View source for Astronomy:Herbig–Haro object

{{Short description|Small patches of nebulosity associated with newly born stars}}
{{Multiple image
| total_width = 400
| image1      = Hubble Sees the Force Awakening in a Newborn Star (23807356641).jpg
| alt1        = HH 24 is located in the Orion B molecular cloud
| image2      = Herbig-Haro object HH32.jpg
| alt2        = HH 32 looks like a star due to its intense brightness. The surrounding gas appears like clouds around a full moon.
| footer      = [[Astronomy:Hubble Space Telescope|''Hubble'' Space Telescope]] images of [[Astronomy:HH 24-26|HH 24]] (left) and HH 32 (right; top) – colourful nebulae are typical of Herbig–Haro objects
}}

'''Herbig–Haro '''('''HH''')''' objects''' are bright patches of [[Astronomy:Nebula|nebulosity]] associated with newborn [[Astronomy:Star|star]]s. They are formed when narrow jets of partially [[Physics:Plasma|ionised gas]] ejected by stars collide with nearby clouds of gas and dust at several hundred kilometres per second. Herbig–Haro objects are commonly found in [[Astronomy:Star formation#Stellar nurseries|star-forming regions]], and several are often seen around a single star, aligned with its [[Physics:Axis of rotation|rotational axis]]. Most of them lie within about one [[Astronomy:Parsec|parsec]] (3.26 [[Astronomy:Light-year|light-year]]s) of the source, although some have been observed several parsecs away. HH objects are transient phenomena that last around a few tens of thousands of years. They can change visibly over timescales of a few years as they move rapidly away from their parent star into the gas clouds of interstellar space (the [[Astronomy:Interstellar medium|interstellar medium]] or ISM). [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]] observations have revealed the complex evolution of HH objects over the period of a few years, as parts of the nebula fade while others brighten as they collide with the clumpy material of the interstellar medium.

First observed in the late 19th century by Sherburne Wesley Burnham, Herbig–Haro objects were recognised as a distinct type of [[Astronomy:Emission nebula|emission nebula]] in the 1940s. The first astronomers to study them in detail were George Herbig and Guillermo Haro, after whom they have been named. Herbig and Haro were working independently on studies of [[Astronomy:Star formation|star formation]] when they first analysed the objects, and recognised that they were a by-product of the star formation process. Although HH objects are visible-[[Wavelength|wavelength]] phenomena, many remain invisible at these wavelengths due to dust and gas, and can only be detected at [[Physics:Infrared|infrared]] wavelengths. Such objects, when observed in near infrared, are called molecular hydrogen emission-line objects (MHOs).

== Discovery and history of observations ==
The first HH object was observed in the late 19th century by Sherburne Wesley Burnham, when he observed the star [[Astronomy:T Tauri|T Tauri]] with the {{convert|36|in|mm|adj=on}} [[Astronomy:Refracting telescope|refracting telescope]] at [[Organization:Lick Observatory|Lick Observatory]] and noted a small patch of nebulosity nearby.<ref name=Burnham1890/> It was thought to be an [[Astronomy:Emission nebula|emission nebula]], later becoming known as Burnham's Nebula, and was not recognized as a distinct class of object.<ref name="Reipurth1997"/> T Tauri was found to be a very young and variable star, and is the prototype of the class of similar objects known as [[Astronomy:T Tauri star|T Tauri star]]s which have yet to reach a state of [[Astronomy:Hydrostatic equilibrium|hydrostatic equilibrium]] between [[Astronomy:Gravitational collapse|gravitational collapse]] and energy generation through [[Physics:Nuclear fusion|nuclear fusion]] at their centres.<ref name="Caroll2014"/> Fifty years after Burnham's discovery, several similar nebulae were discovered with almost star-like appearance. Both George Herbig and Guillermo Haro made independent observations of several of these objects in the Orion Nebula during the 1940s. Herbig also looked at Burnham's Nebula and found it displayed an unusual electromagnetic spectrum, with prominent emission lines of [[Chemistry:Hydrogen|hydrogen]], [[Chemistry:Sulfur|sulfur]] and [[Chemistry:Oxygen|oxygen]]. Haro found that all the objects of this type were invisible in infrared light.<ref name="Reipurth1997"/>

