Life Cycle of the Stars - Coursework Example

Introduction: A star is a massive, luminous ball of plasma held together by gravity. The nearest star to Earth is the Sun, which is the source of energy on Earth. Other stars are visible from Earth during the night when they are not outshone by the Sun or blocked by atmospheric phenomena. Historically, the most prominent stars on the celestial space were grouped together into constellations and asterisms. For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen in its core releasing energy that traverses the stars interior and then radiates into outer space. Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzprung-Russell diagrams (H–R diagram), allows the age and evolutionary state of a star to be determined. 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, astrophysicists 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. Stellar Evolution: A nebula is a cloud of gas (hydrogen) and dust in space. Nebulae are the birthplaces of stars. There are different types of nebula. An Emission Nebula which glows brightly because the gas in it is energized by the stars that have already formed within it. In a Reflection Nebula, starlight reflects on the grains of dust in a nebula. The nebula surrounding the Pleiades Cluster is typical of a reflection nebula. Dark Nebula also exists. These are dense clouds of molecular hydrogen which partially or completely absorb the light from stars behind them. 1st stage of a stars life: PROTOSTAR Stellar evolution begins with the gravitational collapse of a giant molecular cloud (GMC). 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 prostar. Protostars with masses less than roughly 0.08 M (1.6×1029 kg) never reach temperatures high enough for nuclear fusion of hydrogen to begin. These are known as brown dwarfs. For a more massive prostar, the core temperature will eventually reach 10 million kelvins, initiating the proton-proton chain reaction and allowing hydrogen to fuse, first to deuterium and then to helium. The onset of nuclear fusion leads relatively quickly to a hydrostatic equilibrium in which energy released by the core exerts a "radiation pressure" balancing the weight of the stars matter, preventing further gravitational collapse. The star thus evolves rapidly to a stable state, beginning the main sequence phase of its evolution. A new star will fall at a specific point on the main sequence of the Hertzprung-Russell diagrams, with the main sequence spectral type depending upon the mass of the star. Small, relatively cold, low mass red dwarfs burn hydrogen slowly and will remain on the main sequence for hundreds of billions of years, while massive, hot super giants will leave the main sequence after just a few million years. A mid-sized star like the Sun will remain on the main sequence for about 10 billion years. . A star of less than about 0.5 solar mass will never be able to fuse helium even after the core ceases hydrogen fusion. These are the red dwarfs, some of which will live thousands of times longer than the Sun. If a stars core becomes stagnant it will still be surrounded by layers of hydrogen which the star may subsequently draw upon. However, if the star is fully convective, it will not have such surrounding layers. If it does, it will develop into a red giant. But never fuse helium as they do and it will simply contract until electron degeneracy pressure halts its collapse, thus directly turning into a white dwarf. 2nd Stage of Stars Life: MAIN SEQUENCE STAR In either case of the star beginning helium fusion or halting fusion due to hydrostatic equilibrium from electron degeneracy pressure, the accelerated fusion in the hydrogen-containing layer immediately over the core causes the star to expand. This lifts the outer layers away from the core, reducing the gravitational pull on them, and they expand faster than the energy production increases. This causes them to cool, which causes the star to become redder than when it was on the main sequence. Such stars are known as red giants. 3rd Stage of Stars Evolution: GIANTS AND SUPERGIANTS This is a large bright star with a cool surface. It is formed during the later stages of the evolution of a star like the Sun, as it runs out of hydrogen fuel at its centre. Red giants have diameters between 10 and 100 times that of the Sun. They are very bright because they are so large, although their surface temperature is lower than that of the Sun, about 2000-3000?C. Very large stars (red giants) are often called Super Giants. These stars have diameters up to 1000 times that of the Sun and have luminosities often 1,000,000 times greater than the Sun. 4th stage of Stars Evolution: WHITE DWARF STAR These are very cool, faint and small stars, approximately one tenth the mass and diameter of the Sun. They burn very slowly and have estimated lifetimes of 100 billion years. This is very small, hot star, the last stage in the life cycle of a star like the Sun. White dwarfs have a mass similar to that of the Sun, but only 1% of the Suns diameter; approximately the diameter of the Earth. White dwarfs are the wasted remains of normal stars, whose nuclear energy supplies have been used up. White dwarf consists of degenerate matter with a very high density due to gravitational effects, i.e. one spoonful has a mass of several tones. White dwarfs cool and fade over several billion years. 