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39-572: Stratoscope I possessed a 12-inch (30.48 cm) mirror and was first flown in 1957. It was conceived by Martin Schwarzschild and built by the Perkin Elmer Corporation with funding provided by the Office of Naval Research . A small secondary mirror focussed the image from the primary into a 35 mm movie camera, which captured the images on film. Schwarzschild used the telescope to study

78-425: A habitable zone for several billion years at 2 astronomical units (AU) out to around 100 million years at 9 AU out, giving perhaps enough time for life to develop on a suitable world. After the red-giant stage, there would for such a star be a habitable zone between 7 and 22 AU for an additional one billion years. Later studies have refined this scenario, showing how for a 1  M ☉ star

117-449: A planetary nebula and become a white dwarf at the end of its life. A red giant is a star that has exhausted the supply of hydrogen in its core and has begun thermonuclear fusion of hydrogen in a shell surrounding the core. They have radii tens to hundreds of times larger than that of the Sun . However, their outer envelope is lower in temperature, giving them a yellowish-orange hue. Despite

156-404: A type II supernova . The most massive stars can become Wolf–Rayet stars without becoming giants or supergiants at all. Although traditionally it has been suggested the evolution of a star into a red giant will render its planetary system , if present, uninhabitable, some research suggests that, during the evolution of a 1  M ☉ star along the red-giant branch, it could harbor

195-439: A much larger effect would be Roche lobe overflow causing mass-transfer from the star to the planet when the giant expands out to the orbital distance of the planet. (A similar process in multiple star systems is believed to be the cause of most novas and type Ia supernovas .) Many of the well-known bright stars are red giants, because they are luminous and moderately common. The red-giant branch variable star Gamma Crucis

234-404: A situation that has been described as the mirror principle : when the core within the shell contracts, the layers of the star outside the shell must expand. The detailed physical processes that cause this are complex. Still, the behavior is necessary to satisfy simultaneous conservation of gravitational and thermal energy in a star with the shell structure. The core contracts and heats up due to

273-458: A star's life is called the horizontal branch in metal-poor stars , so named because these stars lie on a nearly horizontal line in the H–R diagram of many star clusters. Metal-rich helium-fusing stars instead lie on the so-called red clump in the H–R diagram. An analogous process occurs when the core helium is exhausted, and the star collapses once again, causing helium in a shell to begin fusing. At

312-403: A white dwarf. Very-low-mass stars are fully convective and may continue to fuse hydrogen into helium for up to a trillion years until only a small fraction of the entire star is hydrogen. Luminosity and temperature steadily increase during this time, just as for more-massive main-sequence stars, but the length of time involved means that the temperature eventually increases by about 50% and

351-485: Is the nearest M-class giant star at 88 light-years. The K1.5 red-giant branch star Arcturus is 36 light-years away. The Sun will exit the main sequence in approximately 5 billion years and start to turn into a red giant. As a red giant, the Sun will grow so large (over 200 times its present-day radius : ~ 215   R ☉ ; ~ 1  AU ) that it will engulf Mercury , Venus , and likely Earth. It will lose 38% of its mass growing, then will die into

390-837: The University of Göttingen and took his doctoral examination in December 1936. He left Germany in 1936 for Norway and then the United States. Schwarzschild served in the US army intelligence. He was awarded the Legion of Merit and the Bronze Star for his wartime service. After returning to the US, he married fellow astronomer Barbara Cherry (1914–2008). In 1947, Martin Schwarzschild joined his lifelong friend, Lyman Spitzer at Princeton University. Spitzer died 10 days before Schwarzschild. Schwarzschild's work in

429-414: The main sequence and will not have become giants yet) and more massive stars are expected to have more massive planets. However, the masses of the planets that have been found around giant stars do not correlate with the masses of the stars; therefore, the planets could be growing in mass during the stars' red giant phase. The growth in planet mass could be partly due to accretion from stellar wind, although

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468-414: The red-giant branch by steadily burning hydrogen in a shell around the core. He and Härm were the first to compute stellar models going through thermal pulses on the asymptotic giant branch and later showed that these models develop convective zones between the helium- and hydrogen-burning shells, which can bring nuclear ashes to the visible surface. Schwarzschild's 1958 book Structure and Evolution of

507-399: The triple-alpha process . Once the degenerate core reaches this temperature, the entire core will begin helium fusion nearly simultaneously in a so-called helium flash . In more-massive stars, the collapsing core will reach these temperatures before it is dense enough to be degenerate, so helium fusion will begin much more smoothly, and produce no helium flash. The core helium fusing phase of