Following their independent discoveries, Herbig and Haro met at an astronomy conference in Tucson, Arizona in December 1949. Herbig had initially paid little attention to the objects he had discovered, being primarily concerned with the nearby stars, but on hearing Haro's findings he carried out more detailed studies of them. The [[Place:Soviet Union|Soviet]] astronomer Viktor Ambartsumian gave the objects their name (Herbig–Haro objects, normally shortened to HH objects), and based on their occurrence near young stars (a few hundred thousand years old), suggested they might represent an early stage in the formation of T Tauri stars.<ref name="Reipurth1997"/> Studies of the HH objects showed they were highly [[Physics:Ionization|ionised]], and early theorists speculated that they were reflection nebulae containing low-luminosity hot stars deep inside. But the absence of infrared radiation from the nebulae meant there could not be stars within them, as these would have emitted abundant infrared light. In 1975 American astronomer R. D. Schwartz theorized that [[Astronomy:Solar wind|winds]] from T Tauri stars produce [[Physics:Shock waves in astrophysics|shocks]] in the ambient medium on encounter, resulting in generation of visible light.<ref name="Reipurth1997"/> With the discovery of the first proto-stellar [[Astronomy:Astrophysical jet|jet]] in HH 46/47, it became clear that HH objects are indeed shock-induced phenomena with shocks being driven by a collimated jet from protostars.<ref name="Reipurth1997"/><ref name=Dopita1982/>

An image of a Question Mark associated with the object was reported on 18 August 2023 in ''{{wipe|The New York Times}}''.<ref name="NYT20230818">{{cite news |last=Overbye |first=Dennis |title=The Biggest Question Mark in Astronomy? You're Looking at It. - Close scrutiny of a recent image from the Webb Space Telescope revealed some questionable punctuation. |url=https://www.nytimes.com/2023/08/18/science/space/question-mark-space-webb-telescope.html |date=18 August 2023 |work={{wipe|The New York Times}} |url-status=live |archiveurl=https://archive.today/20230818130716/https://www.nytimes.com/2023/08/18/science/space/question-mark-space-webb-telescope.html |archivedate=18 August 2023 |accessdate=19 August 2023 }}</ref>

== Formation ==
{{main|Astronomy:Star formation|Astronomy:Astrophysical jet}}
{{Multiple image
| total_width = 300
| image1      = HH object diagram.svg
| alt1        = Illustration depicting two arrows of matter moving outwards in opposite directions from a star-disk system, and creating bright emission caps at the ends, where they collide with the surrounding medium
| image2      = Protostar HH-34.jpg
| alt2        = Yellow-green emission cap produced by red jet from a star in a deep green nebula
| footer      = HH objects are formed when accreted material is ejected by a protostar as ionized gas along the star's axis of rotation, as exemplified by [[Astronomy:HH 34|HH 34]] (right).
}}

Stars form by gravitational collapse of [[Astronomy:Interstellar cloud|interstellar gas clouds]]. As the collapse increases the density, [[Physics:Radiation|radiative energy]] loss decreases due to increased [[Physics:Opacity (optics)|opacity]]. This raises the temperature of the cloud which prevents further collapse, and a hydrostatic equilibrium is established. Gas continues to fall towards the core in a [[Astronomy:Accretion disk|rotating disk]]. The core of this system is called a [[Astronomy:Protostar|protostar]].<ref name="Dina2000"/> Some of the [[Physics:Accretion (astrophysics)|accreting]] material is ejected out along the star's [[Physics:Axis of rotation|axis of rotation]] in two [[Astronomy:Astrophysical jet|jets]] of partially ionised gas ([[Physics:Plasma|plasma]]).<ref name=EAA/> The mechanism for producing these collimated bipolar jets is not entirely understood, but it is believed that interaction between the accretion disk and the [[Physics:Stellar magnetic field|stellar magnetic field]] accelerates some of the accreting material from within a few [[Astronomy:Astronomical unit|astronomical unit]]s of the star away from the disk plane. At these distances the outflow is divergent, fanning out at an angle in the range of 10−30°, but it becomes increasingly collimated at distances of tens to hundreds of astronomical units from the source, as its expansion is constrained.<ref name="JBally2016"/><ref name=Frank2014/> The jets also carry away the excess [[Physics:Angular momentum|angular momentum]] resulting from accretion of material onto the star, which would otherwise cause the star to rotate too rapidly and disintegrate.<ref name=Frank2014/> When these jets collide with the interstellar medium, they give rise to the small patches of bright [[Physics:Emission spectrum|emission]] which comprise HH objects.<ref name="J. Bally 1996"/>