5th Stage of Stars Evolution: NEUTRON STAR In massive stars, the core is already large enough at the onset of hydrogen burning shell that helium ignition will occur before electron degeneracy pressure has a chance to become prevalent. Thus, when these stars expand and cool, they do not brighten as much as lower mass stars; however, they were much brighter than lower mass stars to begin with, and are thus still brighter than the red giants formed from less massive stars. These stars are unlikely to survive as red super giants; instead they will destroy themselves as Supernovae Type II. These stars are composed mainly of neutrons and are produced when a supernova explodes, forcing the protons and electrons to combine to produce a neutron star. Neutron stars are very dense. Typical stars having a mass of three times the Sun but a diameter of only 20 km. If its mass is any greater, its gravity will be so strong that it will shrink further to become a black hole. Pulsars are believed to be neutron stars that are spinning very rapidly. BLACK HOLES Black holes are believed to form from massive stars at the end of their life times. The gravitational pull in a black hole is so great that nothing can escape from it, not even light. The density of matter in a black hole cannot be measured. Black holes distort the space around them, and can often suck neighboring matter into them including stars. The figure below shows the Life Cycle of the Sun: Projected timeline of the Suns life. (Retreived on 14th June 2011 from http://en.wikipedia.org/wiki/File:Solar_Life_Cycle.svg) Stellar Properties; such as luminosity, temperature, density, and chemical composition: The most important properties of a star are its size, mass, luminosity, chemical composition, and the temperature, pressure, and density at all distances from its center to its surface. Stars vary widely in size and luminosity but have masses only within the range from about 0.08 to 100 times the mass of the sun, with few exceptions; less massive bodies cannot support the energy-producing processes of a star, while more massive bodies are generally unstable. An ordinary star has a surface temperature of thousands of degrees, implying central temperatures of millions of degrees. The central pressure and density are also extremely high, but the temperature is such that the material will still remain in the gaseous state. At these temperatures, energy is produced by thermonuclear fusion, in which two or more nuclei are fused to form a single heavier nucleus. As such fusion processes proceed within the star, its chemical composition necessarily changes, with heavier elements increasing at the expense of lighter elements. The mass and chemical composition of the star together determines all of its other properties, e.g., size, luminosity, and temperature. Astronomers can determine the temperature and chemical composition of the stars surface from analysis of the spectrum of light from the star. Such a spectrum consists of a continuous black body spectrum produced by complex conditions within the star superimposed on which is a series of dark lines due to absorption of energy by the cooler stellar atmosphere. Different layers of the stars transport heat up and outwards in different ways, primarily convection and radiative transfer but thermal condition is important in white dwarfs. The internal structure of a main sequence star depends upon the mass of the star. In massive stars, the core temperature is above about 1.8×107K, so hydrogen to helium fusion occurs primarily via the CNO cycle. In the outer portion of the star, the temperature gradient is shallower but the temperature is high enough that the hydrogen is nearly fully ionized, so the star remains transparent to ultraviolet radiation. Thus, massive stars have a radiative envelope. The lowest mass main sequence stars have no radiation zone; the dominant energy transport mechanism throughout the star is convection. Giants are also fully convective.] Age: Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old—the observed age of the universe. The oldest star yet discovered, HE 1523–0901, is an estimated 13.2 billion years old. The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of about one million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years. Chemical composition: When stars form in the present Milky Way galaxy they are composed of about 71% hydrogen and 27% helium, as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. Because the molecular clouds where stars form are steadily enriched by heavier elements from supernovae explosions, a measurement of the chemical composition of a star can be used to infer its age. The portion of heavier elements may also be an indicator of the likelihood that the star has a planetary system. The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun. By contrast, the super-metal-rich star µ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron. There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements. Diameter: Stars vary widely in size. Due to their great distance from the Earth, all stars except the Sun appear to the human eye as shining points in the night sky that twinkle because of the effect of the Earths atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds. Magnetic field: The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, generating magnetic fields that extend throughout the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the stars rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Mass: One of the most massive stars known is Eta Carinae with 100–150 times as much mass as the Sun; its lifespan is very short—only several million years at most. A study of the Arches cluster suggests that 150 solar masses is the upper limit for stars in the current era of the universe. A star named R136a1 in the RMC 136a star cluster has been measured at 265 solar masses. Temperature: The surface temperature of a main sequence star is determined by the rate of energy production at the core and the radius of the star and is often estimated from the stars color index. The stellar temperature will determine the rate of energization or ionization of different elements, resulting in characteristic absorption lines in the spectrum. Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3,600 K, but they also have a high luminosity due to their large exterior surface area. Below is a table of the nearest stars: Name Apparent Magnitude Parallax () Distance (pc) Distance (LY) Notes Proxima Centauri +10.7 0.763 1.31 4.27 A companion to Alpha Centauri - this is the nearest star but is nearly 5 magnitudes too faint to be visible with the naked eye. Alpha Centauri 0.0 and 1.4 0.752 1.33 4.33 A binary star consisting of two stars that appear as one to the naked eye. This is the third brightest star in the sky. Barnards Star +9.5 0.545 1.83 5.98 Another star too faint to be seen with the naked eye. Wolf 359 +13.7 0.425 2.35 7.67 This is the faintest star in the table. Lalande 21185 +7.5 0.398 2.51 8.19 Yet another faint star. Sirius -1.5 and 8.7 0.375 2.67 8.69 The brightest star is also one of the closest. It has a faint companion. The following table shows the properties of stars of different spectral types. Spectral Type Prominent Lines Colour Colour Index Temperature (K) Examples O He+, He, H, O2+, N2+, C2+, Si3+ Blue -0.45 45,000 Zeta Puppis B He, H, C+, O+, N+, Fe2+, Mg2+ Bluish White -0.20 30,000 Rigel A H, ionised metals White 0.00 12,000 Sirius F H, Ca+, Ti+, Fe+ Yellowish White +0.40 8,000 Procyon G Ca+, Fe, Ti, Mg, H, some molecular bands Yellow +0.60 6,500 Capella K Ca+, H, molecular bands Orange +1.00 5,000 Aldebaran M TiO, Ca, molecular bands Red +1.5 3,500 Betelgeux Energy output of main-sequence stars: Main sequence stars are characterized by the source of their energy. They are all undergoing fusion of hydrogen into helium within their cores. The rate at which they do this and the amount of fuel available depends upon the mass of the star. All main sequence stars have a core region where energy is generated by nuclear fusion. The temperature and density of this core are at the levels necessary to sustain the energy production that will support the remainder of the star. A reduction of energy production would cause the overlaying mass to compress the core, resulting in an increase in the fusion rate because of higher temperature and pressure. Likewise an increase in energy production would cause the star to expand, lowering the pressure at the core. Thus the star forms a self-regulating system in hydrostatic equilibrium that is stable over the course of its main sequence lifetime. Main sequence stars employ two types of hydrogen fusion processes, and the rate of energy generation from each type depends on the temperature in the core region. Astronomers divide the main sequence into upper and lower parts, based on which of the two is the dominant fusion process. In the lower main sequence, energy is primarily generated as the result of the proton-proton chain, which directly fuses hydrogen together in a series of stages to produce helium. Stars in the upper main sequence have sufficiently high core temperatures to efficiently use the CNO cycle. This process uses atoms of carbon, nitrogen and oxygen as intermediaries in the process of fusing hydrogen into helium. The more massive the star, the greater its gravitational pull inwards. This in turn compresses the gas more. In stars this gas pressure alone is not sufficient to withstand the gravitational collapse. Once the core temperature has reached about 10 million K, fusion of hydrogen occurs, releasing energy. This energy exerts an outwards radiation pressure due to the action of the photons on the extremely dense matter in the core. The radiation pressure combined with the gas pressure balances the inward pull of gravity preventing further collapse. Comparison between the physics of energy of main-sequence stars, red giants, and white dwarfs: Our Sun is a main sequence star. The core temperature of main sequence stars is hot enough (> about 10 million K) that hydrogen nuclei (protons) can fuse together. The net result of this is that, hydrogen is fused to form helium nuclei. In the process a small amount of mass is converted into energy, released in the form of high-energy gamma photons. This hydrogen fusion provides the radiation pressure that supports main sequence stars against further gravitational collapse and ultimately is the source of energy fuelling life on Earth. The proton-proton chain is the main hydrogen fusion sequence powering main sequence stars such as our Sun and those of lower mass. The net result is that four protons are fused to