View source for Homunculus Nebula

{{Short description|Bipolar emission nebula in the constellation Carina}}
{{Infobox nebula
| name = Homunculus Nebula
| type = Emission
| type2 = [[Reflection nebula]]
| image = EtaCarinae.jpg
| epoch = [[J2000]]
| ra = {{RA|10|45|03.6}}<ref name=hog>{{cite journal|bibcode=2000A&A...355L..27H|title=The Tycho-2 catalogue of the 2.5 million brightest stars|journal=Astronomy and Astrophysics|volume=355|pages=L27|last1=Høg|first1=E.|last2=Fabricius|first2=C.|last3=Makarov|first3=V. V.|last4=Urban|first4=S.|last5=Corbin|first5=T.|last6=Wycoff|first6=G.|last7=Bastian|first7=U.|last8=Schwekendiek|first8=P.|last9=Wicenec|first9=A.|year=2000|doi=10.1888/0333750888/2862}}</ref>
| dec = {{DEC|-59|41|04}}<ref name=hog/>
| dist_ly = 7,500
| z =
| appmag_v=6.21 (-0.8–7.9) (including the central star)
| size_v = 18"<ref name=abraham>{{cite journal|bibcode=2014ApJ...791...95A|title=Η Carinae Baby Homunculus Uncovered by A/km/kmLMA|journal=The Astrophysical Journal|volume=791|issue=2|pages=95|last1=Abraham|first1=Zulema|last2=Falceta-Gonçalves|first2=Diego|last3=Beaklini|first3=Pedro P. B.|year=2014|doi=10.1088/0004-637X/791/2/95|arxiv = 1406.6297 |s2cid=62893264}}</ref>
| constellation = [[Carina (constellation)|Carina]]
| radius_ly = 0.29<ref name=abraham/>
| absmag_v =
| notes = [[Bipolar nebula]]
| names =
}}
The '''Homunculus Nebula''' is a [[bipolar nebula|bipolar]] [[Emission nebula|emission]] and [[reflection nebula]] surrounding the massive star system [[Eta Carinae]], about {{val|7,500|ul=light-years|fmt=commas}} ({{val|2,300|ul=parsecs|fmt=commas}}) from Earth.  The nebula is embedded within the much larger [[Carina Nebula]], a large [[Star-forming region|star-forming]] [[H II region]]. From the [[Latin]] ''[[homunculus]]'' meaning ''Little Man'', the nebula consists of gas which was ejected from Eta Carinae during the Great Eruption, which occurred ~7,500 years before it was observed on Earth, from 1838 to 1845.<ref name=teodoro>{{cite journal|bibcode=2008MNRAS.387..564T|title=Near-infrared integral field spectroscopy of the Homunculus nebula around η Carinae using Gemini/CIRPASS|journal=Monthly Notices of the Royal Astronomical Society|volume=387|issue=2|pages=564|last1=Teodoro|first1=M.|last2=Damineli|first2=A.|last3=Sharp|first3=R. G.|last4=Groh|first4=J. H.|last5=Barbosa|first5=C. L.|year=2008|doi=10.1111/j.1365-2966.2008.13264.x|doi-access=free |arxiv = 0804.0240 |s2cid=2460614}}</ref> It also contains dust which absorbs much of the light from the extremely luminous central stellar system and re-radiates it as [[infra-red]] (IR).  It is the brightest object in the sky at mid-IR wavelengths.<ref name=imposters>{{cite book|doi=10.1007/978-1-4614-2275-4_7|chapter=All Things Homunculus|title=Eta Carinae and the Supernova Impostors|volume=384|pages=145–169|series=Astrophysics and Space Science Library|year=2012|last1=Smith|first1=Nathan|arxiv=0906.2204 |isbn=978-1-4614-2274-7|bibcode=2012ASSL..384..145S|s2cid=6275803 }}</ref>

Within the Homunculus is a smaller ''Little Homunculus'', and within that a shell of shocked material from stellar winds that has been called ''Baby Homunculus''.<ref name=abraham/>

