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94-807: Baade observed that bluer stars were strongly associated with the spiral arms, and yellow stars dominated near the central galactic bulge and within globular star clusters . Two main divisions were defined as Population I star and population II , with another newer, hypothetical division called population III added in 1978. Among the population types, significant differences were found with their individual observed stellar spectra. These were later shown to be very important and were possibly related to star formation, observed kinematics , stellar age, and even galaxy evolution in both spiral and elliptical galaxies. These three simple population classes usefully divided stars by their chemical composition or metallicity . By definition, each population group shows

188-422: A spheroidal galactic bulge around the galactic core. However, some stars inhabit a spheroidal halo or galactic spheroid , a type of galactic halo . The orbital behaviour of these stars is disputed, but they may exhibit retrograde and/or highly inclined orbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example,

282-588: A supermassive black hole at their centers. In our own galaxy, for instance, the object called Sagittarius A* is a supermassive black hole. There are many lines of evidence for the existence of black holes in spiral galaxy centers, including the presence of active nuclei in some spiral galaxies, and dynamical measurements that find large compact central masses in galaxies such as Messier 106 . Bar-shaped elongations of stars are observed in roughly two-thirds of all spiral galaxies. Their presence may be either strong or weak. In edge-on spiral (and lenticular) galaxies,

376-554: A "metal", including chemical non-metals such as oxygen. Observation of stellar spectra has revealed that stars older than the Sun have fewer heavy elements compared with the Sun. This immediately suggests that metallicity has evolved through the generations of stars by the process of stellar nucleosynthesis . Under current cosmological models, all matter created in the Big Bang was mostly hydrogen (75%) and helium (25%), with only

470-493: A bright pocket of early population stars in the very bright galaxy Cosmos Redshift 7 from the reionization period around 800 million years after the Big Bang, at z = 6.60 . The rest of the galaxy has some later redder population II stars. Some theories hold that there were two generations of population III stars. Current theory is divided on whether the first stars were very massive or not. One possibility

564-425: A density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms. The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964, attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of

658-401: A dramatic rise in stellar luminosity, where the released energy is distributed over a much larger surface area, which in fact causes the average surface temperature to be lower. In stellar evolution terms, stars undergoing such increases in luminosity are known as asymptotic giant branch stars (AGB). During this phase, the star can lose 50–70% of its total mass from its stellar wind . For

752-443: A final stage of stellar evolution . Spectroscopic observations show that all planetary nebulae are expanding. This led to the idea that planetary nebulae were caused by a star's outer layers being thrown into space at the end of its life. Towards the end of the 20th century, technological improvements helped to further the study of planetary nebulae. Space telescopes allowed astronomers to study light wavelengths outside those that

846-488: A growing inner core of inert carbon and oxygen. Above it is a thin helium-burning shell, surrounded in turn by a hydrogen-burning shell. However, this new phase lasts only 20,000 years or so, a very short period compared to the entire lifetime of the star. The venting of atmosphere continues unabated into interstellar space, but when the outer surface of the exposed core reaches temperatures exceeding about 30,000 K, there are enough emitted ultraviolet photons to ionize

940-450: A hypothetical population of extremely massive, luminous and hot stars with virtually no "metals" , except possibly for intermixing ejecta from other nearby, early population III supernovae. The term was first introduced by Neville J. Woolf in 1965. Such stars are likely to have existed in the very early universe (i.e., at high redshift) and may have started the production of chemical elements heavier than hydrogen , which are needed for

1034-577: A later stage in the universe's development. Scientists have targeted these oldest stars in several different surveys, including the HK objective-prism survey of Timothy C. Beers et al . and the Hamburg- ESO survey of Norbert Christlieb et al., originally started for faint quasars . Thus far, they have uncovered and studied in detail about ten ultra-metal-poor (UMP) stars (such as Sneden's Star , Cayrel's Star , BD +17° 3248 ) and three of

