG" />

In 1734, philosopher [[Emanuel Swedenborg]] in his ''Principia'' speculated that there might be other galaxies outside that were formed into galactic clusters that were minuscule parts of the universe that extended far beyond what could be seen. Swedenborg's views "are remarkably close to the present-day views of the cosmos."<ref name="Gordon2002"/>
In 1745, [[Pierre Louis Maupertuis]] conjectured that some [[nebula]]-like objects were collections of stars with unique properties, including a [[Relativistic jets|glow exceeding the light]] its stars produced on their own, and repeated [[Johannes Hevelius]]'s view that the bright spots were massive and flattened due to their rotation.<ref name=Kant_1755/>
In 1750, [[Thomas Wright (astronomer)|Thomas Wright]] correctly speculated that the Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways.<ref name="our_galaxy"/><ref name=Dyson_1979/>
[[File:Pic iroberts1.jpg|thumb|right|Photograph of the "Great Andromeda Nebula" by [[Isaac Roberts]], 1899, later identified as the [[Andromeda Galaxy]]]]

Toward the end of the 18th century, [[Charles Messier]] compiled a [[Messier object|catalog]] containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae.<ref name="our_galaxy" /> In 1845, [[William Parsons, 3rd Earl of Rosse|Lord Rosse]] examined the nebulae catalogued by Herschel and observed the spiral structure of [[Whirlpool Galaxy|Messier object M51]], now known as the Whirlpool Galaxy.<ref>{{Cite magazine |last=Payne-Gaposchkin |first=Cecilia H. |author-link=Cecilia Payne-Gaposchkin |date=1953 |title=Why Do Galaxies Have a Spiral Form? |url=https://www.jstor.org/stable/24944338 |magazine=[[Scientific American]] |volume=189 |issue=3 |pages=89–99 |jstor=24944338 |issn=0036-8733}}</ref><ref>{{Cite journal |last=Steinicke |first=Wolfgang |date=2012 |title=The M51 mystery: Lord Rosse, Robinson, South and the discovery of spiral structure in 1845 |url=https://adsabs.harvard.edu/full/2012JAHH...15...19S |journal=[[Journal of Astronomical History and Heritage]] |volume=15 |issue=1 |pages=19–29 |doi=10.3724/SP.J.1440-2807.2012.01.03 |bibcode=2012JAHH...15...19S}}</ref>

In 1912, [[Vesto M. Slipher]] made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher discovered that the spiral nebulae have high [[Doppler shift]]s, indicating that they are moving at a rate exceeding the velocity of the stars he had measured. He found that the majority of these nebulae are moving away from us.<ref name=Slipher_1913/><ref name=Slipher_1915/>

In 1917, [[Heber Doust Curtis]] observed nova [[S Andromedae]] within the "Great [[Andromeda (constellation)|Andromeda]] Nebula", as the Andromeda Galaxy, [[Messier object]] [[Andromeda Galaxy|M31]], was then known. Searching the photographic record, he found 11 more [[nova]]e. Curtis noticed that these novae were, on average, 10 [[magnitude (astronomy)|magnitudes]] fainter than those that occurred within this galaxy. As a result, he was able to come up with a distance estimate of 150,000&nbsp;[[parsec]]s. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies.<ref>{{cite journal
 |last1=Curtis |first1=Heber D.
 |author-link1 = Heber Doust Curtis
 |date=1988
 |title=Novae in Spiral Nebulae and the Island Universe Theory
 |journal=[[Publications of the Astronomical Society of the Pacific]]
 |volume=100 |page=6
 |bibcode=1988PASP..100....6C
 |doi=10.1086/132128
|doi-access=free
 }}</ref>

In 1920 a debate took place between [[Harlow Shapley]] and [[Heber Curtis]], the [[Great Debate (astronomy)|Great Debate]], concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.<ref>{{cite web
 |last1=Weaver
 |first1=Harold. F.
 |title=Robert Julius Trumpler
 |url=http://www.nap.edu/readingroom/books/biomems/rtrumpler.html
 |publisher=[[United States National Academy of Sciences|US National Academy of Sciences]]
 |access-date=January 5, 2007
 |archive-date=December 24, 2013
 |archive-url=https://web.archive.org/web/20131224112329/http://www.nap.edu/readingroom/books/biomems/rtrumpler.html
 |url-status=dead
 }}</ref>