== Properties ==
[[File:Ssc2003-06g.jpg|thumb|left|upright=1.3|alt=Plot of light intensity vs wavelength featuring several dips, caused by absorption of light emitted from the star by the molecules in surrounding medium|[[Physics:Infrared spectroscopy|Infrared spectrum]] of HH 46/47 obtained by the ''Spitzer'' Space Telescope, showing the medium in immediate vicinity of the star being silicate-rich]]
Electromagnetic emission from HH objects is caused when their associated [[Physics:Shock wave|shock wave]]s collide with the [[Astronomy:Interstellar medium|interstellar medium]], creating what is called the "terminal working surfaces".<ref name="ReipurthBally2001"/> The [[Physics:Continuous spectrum|spectrum is continuous]], but also has intense emission lines of neutral and ionized species.<ref name=EAA/> Spectroscopic observations of HH objects' doppler shifts indicate velocities of several hundred kilometers per second, but the emission lines in those [[Physics:Electromagnetic spectrum|spectra]] are weaker than what would be expected from such high-speed collisions. This suggests that some of the material they are colliding with is also moving along the beam, although at a lower speed.<ref name=Dopita1978/><ref name="Schwartz1983"/> Spectroscopic observations of HH objects show they are moving away from the source stars at speeds of several hundred kilometres per second.<ref name="Reipurth1997"/><ref name="Heathcote1998"/> In recent years, the high [[Physics:Optical resolution|optical resolution]] of the Hubble Space Telescope has revealed the [[Astronomy:Proper motion|proper motion]] (movement along the sky plane) of many HH objects in observations spaced several years apart.<ref name="Hartigan2001"/><ref name="Raga2016"/> As they move away from the parent star, HH objects evolve significantly, varying in brightness on timescales of a few years. Individual compact knots or clumps within an object may brighten and fade or disappear entirely, while new knots have been seen to appear.<ref name=Frank2014/><ref name="ReipurthBally2001"/> These arise likely because of the [[Physics:Precession|precession]] of their jets,<ref name="Zealey1992"/><ref name=Bally2007_2/> along with the pulsating and intermittent eruptions from their parent stars.<ref name="J. Bally 1996"/> Faster jets catch up with earlier slower jets, creating the so-called "internal working surfaces", where streams of gas collide and generate shock waves and consequent emissions.<ref name="Raga2017"/>

The total mass being ejected by stars to form typical HH objects is estimated to be of the order of 10<sup>−8</sup> to 10<sup>−6</sup> [[Astronomy:Solar mass|{{Solar mass}}]] per year,<ref name="Zealey1992"/> a very small amount of material compared to the mass of the stars themselves<ref name="brugel"/> but amounting to about 1–10% of the total mass accreted by the source stars in a year.<ref name="Hartigan1994"/> Mass loss tends to decrease with increasing age of the source.<ref name=Bally2007/> The temperatures observed in HH objects are typically about 9,000–12,000&nbsp;[[Kelvin|K]],<ref name="Bacciotti 1999"/> similar to those found in other ionized nebulae such as [[Astronomy:H II region|H II region]]s and [[Astronomy:Planetary nebula|planetary nebula]]e.<ref name=EAAHII/> Densities, on the other hand, are higher than in other nebulae, ranging from a few thousand to a few tens of thousands of particles per cm<sup>3</sup>,<ref name="Bacciotti 1999"/> compared to a few thousand particles per cm<sup>3</sup> in most H II regions and planetary nebulae.<ref name=EAAHII/>