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1128-481: A low relative velocity . It was earlier hypothesized that the high metallicity of population I stars makes them more likely to possess planetary systems than the other two populations, because planets , particularly terrestrial planets , are thought to be formed by the accretion of metals. However, observations of the Kepler Space Telescope data have found smaller planets around stars with

1222-457: A planet, that is to say, of equal brightness all over, round or somewhat oval, and about as well defined in outline as the disk of the planets, of a light strong enough to be visible with an ordinary telescope of only one foot, yet they have only the appearance of a star of about ninth magnitude. He assigned these to Class IV of his catalogue of "nebulae", eventually listing 78 "planetary nebulae", most of which are in fact galaxies. Herschel used

1316-562: A planetary nebula (i.e., a 4% distance solution). The cases of NGC 2818 and NGC 2348 in Messier 46 , exhibit mismatched velocities between the planetary nebulae and the clusters, which indicates they are line-of-sight coincidences. A subsample of tentative cases that may potentially be cluster/PN pairs includes Abell 8 and Bica 6, and He 2-86 and NGC 4463. Theoretical models predict that planetary nebulae can form from main-sequence stars of between one and eight solar masses, which puts

1410-480: A quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe ( Hubble volume ) have bars. The Milky Way is a barred spiral, although the bar itself is difficult to observe from Earth's current position within the galactic disc. The most convincing evidence for the stars forming a bar in the Galactic Center comes from several recent surveys, including

1504-406: A range of metallicities, while only larger, potential gas giant planets are concentrated around stars with relatively higher metallicity – a finding that has implications for theories of gas-giant formation. Between the intermediate population I and the population II stars comes the intermediate disc population. Population II, or metal-poor, stars are those with relatively little of

1598-482: A relatively short time, typically from 100 to 600 million years. The distances to planetary nebulae are generally poorly determined, but the Gaia mission is now measuring direct parallactic distances between their central stars and neighboring stars. It is also possible to determine distances to nearby planetary nebula by measuring their expansion rates. High resolution observations taken several years apart will show

1692-466: A very tiny fraction consisting of other light elements such as lithium and beryllium . When the universe had cooled sufficiently, the first stars were born as population III stars, without any contaminating heavier metals. This is postulated to have affected their structure so that their stellar masses became hundreds of times more than that of the Sun. In turn, these massive stars also evolved very quickly, and their nucleosynthetic processes created

1786-510: Is a goal of NASA's James Webb Space Telescope . On 8 December 2022, astronomers reported the possible detection of Population III stars, in a high- redshift galaxy called RX J2129–z8He II. Spiral galaxy#Structure#Galactic bulge Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of

1880-584: Is an extremely old spiral galaxy located in the Abell 1689 galaxy cluster in the Virgo constellation. A1689B11 is 11 billion light years from the Earth, forming 2.6 billion years after the Big Bang. In June 2019, citizen scientists through Galaxy Zoo reported that the usual Hubble classification , particularly concerning spiral galaxies , may not be supported, and may need updating. The pioneer of studies of

1974-433: Is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits. The following hypotheses exist for star formation caused by density waves: Spiral arms appear visually brighter because they contain both young stars and more massive and luminous stars than

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2068-445: Is known as the main sequence , which can last for tens of millions to billions of years, depending on the mass. When the hydrogen in the core starts to run out, nuclear fusion generates less energy and gravity starts compressing the core, causing a rise in temperature to about 100 million K. Such high core temperatures then make the star's cooler outer layers expand to create much larger red giant stars. This end phase causes

2162-509: Is still used. All planetary nebulae form at the end of the life of a star of intermediate mass, about 1-8 solar masses. It is expected that the Sun will form a planetary nebula at the end of its life cycle. They are relatively short-lived phenomena, lasting perhaps a few tens of millennia, compared to considerably longer phases of stellar evolution . Once all of the red giant's atmosphere has been dissipated, energetic ultraviolet radiation from

2256-466: Is that despite their lower overall metallicity, they often have a higher ratio of " alpha elements " (elements produced by the alpha process , like oxygen and neon ) relative to iron (Fe) as compared with population I stars; current theory suggests that this is the result of type II supernovas being more important contributors to the interstellar medium at the time of their formation, whereas type Ia supernova metal-enrichment came at

