Galactic habitable zone

In astrobiology and planetary astrophysics, the galactic habitable zone is the region of a galaxy in which life might most likely develop. More specifically, the concept of a galactic habitable zone incorporates various factors, such as metallicity and the rate of major catastrophes such as supernovae, to calculate which regions of the galaxy are more likely to form terrestrial planets, initially develop simple life, and provide a suitable environment for this life to evolve and advance.[1] According to research published in August 2015, very large galaxies may favor the birth and development of habitable planets more than smaller galaxies such as the Milky Way.[2] In the case of the Milky Way, its galactic habitable zone is commonly believed to be an annulus with an outer radius of about 10 kiloparsecs and an inner radius close to the Galactic Center (with both radii lacking hard boundaries).[1][3]

Galactic habitable-zone theory has been criticized due to an inability to quantify accurately the factors making a region of a galaxy favorable for the emergence of life.[3] In addition, computer simulations suggest that stars may change their orbits around the galactic center significantly, therefore challenging at least part of the view that some galactic areas are necessarily more life-supporting than others.[4][5][6]

Background

The idea of the circumstellar habitable zone was introduced in 1953 by Hubertus Strughold and Harlow Shapley[7][8] and in 1959 by Su-Shu Huang[9] as the region around a star in which an orbiting planet could retain water at its surface. From the 1970s, planetary scientists and astrobiologists began to consider various other factors required for the creation and sustenance of life, including the impact that a nearby supernova may have on life's development.[10] In 1981, Jim Clarke proposed that the apparent lack of extraterrestrial civilizations in the Milky Way could be explained by Seyfert-type outbursts from an active galactic nucleus, with Earth alone being spared from this radiation by virtue of its location in the galaxy.[11] In the same year, Wallace Hampton Tucker analyzed galactic habitability in a more general context, but later work superseded his proposals.[12]

Modern galactic habitable-zone theory was introduced in 1986 by L.S. Marochnik and L.M. Mukhin, who defined the zone as the region in which intelligent life could flourish.[13] Donald Brownlee and palaeontologist Peter Ward expanded upon the concept of a galactic habitable zone, as well as the other factors required for the emergence of complex life, in their 2000 book Rare Earth: Why Complex Life is Uncommon in the Universe.[14] In that book, the authors used the galactic habitable zone, among other factors, to argue that intelligent life is not a common occurrence in the Universe.

The idea of a galactic habitable zone was further developed in 2001 in a paper by Ward and Brownlee, in collaboration with Guillermo Gonzalez of the University of Washington.[15][16] In that paper, Gonzalez, Brownlee, and Ward stated that regions near the galactic halo would lack the heavier elements required to produce habitable terrestrial planets, thus creating an outward limit to the size of the galactic habitable zone.[10] Being too close to the galactic center, however, would expose an otherwise habitable planet to numerous supernovae and other energetic cosmic events, as well as excessive cometary impacts caused by perturbations of the host star's Oort cloud. Therefore, the authors established an inner boundary for the galactic habitable zone, located just outside the galactic bulge.[10]

Considerations

In order to identify a location in the galaxy as being a part of the galactic habitable zone, a variety of factors must be accounted for. These include the distribution of stars and spiral arms, the presence or absence of an active galactic nucleus, the frequency of nearby supernovae that can threaten the existence of life, the metallicity of that location, and other factors.[10] Without fulfilling these factors, a region of the galaxy cannot create or sustain life with efficiency.

Chemical evolution

Starpop
The metallicity of the thin galactic disk is far greater than that of the outlying galactic halo.

One of the most basic requirements for the existence of life around a star is the ability of that star to produce a terrestrial planet of sufficient mass to sustain it. Various elements, such as iron, magnesium, titanium, carbon, oxygen, silicon, and others, are required to produce habitable planets, and the concentration and ratios of these vary throughout the galaxy.[10]

