James Linzel's List: Assignment 2: Origins and Fate of Stars

• hey have collapsed so much that their atoms have been crushed, squashing the protons and electrons together until they merge to leave only neutrons.

• The dying core eventually forms a white dwarf - a spherical diamond the size of the Earth, made of carbon and oxygen. From this point on the Sun will gradually fade away, becoming dimmer and dimmer until its light is finally snuffed out.

• it took 300,000 years for stable hydrogen and helium atoms to form. Gradually these atoms began to clump together into gas clouds called nebulae.

• Hydrogen and helium and some lithium, boron, and beryllium were created when the universe was created. All of the rest of the elements of the universe were produced by the stars in nuclear fusion reactions. These reactions created the heavier elements from fusing together lighter elements in the central regions of the stars. When the outer layers of a star are thrown back into space, the processed material  can be incorporated into gas clouds that will later form stars and planets. The material that formed our solar system incorporated some of the remains of previous stars. All of the atoms on the Earth except hydrogen and most of the helium are  recycled material---they were not created on the Earth. They were created in the stars.
• The atoms heavier than helium up to the iron and nickel atoms were made in the  cores of stars (the process that creates iron also creates a smaller amount  of nickel too). The lowest mass stars can only synthesize helium. Stars around  the  mass of our Sun can synthesize helium, carbon, and oxygen. Massive stars (M_* > 8 solar masses) can  synthesize  helium, carbon, oxygen, neon, magnesium, silicon, sulfur, argon, calcium,  titanium, chromium, and iron (and nickel). Elements heavier than iron are made in supernova explosions from the rapid combination of the abundant neutrons with heavy nuclei. Massive red giants are also able to make small amounts of elements heavier than iron (up to mercury and lead) through a slower combination of neutrons with heavy nuclei, but supernova probably generate the majority of elements heavier than  iron and nickel (and certainly those heavier than lead up to uranium). The  synthesized elements are dispersed into the interstellar  medium during the planetary nebula supernova stage (with supernova being the  best way to distribute the heavy elements far and wide). These elements will  be later incorporated into giant molecular clouds and eventually become part  of future stars and planets (and life forms?)

• During the  supernova outburst, elements heavier than iron are produced as free  neutrons produced in the explosion rapidly combine with heavy nuclei to  produce heavier and very rare nuclei like gold, platinum, uranium among  others. This happens in about the first 15 minutes of the supernova. The most massive stars may also produce very powerful bursts of gamma-rays that stream out in jets at the poles of the stars at the moment their cores collapse to form a black hole.

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• What remains after the outer layers are thrown off depends on the mass of  the core. The core remnants are described fully in the next section. Here a  brief description of each type of core remnant will be given. If the core has a mass less than 1.4 solar masses, it will shrink down to a white dwarf the size of the Earth. The electrons in the  compressed gas bump right against each other to form a strange sort of gas called a  degenerate gas. The electrons prevent  further collapse of the core. 

   If the core has a mass between 1.4 and 3 solar masses, the neutrons will  bump up against each other to form a degenerate gas in a neutron star  about the size of small city. The neutrons prevent further collapse of the core. Nothing can prevent the highest mass cores (greater than 3 solar  masses) from collapsing to a point. On the way to total collapse, it may  momentarily create a neutron star and the resulting supernova rebound explosion and powerful
   bursts of gamma-rays in bipolar jets (possibly the source of some of the ``gamma-ray burst'' objects). Gravity finally wins. Nothing  holds  it up. The gravity around the collapsed core becomes so great that Newton's law of gravity becomes inadequate and the gravity must be described by the  more powerful theory of General Relativity developed by Albert Einstein. This  will be discussed further below.  

   The supercompact point mass is called a black hole because the  escape velocity around the point mass is greater than the speed of light. Since the speed of light is the fastest that any radiation or any other information can travel, the region is totally black. The  distance at which the escape velocity equals the speed of light is called the event horizon because no information of events occurring inside the  event horizon can get to the outside. The radius of the event horizon in  kilometers = 3 × core mass in solar masses.

