The Life Cycle of a Star

9 Key excerpts on "The Life Cycle of a Star"

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  Future of the Earth & Its Decisive Factors

  • (Author)
  • 2014(Publication Date)
  • Learning Press

    (Publisher)

  ________________________ WORLD TECHNOLOGIES ________________________ Chapter-5 Stellar Evolution Life cycle of a Sun-like star. Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years (for the most massive) to trillions of years (for the least massive, which is considerably more than the age of the universe). Stellar evolution is not studied by observing the life of a single star, as most stellar changes occur too slowly to be detected, even over many centuries. Instead, astro-physicists come to understand how stars evolve by observing numerous stars at the various points in their life, and by simulating stellar structure with computer models. ________________________ WORLD TECHNOLOGIES ________________________ Projected timeline of the Sun's life Birth A dense starfield in Sagittarius. ________________________ WORLD TECHNOLOGIES ________________________ Stellar evolution begins with the gravitational collapse of a giant molecular cloud (GMC). Typical GMCs are roughly 100 light-years (9.5×10 14 km) across and contain up to 6,000,000 solar masses (1.2×10 37 kg). As it collapses, a GMC breaks into smaller and smaller pieces. In each of these fragments, the collapsing gas releases gravitational potential energy as heat. As its temperature and pressure increase, a fragment condenses into a rotating sphere of superhot gas known as a protostar. Protostars with masses less than roughly 0.08 M ⊙ (1.6×10 29 kg) never reach temperatures high enough for nuclear fusion of hydrogen to begin. These are known as brown dwarfs. Brown dwarfs heavier than 13 Jupiter masses (2.5 × 10 28 kg) do fuse deuterium, and some astronomers prefer to call only these objects brown dwarfs, classifying anything larger than a planet but smaller than this a sub-stellar object.

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  Gravity from the Ground Up

  An Introductory Guide to Gravity and General Relativity

  • Bernard Schutz(Author)
  • 2003(Publication Date)
  • Cambridge University Press

    (Publisher)

  Birth to death: the life cycle of the stars 12 12 T he cycle of birth, aging, death, and re-birth of stars dominates the activity of In this chapter: stars form in molecular clouds and die when they burn up their fuel. Small stars die quietly as white dwarfs, larger stars explode as supernovae. In both cases, they return some of their material to the interstellar medium so that new stars and planets can form. White dwarfs, and the neutron stars that usually form in supernova explosions, are remarkable objects. They are supported against gravity by purely quantum effects, so they do not need nuclear reactions or heat to keep their structure. We learn about the quantum principles involved and use them to calculate the size and maximum mass of white dwarfs. ordinary galaxies like our own Milky Way. The cycle generates the elements of which our own bodies are made, produces spectacular explosions called supernovae, and leaves behind “cinders”: remnants of stars that will usually no longer participate in the cycle. We call these white dwarfs, neutron stars, and black holes. Governing this cycle is, as everywhere, gravity. An imbalance between gravity and heat in a transparent gas cloud leads to star formation. The long stable life of a star is a robust balance between nuclear energy generation and gravity. This balance is finally lost when the star runs out of nuclear fuel, leading to a quiet death as a white dwarf or to a violent death as a supernova. Even the cinders, all unusual objects, can be understood from simple calculations based on elementary physical ideas. We have already met black holes. White dwarfs and neutron stars exist in a balance between gravity and quantum effects: they il- lustrate the deepest principles of quantum theory. Exotic as these may seem, life on Earth would not exist without the neutron stars and white dwarfs of our Galaxy.