Densities also decrease as the source evolves over time.<ref name=Bally2007/> HH objects consist mostly of hydrogen and [[Chemistry:Helium|helium]], which account for about 75% and 24% of their mass respectively. Around 1% of the mass of HH objects is made up of heavier [[Chemistry:Chemical element|chemical element]]s, including oxygen, sulfur, [[Chemistry:Nitrogen|nitrogen]], [[Chemistry:Iron|iron]], [[Chemistry:Calcium|calcium]] and [[Chemistry:Magnesium|magnesium]]. Abundances of these elements, determined from emission lines of respective ions, are generally similar to their cosmic abundances.<ref name="brugel"/> Many chemical compounds found in the surrounding interstellar medium, but not present in the source material, such as metal hydrides, are believed to have been produced by shock-induced chemical reactions.<ref name="JBally2016"/> Around 20–30% of the gas in HH objects is ionized near the source star, but this proportion decreases at increasing distances. This implies the material is ionized in the polar jet, and recombines as it moves away from the star, rather than being ionized by later collisions.<ref name="Bacciotti 1999"/> Shocking at the end of the jet can re-ionise some material, giving rise to bright "caps".<ref name=EAA/>

== Numbers and distribution ==
[[File:HH 47 HH 34 and HH 2.jpg|thumb|alt=Blue- and orange-appearing turbulent caps of emission|HH 2 (lower right), HH 34 (lower left), and HH 47 (top) were numbered in order of their discovery; it is estimated that there are up to 150,000 such objects in the Milky Way.]]
HH objects are named approximately in order of their identification; HH 1/2 being the earliest such objects to be identified.<ref name=HerbigCat/> More than a thousand individual objects are now known.<ref name="JBally2016"/> They are always present in star-forming H II regions, and are often found in large groups.<ref name="J. Bally 1996"/> They are typically observed near [[Astronomy:Bok globule|Bok globule]]s ([[Astronomy:Dark nebula|dark nebula]]e which contain very young stars) and often emanate from them. Several HH objects have been seen near a single energy source, forming a string of objects along the line of the [[Physics:Rotation#Astronomy|polar axis]] of the parent star.<ref name="JBally2016"/> The number of known HH objects has increased rapidly over the last few years, but that is a very small proportion of the estimated up to 150,000 in the [[Astronomy:Milky Way|Milky Way]],<ref name=Giulbudagian1984/> the vast majority of which are too far away to be resolved. Most HH objects lie within about one [[Astronomy:Parsec|parsec]] of their parent star. Many, however, are seen several parsecs away.<ref name=Bally2007/><ref name="Bacciotti 1999"/>

HH 46/47 is located about {{Convert|450|pc|abbr=off}} away from the Sun and is powered by a [[Astronomy:Protostar#Observed classes of young stars|class I protostar]] [[Astronomy:Binary star|binary]]. The bipolar jet is slamming into the surrounding medium at a velocity of 300&nbsp;kilometers per second, producing two emission caps about {{Convert|2.6|pc|abbr=off}} apart. Jet outflow is accompanied by a {{Convert|0.3|pc|abbr=off}} long molecular gas outflow which is swept up by the jet itself.<ref name="JBally2016"/> Infrared studies by [[Astronomy:Spitzer Space Telescope|Spitzer Space Telescope]] have revealed a variety of chemical compounds in the molecular outflow, including water (ice), [[Chemistry:Methanol|methanol]], [[Chemistry:Methane|methane]], [[Chemistry:Carbon dioxide|carbon dioxide]] ([[Chemistry:Dry ice|dry ice]]) and various [[Chemistry:Silicate|silicate]]s.<ref name="JBally2016"/><ref name=Spitzeroutflow/> Located around {{Convert|460|pc|abbr=off}} away in the [[Astronomy:Orion molecular cloud complex|Orion A molecular cloud]], [[Astronomy:HH 34|HH 34]] is produced by a highly collimated bipolar jet powered by a class I protostar. Matter in the jet is moving at about 220&nbsp;kilometers per second. Two bright [[Physics:Bow shocks in astrophysics|bow shocks]], separated by about {{Convert|0.44|pc|abbr=off}}, are present on the opposite sides of the source, followed by series of fainter ones at larger distances, making the whole complex about {{Convert|3|pc|abbr=off}} long. The jet is surrounded by a {{Convert|0.3|pc|abbr=off}} long weak molecular outflow near the source.<ref name="JBally2016"/><ref name=HH342002/>