2350-400: Is that these stars were much larger than current stars: several hundred solar masses , and possibly up to 1,000 solar masses. Such stars would be very short-lived and last only 2–5 million years. Such large stars may have been possible due to the lack of heavy elements and a much warmer interstellar medium from the Big Bang. Conversely, theories proposed in 2009 and 2011 suggest that

2444-524: Is the central value; it is useful to define: R o p t = 3.2 h {\displaystyle R_{opt}=3.2h} as the size of the stellar disk, whose luminosity is L t o t = 2 π I 0 h 2 {\displaystyle L_{tot}=2\pi I_{0}h^{2}} . The spiral galaxies light profiles, in terms of the coordinate R / h {\displaystyle R/h} , do not depend on galaxy luminosity. Before it

2538-560: Is the oldest and most distant known spiral galaxy, as of 2024.The galaxy has a redshift of 4.4, meaning its light took 12.4 billion years to reach Earth. The oldest grand design spiral galaxy on file is BX442 . At eleven billion years old, it is more than two billion years older than any previous discovery. Researchers believe the galaxy's shape is caused by the gravitational influence of a companion dwarf galaxy . Computer models based on that assumption indicate that BX442's spiral structure will last about 100 million years. A1689B11

2632-445: The Big Bang – are observed in quasar emission spectra . They are also thought to be components of faint blue galaxies . These stars likely triggered the universe's period of reionization , a major phase transition of the hydrogen gas composing most of the interstellar medium. Observations of the galaxy UDFy-38135539 suggest that it may have played a role in this reionization process. The European Southern Observatory discovered

2726-513: The Hubble sequence . Most spiral galaxies consist of a flat, rotating disk containing stars , gas and dust , and a central concentration of stars known as the bulge . These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters . Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than

2820-460: The Milky Way galaxy. The Sun is considered as an intermediate population I star, while the sun-like μ Arae is much richer in metals. (The term "metal rich star" is used to describe stars with a significantly higher metallicity than the Sun; higher than can be explained by measurement error.) Population I stars usually have regular elliptical orbits of the Galactic Center , with

2914-591: The Ring Nebula , "a very dull nebula, but perfectly outlined; as large as Jupiter and looks like a fading planet". The nature of these objects remained unclear. In 1782, William Herschel , discoverer of Uranus, found the Saturn Nebula (NGC 7009) and described it as "A curious nebula, or what else to call it I do not know". He later described these objects as seeming to be planets "of the starry kind". As noted by Darquier before him, Herschel found that

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3008-555: The Sagittarius Dwarf Spheroidal Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it. Unlike the galactic disc, the halo seems to be free of dust , and in further contrast, stars in the galactic halo are of Population II , much older and with much lower metallicity than their Population I cousins in

3102-554: The Spitzer Space Telescope . Together with irregular galaxies , spiral galaxies make up approximately 60% of galaxies in today's universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters. Spiral galaxies may consist of several distinct components: The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy. Spiral arms are regions of stars that extend from

3196-521: The asymptotic giant branch phase, they create heavier elements via nuclear fusion which are eventually expelled by strong stellar winds . Planetary nebulae usually contain larger proportions of elements such as carbon , nitrogen and oxygen , and these are recycled into the interstellar medium via these powerful winds. In this way, planetary nebulae greatly enrich the Milky Way and their nebulae with these heavier elements – collectively known by astronomers as metals and specifically referred to by

3290-415: The interstellar medium via planetary nebulae and supernovae, enriching further the nebulae, out of which the newer stars formed. These youngest stars, including the Sun , therefore have the highest metal content, and are known as population I stars. Population I stars are young stars with the highest metallicity out of all three populations and are more commonly found in the spiral arms of

3384-406: The metallicity parameter Z . Subsequent generations of stars formed from such nebulae also tend to have higher metallicities. Although these metals are present in stars in relatively tiny amounts, they have marked effects on stellar evolution and fusion reactions. When stars formed earlier in the universe they theoretically contained smaller quantities of heavier elements. Known examples are