One important elemental ratio is that of [Fe/H], one of the factors determining the propensity of a region of the galaxy to produce terrestrial planets. The galactic bulge, the region of the galaxy closest to the galactic center, has an [Fe/H] distribution peaking at −0.2 decimal exponent units (dex) relative to the Sun's ratio; the thin disk, where the Sun is located, has an average metallicity of −0.02 dex at the orbital distance of the Sun around the galactic center, reducing by 0.07 dex for every additional kiloparsec of orbital distance. The extended thick disk has an average [Fe/H] of −0.6 dex, while the halo, the region farthest from the galactic center, has the lowest [Fe/H] distribution peak, at around −1.5 dex.[10] In addition, ratios such as [C/O], [Mg/Fe], [Si/Fe], and [S/Fe] may be relevant to the ability of a region of a galaxy to form habitable terrestrial planets, and of these [Mg/Fe] and [Si/Fe] are slowly reducing over time, meaning that future terrestrial planets are more likely to possess larger iron cores.[10]

In addition to specific amounts of the various stable elements that comprise a terrestrial planet's mass, an abundance of radionuclides such as 40K, 235U, 238U, and 232Th is required in order to heat the planet's interior and power life-sustaining processes such as plate tectonics, volcanism, and a geomagnetic dynamo.[10] The [U/H] and [Th/H] ratios are dependent on the [Fe/H] ratio; however, a general function for the abundance of 40K cannot be created with existing data.[10]

Even on a habitable planet with enough radioisotopes to heat its interior, various prebiotic molecules are required in order to produce life; therefore, the distribution of these molecules in the galaxy is important in determining the galactic habitable zone.[13] A 2008 study by Samantha Blair and colleagues attempted to determine the outer edge of the galactic habitable zone by means of analyzing formaldehyde and carbon monoxide emissions from various giant molecular clouds scattered throughout the Milky Way; however, the data is neither conclusive nor complete.

While high metallicity is beneficial for the creation of terrestrial extrasolar planets, an excess amount can be harmful for life. Excess metallicity may lead to the formation of a large number of gas giants in a given system, which may subsequently migrate from beyond the system's frost line and become hot Jupiters, disturbing planets that would otherwise have been located in the system's circumstellar habitable zone.[17] Thus, it was found that the Goldilocks principle applies to metallicity as well; low-metallicity systems have low probabilities of forming terrestrial-mass planets at all, while excessive metallicities cause a large number of gas giants to develop, disrupting the orbital dynamics of the system and altering the habitability of terrestrial planets in the system.

Catastrophic events

Crab Nebula
The impact of supernovae on the extent of the galactic habitable zone has been extensively studied.

As well as being located in a region of the galaxy that is chemically advantageous for the development of life, a star must also avoid an excessive number of catastrophic cosmic events with the potential to damage life on its otherwise habitable planets.[17] Nearby supernovae, for example, have the potential to severely harm life on a planet; with excessive frequency, such catastrophic outbursts have the potential to sterilize an entire region of a galaxy for billions of years. The galactic bulge, for example, experienced an initial wave of extremely rapid star formation,[10] triggering a cascade of supernovae that for five billion years left that area almost completely unable to develop life.

In addition to supernovae, gamma-ray bursts,[18] excessive amounts of radiation, gravitational perturbations[17] and various other events have been proposed to affect the distribution of life within the galaxy. These include, controversially, such proposals as "galactic tides" with the potential to induce cometary impacts or even cold bodies of dark matter[18] that pass through organisms and induce genetic mutations.[19] However, the impact of many of these events may be difficult to quantify.[17]

Galactic morphology

Various morphological features of galaxies can affect their potential for habitability. Spiral arms, for example, are the location of star formation, but they contain numerous giant molecular clouds and a high density of stars that can perturb a star's Oort cloud, sending avalanches of comets and asteroids toward any planets further in.[20] In addition, the high density of stars and rate of massive star formation can expose any stars orbiting within the spiral arms for too long to supernova explosions, reducing their prospects for the survival and development of life.[20] Considering these factors, the Sun is advantageously placed within the galaxy because, in addition to being outside a spiral arm, it orbits near the corotation radius, maximizing the interval between spiral-arm crossings.[20][21]

Spiral arms also have the ability to cause climatic changes on a planet. Passing through the dense molecular clouds of galactic spiral arms, stellar winds may be pushed back to the point that a reflective hydrogen layer accumulates in an orbiting planet's atmosphere, perhaps leading to a snowball Earth scenario.[6][22]

A galactic bar also has the potential to affect the size of the galactic habitable zone. Galactic bars are thought to grow over time, eventually reaching the corotation radius of the galaxy and perturbing the orbits of the stars located there.[21] High-metallicity stars like our Sun, for example, located at an intermediate location between the low-metallicity galactic halo and the high-radiation galactic center, may be scattered throughout the galaxy, affecting the definition of the galactic habitable zone. It has been suggested that for this reason, it may be impossible to properly define a galactic habitable zone.[21]

Boundaries

Milky Way galactic habitable zone
The galactic habitable zone is often viewed as an annulus 4–10 kpc from the galactic center, shown in green here, though recent research has called this into question.