== Source stars ==
[[File:HH 34 jet video HST.ogv|thumb|left|Thirteen-year timelapse of material ejecting from a class I protostar, forming the Herbig–Haro object HH 34]]
The stars from which HH jets are emitted are all very young stars, a few tens of thousands to about a million years old. The youngest of these are still protostars in the process of collecting from their surrounding gases. Astronomers divide these stars into classes 0, I, II and III, according to how much infrared radiation the stars emit.<ref name="McKee2007"/> A greater amount of infrared radiation implies a larger amount of cooler material surrounding the star, which indicates it is still coalescing. The numbering of the classes arises because class 0 objects (the youngest) were not discovered until classes I, II and III had already been defined.<ref name="Andre1994"/><ref name="McKee2007"/>

Class 0 objects are only a few thousand years old; so young that they are not yet undergoing nuclear fusion reactions at their centres. Instead, they are powered only by the gravitational potential energy released as material falls onto them.<ref name=Andre1993/> They mostly contain molecular outflows with low velocities (less than a hundred kilometres per second) and weak emissions in the outflows.<ref name=Bally2007_2/> Nuclear fusion has begun in the cores of Class I objects, but gas and dust are still falling onto their surfaces from the surrounding nebula, and most of their luminosity is accounted for by gravitational energy. They are generally still shrouded in dense clouds of dust and gas, which obscure all their [[Physics:Visible light|visible light]] and as a result can only be observed at infrared and [[Social:Radio|radio]] wavelengths.<ref name="Stahler2004"/> Outflows from this class are dominated by ionized species and velocities can range up to 400 kilometres per second.<ref name=Bally2007_2/> The in-fall of gas and dust has largely finished in Class II objects (Classical T Tauri stars), but they are still surrounded by disks of dust and gas, and produce weak outflows of low luminosity.<ref name=Bally2007_2/> Class III objects (Weak-line T Tauri stars) have only trace remnants of their original accretion disk.<ref name="McKee2007"/>

About 80% of the stars giving rise to HH objects are binary or multiple systems (two or more stars orbiting each other), which is a much higher proportion than that found for low mass stars on the [[Astronomy:Main sequence|main sequence]]. This may indicate that binary systems are more likely to generate the jets which give rise to HH objects, and evidence suggests the largest HH outflows might be formed when multiple–star systems disintegrate.<ref name="Reipurth2000"/> It is thought that most stars originate from multiple star systems, but that a sizable fraction of these systems are disrupted before their stars reach the main sequence due to gravitational interactions with nearby stars and dense clouds of gas.<ref name="Reipurth2000"/><ref name=Reipurth2004/>

The first and currently only (as of May 2017) large-scale Herbig-Haro object around a proto-[[Astronomy:Brown dwarf|brown dwarf]] is HH 1165, which is connected to the proto-brown dwarf Mayrit 1701117. HH 1165 has a length of 0.8 [[Astronomy:Light-year|light-years]] (0.26 [[Astronomy:Parsec|parsec]]) and is located in the vicinity of the [[Astronomy:Sigma Orionis|sigma Orionis]] cluster. Previously only small mini-jets (≤0.03 parsec) were found around proto-brown dwarfs.<ref>{{Cite web|url=http://www.noao.edu/news/2017/pr1701.php|title=Punching Above Its Weight, a Brown Dwarf Launches a Parsec-Scale Jet|website=National Optical Astronomy Observatory|access-date=2020-03-06}}</ref><ref>{{Cite journal|last1=Riaz|first1=B.|last2=Briceño|first2=C.|last3=Whelan|first3=E. T.|last4=Heathcote|first4=S.|date=July 2017|title=First Large-scale Herbig-Haro Jet Driven by a Proto-brown Dwarf|journal= The Astrophysical Journal|language=en|volume=844|issue=1|pages=47|doi=10.3847/1538-4357/aa70e8|arxiv=1705.01170|bibcode=2017ApJ...844...47R|s2cid=119080074 |issn=0004-637X |doi-access=free }}</ref>