In 1922, the [[Estonia]]n astronomer [[Ernst Öpik]] gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object.<ref>{{cite journal
 |last=Öpik |first=Ernst
 |author-link=Ernst Öpik
 |date=1922
 |title=An estimate of the distance of the Andromeda Nebula
 |journal=[[The Astrophysical Journal]]
 |volume=55 |page=406
 |bibcode=1922ApJ....55..406O
 |doi=10.1086/142680
}}</ref> Using the new 100-inch [[Mount Wilson Observatory|Mt. Wilson]] telescope, [[Edwin Hubble]] was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some [[Cepheid variable]]s, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.<ref>{{cite journal
 |last1=Hubble |first1=Edwin P.
 |author-link1 = Edwin Hubble
 |date=1929
 |title=A spiral nebula as a stellar system, Messier 31
 |journal=[[The Astrophysical Journal]]
 |volume=69 |pages=103–158
 |bibcode=1929ApJ....69..103H
 |doi=10.1086/143167
|doi-access=free
 }}</ref> In 1926 Hubble produced a classification of [[Galaxy morphological classification|galactic morphology]] that is used to this day.<ref>{{cite journal
 | title=No. 324. Extra-galactic nebulae
 | last=Hubble | first=Edwin P.
 | author-link = Edwin Hubble
 | journal=Contributions from the Mount Wilson Observatory
 | publisher=Carnegie Institution of Washington
 | volume=324 | pages=1–49 | year=1926
 | bibcode=1926CMWCI.324....1H
}}</ref><ref>{{cite journal
 | title=Edwin Hubble, 1889–1953
 | last=Sandage | first=Allan R.
 | author-link=Allan Sandage
 | journal=[[Journal of the Royal Astronomical Society of Canada]]
 | volume=83 | issue=6 | pages=351–362 | date=1989
 | bibcode=1989JRASC..83..351S
 | url=https://apod.nasa.gov/diamond_jubilee/1996/sandage_hubble.html
 | access-date=April 21, 2024 | archive-date=February 5, 2024
 | archive-url=https://web.archive.org/web/20240205104433/https://apod.nasa.gov/diamond_jubilee/1996/sandage_hubble.html
 | url-status=live }}</ref>

=== Multi-wavelength observation ===
{{See also|Observational astronomy}}
{{multiple image
| align = right
| direction = vertical
| width = 220
| image1 =
| caption1 = A visual light image of [[Andromeda Galaxy]] shows the emission of ordinary stars and the light reflected by dust.
| image2 = Andromeda galaxy.jpg
| caption2 = This ultraviolet image of [[Andromeda Galaxy|Andromeda]] shows blue regions containing young, massive stars.
}}
Advances in astronomy have always been driven by technology. After centuries of success in [[optical astronomy]], recent decades have seen major progress in other regions of the [[electromagnetic spectrum]].<ref>{{Cite book |last=Rieke |first=George Henry |author-link=George H. Rieke
|title=Measuring the Universe: A Multiwavelength Perspective |date=2012 |publisher=[[Cambridge University Press]] |isbn=978-0-521-76229-8 |page=xi}}</ref>

The [[cosmic dust|dust]] present in the interstellar medium is opaque to visual light. It is more transparent to [[far infrared astronomy|far-infrared]], which can be used to observe the interior regions of giant molecular clouds and [[Bulge (astronomy)|galactic cores]] in great detail.<ref>{{cite web
 |title=Near, Mid & Far Infrared
 |url=http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html
 |website=[[Infrared Processing and Analysis Center]]
 |publisher=[[California Institute of Technology]]
 |access-date=January 2, 2007
 |url-status=dead
 |archive-url=https://web.archive.org/web/20061230203454/http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html
 |archive-date=December 30, 2006
}}</ref> Infrared is also used to observe distant, [[redshift|red-shifted]] galaxies that were formed much earlier. Water vapor and [[carbon dioxide]] absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for [[infrared astronomy]].{{sfn|Fraknoi et al.|2023|pp=[https://openstax.org/books/astronomy-2e/pages/5-2-the-electromagnetic-spectrum 144], [https://openstax.org/books/astronomy-2e/pages/6-5-observations-outside-earths-atmosphere 202]}}

The first non-visual study of galaxies, particularly active galaxies, was made using [[radio astronomy|radio frequencies]]. The Earth's atmosphere is nearly transparent to radio between 5&nbsp;[[Hertz|MHz]] and 30&nbsp;GHz. The [[ionosphere]] blocks signals below this range.<ref>{{cite web
 |title=The Effects of Earth's Upper Atmosphere on Radio Signals
 |url=http://radiojove.gsfc.nasa.gov/education/educ/radio/tran-rec/exerc/iono.htm
 |publisher=[[NASA]]
 |access-date=August 10, 2006
 |archive-date=May 7, 2012
 |archive-url=https://web.archive.org/web/20120507101534/http://radiojove.gsfc.nasa.gov/education/educ/radio/tran-rec/exerc/iono.htm
 |url-status=dead
 }}</ref> Large radio [[interferometry|interferometers]] have been used to map the active jets emitted from active nuclei.

