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Nebula
Interstellar cloud of dust, hydrogen, helium and other ionized gases

A nebula is a luminous region within the interstellar medium composed of ionized or neutral hydrogen and cosmic dust, often serving as star-forming zones such as the Pillars of Creation in the Eagle Nebula. Nebulae gather matter to birth stars and form planetary systems. Some stretch hundreds of light-years across, like the Orion Nebula, visible to the naked eye from Earth. Though massive, nebulae are extremely diffuse, much less dense than Earth’s atmosphere. Historically, “nebula” also referred to distant galaxies such as the Andromeda Galaxy, before astronomers like Edwin Hubble revealed their true nature and helped classify nebulae by their light spectra.

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Observational history

Around 150 AD, Ptolemy recorded, in books VII–VIII of his Almagest, five stars that appeared nebulous. He also noted a region of nebulosity between the constellations Ursa Major and Leo that was not associated with any star.9 The first true nebula, as distinct from a star cluster, was mentioned by the Muslim Persian astronomer Abd al-Rahman al-Sufi in his Book of Fixed Stars (964).10 He noted "a little cloud" where the Andromeda Galaxy is located.11 He also cataloged the Omicron Velorum star cluster as a "nebulous star" and other nebulous objects, such as Brocchi's Cluster.12 The supernovas that created the Crab Nebula, SN 1054, was observed by Arabic and Chinese astronomers in 1054.1314

In 1610, Nicolas-Claude Fabri de Peiresc discovered the Orion Nebula using a telescope. This nebula was also observed by Johann Baptist Cysat in 1618. However, the first detailed study of the Orion Nebula was not performed until 1659 by Christiaan Huygens, who also believed he was the first person to discover this nebulosity.15

In 1715, Edmond Halley published a list of six nebulae.16 This number steadily increased during the century, with Jean-Philippe de Cheseaux compiling a list of 20 (including eight not previously known) in 1746. From 1751 to 1753, Nicolas-Louis de Lacaille cataloged 42 nebulae from the Cape of Good Hope, most of which were previously unknown. Charles Messier then compiled a catalog of 103 "nebulae" (now called Messier objects, which included what are now known to be galaxies) by 1781; his interest was detecting comets, and these were objects that might be mistaken for them.17

The number of nebulae was then greatly increased by the efforts of William Herschel and his sister, Caroline Herschel. Their Catalogue of One Thousand New Nebulae and Clusters of Stars18 was published in 1786. A second catalog of a thousand was published in 1789, and the third and final catalog of 510 appeared in 1802. During much of their work, William Herschel believed that these nebulae were merely unresolved clusters of stars. In 1790, however, he discovered a star surrounded by nebulosity and concluded that this was a true nebulosity rather than a more distant cluster.19

Beginning in 1864, William Huggins examined the spectra of about 70 nebulae. He found that roughly a third of them had the emission spectrum of a gas. The rest showed a continuous spectrum and were thus thought to consist of a mass of stars.2021 A third category was added in 1912 when Vesto Slipher showed that the spectrum of the nebula that surrounded the star Merope matched the spectra of the Pleiades open cluster. Thus, the nebula radiates by reflected star light.22

In 1923, following the Great Debate, it became clear that many "nebulae" were in fact galaxies far from the Milky Way.

Slipher and Edwin Hubble continued to collect the spectra from many different nebulae, finding 29 that showed emission spectra and 33 that had the continuous spectra of star light.23 In 1922, Hubble announced that nearly all nebulae are associated with stars and that their illumination comes from star light. He also discovered that the emission spectrum nebulae are nearly always associated with stars having spectral classifications of B or hotter (including all O-type main sequence stars), while nebulae with continuous spectra appear with cooler stars.24 Both Hubble and Henry Norris Russell concluded that the nebulae surrounding the hotter stars are transformed in some manner.25

Formation

There are a variety of formation mechanisms for the different types of nebulae. Some nebulae form from gas that is already in the interstellar medium while others are produced by stars. Examples of the former case are giant molecular clouds, the coldest, densest phase of interstellar gas, which can form by the cooling and condensation of more diffuse gas. Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of its stellar evolution.