== Infrared counterparts ==
[[File:Herbig-Haro 49 50.jpg|thumb|HH 49/50 seen in infrared by the Spitzer Space Telescope]]
HH objects associated with very young stars or very massive protostars are often hidden from view at optical wavelengths by the cloud of gas and dust from which they form. The intervening material can diminish the visual magnitude by factors of tens or even hundreds at optical wavelengths. Such deeply embedded objects can only be observed at infrared or radio wavelengths,<ref name=Davis1995/> usually in the frequencies of hot molecular hydrogen or warm [[Chemistry:Carbon monoxide|carbon monoxide]] emission.<ref name=Giannini2006/> In recent years, infrared images have revealed dozens of examples of "infrared HH objects". Most look like bow waves (similar to the waves at the head of a ship), and so are usually referred to as molecular "bow shocks". The physics of infrared bow shocks can be understood in much the same way as that of HH objects, since these objects are essentially the same&nbsp;– supersonic shocks driven by collimated jets from the opposite poles of a protostar.<ref name="MHOcat"/> It is only the conditions in the jet and surrounding cloud that are different, causing infrared emission from molecules rather than optical emission from atoms and ions.<ref name=Smith2003/> 

In 2009 the acronym "MHO", for Molecular Hydrogen emission-line Object, was approved for such objects, detected in near infrared, by the [[Organization:International Astronomical Union|International Astronomical Union]] Working Group on Designations, and has been entered into their on-line Reference Dictionary of Nomenclature of Celestial Objects.<ref name="MHOcat"/> The MHO catalog contains over 2000 objects.

== Ultraviolet Herbig-Haro objects ==
HH objects have been observed in the ultraviolet spectrum.<ref>{{Citation |last=Böhm |first=Karl-Heinz |title=Herbig-Haro objects |date=1989 |url=http://link.springer.com/10.1007/BFb0114879 |work=Structure and Dynamics of the Interstellar medium |series=Lecture Notes in Physics |volume=350 |pages=282–294 |editor-last=Tenorio-Tagle |editor-first=Guillermo |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |language=en |doi=10.1007/bfb0114879 |isbn=978-3-540-51956-0 |s2cid=222245602 |access-date=2022-10-18 |editor2-last=Moles |editor2-first=Mariano |editor3-last=Melnick |editor3-first=Jorge|url-access=subscription }}</ref>

== See also ==
* [[Astronomy:Bipolar outflow|Bipolar outflow]]
* [[Astronomy:Protostar|Protostar]]
* [[Astronomy:Protoplanetary disk|Protoplanetary disk]]
{{Clear}}

== References ==
{{Reflist|colwidth=30em|refs=
<ref name=Burnham1890>{{cite journal |author=Burnham, S. W. |date=1890 |title=Note on Hind's Variable Nebula in Taurus |journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]] |volume=51 |issue=2 |pages=94–95 |bibcode=1890MNRAS..51...94B |doi=10.1093/mnras/51.2.94|doi-access=free }}</ref>

<ref name="Reipurth1997">{{cite conference |date=1997 |title=50 Years of Herbig–Haro Research. From discovery to HST |book-title=Herbig–Haro Flows and the Birth of Stars |conference=IAU Symposium No. 182 |editor1=Reipurth, B. |editor2=Bertout, C. |location=Dordrecht|publisher=Kluwer Academic Publishers |pages=3–18 |bibcode=1997IAUS..182....3R}}</ref>

<ref name="Caroll2014">{{cite book |last1=Carroll |first1=Bradley W. |last2=Ostlie |first2=Dale A. |date=2014 |title=An Introduction to Modern Astrophysics |location= Harlow |publisher=Pearson Education Limited |page=478 |isbn=978-1-292-02293-2}}</ref>

<ref name="Dina2000">{{cite book |author=Prialnik, D. |date=2000 |title=An Introduction to the Theory of Stellar Structure and Evolution |location=Cambridge, United Kingdom |publisher={{wipe|Cambridge University Press}} |pages=198–199 |isbn=978-0-521-65937-6}}</ref>