3478-511: The 1780s with the English astronomer William Herschel who described these nebulae as resembling planets; however, as early as January 1779, the French astronomer Antoine Darquier de Pellepoix described in his observations of the Ring Nebula , "very dim but perfectly outlined; it is as large as Jupiter and resembles a fading planet". Though the modern interpretation is different, the old term

3572-467: The 1990s, Hubble Space Telescope images revealed that many planetary nebulae have extremely complex and varied morphologies. About one-fifth are roughly spherical, but the majority are not spherically symmetric. The mechanisms that produce such a wide variety of shapes and features are not yet well understood, but binary central stars , stellar winds and magnetic fields may play a role. The first planetary nebula discovered (though not yet termed as such)

3666-419: The 500.7 nm emission line and others. These spectral lines, which can only be seen in very low-density gases, are called forbidden lines . Spectroscopic observations thus showed that nebulae were made of extremely rarefied gas. The central stars of planetary nebulae are very hot. Only when a star has exhausted most of its nuclear fuel can it collapse to a small size. Planetary nebulae are understood as

3760-464: The AGB. As the gases expand, the central star undergoes a two-stage evolution, first growing hotter as it continues to contract and hydrogen fusion reactions occur in the shell around the core and then slowly cooling when the hydrogen shell is exhausted through fusion and mass loss. In the second phase, it radiates away its energy and fusion reactions cease, as the central star is not heavy enough to generate

3854-457: The Earth's atmosphere transmits. Infrared and ultraviolet studies of planetary nebulae allowed much more accurate determinations of nebular temperatures , densities and elemental abundances. Charge-coupled device technology allowed much fainter spectral lines to be measured accurately than had previously been possible. The Hubble Space Telescope also showed that while many nebulae appear to have simple and regular structures when observed from

Stellar population - Misplaced Pages Continue

3948-515: The Milky Way's central bar is larger than what was previously suspected. Planetary nebula A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated to planets . The term originates from the planet-like round shape of these nebulae observed by astronomers through early telescopes. The first usage may have occurred during

4042-424: The Sun. The huge variety of the shapes is partially the projection effect—the same nebula when viewed under different angles will appear different. Nevertheless, the reason for the huge variety of physical shapes is not fully understood. Gravitational interactions with companion stars if the central stars are binary stars may be one cause. Another possibility is that planets disrupt the flow of material away from

4136-415: The bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, blue Population I stars . Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity); others simply appear as higher density centers of disks, with properties similar to disk galaxies. Many bulges are thought to host

4230-446: The center of barred and unbarred spiral galaxies . These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies have tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain many young, blue stars (due to

4324-414: The core temperatures required for carbon and oxygen to fuse. During the first phase, the central star maintains constant luminosity, while at the same time it grows ever hotter, eventually reaching temperatures around 100,000 K. In the second phase, it cools so much that it does not give off enough ultraviolet radiation to ionize the increasingly distant gas cloud. The star becomes a white dwarf , and

4418-412: The disk resembled a planet but it was too faint to be one. In 1785, Herschel wrote to Jérôme Lalande : These are celestial bodies of which as yet we have no clear idea and which are perhaps of a type quite different from those that we are familiar with in the heavens. I have already found four that have a visible diameter of between 15 and 30 seconds. These bodies appear to have a disk that is rather like

4512-493: The earlier history of the universe. Scientists have found evidence of an extremely small ultra metal-poor star , slightly smaller than the Sun, found in a binary system of the spiral arms in the Milky Way . The discovery opens up the possibility of observing even older stars. Stars too massive to produce pair-instability supernovae would have likely collapsed into black holes through a process known as photodisintegration . Here some matter may have escaped during this process in

4606-505: The early universe. Unlike high-mass black hole seeds, such as direct collapse black holes , they would have produced light ones. If they could have grown to larger than expected masses, then they could have been quasi-stars , other hypothetical seeds of heavy black holes which would have existed in the early development of the Universe before hydrogen and helium were contaminated by heavier elements. Detection of population III stars