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  The Sciences

  An Integrated Approach

  • James Trefil, Robert M. Hazen(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  319 14.4 THE LIFE CYCLES OF STARS The Main Sequence and the Death of Stars Every star begins as an immense ball of hydrogen and helium, formed by the gravitational collapse of a nebula. he ultimate fate of any given star, however, depends on the total mass of hydrogen and helium. he Hertzsprung-Russell dia- gram provides the key to understanding the very diferent fates of stars. Stars Much Less Massive Than the Sun All stars begin their lives in the hydrogen-burning stage—the stage represented by main sequence stars on the Hertzsprung- Russell diagram. If a star is much less massive than the Sun— perhaps only 10% of the Sun’s mass—it will be just barely large enough to begin hydrogen burning in its core. Such a small star, called a brown dwarf, shines faintly compared to the Sun, with surface temperatures of only a few thousand degrees. Nuclear fusion proceeds relatively slowly, so such a star will continue to glow steadily for a hundred billion years without any signiicant change in size, temperature, or energy output. Stars about the Mass of the Sun he Sun and other stars of similar mass enjoy a more central position on the Herzsprung- Russell main sequence (Figure 14-17). he greater mass of the Sun, relative to brown dwarf stars, means that core temperatures and pressures are much higher and hydrogen burn- ing proceeds at a much faster rate. Consequently, the Sun has a higher surface tempera- ture, and it completes its hydrogen-burning phase much more quickly—in a matter of a few billion years. One way to look at the life of a star like the Sun is to think of it as a continual battle against the force of gravity. From the moment when the Sun’s original gas cloud started to contract, the force of gravity acted on every particle, forcing it inward and trying to make the entire structure collapse on itself. When the nuclear ires ignited in the core of the Sun 4.5 billion years ago, gravity was held at bay.

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  The Sciences

  An Integrated Approach

  • James Trefil, Robert M. Hazen(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  Our solar system would appear as a small dot next to this nebula. 14.4 | The Life Cycles of Stars 385 region of the nebula, the pressure and temperature at the center of the proto-star begins to climb. Once this central mass achieves a critical size, the pressure and temperature deep inside will become high enough to initiate nuclear fusion reactions. At that moment, a star is born. The Main Sequence and the Death of Stars Every star begins as an immense ball of hydrogen and helium, formed by the gravitational collapse of a nebula. The ultimate fate of any given star, however, depends on the total mass of hydrogen and helium. The Hertzsprung-Russell diagram provides the key to understanding the very different fates of stars. Stars Much Less Massive Than the Sun All stars begin their lives in the hydrogen-burning stage—the stage represented by main sequence stars on the Hertzsprung-Russell diagram. If a star is much less massive than the Sun—perhaps only 10% of the Sun’s mass—it will be just barely large enough to begin hydrogen burning in its core. Such a small star, called a brown dwarf, shines faintly compared to the Sun, with surface temperatures of only a few thousand degrees. Nuclear fusion proceeds relatively slowly, so such a star will continue to glow steadily for a hun- dred billion years without any significant change in size, temperature, or energy output. Stars about the Mass of the Sun The Sun and other stars of similar mass enjoy a more central position on the Herzsprung- Russell main sequence (Figure 14.16). The greater mass of the Sun, relative to brown dwarf stars, means that core temperatures and pressures are much higher and hydrogen burning proceeds at a much faster rate. Consequently, the Sun has a higher surface tem- perature, and it completes its hydrogen-burning phase much more quickly—in a matter of a few billion years. One way to look at the life of a star like the Sun is to think of it as a continual battle against the force of gravity.

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  An Introduction to the Theory of Stellar Structure and Evolution

  • Dina Prialnik(Author)
  • 2009(Publication Date)
  • Cambridge University Press

    (Publisher)