Star-forming regions are a class of emission nebula associated with giant molecular clouds. These form as a molecular cloud collapses under its own weight, producing stars. Massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths. The region of ionized hydrogen surrounding the massive stars is known as an H II region while the shells of neutral hydrogen surrounding the H II region are known as photodissociation region. Examples of star-forming regions are the Orion Nebula, the Rosette Nebula and the Omega Nebula. Feedback from star-formation, in the form of supernova explosions of massive stars, stellar winds or ultraviolet radiation from massive stars, or outflows from low-mass stars may disrupt the cloud, destroying the nebula after several million years.

Other nebulae form as the result of supernova explosions; the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are then ionized by the energy and the compact object that its core produces. One of the best examples of this is the Crab Nebula, in Taurus. The supernova event was recorded in the year 1054 and is labeled SN 1054. The compact object that was created after the explosion lies in the center of the Crab Nebula and its core is now a neutron star.

Still other nebulae form as planetary nebulae. This is the final stage of a low-mass star's life, like Earth's Sun. Stars with a mass up to 8–10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost enough material, its temperature increases and the ultraviolet radiation it emits can ionize the surrounding nebula that it has thrown off. The Sun will produce a planetary nebula and its core will remain behind in the form of a white dwarf.

Types

Classical types

Objects named nebulae belong to four major groups. Before their nature was understood, galaxies ("spiral nebulae") and star clusters too distant to be resolved as stars were also classified as nebulae, but no longer are.

Not all cloud-like structures are nebulae; Herbig–Haro objects are an example.

Flux Nebulae

Main article: Integrated Flux Nebula

This section is an excerpt from Integrated Flux Nebula.[edit]

Integrated flux nebulae are a relatively recently identified astronomical phenomenon. In contrast to the typical and well known gaseous nebulae within the plane of the Milky Way galaxy, IFNs lie beyond the main body of the galaxy.

The term was coined by Steve Mandel who defined them as "high galactic latitude nebula that are illuminated not by a single star (as most nebula in the plane of the Galaxy are) but by the energy from the integrated flux of all the stars in the Milky Way. As a result, these nebulae are incredibly faint, taking hours of exposure to capture. These nebulae clouds, an important component of the interstellar medium, are composed of dust particles, hydrogen and carbon monoxide and some other elements."26 They are particularly prominent in the direction of both the north and south celestial poles. The vast nebula close to the south celestial pole is MW9, commonly known as the South Celestial Serpent.27

Diffuse nebulae

Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries.28 Diffuse nebulae can be divided into emission nebulae, reflection nebulae and dark nebulae.

Visible light nebulae may be divided into emission nebulae, which emit spectral line radiation from excited or ionized gas (mostly ionized hydrogen);29 they are often called H II regions, H II referring to ionized hydrogen), and reflection nebulae which are visible primarily due to the light they reflect.

Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them.30 Similar nebulae not illuminated by stars do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them; they are called dark nebulae.31

Although these nebulae have different visibility at optical wavelengths, they are all bright sources of infrared emission, chiefly from dust within the nebulae.32

Planetary nebulae

Main article: Planetary nebula

Planetary nebulae are the remnants of the final stages of stellar evolution for mid-mass stars (varying in size between 0.5-~8 solar masses). Evolved asymptotic giant branch stars expel their outer layers outwards due to strong stellar winds, thus forming gaseous shells while leaving behind the star's core in the form of a white dwarf.33 Radiation from the hot white dwarf excites the expelled gases, producing emission nebulae with spectra similar to those of emission nebulae found in star formation regions.34 They are H II regions, because mostly hydrogen is ionized, but planetary are denser and more compact than nebulae found in star formation regions.35

Planetary nebulae were given their name by the first astronomical observers who were initially unable to distinguish them from planets, which were of more interest to them. The Sun is expected to spawn a planetary nebula about 12 billion years after its formation.36