4700-451: The ejected atmosphere, causing the gas to shine as a planetary nebula. After a star passes through the asymptotic giant branch (AGB) phase, the short planetary nebula phase of stellar evolution begins as gases blow away from the central star at speeds of a few kilometers per second. The central star is the remnant of its AGB progenitor, an electron-degenerate carbon-oxygen core that has lost most of its hydrogen envelope due to mass loss on

4794-424: The elements heavier than helium. These objects were formed during an earlier time of the universe. Intermediate population II stars are common in the bulge near the centre of the Milky Way , whereas population II stars found in the galactic halo are older and thus more metal-deficient. Globular clusters also contain high numbers of population II stars. A characteristic of population II stars

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4888-440: The end of the lives of intermediate and low mass stars between 0.8 M ⊙ to 8.0 M ⊙ . Progenitor stars that form planetary nebulae will spend most of their lifetimes converting their hydrogen into helium in the star's core by nuclear fusion at about 15 million K . This generates energy in the core, which creates outward pressure that balances the crushing inward pressures of gravity. This state of equilibrium

4982-401: The expanding gas cloud becomes invisible to us, ending the planetary nebula phase of evolution. For a typical planetary nebula, about 10,000 years passes between its formation and recombination of the resulting plasma . Planetary nebulae may play a very important role in galactic evolution. Newly born stars consist almost entirely of hydrogen and helium , but as stars evolve through

5076-509: The expansion of the nebula perpendicular to the line of sight, while spectroscopic observations of the Doppler shift will reveal the velocity of expansion in the line of sight. Comparing the angular expansion with the derived velocity of expansion will reveal the distance to the nebula. The issue of how such a diverse range of nebular shapes can be produced is a debatable topic. It is theorised that interactions between material moving away from

5170-593: The exposed hot luminous core, called a planetary nebula nucleus (P.N.N.), ionizes the ejected material. Absorbed ultraviolet light then energizes the shell of nebulous gas around the central star, causing it to appear as a brightly coloured planetary nebula. Planetary nebulae probably play a crucial role in the chemical evolution of the Milky Way by expelling elements into the interstellar medium from stars where those elements were created. Planetary nebulae are observed in more distant galaxies , yielding useful information about their chemical abundances. Starting from

5264-641: The first 26 elements (up to iron in the periodic table ). Many theoretical stellar models show that most high-mass population III stars rapidly exhausted their fuel and likely exploded in extremely energetic pair-instability supernovae . Those explosions would have thoroughly dispersed their material, ejecting metals into the interstellar medium (ISM), to be incorporated into the later generations of stars. Their destruction suggests that no galactic high-mass population III stars should be observable. However, some population III stars might be seen in high- redshift galaxies whose light originated during

5358-587: The first drawing of Andromeda Galaxy 's spiral structure. In 1852 Stephen Alexander supposed that Milky Way is also a spiral nebula. The question of whether such objects were separate galaxies independent of the Milky Way, or a type of nebula existing within our own galaxy, was the subject of the Great Debate of 1920, between Heber Curtis of Lick Observatory and Harlow Shapley of Mount Wilson Observatory . Beginning in 1923, Edwin Hubble observed Cepheid variables in several spiral nebulae, including

5452-406: The first star groups might have consisted of a massive star surrounded by several smaller stars. The smaller stars, if they remained in the birth cluster, would accumulate more gas and could not survive to the present day, but a 2017 study concluded that if a star of 0.8 solar masses ( M ☉ ) or less was ejected from its birth cluster before it accumulated more mass, it could survive to

5546-508: The form of relativistic jets , and this could have distributed the first metals into the universe. The oldest stars observed thus far, known as population II, have very low metallicities; as subsequent generations of stars were born, they became more metal-enriched, as the gaseous clouds from which they formed received the metal-rich dust manufactured by previous generations of stars from population III. As those population II stars died, they returned metal-enriched material to