<ref name="JBally2016">{{cite journal |author=Bally, J. |date=September 2016 |title=Protostellar Outflows |journal=[[Physics:Annual Review of Astronomy and Astrophysics|Annual Review of Astronomy and Astrophysics]] |volume=54 |pages=491–528 |bibcode=2016ARA&A..54..491B |doi=10.1146/annurev-astro-081915-023341|doi-access=free }}</ref>

<ref name=Frank2014>{{cite book |author=Frank, A. |author2=Ray, T. P. |author3=Cabrit, S. |display-authors=et al. |editor=Beuther, S. |editor2=Klessen, R. S. |editor3=Dullemond, C. P. |editor4=Henning, T. |date=2014 |title=Protostars and Planets VI |chapter=Jets and Outflows from Star to Cloud: Observations Confront Theory |pages=451–474 |publisher=University of Arizona Press |location=Tucson |bibcode=2014prpl.conf..451F |doi=10.2458/azu_uapress_9780816531240-ch020 |arxiv=1402.3553 |isbn=9780816531240 |s2cid=118539135 }}</ref>

<ref name="J. Bally 1996">{{cite conference |date=1996 |title=The Birth of Stars: Herbig–Haro Jets, Accretion and Proto-Planetary Disks |book-title=Science with the Hubble Space Telescope&nbsp;– II |editor1=P. Benvenuti |editor2=F. D. Macchetto |editor3=E. J. Schreier |location=Baltimore|publisher=[[Organization:Space Telescope Science Institute|Space Telescope Science Institute]] |bibcode=1996swhs.conf..491B}} ([http://www.stsci.edu/stsci/meetings/shst2/ballyj.html HTML version])</ref>

<ref name="ReipurthBally2001">{{cite journal |author=Reipurth, B. |author2=Bally, J. |date=2001 |title=Herbig–Haro Flows: Probes of Early Stellar Evolution |journal=Annual Review of Astronomy and Astrophysics |volume=39 |issue=1–2 |pages=403–455 |bibcode=2001ARA&A..39..403R |doi=10.1146/annurev.astro.39.1.403}}</ref>

<ref name=Dopita1978>{{cite journal |author=Dopita, M. |date=February 1978 |title=The Herbig–Haro objects in the GUM Nebula |journal=Astronomy and Astrophysics |volume=63 |issue=1–2 |pages=237–241 |bibcode=1978A&A....63..237D}}</ref>

<ref name="Schwartz1983">{{cite journal |author=Schwartz, R. D. |date=1983 |title=Herbig–Haro Objects |journal=Annual Review of Astronomy and Astrophysics |volume=21 |pages=209–237 |bibcode=1983ARA&A..21..209S |doi=10.1146/annurev.aa.21.090183.001233}}</ref>

<ref name="Heathcote1998">{{cite journal |author=Heathcote, S. |author2=Reipurth, B. |author3=Raga, A. C. |date=July 1998 |title=Structure, Excitation, and Kinematics of the Luminous Herbig–Haro Objects 80/81 |journal=Astronomical Journal |volume=116 |issue=4 |pages=1940–1960 |bibcode=1998AJ....116.1940H |doi=10.1086/300548 |doi-access=free }}</ref>

<ref name="Hartigan2001">{{cite journal |author=Hartigan, P. |author2=Morse, J. |author3=Reipurth, B. |author4=Heathcote, S. |author5=Bally, J. |display-authors=3 |date=September 2001 |title=Proper Motions of the HH 111 Jet Observed with the Hubble Space Telescope |journal=Astrophysical Journal Letters |volume=559 |issue=2 |pages=L157–L161 |bibcode=2001ApJ...559L.157H |doi=10.1086/323976|doi-access=free }}</ref>

<ref name="Raga2016">{{cite journal |author=Raga, A. |author2=Reipurth, B. |author3=Velázquez, P. |author4=Esquivel, A. |author5=Bally, J. |display-authors=3 |date=December 2016 |title=The time evolution of HH 2 from four epochs of HST images |journal=Astronomical Journal |volume=152 |issue=6 |pages=186 |id=186 |arxiv=1610.01951 |bibcode=2016AJ....152..186R |doi=10.3847/0004-6256/152/6/186|s2cid=58923690 |doi-access=free }}</ref>