Early research on the galactic habitable zone, including the 2001 paper by Gonzalez, Brownlee, and Ward, did not demarcate any specific boundaries, merely stating that the zone was an annulus encompassing a region of the galaxy that was both enriched with metals and spared from excessive radiation, and that habitability would be more likely in the galaxy's thin disk.[10] However, later research conducted in 2004 by Lineweaver and colleagues did create boundaries for this annulus, in the case of the Milky Way ranging from 4 kpc to 10 kpc from the galactic center.

The Lineweaver team also analyzed the evolution of the galactic habitable zone with respect to time, finding, for example, that stars close to the galactic bulge had to form within a time window of about two billion years in order to have habitable planets.[17] Before that window, galactic-bulge stars would be prevented from having life-sustaining planets from frequent supernova events. After the supernova threat had subsided, though, the increasing metallicity of the galactic core would eventually mean that stars there would have a high number of giant planets, with the potential to destabilize star systems and radically alter the orbit of any planet located in a star's circumstellar habitable zone.[17] Simulations conducted in 2005 at the University of Washington, however, show that even in the presence of hot Jupiters, terrestrial planets may remain stable over long timescales.[23]

A 2006 study by Milan Ćirković and colleagues extended the notion of a time-dependent galactic habitable zone, analyzing various catastrophic events as well as the underlying secular evolution of galactic dynamics.[18] The paper considers that the number of habitable planets may fluctuate wildly with time due to the unpredictable timing of catastrophic events, thereby creating a punctuated equilibrium in which habitable planets are more likely at some times than at others.[18] Based on the results of Monte Carlo simulations on a toy model of the Milky Way, the team found that the number of habitable planets is likely to increase with time, though not in a perfectly linear pattern.[18]

Subsequent studies saw more fundamental revision of the old concept of the galactic habitable zone as an annulus. In 2008, a study by Nikos Prantzos revealed that, while the probability of a planet escaping sterilization by supernova was highest at a distance of about 10 kpc from the galactic center, the sheer density of stars in the inner galaxy meant that the highest number of habitable planets could be found there.[3] The research was corroborated in a 2011 paper by Michael Gowanlock, who calculated the frequency of supernova-surviving planets as a function of their distance from the galactic center, their height above the galactic plane, and their age, ultimately discovering that about 0.3% of stars in the galaxy could today support complex life, or 1.2% if one does not consider the tidal locking of red dwarf planets as precluding the development of complex life.[1]

Criticism

The idea of the galactic habitable zone has been criticized by Nikos Prantzos, on the grounds that the parameters to create it are impossible to define even approximately, and that thus the galactic habitable zone may merely be a useful conceptual tool to enable a better understanding of the distribution of life, rather than an end to itself.[3] For these reasons, Prantzos has suggested that the entire galaxy may be habitable, rather than habitability being restricted to a specific region in space and time.[3] In addition, stars "riding" the galaxy's spiral arms may move tens of thousands of light years from their original orbits, thus supporting the notion that there may not be one specific galactic habitable zone.[4][5][6] A Monte Carlo simulation, improving on the mechanisms used by Ćirković in 2006, was conducted in 2010 by Duncan Forgan of Royal Observatory Edinburgh. The data collected from the experiments support Prantzos's notion that there is no solidly defined galactic habitable zone, indicating the possibility of hundreds of extraterrestrial civilizations in the Milky Way, though further data will be required in order for a definitive determination to be made.[24]