5640-483: The galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters. The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarfs close to the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830 . Due to their irregular movement around the center of the galaxy, these stars often display unusually high proper motion . BRI 1335-0417

5734-404: The galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem . Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain

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5828-411: The galaxy's gas and stars. They suggested that the spiral arms were manifestations of spiral density waves – they assumed that the stars travel in slightly elliptical orbits, and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center. This is illustrated in the diagram to the right. It

5922-582: The ground, the very high optical resolution achievable by telescopes above the Earth's atmosphere reveals extremely complex structures. Under the Morgan-Keenan spectral classification scheme, planetary nebulae are classified as Type- P , although this notation is seldom used in practice. Stars greater than 8  solar masses (M ⊙ ) will probably end their lives in dramatic supernovae explosions, while planetary nebulae seemingly only occur at

6016-501: The high mass density and the high rate of star formation), which make the arms so bright. A bulge is a large, tightly packed group of stars. The term refers to the central group of stars found in most spiral galaxies, often defined as the excess of stellar light above the inward extrapolation of the outer (exponential) disk light. Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars , which are old, red stars with low metal content. Further,

6110-431: The highest densities, sometimes as high as 10 particles per cm . As nebulae age, their expansion causes their density to decrease. The masses of planetary nebulae range from 0.1 to 1  solar masses . Radiation from the central star heats the gases to temperatures of about 10,000  K . The gas temperature in central regions is usually much higher than at the periphery reaching 16,000–25,000 K. The volume in

6204-418: The later formation of planets and life as we know it. The existence of population III stars is inferred from physical cosmology , but they have not yet been observed directly. Indirect evidence for their existence has been found in a gravitationally lensed galaxy in a very distant part of the universe. Their existence may account for the fact that heavy elements – which could not have been created in

6298-455: The line at 500.7 nm was due to a familiar element in unfamiliar conditions. Physicists showed in the 1920s that in gas at extremely low densities, electrons can occupy excited metastable energy levels in atoms and ions that would otherwise be de-excited by collisions that would occur at higher densities. Electron transitions from these levels in nitrogen and oxygen ions ( O , O (a.k.a. O  iii ), and N ) give rise to

6392-404: The majority of them belong to just three types: spherical, elliptical and bipolar. Bipolar nebulae are concentrated in the galactic plane , probably produced by relatively young massive progenitor stars; and bipolars in the galactic bulge appear to prefer orienting their orbital axes parallel to the galactic plane. On the other hand, spherical nebulae are probably produced by old stars similar to

6486-422: The metal poor Population II stars. (See Stellar population .) Identification of stellar metallicity content is found by spectroscopy . A typical planetary nebula is roughly one light year across, and consists of extremely rarefied gas, with a density generally from 100 to 10,000 particles per cm . (The Earth's atmosphere, by comparison, contains 2.5 × 10 particles per cm .) Young planetary nebulae have

6580-425: The more massive asymptotic giant branch stars that form planetary nebulae, whose progenitors exceed about 0.6M ⊙ , their cores will continue to contract. When temperatures reach about 100 million K, the available helium nuclei fuse into carbon and oxygen , so that the star again resumes radiating energy, temporarily stopping the core's contraction. This new helium burning phase (fusion of helium nuclei) forms

6674-673: The oldest stars known to date: HE 0107-5240 , HE 1327-2326 and HE 1523-0901 . Caffau's star was identified as the most metal-poor star yet when it was found in 2012 using Sloan Digital Sky Survey data. However, in February ;2014 the discovery of an even lower-metallicity star was announced, SMSS J031300.36-670839.3 located with the aid of SkyMapper astronomical survey data. Less extreme in their metal deficiency, but nearer and brighter and hence longer known, are HD 122563 (a red giant ) and HD 140283 (a subgiant ). Population III stars are

6768-405: The plane of the Milky Way , with the greatest concentration near the Galactic Center . Only about 20% of planetary nebulae are spherically symmetric (for example, see Abell 39 ). A wide variety of shapes exist with some very complex forms seen. Planetary nebulae are classified by different authors into: stellar, disk, ring, irregular, helical, bipolar , quadrupolar, and other types, although