546-502: The Stars taught a generation of astrophysicists how to apply electronic computers to the computation of stellar models. In the 1950s and ’60s he headed the Stratoscope projects, which took instrumented balloons to unprecedented heights. The first Stratoscope produced high resolution images of solar granules and sunspots , confirming the existence of convection in the solar atmosphere, and

585-486: The Sun ( R ☉ ). Stars on the horizontal branch are hotter, with only a small range of luminosities around 75  L ☉ . Asymptotic-giant-branch stars range from similar luminosities as the brighter stars of the red-giant branch, up to several times more luminous at the end of the thermal pulsing phase. Among the asymptotic-giant-branch stars belong the carbon stars of type C-N and late C-R, produced when carbon and other elements are convected to

624-405: The Sun. After some billions more years, they start to become less luminous and cooler even though hydrogen shell burning continues. These become cool helium white dwarfs. Very-high-mass stars develop into supergiants that follow an evolutionary track that takes them back and forth horizontally over the H–R diagram, at the right end constituting red supergiants . These usually end their life as

663-479: The fields of stellar structure and stellar evolution led to improved understanding of pulsating stars, differential solar rotation, post-main sequence evolutionary tracks on the Hertzsprung-Russell diagram (including how stars become red giants), hydrogen shell sources , the helium flash , and the ages of star clusters . With Fred Hoyle , he computed some of the first stellar models to correctly ascend

702-440: The habitable zone for 5.8 billion years and 2.1 billion years, respectively; for stars more massive than the Sun, the times are considerably shorter. As of 2023, several hundred giant planets have been discovered around giant stars. However, these giant planets are more massive than the giant planets found around solar-type stars. This could be because giant stars are more massive than the Sun (less massive stars will still be on

741-408: The habitable zone lasts from 100 million years for a planet with an orbit similar to that of Mars to 210 million years for one that orbits at Saturn 's distance to the Sun, the maximum time (370 million years) corresponding for planets orbiting at the distance of Jupiter . However, planets orbiting a 0.5  M ☉ star in equivalent orbits to those of Jupiter and Saturn would be in

780-504: The heating mechanisms for the chromospheres to form requires 3D simulations of red giants. Another noteworthy feature of red giants is that, unlike Sun-like stars whose photospheres have a large number of small convection cells ( solar granules ), red-giant photospheres, as well as those of red supergiants , have just a few large cells, the features of which cause the variations of brightness so common on both types of stars. Red giants are evolved from main-sequence stars with masses in

819-439: The hydrogen in the core has been fused. For the Sun, the main-sequence lifetime is approximately 10 billion years. More massive stars burn disproportionately faster and so have a shorter lifetime than less massive stars. When the star has mostly exhausted the hydrogen fuel in its core, the core's rate of nuclear reactions declines, and thus so do the radiation and thermal pressure the core generates, which are what support

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858-505: The lack of fusion, and so the outer layers of the star expand greatly, absorbing most of the extra energy from shell fusion. This process of cooling and expanding is the subgiant stage. When the envelope of the star cools sufficiently it becomes convective , the star stops expanding, its luminosity starts to increase, and the star is ascending the red-giant branch of the Hertzsprung–Russell (H–R) diagram . The evolutionary path

897-470: The lower energy density of their envelope, red giants are many times more luminous than the Sun because of their great size. Red-giant-branch stars have luminosities up to nearly three thousand times that of the Sun ( L ☉ ); spectral types of K or M have surface temperatures of 3,000–4,000  K (compared with the Sun's photosphere temperature of nearly 6,000 K ) and radii up to about 200 times

936-464: The luminosity by around 10 times. Eventually the level of helium increases to the point where the star ceases to be fully convective and the remaining hydrogen locked in the core is consumed in only a few billion more years. Depending on mass, the temperature and luminosity continue to increase for a time during hydrogen shell burning, the star can become hotter than the Sun and tens of times more luminous than when it formed although still not as luminous as

975-456: The main sequence when its core reaches a temperature (several million kelvins ) high enough to begin fusing hydrogen-1 (the predominant isotope), and establishes hydrostatic equilibrium . (In astrophysics, stellar fusion is often referred to as "burning", with hydrogen fusion sometimes termed " hydrogen burning ".) Over its main sequence life, the star slowly fuses the hydrogen in the core into helium; its main-sequence life ends when nearly all

1014-417: The range from about 0.3  M ☉ to around 8  M ☉ . When a star initially forms from a collapsing molecular cloud in the interstellar medium , it contains primarily hydrogen and helium, with trace amounts of " metals " (in astrophysics, this refers to all elements heavier than hydrogen and helium). These elements are all uniformly mixed throughout the star. The star "enters"