See also

References

  1. ^ a b c Gowanlock, M. G.; Patton, D. R.; McConnell, S. M. (2011). "A Model of Habitability Within the Milky Way Galaxy". Astrobiology. 11 (9): 855–873. arXiv:1107.1286. Bibcode:2011AsBio..11..855G. doi:10.1089/ast.2010.0555. PMID 22059554.
  2. ^ Choi, Charles Q. (21 August 2015). "Giant Galaxies May Be Better Cradles for Habitable Planets". Space.com. Retrieved 24 August 2015.
  3. ^ a b c d e Prantzos, Nikos (2006). "On the "Galactic Habitable Zone"". Space Science Reviews. 135 (1–4): 313–322. arXiv:astro-ph/0612316. Bibcode:2008SSRv..135..313P. doi:10.1007/s11214-007-9236-9.
  4. ^ a b Rok Roškar; Debattista; Quinn; Stinson; James Wadsley (2008). "Riding the Spiral Waves: Implications of Stellar Migration for the Properties of Galactic Disks". The Astrophysical Journal. 684 (2): L79. arXiv:0808.0206. Bibcode:2008ApJ...684L..79R. doi:10.1086/592231.
  5. ^ a b "Immigrant Sun: Our Star Could be Far from Where It Started in Milky Way". Newswise, Retrieved on September 15, 2008.
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  7. ^ Strughold, Hubertus (1953). The Green and Red Planet: A Physiological Study of the Possibility of Life on Mars. University of New Mexico Press.
  8. ^ James Kasting (2010). How to Find a Habitable Planet. Princeton University Press. p. 127. ISBN 978-0-691-13805-3. Retrieved 4 May 2013.
  9. ^ Huang, Su-Shu (April 1960). "Life-Supporting Regions in the Vicinity of Binary Systems". Publications of the Astronomical Society of the Pacific. 72 (425): 106–114. Bibcode:1960PASP...72..106H. doi:10.1086/127489.
  10. ^ a b c d e f g h i j k Gonzalez, Guillermo; Brownlee, Donald; Peter, Ward (2001). "The Galactic Habitable Zone: Galactic Chemical Evolution". Icarus. 152 (1): 185. arXiv:astro-ph/0103165. Bibcode:2001Icar..152..185G. doi:10.1006/icar.2001.6617.
  11. ^ Clarke, J. N. (1981). "Extraterrestrial intelligence and galactic nuclear activity". Icarus. 46 (1): 94–96. Bibcode:1981Icar...46...94C. doi:10.1016/0019-1035(81)90078-6.
  12. ^ Tucker, Wallace H. (1981). Billingham, John, ed. Life in the Universe. Cambridge: The MIT Press. pp. 187–296. ISBN 9780262520621.
  13. ^ a b Blair, S. K.; Magnani, L.; Brand, J.; Wouterloot, J. G. A. (2008). "Formaldehyde in the Far Outer Galaxy: Constraining the Outer Boundary of the Galactic Habitable Zone". Astrobiology. 8 (1): 59–73. Bibcode:2008AsBio...8...59B. doi:10.1089/ast.2007.0171. PMID 18266563.
  14. ^ Ward, Peter; Brownlee, Donald (2003-12-10). Rare Earth: Why Complex Life is Uncommon in the Universe. Springer. pp. 191–220. ISBN 9780387952895.
  15. ^ Gonzalez, G (2001). "The Galactic Habitable Zone: Galactic Chemical Evolution". Icarus. 152: 185–200. arXiv:astro-ph/0103165. Bibcode:2001Icar..152..185G. doi:10.1006/icar.2001.6617.
  16. ^ Charles H. Lineweaver, Yeshe Fenner and Brad K. Gibson (January 2004). "The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way". Science. 303 (5654): 59–62. arXiv:astro-ph/0401024. Bibcode:2004Sci...303...59L. doi:10.1126/science.1092322. PMID 14704421.
  17. ^ a b c d e f Lineweaver, C. H.; Fenner, Y.; Gibson, B. K. (2004). "The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way". Science. 303 (5654): 59–62. arXiv:astro-ph/0401024. Bibcode:2004Sci...303...59L. doi:10.1126/science.1092322. PMID 14704421.
  18. ^ a b c d e Vukotic, B.; Cirkovic, M. M. (2007). "On the timescale forcing in astrobiology". Serbian Astronomical Journal. 175 (175): 45. arXiv:0712.1508. Bibcode:2007SerAJ.175...45V. doi:10.2298/SAJ0775045V.
  19. ^ Collar, J. I. (1996). "Clumpy Cold Dark Matter and biological extinctions". Physics Letters B. 368 (4): 266–269. arXiv:astro-ph/9512054. Bibcode:1996PhLB..368..266C. doi:10.1016/0370-2693(95)01469-1.
  20. ^ a b c Mullen, Leslie (May 18, 2001). "Galactic Habitable Zones". NAI Features Archive. Nasa Astrobiology Institute. Archived from the original on April 9, 2013. Retrieved May 9, 2013.
  21. ^ a b c Sundin, M. (2006). "The galactic habitable zone in barred galaxies". International Journal of Astrobiology. 5 (4): 325. Bibcode:2006IJAsB...5..325S. doi:10.1017/S1473550406003065.
  22. ^ Pavlov, Alexander A. (2005). "Passing through a giant molecular cloud: "Snowball" glaciations produced by interstellar dust". Geophysical Research Letters. 32 (3): L03705. Bibcode:2005GeoRL..32.3705P. doi:10.1029/2004GL021890.
  23. ^ Raymond, Sean N.; Quinn, Thomas; Lunine, Jonathan I. (2005). "The formation and habitability of terrestrial planets in the presence of close-in giant planets". Icarus. 177 (1): 256–263. arXiv:astro-ph/0407620. Bibcode:2005Icar..177..256R. doi:10.1016/j.icarus.2005.03.008.
  24. ^ Forgan, D. H. (2009). "A numerical testbed for hypotheses of extraterrestrial life and intelligence". International Journal of Astrobiology. 8 (2): 121. arXiv:0810.2222. Bibcode:2009IJAsB...8..121F. doi:10.1017/S1473550408004321.