  This is sketched in Figure 12.7 . The term ‘generation of stars’ is somewhat misleading, for we have seen that stellar lifetimes differ by as much as four orders of magnitude, depending on the initial mass. Thus a succession of a great many generations of massive stars may coincide with only one single generation of low-mass stars. The different ways by which stars return material to the interstellar medium are illustrated by the images of Figure 12.8 , where the shell ejected by a nova outburst ( Section 11.6 ) is shown in addition to the wind from a massive star ( Section 9.9 ), another example of a planetary nebula ( Section 9.7 ), and the shell ejected by supernova SN1987A ( Section 10.3 ). We note the conspicuous similarity of these images, despite the huge differences in length and time scales. The ejected material has been pro-cessed, however, and its composition differs from the prevailing composition of the galactic gas. Thus, later generations of stars have, at birth, increasingly larger abundances of heavy elements (or metals ). The survivors of the entire evolution process are dense compact stars – white dwarfs, neutron stars, and, possibly, black holes – as well as brown dwarfs and low-mass main-sequence stars, whose main-sequence life spans exceed the present age of the universe. In the end, when the entire gas reservoir will have been locked up in these small and mostly faint stars, star formation will cease. 244 12 The stellar life cycle Figure 12.8 Illustration of mass loss by images taken with NASA’s Hubble Space Tele-scope: (a) nebula (Pistol) ejected by a massive star (estimated at ∼ 100 M ) extending in radius to ∼ 4 ly (photograph by D. F. Figer, University of California at Los Angeles); (b) mass ejected by SN1987A: the ring of gas, about 1.5 ly in diameter, was expelled by the progenitor star some 2 × 10 4 yr before the supernova explosion.

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  Investigating Life in the Universe

  Astrobiology and the Search for Extraterrestrial Life

  • Christopher K. Walker(Author)
  • 2023(Publication Date)
  • CRC Press

    (Publisher)

  The entire star formation process, from being an interstellar molecular cloud clump with a gas density far less than that of the air you breathe, to being a solar mass star with a young planetary system, takes only ~10 million years; about 0.1% of the star’s main sequence lifetime (see Figure 4.11). FIGURE 4.11 Stages of protostellar evolution. In just ~10 7 years a young planetary system can form from the collapse of a clump within a Giant Molecular Cloud (GMC), providing an oasis for life in a vast cosmic desert. Surrounding the system at a distance of ~1 ly is a cloud (i.e., the Oort Cloud) of cometary debris left over from the planet formation process. Occasionally gravitational perturbations will send one of these comets into the inner solar system, providing a fossil record of the primordial solar system for Earth-bound observers. Image credits: M16: NASA, Jeff Hester, and Paul Scowen; HH-30: NASA Hubble; Fomalhaut Protoplanetary system: David A. Hardy. Where exactly a star will end up on the main sequence depends primarily on its mass, with higher mass stars on the upper left and lower mass stars on the lower right (see Figure 4.9). While on the main sequence the star is fusing hydrogen into helium in its core. Hydrogen is the fuel the star burns and helium is the ash left behind. In the case of a nuclear bomb the initiation of a fusion reaction leads to an outward explosion. In the case of a main sequence star, the outward pressure generated from the heat of the nuclear reaction in its core is held in check by the inward pressure exerted by gravity trying to pull everything toward the center. When the tug of war between a star’s gravitational energy and thermal energy is a stalemate, the star is said to be in hydrostatic equilibrium. A star will remain on the main sequence until a significant fraction of the hydrogen in its core is consumed. For a solar mass star, this will not happen for ~10 billion years (Eq. 4.7)

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  Stellar Evolution Physics: Volume 1, Physical Processes in Stellar Interiors

  • Icko Iben(Author)
  • 2012(Publication Date)
  • Cambridge University Press

    (Publisher)

  Two additional primary avenues of stellar evolution require mention. Single stars ini- tially more massive than about 10 times the mass of the Sun continue to burn nuclear fuels until they form a core composed of iron-peak elements. The core collapses into a neutron star or black hole, depending upon the mass of the initial main sequence precursor, and the heavy-element-rich envelope is ejected into the interstellar medium, appearing as a type II supernova. These stars are the source of perhaps half of all of the iron and of most of the other elements heavier than helium in the Universe other than the nitrogen, carbon, and s-process elements produced by less massive stars. The neutron star remnants occupy approximately one tenth of the graveyard of “dead” stars and the black holes occupy one one hundredth. The label “dead” has here been applied to the compact objects into which nuclear burn- ing stars ultimately evolve for poetic effect. The label does such stars a serious injustice. Although they are not burning a nuclear fuel, they are releasing from their surfaces energy that is produced in most interesting ways: in the case of white dwarfs, by cooling of heavy ions in their interiors; in the case of neutron stars known as millisecond pulsars, by the interaction of energetic electrons with a magnetic field which transforms rotational kinetic energy into beamed radiation at radio wavelengths. In the interiors of these stars, which are very much alive, conditions are so far from those achievable in terrestial laboratories that considerable imagination must be exercised in contemplating their behavior, and the exercise leads to a more complete understanding of the physics of matter. The last avenue of evolution which deserves mention is that followed by close binary stars which can transfer mass back and forth and evolve into configurations not accessible to single stars.