==Observational history==

In 1914, Eta Carinae was reported to have a faint companion and also to be non-stellar.<ref name=innes>{{cite journal|bibcode=1914MNRAS..74..697I|title=Η Argûs, Magnitude of, in 1914, and the discovery of a close companion to it|journal=Monthly Notices of the Royal Astronomical Society|volume=74|issue=8|pages=697|last1=Innes|first1=R. T. A.|year=1914|doi=10.1093/mnras/74.8.697|doi-access=free }}</ref> Observations in 1944 and 1945 showed a somewhat elongated nebulosity around {{val|5|ul="}} wide and {{val|10|u="}} long.  It was measured to be expanding at a rate which was consistent with having originated in an explosion in the mid 19th century.  At that time the shape of the nebula showed a central bulge with a single large lump to the northwest and two smaller extensions to the southeast, which was described as a ''Homunculus''.<ref name=gaviola>{{cite journal|bibcode=1950ApJ...111..408G|title=Eta Carinae. I. The Nebulosity|journal=Astrophysical Journal|volume=111|pages=408|last1=Gaviola|first1=E.|year=1950|doi=10.1086/145274|doi-access=free}}</ref>  Other observations at around the same time described a strongly orange central region in a larger fainter green nebulosity. One paper described it as looking like a "red spade-beard".<ref name=thackeray>{{cite journal|bibcode=1949Obs....69...31T|title=Nebulosity surrounding eta Carinae|journal=The Observatory|volume=69|pages=31|last1=Thackeray|first1=A. D.|year=1949}}</ref>

==Shape==
[[File:3D Homunculus Nebula.jpg|left|thumb|upright=1.2|A 3D model of the Homunculus Nebula.]]
The Homunculus consists of two lobes, referred to as northwest (NW) and southeast (SE) based on their orientation as seen from Earth. Each is approximately {{val|7|u="}} wide by {{val|5|u="}} long.  There is also a ragged equatorial skirt of material which can be seen faintly in deep images at certain wavelengths.  The lobes are mostly hollow with the material strongly concentrated towards the poles.<ref name=abraham/><ref name=steffen/>

The equatorial skirt appears to contain material of the same age and younger than the lobes.  It contains a much smaller mass of material than the lobes, shining mainly by reflected light which escapes most easily at equatorial latitudes.  There is less dust and little molecular hydrogen compared to the lobes.<ref name=smith2006>{{cite journal|doi=10.1086/503766|title=The Structure of the Homunculus. I. Shape and Latitude Dependence from H2and [Feii] Velocity Maps of η Carinae|journal=The Astrophysical Journal|volume=644|issue=2|pages=1151–1163|year=2006|last1=Smith|first1=Nathan|bibcode=2006ApJ...644.1151S|arxiv = astro-ph/0602464 |s2cid=12453761}}</ref>

The bipolar nebula is angled so that the NW lobe is further away from Earth than the SE lobe is.  The whole nebula is expanding so that the SE lobe is blue-shifted and the NW lobe is red-shifted, relative to the central source.  The lobes contain the majority of the material in the Homunculus Nebula, in relatively thin shells concentrated towards the poles.  The shells consist of two components, an inner warm region and a more massive outer cool skin.  The shells are smooth and thin suggesting they were ejected in as little as five years, but there are streaks of thicker dust detectable within the shells.<ref name=smith2006/>

[[File:Detailed look on Eta Carinae.jpg|right|thumb|upright=1.2|Detailed look on [[Eta Carinae]]. Carina Nebula (left), Homunculus Nebula (center), and high resolution image of Eta Carinae (right).<ref>{{cite web|title=Highest Resolution Image of Eta Carinae - VLT Interferometer captures raging winds in famous massive stellar system|url=https://www.eso.org/public/news/eso1637/|website=www.eso.org|access-date=20 October 2016}}</ref>]]

[[File:Homunculus Nebula Expansion.gif|thumb|The Homunculus Nebula imaged with an 8" untracked Dobsonian telescope, showing the expansion of its lobes between 2021 and 2025.]]

Each lobe has polar "hole" although it is not known whether it is an actual gap in the shell of the lobe or just a deep indentation. Surrounding each polar hole is a "trench".  The trenches are visible as approximate semicircles centred on the axis of the lobes but may form complete circles.  There are other smaller irregular indentations and protrusions to the lobes, which are symmetrical with the same features appearing on each lobe.  These include flattened protrusions at about 10° latitude, one on each lobe (labeled "Protrusions" in the [https://upload.wikimedia.org/wikipedia/commons/f/f8/3D_Homunculus_Nebula.jpg illustrated model]), with other smaller protrusions near the equatorial skirt.<ref name=steffen>{{cite journal|bibcode=2014MNRAS.442.3316S|title=The three-dimensional structure of the Eta Carinae Homunculus|journal=Monthly Notices of the Royal Astronomical Society|volume=442|issue=4|pages=3316|last1=Steffen|first1=W.|last2=Teodoro|first2=M.|last3=Madura|first3=T. I.|last4=Groh|first4=J. H.|last5=Gull|first5=T. R.|last6=Mehner|first6=A.|last7=Corcoran|first7=M. F.|last8=Damineli|first8=A.|last9=Hamaguchi|first9=K.|year=2014|doi=10.1093/mnras/stu1088|doi-access=free |arxiv = 1407.4096 }}</ref>