[[UV astronomy|Ultraviolet]] and [[X-ray astronomy|X-ray telescopes]] can observe highly energetic galactic phenomena. Ultraviolet flares are sometimes observed when a star in a distant galaxy is torn apart from the tidal forces of a nearby black hole.<ref>{{cite news
 |title=NASA Telescope Sees Black Hole Munch on a Star
 |url=http://www.nasa.gov/mission_pages/galex/galex-20061205.html
 |website=[[Galaxy Evolution Explorer]]
 |publisher=[[NASA]]
 |date=December 5, 2006
 |access-date=January 2, 2007
 |archive-date=April 25, 2010
 |archive-url=https://web.archive.org/web/20100425205637/http://www.nasa.gov/mission_pages/galex/galex-20061205.html
 |url-status=dead
 }}</ref> The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of supermassive black holes at the cores of galaxies was confirmed through X-ray astronomy.<ref>{{cite web
 |last1=Dunn
 |first1=Robert
 |title=An Introduction to X-ray Astronomy
 |url=http://www-xray.ast.cam.ac.uk/xray_introduction/
 |publisher=[[Institute of Astronomy, Cambridge|Institute of Astronomy]] X-Ray Group
 |date=August 9, 2005
 |access-date=January 2, 2007
 |archive-date=January 1, 2007
 |archive-url=https://web.archive.org/web/20070101105849/http://www-xray.ast.cam.ac.uk/xray_introduction/
 |url-status=live
 }}</ref>

=== Modern research ===
[[File:Rotation curve of spiral galaxy Messier 33 (Triangulum).png|thumb|upright=1.8|Rotation curve of spiral galaxy [[Triangulum Galaxy|Messier 33]] (yellow and blue points with error bars), and a predicted one from distribution of the visible matter (gray line). The discrepancy between the two curves can be accounted for by adding a [[dark matter halo]] surrounding the galaxy.<ref>{{cite journal |last1=Corbelli |first1=E. |last2=Salucci |first2=P. |year=2000 |title=The extended rotation curve and the dark matter halo of M33 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=311 |issue=2 |pages=441–447 |arxiv=astro-ph/9909252 |bibcode=2000MNRAS.311..441C |bibcode-access=free |doi=10.1046/j.1365-8711.2000.03075.x |doi-access=free }}</ref>]]
In 1944, [[Hendrik C. van de Hulst|Hendrik van de Hulst]] predicted that [[microwave]] radiation with [[hydrogen line|wavelength of 21 cm]] would be detectable from interstellar atomic [[hydrogen]] gas;<ref>{{cite web
 |last1=Tenn
 |first1=Joe
 |title=Hendrik Christoffel van de Hulst
 |url=http://www.phys-astro.sonoma.edu/BruceMedalists/vandeHulst/
 |website=The Bruce Medalists
 |publisher=[[Sonoma State University]]
 |access-date=January 5, 2007
 |archive-date=January 14, 2012
 |archive-url=https://web.archive.org/web/20120114042216/http://www.phys-astro.sonoma.edu/BruceMedalists/vandeHulst/
 |url-status=dead
 }}</ref> and in 1951 it was observed. This radiation is not affected by dust absorption, and so its Doppler shift can be used to map the motion of the gas in this galaxy. These observations led to the hypothesis of a rotating [[barred spiral galaxy|bar structure]] in the center of this galaxy.<ref>{{cite journal
 |last1=López-Corredoira |first1=M.
 |s2cid=18399375
 |display-authors=etal
 |date=2001
 |title=Searching for the in-plane Galactic bar and ring in DENIS
 |journal=[[Astronomy & Astrophysics]]
 |volume=373
 |issue=1 |pages=139–152
 |bibcode=2001A&A...373..139L
 |doi=10.1051/0004-6361:20010560
|arxiv = astro-ph/0104307 }}</ref> With improved [[radio telescope]]s, hydrogen gas could also be traced in other galaxies.
In the 1970s, [[Vera Rubin]] uncovered a discrepancy between observed galactic [[galaxy rotation curve|rotation speed]] and that predicted by the visible mass of stars and gas. Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen [[dark matter]].<ref>{{cite magazine
 |last=Rubin |first=Vera C.
 |author-link=Vera Rubin
 |date=1983
 |title=Dark matter in spiral galaxies
 |magazine=[[Scientific American]]
 |volume=248
 |issue=6 |pages=96–106
 |bibcode=1983SciAm.248f..96R
 |doi=10.1038/scientificamerican0683-96
}}</ref><ref>{{cite journal
 |last=Rubin |first=Vera C.
 |author-link=Vera Rubin
 |date=2000
 |title=One Hundred Years of Rotating Galaxies
 |journal=[[Publications of the Astronomical Society of the Pacific]]
 |volume=112 |issue=772 |pages=747–750
 |bibcode=2000PASP..112..747R
 |doi=10.1086/316573
|s2cid=122927800
 |doi-access=free
 }}</ref>