6862-440: The potential discovery of planetary nebulae in globular clusters in the galaxy M31 . However, there is currently only one case of a planetary nebula discovered in an open cluster that is agreed upon by independent researchers. That case pertains to the planetary nebula PHR 1315-6555 and the open cluster Andrews-Lindsay 1. Indeed, through cluster membership, PHR 1315-6555 possesses among the most precise distances established for

6956-418: The presence of the bar can sometimes be discerned by the out-of-plane X-shaped or (peanut shell)-shaped structures which typically have a maximum visibility at half the length of the in-plane bar. The bulk of the stars in a spiral galaxy are located either close to a single plane (the galactic plane ) in more or less conventional circular orbits around the center of the galaxy (the Galactic Center ), or in

7050-1050: The present day, possibly even in our Milky Way galaxy. Analysis of data of extremely low- metallicity population II stars such as HE 0107-5240 , which are thought to contain the metals produced by population III stars, suggest that these metal-free stars had masses of 20~130 solar masses. On the other hand, analysis of globular clusters associated with elliptical galaxies suggests pair-instability supernovae , which are typically associated with very massive stars, were responsible for their metallic composition. This also explains why there have been no low-mass stars with zero metallicity observed, despite models constructed for smaller population III stars. Clusters containing zero-metallicity red dwarfs or brown dwarfs (possibly created by pair-instability supernovae) have been proposed as dark matter candidates, but searches for these types of MACHOs through gravitational microlensing have produced negative results. Population III stars are considered seeds of black holes in

7144-491: The progenitor star's age at greater than 40 million years. Although there are a few hundred known open clusters within that age range, a variety of reasons limit the chances of finding a planetary nebula within. For one reason, the planetary nebula phase for more massive stars is on the order of millennia, which is a blink of the eye in astronomic terms. Also, partly because of their small total mass, open clusters have relatively poor gravitational cohesion and tend to disperse after

7238-412: The rest of the galaxy. As massive stars evolve far more quickly, their demise tends to leave a darker background of fainter stars immediately behind the density waves. This make the density waves much more prominent. Spiral arms simply appear to pass through the older established stars as they travel in their galactic orbits, so they also do not necessarily follow the arms. As stars move through an arm,

7332-462: The rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as

7426-417: The so-called "Andromeda Nebula" , proving that they are, in fact, entire galaxies outside our own. The term spiral nebula has since fallen out of use. The Milky Way was once considered an ordinary spiral galaxy. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1960s. Their suspicions were confirmed by Spitzer Space Telescope observations in 2005, which showed that

7520-568: The space velocity of each stellar system is modified by the gravitational force of the local higher density. Also the newly created stars do not remain forever fixed in the position within the spiral arms, where the average space velocity returns to normal after the stars depart on the other side of the arm. Charles Francis and Erik Anderson showed from observations of motions of over 20,000 local stars (within 300 parsecs) that stars do move along spiral arms, and described how mutual gravity between stars causes orbits to align on logarithmic spirals. When

7614-422: The stability of the spiral structure. Since the 1970s, there have been two leading hypotheses or models for the spiral structures of galaxies: These different hypotheses are not mutually exclusive, as they may explain different types of spiral arms. Bertil Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy's stars and gas. As gas enters

7708-521: The star as the nebula forms. It has been determined that the more massive stars produce more irregularly shaped nebulae. In January 2005, astronomers announced the first detection of magnetic fields around the central stars of two planetary nebulae, and hypothesized that the fields might be partly or wholly responsible for their remarkable shapes. Planetary nebulae have been detected as members in four Galactic globular clusters : Messier 15 , Messier 22 , NGC 6441 and Palomar 6 . Evidence also points to

7802-542: The star at different speeds gives rise to most observed shapes. However, some astronomers postulate that close binary central stars might be responsible for the more complex and extreme planetary nebulae. Several have been shown to exhibit strong magnetic fields, and their interactions with ionized gas could explain some planetary nebulae shapes. There are two main methods of determining metal abundances in nebulae. These rely on recombination lines and collisionally excited lines. Large discrepancies are sometimes seen between