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• If the star is massive enough, gravity can compress the core enough to  create high enough temperatures to start fusing helium (or heavier elements if it is repeating this stage).
• When the core fuel runs out again, the core resumes its collapse. If the   star is  massive enough, it will repeat stage 5. The number of times a star can   cycle  through stages 5 to 7 depends on the mass of the star. Each time through   the cycle, the star creates new heavier elements from the ash of fusion   reactions  in the previous cycle. This creation of heavier elements from lighter   elements  is called stellar nucleosynthesis. For the most   massive stars,  this continues up to the production of iron in the core. Stars like our Sun  will synthesize elements only up to carbon and oxygen in their cores.  Each repeat of stages 5 to 7 occurs over a shorter time period than the previous  repeat.
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• Eventually the star becomes stable because  hydrostatic equilibrium has been established. The star settles down to spend about 90% of its life as a main sequence star. It is fusing hydrogen to form helium in  the core. 

   Stars initially begin their lives near other stars in a cluster. After a  few orbits around the galactic center, gravitational tugs from other stars in the galaxy cause the stars in the cluster to wander away from their cluster and live their lives alone or with perhaps one or  two companions.
• Eventually the star becomes stable because  hydrostatic equilibrium has been established. The star settles down to spend about 90% of its life as a main sequence star. It is fusing hydrogen to form helium in  the core. 

   Stars initially begin their lives near other stars in a cluster. After a  few orbits around the galactic center, gravitational tugs from other stars in the galaxy cause the stars in the cluster to wander away from their cluster and live their lives alone or with perhaps one or  two companions.

• A giant molecular cloud is a large, dense gas cloud (with dust)  that is cold enough for molecules to form. Thousands of giant molecular clouds  exist in the disk part of our galaxy. Each giant molecular cloud has 100,000's  to a few million solar masses of material.
• The Orion Complex is about 1500 light years away, several hundred  light years across, and has enough material to form many tens of thousands of suns
• In addition to the most common molecule, molecular hydrogen,  over 80 other molecules have been discovered in the clouds from simple ones  like carbon monoxide to complex organic molecules such as methanol and acetone.  Radio telescopes are used to observe these very dark, cold clouds. The clouds  are dense relative to the rest of the gas between the stars but are still much  less dense than the atmosphere of a planet.
• These shock waves compress the gas clouds enough for them to gravitationally collapse. Gas clouds may  start to collapse without any outside force if they are cool enough and massive enough to spontaneously collapse. Whatever the reason, the result is the  same: gas clumps compress to become protostars.
• As a gas clump collapses it heats up because the gas particles run into  each other. The energy the gas particles had from falling under the force of gravity  gets converted to heat energy. The gas clump becomes warm enough to produce a  lot of infrared and microwave radiation. At this stage the warm clump is called a protostar. The gas clump forms a disk with the protostar in the center. Other material in the disk may coalesce to form another star or  planets.
• A protostar will reach a temperature of 2000 to 3000 K, hot enough to glow  a dull red with most of its energy in the infrared.
• Fusion starts in the core and the outward pressure from  those reactions stops the core from collapsing any further. But material from the surrounding  cloud  continues to fall onto the protostar. Most of the energy produced by the  protostar is from the gravitational collapse of the cloud material.
• Stars are  observed to be born in clusters.
• Selecting the figure will bring up an expanded view of the Hubble Space Telescope image in another window (courtesy of Space Telescope Science Institute). Note that the tiniest fingers you see sticking out of the sides of the pillars are larger than our entire solar system!

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• The figure below illustrates the inter-dependence of measurable  quantities with the derived values that have been discussed so far. In the left triangular  relationship, the apparent brightness, distance, and luminosity are tied together  such that if you know any two of the sides, you can derive the third side.  For example, if you measure a glowing object's apparent brightness (how bright  it appears from your location) and its distance (with trigonometric parallax),  then you can derive the glowing object's luminosity. Or if you measure a glowing  object's apparent brightness and you know the object's luminosity without knowing  its distance, you can derive the distance (using the inverse square law). In  the right triangular relationship, the luminosity, temperature, and size of  the glowing object are tied together. If you measure the object's temperature  and know its luminosity, you can derive the object's size. Or if you measure  the glowing object's size and its temperature, you can derive the glowing object's  luminosity---its electromagnetic energy output.
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• Finally, note that a small, hot object can have the same luminosity  as a large, cool object. So if the luminosity remains the same,  an increase in the size (surface area) of the object must result in a DEcrease  in the temperature to compensate.
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• Gravity Holds a Star Together

    Stars are held together by gravity. Gravity tries to compress everything to the center. What holds an ordinary star up and prevents total collapse is thermal and radiation pressure. The thermal and radiation pressure tries to expand the star layers outward to infinity. 