1053-479: The red giant is from yellow-white to reddish-orange, including the spectral types K and M, sometimes G, but also class S stars and most carbon stars . Red giants vary in the way by which they generate energy: Many of the well-known bright stars are red giants because they are luminous and moderately common. The K0 RGB star Arcturus is 36 light-years away, and Gacrux is the nearest M-class giant at 88 light-years' distance. A red giant will usually produce

1092-403: The same time, hydrogen may begin fusion in a shell just outside the burning helium shell. This puts the star onto the asymptotic giant branch , a second red-giant phase. The helium fusion results in the build-up of a carbon–oxygen core. A star below about 8  M ☉ will never start fusion in its degenerate carbon–oxygen core. Instead, at the end of the asymptotic-giant-branch phase

1131-432: The second obtained infrared spectra of planets, red giant stars, and the nuclei of galaxies. In his later years he made significant contributions toward understanding the dynamics of elliptical galaxies. Schwarzschild was renowned as a teacher and held major leadership positions in several scientific societies. In the 1980s, Schwarzschild applied his numerical skills to building models for triaxial galaxies. Schwarzschild

1170-435: The star against gravitational contraction . The star further contracts, increasing the pressures and thus temperatures inside the star (as described by the ideal gas law ). Eventually a "shell" layer around the core reaches temperatures sufficient to fuse hydrogen and thus generate its own radiation and thermal pressure, which "re-inflates" the star's outer layers and causes them to expand. The hydrogen-burning shell results in

1209-446: The star has about 0.2 to 0.5  M ☉ , it is massive enough to become a red giant but does not have enough mass to initiate the fusion of helium. These "intermediate" stars cool somewhat and increase their luminosity but never achieve the tip of the red-giant branch and helium core flash. When the ascent of the red-giant branch ends they puff off their outer layers much like a post-asymptotic-giant-branch star and then become

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1248-428: The star takes as it moves along the red-giant branch depends on the mass of the star. For the Sun and stars of less than about 2  M ☉ the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further. Once the core is degenerate , it will continue to heat until it reaches a temperature of roughly 1 × 10  K , hot enough to begin fusing helium to carbon via

1287-509: The star will eject its outer layers, forming a planetary nebula with the core of the star exposed, ultimately becoming a white dwarf . The ejection of the outer mass and the creation of a planetary nebula finally ends the red-giant phase of the star's evolution. The red-giant phase typically lasts only around a billion years in total for a solar mass star, almost all of which is spent on the red-giant branch. The horizontal-branch and asymptotic-giant-branch phases proceed tens of times faster. If

1326-488: The surface in what is called a dredge-up . The first dredge-up occurs during hydrogen shell burning on the red-giant branch, but does not produce a large carbon abundance at the surface. The second, and sometimes third, dredge-up occurs during helium shell burning on the asymptotic-giant branch and convects carbon to the surface in sufficiently massive stars. The stellar limb of a red giant is not sharply defined, contrary to their depiction in many illustrations. Rather, due to

1365-492: The turbulence and granulation in the Sun 's photosphere . Stratoscope II, a 36-inch (91.4 cm) reflecting telescope, flew from 1963 to 1971. This larger project proved to be beyond the ability of the university-led research team funded by ONR and, later, the National Science Foundation , so was managed by NASA as a beginning of its scientific ballooning program led by Nancy Grace Roman . The gondola it

1404-440: The very low mass density of the envelope, such stars lack a well-defined photosphere , and the body of the star gradually transitions into a ' corona '. The coolest red giants have complex spectra, with molecular lines , emission features, and sometimes masers , particularly from thermally pulsing AGB stars. Observations have also provided evidence of a hot chromosphere above the photosphere of red giants, where investigating

1443-568: Was a German-American astrophysicist. Schwarzschild was born in Potsdam into a distinguished German Jewish academic family. His father was the physicist Karl Schwarzschild and his uncle the astrophysicist Robert Emden . His sister, Agathe Thornton , became a classics scholar in New Zealand. In line with a request in his father's will, his family moved to Göttingen in 1916. Schwarzschild studied at

1482-506: Was mounted on weighed 3.5 tons. It studied planetary atmospheres , the atmospheres of red giant stars, and galaxies . On early flights of Stratoscope II, photographic film was used, but this was soon replaced by television detectors. This telescope -related article is a stub . You can help Misplaced Pages by expanding it . Martin Schwarzschild Martin Schwarzschild (May 31, 1912 – April 10, 1997)

1521-572: Was the Eugene Higgins Professor Emeritus of Astronomy at Princeton University , where he spent most of his professional life. Red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ( M ☉ )) in a late phase of stellar evolution . The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000  K [K] (4,700 °C; 8,500 °F) or lower. The appearance of

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