Protoplanetary nebulae

Main article: Protoplanetary nebula

This section is an excerpt from Protoplanetary nebula.[edit]

A protoplanetary nebula or preplanetary nebula37 (PPN, plural PPNe) is an astronomical object which is at the short-lived episode during a star's rapid evolution between the late asymptotic giant branch (LAGB)[a] phase and the subsequent planetary nebula (PN) phase. A PPN emits strongly in infrared radiation, and is a kind of reflection nebula. It is the second-from-the-last high-luminosity evolution phase in the life cycle of intermediate-mass stars (1–8 M☉).38: 469 

Supernova remnants

A supernova occurs when a high-mass star reaches the end of its life. When nuclear fusion in the core of the star stops, the star collapses. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode.39 The expanding shell of gas forms a supernova remnant, a special diffuse nebula.40 Although much of the optical and X-ray emission from supernova remnants originates from ionized gas, a great amount of the radio emission is a form of non-thermal emission called synchrotron emission.41 This emission originates from high-velocity electrons oscillating within magnetic fields.

Examples

Catalogs

See also

References

  1. Harper, Douglas. "nebula". Online Etymology Dictionary. https://www.etymonline.com/?term=nebula ↩

  2. American Heritage Dictionary of the English Language, Fifth Edition. S.v. "nebula." Retrieved November 23, 2019, via https://thefreedictionary.com/nebula https://thefreedictionary.com/nebula ↩

  3. Collins English Dictionary – Complete and Unabridged, 12th Edition 2014. S.v. "nebula." Retrieved November 23, 2019, via https://thefreedictionary.com/nebula https://thefreedictionary.com/nebula ↩

  4. Random House Kernerman Webster's College Dictionary. S.v. "nebula." Retrieved November 23, 2019, via https://thefreedictionary.com/nebula https://thefreedictionary.com/nebula ↩

  5. The American Heritage Dictionary of Student Science, Second Edition. S.v. "nebula." Retrieved November 23, 2019, via https://thefreedictionary.com/nebula https://thefreedictionary.com/nebula ↩

  6. Howell, Elizabeth (2013-02-22). "In Reality, Nebulae Offer No Place for Spaceships to Hide". Universe Today. http://www.universetoday.com/99989/in-reality-nebulae-offer-no-place-for-spaceships-to-hide/ ↩

  7. Clark, Roger N. (1990). Visual astronomy of the deep sky. Cambridge University Press. p. 98. ISBN 9780521361552. 9780521361552 ↩

  8. "What is a nebula?". Space Center Houston. March 19, 2020. Retrieved June 27, 2021. https://spacecenter.org/what-is-a-nebula/ ↩

  9. Kunitzsch, P. (1987), "A Medieval Reference to the Andromeda Nebula" (PDF), ESO Messenger, 49: 42–43, Bibcode:1987Msngr..49...42K, retrieved 2009-10-31 http://www.eso.org/sci/publications/messenger/archive/no.49-sep87/messenger-no49-42-43.pdf ↩

  10. Jones, Kenneth Glyn (1991). Messier's nebulae and star clusters. Cambridge University Press. p. 1. ISBN 0-521-37079-5. 0-521-37079-5 ↩

  11. Harrison, T. G. (March 1984). "The Orion Nebula – where in History is it". Quarterly Journal of the Royal Astronomical Society. 25 (1): 70–73. Bibcode:1984QJRAS..25...65H. /wiki/Quarterly_Journal_of_the_Royal_Astronomical_Society ↩

  12. Jones, Kenneth Glyn (1991). Messier's nebulae and star clusters. Cambridge University Press. p. 1. ISBN 0-521-37079-5. 0-521-37079-5 ↩

  13. Lundmark, K (1921). "Suspected New Stars Recorded in the Old Chronicles and Among Recent Meridian Observations". Publications of the Astronomical Society of the Pacific. 33 (195): 225. Bibcode:1921PASP...33..225L. doi:10.1086/123101. https://doi.org/10.1086%2F123101 ↩