External links

Media related to Habitable zone at Wikimedia Commons

Alien Planet

Alien Planet is a 94-minute docufiction, originally airing on the Discovery Channel, about two internationally built robot probes searching for alien life on the fictional planet Darwin IV. It was based on the book Expedition, by sci-fi/fantasy artist and writer Wayne Douglas Barlowe, who was also executive producer on the special. It premiered on May 14, 2005.

The show uses computer-generated imagery, which is interspersed with interviews from such notables as Stephen Hawking, George Lucas, Michio Kaku and Jack Horner. The show was filmed in Iceland and Mono Lake in California.

Andrew Siemion

Andrew Patrick Vincent Siemion is an astrophysicist and director of the Berkeley SETI Research Center. His research interests include high energy time-variable celestial phenomena, astronomical instrumentation and the search for extraterrestrial intelligence (SETI).. Andrew Siemion is the Principal Investigator for the Breakthrough Listen program.Siemion received his B.A. (2008) M.A. (2010) and Ph.D. (2012) in astrophysics from the University of California, Berkeley. In 2018, Siemion was named the Bernard M. Oliver Chair for SETI at the SETI Institute. Siemion is jointly affiliated with Radboud University Nijmegen and the University of Malta. Also in 2018, he was elected to the International Academy of Astronautics as a Corresponding Member for Basic Sciences. In September 2015, Siemion testified on the current status of astrobiology to the House Committee on Science, Space, and Technology of the United States Congress.

Circumstellar habitable zone

In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The bounds of the CHZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the CHZ and the objects within it may be instrumental in determining the scope and distribution of Earth-like extraterrestrial life and intelligence.

The habitable zone is also called the Goldilocks zone, a metaphor of the children's fairy tale of "Goldilocks and the Three Bears", in which a little girl chooses from sets of three items, ignoring the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right".