form a He-4 nucleus, gamma photons, positrons and neutrinos. The total mass of the products is slightly less than the constituents - the difference being converted to and released as energy. A different sequence, the CNO cycle (for carbon-nitrogen-oxygen) dominates in higher mass main sequence stars. In the CNO cycle carbon-12 nuclei act as nuclear catalysts but the overall result is much the same as for the proton-proton chain, four protons are converted into a He-4 nucleus, releasing energy, primarily as high-energy gamma photons. A star with a mass of about one-tenth that of the Sun has just enough gravitational force to heat the core to about 10 million K, the temperature needed for hydrogen fusion to start. If a protostar is less massive than this, fusion cannot be triggered and it becomes a brown dwarf or a "failed" star, emitting energy in the infrared. The greater the mass of a main sequence star, the higher its core temperature and the greater the rate of its hydrogen fusion. Higher-mass stars therefore produce more energy and are thus more luminous than lower mass ones. This comes at a cost though. High mass stars consume their core hydrogen fuel much faster than lower-mass ones. Our Sun has sufficient hydrogen in its core to last about 10 billion years (1010 years) on the main sequence. A five solar-mass star would consume its core hydrogen in about 70 million years whilst an extremely massive star may only last three or four million years. In Red Giants, Inward gravitational attraction causes the helium core to contract, converting gravitational potential energy into thermal energy. Although fusion is no longer taking place in the core, the rise in temperature heats up the shell of hydrogen surrounding the core until it is hot enough to start hydrogen fusion, producing more energy than when it was a main sequence star. Convection transports the energy to the outer layers of the star from the shell-burning region. The stars luminosity eventually increases by a factor of 1000 × or so. During this stage of expansion, the star will move up and to the right on the HR diagram along the Red Giant Branch (RGB). A G (V)-class star such as our Sun may end up as a high-K or low-M giant. Stars such as this are commonly referred to as red giants. In Supergiants & Supernovae Once this fuel is used up, the core contracts due to gravity and heats up. This triggers helium-burning in the core. Unlike lower-mass stars, this helium fusion (triple-alpha process) starts gradually rather than in a helium flash. In moving off the main sequence, the effective temperature of the star drops as its outer layers expand. The decrease in temperature balances the increased radius so that the overall luminosity remains essentially constant. Energy liberated by helium fusion in the core raises the temperature of the surrounding hydrogen shell so that it too begins fusing. Stars with masses similar to our Sun will end up as white dwarfs. A white dwarf is composed of carbon and oxygen ions mixed in with a sea of degenerate electrons. It is the degeneracy pressure provided by the electrons that prevents further collapse. A white dwarf, with a mass roughly that of the Sun packed into a volume not much greater than the Earth must have an extremely high density. At 109 kg m-3 its density is one million times greater than that of water. Although its surface temperature is about 10,000 K, the core temperature may be as high as 107 K. The heat trapped within a white dwarf will gradually be radiated away by it but with its small radius, a white dwarf has only a small surface area. Heat therefore cannot escape quickly. The evolution of a star and where the Sun is in its life cycle: The life cycle of a star, from birth in a dense interstellar cloud to its final end, is a process that lasts anywhere from a few Million years for the most massive stars to many Billions of years for stars like the Sun.. By observing many different star clusters, each with a different age, we have pieced together how the clusters age and work out the timescales of "stellar evolution". The sun formed out of a dark cloud of cold molecular Hydrogen gas and dust. Starts out as a dense globule of gas. Breaks away from the parent cloud. Slowly collapses and heats up. Overall, the formation process for a star the mass of the Sun takes about 50 Myr. Molecular gas cloud fragments in the Eagle Nebula (M16) star-formation complex. This spectacular Hubble Space Telescope image shows regions of molecular gas and dust, most with faint embedded protostars, being illuminated by a hot young star off the edge of this field of view. The individual lumps breaking off the ends of these "elephant trunks" of gas are comparable in size and mass to the pre-stellar clumps out of which stars like the Sun might form. In this region, however, it appears that many of these clumps are being evaporated by the hot UV radiation from the nearby hot stars before they have a chance to form into low-mass stars. Collapse of the proto-Sun continued until the core temperature reached 15 Million K: At this point: Fusion of Hydrogen into Helium ignited in the core, releasing energy. The extra fusion energy raised the internal pressure enough to stop the gravitational collapse of proto-Sun. Sunlight and a fast solar wind streaming off the infant Sun blew away the remaining gas cloud, except