  14. Mayall, N.U. (1939). "The Crab Nebula, a Probable Supernova". Astronomical Society of the Pacific Leaflets. 3 (119): 145. Bibcode:1939ASPL....3..145M. http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1939ASPL....3..145M&link_type=ARTICLE&db_key=AST ↩

  15. Harrison, T. G. (March 1984). "The Orion Nebula – where in History is it". Quarterly Journal of the Royal Astronomical Society. 25 (1): 70–73. Bibcode:1984QJRAS..25...65H. /wiki/Quarterly_Journal_of_the_Royal_Astronomical_Society ↩

  16. Halley, E. (1714–1716). "An account of several nebulae or lucid spots like clouds, lately discovered among the fixed stars by help of the telescope". Philosophical Transactions. XXXIX: 390–92. /wiki/Philosophical_Transactions ↩

  17. Hoskin, Michael (2005). "Unfinished Business: William Herschel's Sweeps for Nebulae". British Journal for the History of Science. 43 (3): 305–320. Bibcode:2005HisSc..43..305H. doi:10.1177/007327530504300303. S2CID 161558679. /wiki/British_Journal_for_the_History_of_Science ↩

  18. Philosophical Transactions. T.N. 1786. p. 457. https://archive.org/details/philosophicaltr04unkngoog ↩

  19. Hoskin, Michael (2005). "Unfinished Business: William Herschel's Sweeps for Nebulae". British Journal for the History of Science. 43 (3): 305–320. Bibcode:2005HisSc..43..305H. doi:10.1177/007327530504300303. S2CID 161558679. /wiki/British_Journal_for_the_History_of_Science ↩

  20. Watts, William Marshall; Huggins, Sir William; Lady Huggins (1904). An introduction to the study of spectrum analysis. Longmans, Green, and Co. pp. 84–85. Retrieved 2009-10-31. https://archive.org/details/anintroductiont00hugggoog ↩

  21. Struve, Otto (1937). "Recent Progress in the Study of Reflection Nebulae". Popular Astronomy. 45: 9–22. Bibcode:1937PA.....45....9S. /wiki/Bibcode_(identifier) ↩

  22. Slipher, V. M. (1912). "On the spectrum of the nebula in the Pleiades". Lowell Observatory Bulletin. 1: 26–27. Bibcode:1912LowOB...2...26S. /wiki/Bibcode_(identifier) ↩

  23. Struve, Otto (1937). "Recent Progress in the Study of Reflection Nebulae". Popular Astronomy. 45: 9–22. Bibcode:1937PA.....45....9S. /wiki/Bibcode_(identifier) ↩

  24. Hubble, E. P. (December 1922). "The source of luminosity in galactic nebulae". Astrophysical Journal. 56: 400–438. Bibcode:1922ApJ....56..400H. doi:10.1086/142713. https://doi.org/10.1086%2F142713 ↩

  25. Struve, Otto (1937). "Recent Progress in the Study of Reflection Nebulae". Popular Astronomy. 45: 9–22. Bibcode:1937PA.....45....9S. /wiki/Bibcode_(identifier) ↩

  26. http://www.aicccd.com/archive/aic2005/The_unexplored_nebula_project-smandel.pdf http://www.aicccd.com/archive/aic2005/The_unexplored_nebula_project-smandel.pdf ↩

  27. Chadwick, Stephen; Cooper, Ian (11 December 2012). Imaging the Southern Sky. Springer. p. 248. ISBN 978-1461447498. 978-1461447498 ↩

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  37. Sahai, Raghvendra; Sánchez Contreras, Carmen; Morris, Mark (2005). "A Starfish Preplanetary Nebula: IRAS 19024+0044" (PDF). The Astrophysical Journal. 620 (2): 948–960. Bibcode:2005ApJ...620..948S. doi:10.1086/426469. https://trs.jpl.nasa.gov/bitstream/2014/40924/1/04-3294.pdf ↩

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