7896-472: The surrounding disc because of the young, hot OB stars that inhabit them. Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. The proportion of barred spirals relative to barless spirals has likely changed over the history of the universe , with only about 10% containing bars about 8 billion years ago, to roughly

7990-440: The term "planetary nebulae" for these objects. The origin of this term not known. The label "planetary nebula" became ingrained in the terminology used by astronomers to categorize these types of nebulae, and is still in use by astronomers today. The nature of planetary nebulae remained unknown until the first spectroscopic observations were made in the mid-19th century. Using a prism to disperse their light, William Huggins

8084-422: The theory is applied to gas, collisions between gas clouds generate the molecular clouds in which new stars form, and evolution towards grand-design bisymmetric spirals is explained. The stars in spirals are distributed in thin disks radial with intensity profiles such that with h {\displaystyle h} being the disk scale-length; I 0 {\displaystyle I_{0}}

8178-458: The trend where lower metal content indicates higher age of stars. Hence, the first stars in the universe (very low metal content) were deemed population III, old stars (low metallicity) as population II, and recent stars (high metallicity) as population I. The Sun is considered population I, a recent star with a relatively high 1.4% metallicity. Note that astrophysics nomenclature considers any element heavier than helium to be

8272-458: The vicinity of the central star is often filled with a very hot (coronal) gas having the temperature of about 1,000,000 K. This gas originates from the surface of the central star in the form of the fast stellar wind. Nebulae may be described as matter bounded or radiation bounded . In the former case, there is not enough matter in the nebula to absorb all the UV photons emitted by the star, and

8366-462: The visible nebula is fully ionized. In the latter case, there are not enough UV photons being emitted by the central star to ionize all the surrounding gas, and an ionization front propagates outward into the circumstellar envelope of neutral atoms. About 3000 planetary nebulae are now known to exist in our galaxy, out of 200 billion stars. Their very short lifetime compared to total stellar lifetime accounts for their rarity. They are found mostly near

8460-403: Was hypothesized that the line might be due to an unknown element, which was named nebulium . A similar idea had led to the discovery of helium through analysis of the Sun 's spectrum in 1868. While helium was isolated on Earth soon after its discovery in the spectrum of the Sun, "nebulium" was not. In the early 20th century, Henry Norris Russell proposed that, rather than being a new element,

8554-492: Was one of the earliest astronomers to study the optical spectra of astronomical objects. On August 29, 1864, Huggins was the first to analyze the spectrum of a planetary nebula when he observed Cat's Eye Nebula . His observations of stars had shown that their spectra consisted of a continuum of radiation with many dark lines superimposed. He found that many nebulous objects such as the Andromeda Nebula (as it

8648-497: Was the Dumbbell Nebula in the constellation of Vulpecula . It was observed by Charles Messier on July 12, 1764 and listed as M27 in his catalogue of nebulous objects. To early observers with low-resolution telescopes, M27 and subsequently discovered planetary nebulae resembled the giant planets like Uranus . As early as January 1779, the French astronomer Antoine Darquier de Pellepoix described in his observations of

8742-485: Was then known) had spectra that were quite similar. However, when Huggins looked at the Cat's Eye Nebula, he found a very different spectrum. Rather than a strong continuum with absorption lines superimposed, the Cat's Eye Nebula and other similar objects showed a number of emission lines . Brightest of these was at a wavelength of 500.7  nanometres , which did not correspond with a line of any known element. At first, it

8836-518: Was understood that spiral galaxies existed outside of our Milky Way galaxy, they were often referred to as spiral nebulae , due to Lord Rosse , whose telescope Leviathan was the first to reveal the spiral structure of galaxies. In 1845 he discovered the spiral structure of M51, a galaxy nicknamed later as the " Whirlpool Galaxy ", and his drawings of it closely resemble modern photographs. In 1846 and in 1849 Lord Rosse identified similar pattern in Messier 99 and Messier 33 respectively. In 1850 he made

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