for a dense disk of gas and solid particles that formed in the equatorial plane of the proto-Sun. The Sun today is a middle-aged star with the following properties: Age: 4.55 Billion Years Mass: 1 Msun = 1.99x1033 g Radius: 1 Rsun = 700,000 km Luminosity: 1 Lsun = 3.83x1026 Watts Temperature: 5779 K Fuel Supply: 50% of the core Hydrogen has been consumed. The Sun is about halfway through its main-sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than 4 million tonnes of matter are converted into energy within the Suns core, producing neutrinos and solar radiation; at this rate, the Sun will have so far converted around 100 Earth-masses of matter into energy. The Sun will spend a total of approximately 10 billion years as a main sequence star Comparison between the prosperity of Life on a planet that evolves around a yellow dwarf star and a red dwarf star: Planetary habitability of red dwarf star systems is subject to some debate. In spite of their great numbers and long lifespans, there are several factors which may make life difficult on planets around a red dwarf star. First, planets in the habitable zone of a red dwarf would be so close to the parent star that they would likely tidally locked. This would mean that one side would be in perpetual daylight and the other in eternal night. This could create enormous temperature variations from one side of the planet to the other. Such conditions would appear to make it difficult for forms of life similar to those on Earth to evolve. And it appears there is a great problem with the atmosphere of such tidally locked planets: the perpetual night zone would be cold enough to freeze the main gases of its atmosphere, leaving the daylight zone nude and dry. On the other hand, recent theories propose that either a thick atmosphere or planetary ocean could potentially circulate heat around such a planet, or life could survive by migration. Alternatively, a moon in orbit around a gas giant planet may be habitable. It would circumvent the tidal lock problem by becoming tidally locked to its planet. This way there would be a day/night cycle as the moon orbited its primary, and there would be distribution of heat. In addition, red dwarfs emit most of their radiation as infrared light, while on Earth plants use energy mostly in the visible spectrum. Red dwarfs emit almost no ultraviolet light, which would be a problem if UV is required for life to exist. Variability in stellar energy output may also have negative impacts on development of life. Red dwarfs are often covered by star spots, reducing stellar output by as much as 40% for months at a time. At other times, some red dwarfs, called flare stars, can emit gigantic flares, doubling their brightness in minutes. This variability may also make it difficult for life to develop and persist near a red dwarf star. REFERENCE: 1. Stellar evolution, retrieved on 14th of June 2011 from, http://en.wikipedia.org/wiki/Stellar_evolution 2. Stellar evolution, retrieved on 14th of June 2011 from, http://www.telescope.org/pparc/res9.html 3. A stars stellar life cycle, retrieved on 14th of June 2011 from, http://www.williamsclass.com/EighthScienceWork/StellarEvolution.htm 4. The Evolutionary Cycle of Stars, retrieved on 14th of June 2011 from, http://www2.glos.ac.uk/gdn/origins/earth/ch2_3.htm 5. Hansen, Carl J.; Kawaler, Steven D.; Trimble, Virginia (2004), Stellar Interiors (2nd ed.), Springer, 6. Kennedy, Dallas C.; Bludman, Sidney A. (1997), "Variational Principles for Stellar Structure", Astrophysical Journal 484 (1): 329, retrieved on 14th of June 2011 from, arXiv:astro-ph/9610099, Bibcode 1997ApJ...484..329K, doi:10.1086/304333  7. Weiss, Achim; Hillebrandt, Wolfgang; Thomas, Hans-Christoph; Ritter, H. (2004), Cox and Giulis Principles of Stellar Structure, Cambridge Scientific Publishers  8. Zeilik, Michael A.; Gregory, Stephan A. (1998), Introductory Astronomy & Astrophysics (4th ed.), Saunders College Publishing,  9. Measuring The Stars, retrieved on 15th of June 2011 from, http://www.krysstal.com/thestars.html 10. Stars, retrieved on 14th of June 2011 from, http://en.wikipedia.org/wiki/Star#Characteristics 11. Dr. Laura A. Whitlock & Ms. Kara C. Granger Buck ,The life cycle of stars, retrieved on 14th of June 2011 from, http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/Imagine2.pdf 12. Main sequence stars, retrieved on 14th of June 2011 from, http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_mainsequence.html 13. Main sequence, retrieved on 15th of June 2011 from, http://main-sequence.co.tv/#Energy_generation 14. Ancient Astronomy and Constellations, retrieved on 15th of June 2011 from, http://universe-review.ca/F08-star.htm 15. Stars & Their Energy Sources, retrieved on 15th of June 2011 from, http://outreach.atnf.csiro.au/education/senior/cosmicengine/stars_types.html 16. The Once and Future Sun, retrieved on 15th of June 2011 from http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html 17. Red dwarf, retrieved on 15th of June 2011 from http://www.google.co.in/url?q=http://en.wikipedia.org/wiki/Red_dwarf&sa=U&ei=3OL4Td-rEorOrQfgt8WiCA&ved=0CBoQFjAD&usg=AFQjCNGJuMDyNNq2Fc3DXkBl0n2e8SDc-w 18. Can life thrive around a red dwarf star? , retrieved on 15th of June 2011 from http://www.msnbc.msn.com/id/30136580/ns/technology_and_science-space/t/can-life-thrive-around-red-dwarf-star/ Read More