The mass of the nebula cannot be determined directly.  However, the amount of dust can be measured fairly accurately and estimates of the gas to dust ratio used to calculate the total mass.  The total dust mass is calculated at {{solar mass|0.4}}, leading to estimates that up to {{solar mass|40}} of gas are contained in the Homunculus itself.  Nearly as much material is detected within outer ejecta, which formed earlier, but within the last thousand years.  Older calculations had produced consensus estimates of {{solar mass|10-15}}<ref name=gomez>{{cite journal|doi=10.1111/j.1745-3933.2009.00784.x|title=Submillimetre variability of Eta Carinae: Cool dust within the outer ejecta|journal=Monthly Notices of the Royal Astronomical Society: Letters|volume=401|pages=L48–L52|year=2010|last1=Gomez|first1=H. L.|last2=Vlahakis|first2=C.|last3=Stretch|first3=C. M.|last4=Dunne|first4=L.|last5=Eales|first5=S. A.|last6=Beelen|first6=A.|last7=Gomez|first7=E. L.|last8=Edmunds|first8=M. G.|issue=1|doi-access=free |bibcode=2010MNRAS.401L..48G|arxiv = 0911.0176 |s2cid=119295262}}</ref>

==Weigelt Blobs==
Early speckle interferometry showed that the central region of the Homunculus contains four point-like sources, originally designated A1, A2, A3, and A4.<ref name=weigelt>{{cite journal|bibcode=1986A&A...163L...5W|title=Eta Carinae resolved by speckle interferometry|journal=Astronomy and Astrophysics|volume=163|pages=L5|last1=Weigelt|first1=G.|last2=Ebersberger|first2=J.|year=1986}}</ref>  The four speckle objects were later referred to as A, B, C, and D.  Higher resolution studies showed that only the brightest source A was truly stellar, and the other three were small nebular condensations.  The three ''Weigelt Blobs'' are visible primarily in light directly reflected from the Eta Carinae stars.  The blobs are believed to lie near the equatorial plane of the stellar system, but their origin is unclear.  Their speed has been measured, but within uncertainties they could have been emitted in the 1890 outburst or a 1941 event.  The situation is complicated further by the likely acceleration of their slow movement due to the intense stellar winds.<ref name=dorland>{{cite journal|doi=10.1086/380941|title=Did Carinae's Weigelt Blobs Originate circa 1941?|journal=The Astronomical Journal|volume=127|issue=2|pages=1052|year=2004|last1=Dorland|first1=Bryan N.|last2=Currie|first2=Douglas G.|last3=Hajian|first3=Arsen R.|bibcode=2004AJ....127.1052D|doi-access=free}}</ref>

==Spectrum==
[[File:Cosmic Fireworks in Ultraviolet Eta Carinae Nebula.tif|thumb|Ultraviolet image of Homunculus Nebula taken by [[Hubble Space Telescope|Hubble]]]]
The spectrum of the Homunculus is complex, consisting of reflected, thermal, and emission components at wavelengths across the electromagnetic spectrum.  The dominant feature is [[blackbody]] radiation from dust heated by the stars within.  Overlaid on this is some light from the stars themselves reflected mostly from dense features within the nebulosity, showing strong visual and UV spectral lines in emission.  There are also emission lines from ionised gas where it collides with slower moving material or is excited by high energy electromagnetic radiation from the stars.  The ionisation emission is similar to a [[planetary nebula]] but at lower levels of ionisation due to the lower temperatures of the central stars.  The strongest lines are [Fe&nbsp;{{smallcaps|ii}}] and [N&nbsp;{{smallcaps|ii}}], similar to those from the stellar winds of the stars themselves, but with narrower profiles.<ref name="smith2006"/>

Shock waves at the outer edge of the ejecta are heated to millions of [[kelvin]] and emit x-ray radiation.  The lobes of the Homunculus emit copious radio waves, including emission in the [[21&nbsp;cm line]] of hydrogen.

The reflected spectrum of the Homunculus lobes varies with position, due to the central star emitting different radiation at different latitudes on its surface.  This is the only star for which such an effect can be observed.<ref name="smith2006"/>

==Formation==
{{main|Eta Carinae}}
The Homunculus was ejected in an enormous outburst from Eta Carinae. Light from this event reached Earth in 1841, when Eta Carinae briefly became the second-brightest star in the sky, after [[Sirius]]; the ejected gas and dust have since obscured much of its light. The [[Supernova imposter|near-supernova]] explosion produced two polar lobes, and a large but thin [[Non-inclined orbit|equatorial]] disk, all moving outward at up to {{convert|670|km/s|mph|abbr=on}}.