<ref name="Zealey1992">{{cite journal |author=Zealey, W. J. |date=1992 |title=Young Stellar Objects and Herbig–Haro Objects |journal=[[Physics:Australian Journal of Physics|Australian Journal of Physics]] |volume=45 |number=4 |pages=487–499 |bibcode=1992AuJPh..45..487Z |doi=10.1071/PH920487|doi-access=free }}</ref>

<ref name="Raga2017">{{cite journal |author=Raga, A. |author2=Cantó, J. |date=October 2017 |title=The formation of double working surfaces in periodically variable jets |journal=Revista Mexicana de Astronomía y Astrofísica |volume=53 |issue=2 |pages=219–225 |bibcode=2017RMxAA..53..219R}}</ref>

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<ref name="Hartigan1994">{{cite journal |author=Hartigan, P. |author2=Morse, J. A. |author3=Raymond, J. |date=November 1994 |title=Mass-loss rates, ionization fractions, shock velocities, and magnetic fields of stellar jets |journal=Astrophysical Journal |volume=436 |number=1 |pages=125–143 |bibcode=1994ApJ...436..125H |doi=10.1086/174887}}</ref>

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<ref name=Spitzeroutflow>{{cite web |url=http://www.spitzer.caltech.edu/images/1096-ssc2003-06g-Embedded-Outflow-in-HH-46-47 |date=December 18, 2003 |title=Embedded Outflow in HH 46/47 |website=NASA Spitzer Space Telescope |publisher=[[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]], California Institute of Technology |access-date=February 16, 2018 |archive-url=https://web.archive.org/web/20180217082427/http://www.spitzer.caltech.edu/images/1096-ssc2003-06g-Embedded-Outflow-in-HH-46-47 |archive-date=February 17, 2018 |url-status=live }}</ref>

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<ref name=EAA>{{cite encyclopedia|last=Raga|first=A. C.|editor-last=Murdin|editor-first=Paul|encyclopedia=Encyclopedia of Astronomy and Astrophysics|title=Herbig–Haro Objects and Exciting Stars|pages=1654–1657|edition=First|year=2001|publisher=[[Company:Nature Research|Nature Publishing Group]]|location=Hampshire|isbn=978-0333786536}}</ref>

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<ref name=Bally2007_2>{{cite journal |author=Bally, J. |date=October 2007 |title=Jets from young stars |journal=[[Physics:Astrophysics and Space Science|Astrophysics and Space Science]] |volume=311 |issue=1–3 |pages=15–24 |bibcode=2007Ap&SS.311...15B |doi=10.1007/s10509-007-9531-7|s2cid=55887210 }}</ref>

<!---unused<ref name=Cernicharo1996>{{cite journal |author=Cernicharo, J. |author2=Reipurth, B. |date=1996 |title=Herbig–Haro Jets, CO Flows, and CO Bullets: The Case of HH 111 |journal=Astrophysical Journal Letters |volume=460 |pages=L57 |bibcode=1996ApJ...460L..57C |doi=10.1086/309967}}</ref>--->

<!---unused<ref name=Reidel1985>{{cite conference |date=1987 |title=Star formation&nbsp;– From OB associations to protostars |book-title=Star forming regions |conference=Proceedings of the Symposium, Tokyo, Japan, Nov. 11–15, 1985 (A87-45601 20–90) |location=Dordrecht |publisher=D. Reidel Publishing Co. |pages=1–17 |bibcode=1987IAUS..115....1L}}</ref>--->
}}

== External links ==
* [http://vizier.u-strasbg.fr/viz-bin/VizieR-3?-source=V/104/catalog Catalogue of HH Objects at VizieR]
* [http://sparky.rice.edu/~hartigan/movies.html Animations of HH object jets from HST observations]
* [http://astro.kent.ac.uk/~df/MHCat/ A Catalogue of Molecular Hydrogen Emission-Line Objects in Outflows from Young Stars: MHO Catalogue]

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