Since the concept was first presented in 1953, many stars have been confirmed to possess a CHZ planet, including some systems that consist of multiple CHZ planets. Most such planets, being super-Earths or gas giants, are more massive than Earth, because such planets are easier to detect. On November 4, 2013, astronomers reported, based on Kepler data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way. 11 billion of these may be orbiting Sun-like stars. Proxima Centauri b, located about 4.2 light-years (1.3 parsecs) from Earth in the constellation of Centaurus, is the nearest known exoplanet, and is orbiting in the habitable zone of its star. The CHZ is also of particular interest to the emerging field of habitability of natural satellites, because planetary-mass moons in the CHZ might outnumber planets.In subsequent decades, the CHZ concept began to be challenged as a primary criterion for life, so the concept is still evolving. Since the discovery of evidence for extraterrestrial liquid water, substantial quantities of it are now thought to occur outside the circumstellar habitable zone. The concept of deep biospheres, like Earth's, that exist independently of stellar energy, are now generally accepted in astrobiology given the large amount of liquid water known to exist within in lithospheres and asthenospheres of the Solar System. Sustained by other energy sources, such as tidal heating or radioactive decay or pressurized by non-atmospheric means, liquid water may be found even on rogue planets, or their moons. Liquid water can also exist at a wider range of temperatures and pressures as a solution, for example with sodium chlorides in seawater on Earth, chlorides and sulphates on equatorial Mars, or ammoniates, due to its different colligative properties. In addition, other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at the surface, have been proposed.

Coma Filament

Coma Filament is a galaxy filament. The filament contains the Coma Supercluster of galaxies and forms a part of the CfA2 Great Wall.

Drake equation

The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy.The equation was written in 1961 by Frank Drake, not for purposes of quantifying the number of civilizations, but as a way to stimulate scientific dialogue at the first scientific meeting on the search for extraterrestrial intelligence (SETI). The equation summarizes the main concepts which scientists must contemplate when considering the question of other radio-communicative life. It is more properly thought of as a Fermi problem rather than as a serious attempt to nail down a precise number.

Criticism related to the Drake equation focuses not on the equation itself, but on the fact that the estimated values for several of its factors are highly conjectural, the combined effect being that the uncertainty associated with any derived value is so large that the equation cannot be used to draw firm conclusions.

Extraterrestrial (TV program)

Extraterrestrial (also Alien Worlds in the UK) is a British-American two-part television documentary miniseries, aired in 2005 in the UK by Channel 4, by the National Geographic Channel (as Extraterrestrial) in the US on Monday, May 30, 2005 and produced by Blue Wave Productions Ltd. The program focuses on the hypothetical and scientifically feasible evolution of alien life on extrasolar planets, providing model examples of two different fictional worlds, one in each of the series's two episodes.The documentary is based on speculative collaboration of a group of American and British scientists, who were collectively commissioned by National Geographic. For the purposes of the documentary, the team of scientists divides two hypothetical examples of realistic worlds on which extraterrestrial life could evolve: A tidally locked planet orbiting a red dwarf star (dubbed "Aurelia") and a large moon (dubbed "Blue Moon") orbiting a gas giant in a binary star system. The scientific team of the series used a combination of accretion theory, climatology, and xenobiology to imagine the most likely locations for extraterrestrial life and most probable evolutionary path such life would take.The "Aurelia" and "Blue Moon" concepts seen in the series were also featured in the touring exhibition The Science of Aliens.

The show's concept shares basic similarities with The Future is Wild. Both series depict imaginary but scientifically-plausible ecosystems and the species that inhabit them, with commentary by scientists. The key difference is that in The Future is Wild the ecosystems represent the possible future evolution of life on planet Earth, while in Extraterrestrial they are designed from scratch based on possible conditions on extrasolar planets.

Guillermo Gonzalez (astronomer)

Guillermo Gonzalez (born 1963 in Havana, Cuba) is an astrophysicist, a proponent of the principle of intelligent design, and an assistant professor at Ball State University, a public research university, in Muncie, Indiana. He is a senior fellow of the Discovery Institute's Center for Science and Culture, considered the hub of the intelligent design movement, and a fellow with the International Society for Complexity, Information and Design, which also promotes intelligent design.

Lynx–Ursa Major Filament

Lynx–Ursa Major Filament (LUM Filament) is a galaxy filament.The filament is connected to and separate from the Lynx–Ursa Major Supercluster.

NGC 17

NGC 17, also known as NGC 34, is a spiral galaxy in the constellation Cetus. It is the result of a merger between two disk galaxies, resulting in a recent starburst in the central regions and continuing starforming activity. The galaxy is still gas-rich, and has a single galactic nucleus. It lies 250 million light years away. It was discovered in 1886 by Frank Muller and then observed again later that year by Lewis Swift.