Beginning in the 1990s, the [[Hubble Space Telescope]] yielded improved observations. Among other things, its data helped establish that the missing dark matter in this galaxy could not consist solely of inherently faint and small stars.<ref>{{cite press release
 |title=Hubble Rules Out a Leading Explanation for Dark Matter
 |website=Hubble News Desk
 |publisher=[[NASA]]
 |date=October 17, 1994
 |url=http://hubblesite.org/newscenter/archive/releases/1994/41/text/
 |access-date=January 8, 2007
 |archive-date=November 27, 2012
 |archive-url=https://web.archive.org/web/20121127083251/http://hubblesite.org/newscenter/archive/releases/1994/41/text/
 |url-status=dead
 }}</ref> The [[Hubble Deep Field]], an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion ({{val|1.25|e=11}}) galaxies in the observable universe.<ref>{{cite web
 |last=Mattson
 |first=Barbara
 |editor-last=Gibb
 |editor-first=Meredith
 |date=November 27, 2002
 |title=How many galaxies are there?
 |url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/021127a.html
 |website=Imagine the Universe!
 |publisher=[[NASA]]
 |access-date=January 8, 2007
 |archive-date=July 28, 2012
 |archive-url=https://web.archive.org/web/20120728035236/http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/021127a.html
 |url-status=dead
 }}</ref> Improved technology in detecting the [[electromagnetic spectrum|spectra]] invisible to humans (radio telescopes, infrared cameras, and [[x-ray astronomy|x-ray telescopes]]) allows detection of other galaxies that are not detected by Hubble. Particularly, surveys in the [[Zone of Avoidance]] (the region of sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies.<ref>{{cite journal
 |last1=Kraan-Korteweg |first1=R. C.
 |last2=Juraszek |first2=S.
 |s2cid=17900483
 |date=2000
 |title=Mapping the hidden Universe: The galaxy distribution in the Zone of Avoidance
 |journal=[[Publications of the Astronomical Society of Australia]]
 |volume=17 |issue=1 |pages=6–12
 |bibcode=2000PASA...17....6K
|arxiv = astro-ph/9910572
 |doi=10.1071/AS00006 }}</ref>

A 2016 study published in ''[[The Astrophysical Journal]],'' led by [[Christopher Conselice]] of the [[University of Nottingham]], analyzed many sources of data to estimate that the observable universe (up to z=8) contained at least two trillion ({{val|2|e=12}}) galaxies, a factor of 10 more than are directly observed in [[Hubble Space Telescope|Hubble]] images.<ref name="Conselice">{{cite journal|title=The Evolution of Galaxy Number Density at z <{{nbsp}}8 and its Implications|first=Christopher J.|last=Conselice|s2cid=17424588|display-authors=etal|journal=[[The Astrophysical Journal]]|volume=830|issue=2|year=2016|arxiv=1607.03909|bibcode= 2016ApJ...830...83C|doi=10.3847/0004-637X/830/2/83|page=83 |doi-access=free }}</ref>{{rp|12}}<ref name="NYT-20161017">{{cite news |last=Fountain |first=Henry |date=17 October 2016 |title=Two Trillion Galaxies, at the Very Least |work=[[The New York Times]] |url-access=registration |url=https://www.nytimes.com/2016/10/18/science/two-trillion-galaxies-at-the-very-least.html |url-status=live |access-date=17 October 2016 |archive-url=https://web.archive.org/web/20191231233343/https://www.nytimes.com/2016/10/18/science/two-trillion-galaxies-at-the-very-least.html |archive-date=December 31, 2019}}</ref> However, later observations with the [[New Horizons]] space probe from outside the [[zodiacal light]] observed less cosmic optical light than Conselice while still suggesting that direct observations are missing galaxies.<ref name="Lauer"/><ref>{{cite news |title=New Horizons spacecraft answers the question: How dark is space? |work=[[Phys.org]] |url=https://phys.org/news/2021-01-horizons-spacecraft-dark-space.html |access-date=15 January 2021 |language=en |archive-date=January 15, 2021 |archive-url=https://web.archive.org/web/20210115110710/https://phys.org/news/2021-01-horizons-spacecraft-dark-space.html |url-status=live }}</ref>