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  Chemistry in Space

  From Interstellar Matter to the Origin of Life

  • Dieter Rehder(Author)
  • 2011(Publication Date)
  • Wiley-VCH

    (Publisher)

  Stars of about the present mass of our Sun (F-, G-, and K-type stars), condensing out of a gas and dust nebula, start as subgiant (luminosity class IV) pre -main sequence stars, arriving at the main sequence after about 100 million years. These Sun-like stars will spend most of their lifetime on the main sequence, and finally end up as white dwarfs. For the Sun, the dwell time on the main sequence is estimated to cover another 10 billion years, in addition to the 4.6 billion years that already have elapsed. Depending on their mass, old stars will evolve through planetary nebulae or supernovae into white dwarfs, neutron stars, or black holes; for planetary nebulae (PN) and supernovae, see Figures 3.3a and b, for a white dwarf "devouring" a giant companion star, see Figure 3.3c. Figure 3.3 Examples for stars at their evolutionary end stages. (a) Cat’s Eye Nebula, a PN in the constellation of Draco. Credit: NASA, ESA; J. Hester and A. Loll (Arizona State University). (b) A recent photo of the Crab Nebula in the constellation of Taurus, a remnant of a supernova dating back to 1054. The central star is a pulsar (cf. Section 3.1.2). Credit: NASA, ESA, HEIC and the Hubble Heritage Team. (c) The nova Mira (= o Ceti), a variable binary system: a white dwarf, Mira B, drags and accretes matter from its red giant companion Mira A. Credit: NASA/CXC/SAO/M; Karovska et al. The typical stages of the development of a star of approximately the mass of the Sun are represented by tracks 1-6 of the evolution line in the HR diagram, Figure 3.2. In short, these stages are as given below: 1) Accretion of a nebula into a protostar that further develops into a T-Tauri variable and finally commences hydrogen fusion to helium (hydrogen burning)

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  The Milky Way and Beyond: Stars, Nebulae, and Other Galaxies

  • Britannica Educational Publishing, Erik Gregersen(Authors)
  • 2009(Publication Date)
  • Britannica Educational Publishing

    (Publisher)

  TAR FORMATION AND EVOLUTION

  T hroughout the Milky Way Galaxy (and even near the Sun itself), astronomers have discovered stars that are well evolved or even approaching extinction, or both, as well as occasional stars that must be very young or still in the process of formation. Evolutionary effects on these stars are not negligible, even for a middle-aged star such as the Sun. More massive stars must display more spectacular effects because the rate of conversion of mass into energy is higher. While the Sun produces energy at the rate of about two ergs per gram per second, a more luminous main-sequence star can release energy at a rate some 1,000 times greater. Consequently, effects that require billions of years to be easily recognized in the Sun might occur within a few million years in highly luminous and massive stars. A supergiant star such as Antares, a bright main-sequence star such as Rigel, or even a more modest star such as Sirius cannot have endured as long as the Sun has endured. These stars must have been formed relatively recently.

  BIRTH OF STARS AND EVOLUTION TO THE MAIN SEQUENCE

  Detailed radio maps of nearby molecular clouds reveal that they are clumpy, with regions containing a wide range of densities—from a few tens of molecules (mostly hydrogen) per cubic centimetre to more than one million. Stars form only from the densest regions, termed cloud cores, though they need not lie at the geometric centre of the cloud. Large cores (which probably contain subcondensations) up to a few light-years in size seem to give rise to unbound associations of very massive stars (called OB associations after the spectral type of their most prominent members, O and B stars) or to bound clusters of less massive stars. Whether a stellar group materializes as an association or a cluster seems to depend on the efficiency of star formation.

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