Due to the major merger event NGC 17 has no defined spiral arms like the Milky Way galaxy. Unlike the Milky Way, the center bar nucleus is also distorted. The merger destroyed any galactic habitable zone that may have been there before the merger. For the Milky Way, the galactic habitable zone is commonly believed to be an annulus with an outer radius of about 10 kiloparsecs and an inner radius close to the Galactic Center, both of which lack hard boundaries.

NGC 45

NGC 45 is a low surface brightness spiral galaxy in the constellation of Cetus. It was discovered on 11 November 1835 by the English astronomer John Herschel.

Unlike the Milky Way, NGC 45 has no clear defined spiral arms, and its center bar nucleus is also very small and distorted. NGC 45 thus does not have a galactic habitable zone. For the Milky Way, the galactic habitable zone is commonly believed to be an annulus with an outer radius of about 10 kiloparsecs and an inner radius close to the Galactic Center, both of which lack hard boundaries.

NGC 5964

NGC 5964 is a barred spiral galaxy in the constellation Serpens Caput. NGC 5964 is also known by the names IC 4551 and PGC 55637.

NGC 5964 has relatively unwound spiral arms; it lacks the clear defined spiral arms the Milky Way galaxy has. The central bar is very small, long and thin. NGC 5964 thus does not have a galactic habitable zone like the Milky Way. For the Milky Way, the galactic habitable zone is commonly believed to be an annulus with an outer radius of about 10 kiloparsecs and an inner radius close to the Galactic Center, both of which lack hard boundaries.

NGC 6118

NGC 6118 is a grand design spiral galaxy located 83 million light-years away in the constellation Serpens (the Snake). It measures roughly 110,000 light-years across; about the same as our own galaxy, the Milky Way. Its shape is classified as "SA(s)cd," meaning that it is unbarred and has several rather loosely wound spiral arms. The large numbers of bright bluish knots are active star-forming regions where some very luminous and young stars can be perceived.Because NGC 6118 has loosely wound spiral open arms, no clear defined spiral arms like the Milky Way galaxy and lacks a central bar, the galaxy thus does not have a galactic habitable zone like the Milky Way. For the Milky Way, the galactic habitable zone is commonly believed to be an annulus with an outer radius of about 10 kiloparsecs and an inner radius close to the Galactic Center, both of which lack hard boundaries.NGC 6118 is difficult to see with a small telescope. Amateur astronomers have nicknamed it the "Blinking Galaxy", as it has a tendency to flick in and out of view with different eye positions.

Neocatastrophism

Neocatastrophism is the hypothesis that life-exterminating events such as gamma-ray bursts have acted as a galactic regulation mechanism in the Milky Way upon the emergence of complex life in its habitable zone. It is proposed as an explanation of Fermi's paradox since it provides a mechanism which would have delayed the otherwise expected advent of intelligent beings in the local galaxy nearby to Earth. This is an avenue to explain why none so far have been detected by humans.

Perseus–Pegasus Filament

Perseus–Pegasus Filament is a galaxy filament containing the Perseus-Pisces Supercluster and stretching for roughly a billion light years (or over 300/h Mpc). Currently, it is considered to be one of the largest known structures in the universe. This filament is adjacent to the Pisces–Cetus Supercluster Complex.

Postbiological evolution

Postbiological evolution is a form of evolution which has transitioned from a biological paradigm, driven by the propagation of genes, to a nonbiological (e.g., cultural or technological) paradigm, presumably driven by some alternative replicator (e.g., memes or temes), and potentially resulting in the extinction, obsolescence, or trophic reorganization of the former. Researchers anticipating a postbiological universe tend to describe this transition as marked by the maturation and potential convergence of high technologies, such as artificial intelligence or nanotechnology.

Rare Earth (book)

Rare Earth: Why Complex Life Is Uncommon in the Universe is a 2000 popular science book about xenobiology by Peter Ward, a geologist and evolutionary biologist, and Donald E. Brownlee, a cosmologist and astrobiologist, both faculty members at the University of Washington. The book is the origin of the term 'Rare Earth Hypothesis' which, like the book, asserts the concept that complex life is rare in the universe. The book was eventually succeeded by a follow-up book called The Life and Death of Planet Earth: How the New Science of Astrobiology Charts the Ultimate Fate of our World, also by Ward and Brownlee, which talks about the Earth's long term future and eventual demise under a warming and expanding Sun, showing readers the concept that planets like Earth have finite lifespans, and complex life is not just rare in space, but also rare in time, and is more likely to die out within a short time on geological timescales, while microbes dominate most of the planet's history.