The Homunculus Nebula is a virtually unique structure, believed to result from the extremely young age.  This means that the shape and structure is almost entirely due to the original eruption rather than the interaction with surrounding interstellar material.  Suggestions that the waist between the two lobes was formed by "pinching" from a dense surrounding material have been disproved, and the bipolar shells are now believed to be caused by concentrated polar outflows of material, with the equatorial skirt formed by breakout of faster ejected material through the thinnest parts of the shells.<ref name=steffen/>  The ejection of material preferentially along the axis of rotation of the stars, or of the binary orbit, may be due to the rotation of Eta Carinae A itself resulting in stronger mass loss towards the poles.<ref name=smith2006/>

The thinness of the bipolar shells argues for their ejection within approximately five years.<ref name=smith2006/>  Irregularities in the otherwise very smooth structure of the shells are conjectured to result from interactions between the winds of the two central stars, and from their orbital motion.<ref name=steffen/>

Analysis of the expansion of the nebula has given a time for its formation at {{val|1847.1|0.8|ul=yr|fmt=none}}.  This date is inconsistent with the peaks in brightness and with estimates of the [[periastron]] passage of the secondary star.<ref name=smith>{{cite journal|bibcode=2017MNRAS.471.4465S|title=A moderately precise dynamical age for the Homunculus of Eta Carinae based on 13 years of HST imaging|journal=Monthly Notices of the Royal Astronomical Society|volume=471|issue=4|pages=4465–4475|last1=Smith|first1=Nathan|year=2017|arxiv=1709.06210|doi=10.1093/mnras/stx1868|doi-access=free }}</ref>

==Distance==
[[File:Eta Carinae (HST).jpg|left|thumb|Homunculus Nebula around Eta Carinae (HST UV and visual image)]]
The Homunculus and Eta Carinae are assumed to be at approximately the same distance as Trumpler 16 and the Carina Nebula, but the distances to these objects are not known with any great accuracy.  Instead, the distance of the Homunculus Nebula itself can be calculated using measurements of its expansion. The velocity of particular locations within the thin shell of the Homunculus lobes can be measured using the [[doppler shift]] of the spectral lines at that point, assuming the lobes are symmetrical.  There are two different doppler shifts visible in the spectrum: one for direct emission lines; and one for reflected lines from Eta Carinae A.  The direct line doppler shift gives the expansion velocity of the shell project onto the line of sight.  Assuming an origin in Eta Carinae's Great Eruption and a constant expansion velocity, this gives the linear distance of the shell from the central star projected along the line of sight.  The difference between the reflected line doppler shift velocity and the direct line velocity gives the distance of the shell from the central star, again assuming expansion at constant velocity since the Great Eruption.

Observations of the spectrum of the Homunculus at a particular angular distance from the central star has shown the actual linear distance of that point from the central star, which defines the distance.  Values obtained using this method are around {{val|2.3|ul=kpc}} with a margin of error around {{val|100|ul=pc}}.<ref name=smith/><ref name="smith2006"/><ref name=davidson>{{cite journal|doi=10.1086/319419|title=The Shape and Orientation of the Homunculus Nebula Based on Spectroscopic Velocities|journal=The Astronomical Journal|volume=121|issue=3|pages=1569|year=2001|last1=Davidson|first1=Kris|last2=Smith|first2=Nathan|last3=Gull|first3=Theodore R.|last4=Ishibashi|first4=Kazunori|last5=Hillier|first5=D. J.|bibcode=2001AJ....121.1569D|doi-access=free}}</ref>

The same calculations also return the inclination of the axis of the Homunculus relative to the line of sight.  This turns out to be 41°, or 49° relative to the plane of the sky, which means it is slightly more "end on" than "side on".<ref name=smith2006/><ref name=davidson/>

==References==
{{reflist|colwidth=25em}}

==External links==
* [http://apod.nasa.gov/apod/ap140717.html 3D Homunculus Nebula] [[Astronomy Picture of the Day]] 2014 July 17
* [http://apod.nasa.gov/apod/ap141202.html Expanding Homunculus Nebula] Astronomy Picture of the day (animated GIF)
* [http://www.gemini.edu/node/11384 Little Homunculus and Butterfly Nebula]

{{Stars of Carina}}

{{DEFAULTSORT:Homunculus Nebula}}
[[Category:Carina Nebula]]
[[Category:Emission nebulae]]
[[Category:Carina (constellation)]]
[[Category:Reflection nebulae]]