Rare Earth hypothesis

In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the origin of life and the evolution of biological complexity such as sexually reproducing, multicellular organisms on Earth (and, subsequently, human intelligence) required an improbable combination of astrophysical and geological events and circumstances.

According to the hypothesis, complex extraterrestrial life is an improbable phenomenon and likely to be rare. The term "Rare Earth" originates from Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist, both faculty members at the University of Washington.

A contrary view was argued in the 1970s and 1980s by Carl Sagan and Frank Drake, among others. It holds that Earth is a typical rocky planet in a typical planetary system, located in a non-exceptional region of a common barred-spiral galaxy. Given the principle of mediocrity (in the same vein as the Copernican principle), it is probable that we are typical, and the universe teems with complex life. However, Ward and Brownlee argue that planets, planetary systems, and galactic regions that are as friendly to complex life as the Earth, the Solar System, and our galactic region are rare.

Supernova nucleosynthesis

Supernova nucleosynthesis is a theory of the nucleosynthesis of the natural abundances of the chemical elements in supernova explosions, advanced as the nucleosynthesis of elements from carbon to nickel in massive stars by Fred Hoyle in 1954. In massive stars, the nucleosynthesis by fusion of lighter elements into heavier ones occurs during sequential hydrostatic burning processes called helium burning, carbon burning, oxygen burning, and silicon burning, in which the ashes of one nuclear fuel become, after compressional heating, the fuel for the subsequent burning stage. During hydrostatic burning these fuels synthesize overwhelmingly the alpha-nucleus (A = 2Z) products. A rapid final explosive burning is caused by the sudden temperature spike owing to passage of the radially moving shock wave that was launched by the gravitational collapse of the core. W. D. Arnett and his Rice University colleagues demonstrated that the final shock burning would synthesize the non-alpha-nucleus isotopes more effectively than hydrostatic burning was able to do, suggesting that the expected shock-wave nucleosynthesis is an essential component of supernova nucleosynthesis. Together, shock-wave nucleosynthesis and hydrostatic-burning processes create most of the isotopes of the elements carbon (Z = 6), oxygen (Z = 8), and elements with Z = 10–28 (from neon to nickel). As a result of the ejection of the newly synthesized isotopes of the chemical elements by supernova explosions their abundances steadily increased within interstellar gas. That increase became evident to astronomers from the initial abundances in newly born stars exceeding those in earlier-born stars. To explain that temporal increase of the natural abundances of the elements was the main goal of stellar nucleosynthesis. Hoyle's paper was the founding paper of that theory; however, ideas about nuclear reactions in stars providing power for the stars is often confused with stellar nucleosynthesis. Realize that nuclear fusion in stars can occur with negligible impact on the abundances of the chemical elements.

Elements heavier than nickel are comparatively rare owing to the decline with atomic weight of their nuclear binding energies per nucleon, but they too are created in part within supernovae. Of greatest interest historically has been their synthesis by rapid capture of neutrons during the r-process, reflecting the common belief that supernova cores are likely to provide the necessary conditions. But see the r-process below for a recently discovered alternative. The r-process isotopes are roughly a 100,000 times less abundant than the primary chemical elements fused in supernova shells above. Furthermore, other nucleosynthesis processes in supernovae are thought to also be responsible for some nucleosynthesis of other heavy elements, notably, the proton capture process known as the rp-process, the slow capture of neutrons (s-process) in the Helium-burning shells and in the carbon-burning shells of massive stars, and a photodisintegration process known as the γ-process (gamma-process). The latter synthesizes the lightest, most neutron-poor, isotopes of the elements heavier than iron from preexisting heavier isotopes.

Ursa Major Filament

Ursa Major Filament is a galaxy filament. The filament is connected to the CfA Homunculus, a portion of the filament forms a portion of the "leg" of the Homunculus.

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