   

       

  Hydrostatic equilibrium: gravity compression is balanced by pressure outward.

  Greater gravity compresses the gas, making it denser and hotter, so the outward pressure increases.

    

   In any given layer of a star,  there is a balance between the thermal pressure (outward) and the weight of the  material above pressing downward (inward). This balance is called  hydrostatic equilibrium. A star is like a balloon. In a balloon the gas inside the balloon pushes outward and the elastic material supplies just enough inward compression to balance the gas pressure. In a star the star's internal gravity supplies the inward compression. Gravity compresses the star into the most compact shape possible: a sphere. Stars are round because gravity attracts everything in an object to the center. Hydrostatic equilibrium also explains why the Earth's atmosphere does not collapse to a very thin layer on the ground and how the tires on your car or bicyle are able to support the weight of your vehicle.

• Core

   The core is the innermost 10% of the Sun's mass. It is where  the energy from nuclear fusion is generated. Because of the enormous amount of gravity compression from all of the layers above it, the core is very hot and dense. Nuclear fusion requires extremely high temperatures and densities. The Sun's core is about 16 million K and has a density around 160 times the density of water. This is over 20 times denser than the dense metal iron which has a density of ``only'' 7 times that of water. However, the Sun's interior is still gaseous all the way to the very center because of the extreme temperatures. There is no molten rock like that found in the interior of the Earth.

• Convective Zone

   Energy in the outer 15% of the Sun's radius is transported by the bulk motions of gas in a process called convection. At  cooler temperatures, more ions are able to block the outward flow of photon radiation more effectively, so nature kicks in convection to help the  transport of energy from the very hot interior to the cold space. This part of the Sun just below the surface is called the convection zone.

• Astronomers believe that molecular clouds, dense clouds of gas located primarily in the spiral arms of galaxies are the birthplace of stars. Dense regions in the clouds collapse and form "protostars". Initially, the gravitational energy of the collapsing star is the source of its energy. Once the star contracts enough that its central core can burn hydrogen to helium, it becomes a "main sequence" star.

  • To understand this paragraph you need to make yourself familar with the Hertzsprung-Russel diagram. Look up an image of these diagrams that are used to classify stars according to two criteria. What are the criteria? You should also remember that colour is related to temperature.
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• Main sequence stars are stars, like our Sun, that fuse hydrogen atoms together to make helium atoms in their cores. For a given chemical composition and stellar age, a stars' luminosity, the total energy radiated by the star per unit time, depends only on its mass. Stars that are ten times more massive than the Sun are over a thousand times more luminous than the Sun. However, we should not be too embarrassed by the Sun's low luminosity: it is ten times brighter than a star half its mass. The more massive a main sequence star, the brighter and bluer it is. For example, Sirius, the dog star, located to the lower left of the constellation Orion, is more massive than the Sun, and is noticeably bluer. On the other hand, Proxima Centauri, our nearest neighbor, is less massive than the Sun, and is thus redder and less luminous.
• Thus, it is fortunate that our Sun is not more massive than it is since high mass stars rapidly exhaust their core hydrogen supply. Once a star exhausts its core hydrogen supply, the star becomes redder, larger, and more luminous: it becomes a red giant star. This relationship between mass and lifetime enables astronomers to put a lower limit on the age of the universe
• Meanwhile, the core of the star collapses under gravity's pull until it reaches a high enough density to start burning helium to carbon. The helium burning phase will last about 100 million years, until the helium is exhausted in the core and the star becomes a red supergiant. At this stage, the Sun will have an outer envelope extending out towards Jupiter. During this brief phase of its existence, which lasts only a few tens of thousands of years, the Sun will lose mass in a powerful wind. Eventually, the Sun will lose all of the mass in its envelope and leave behind a hot core of carbon embedded in a nebula of expelled gas. Radiation from this hot core will ionize the nebula, producing a striking "planetary nebula", much like the nebulae seen around the remnants of other stars. The carbon core will eventually cool and become a white dwarf, the dense dim remnant of a once bright star.
• Massive stars burn brighter and perish more dramatically than most. When a star ten times more massive than Sun exhaust the helium in the core, the nuclear burning cycle continues. The carbon core contracts further and reaches high enough temperature to burn carbon to oxygen, neon, silicon, sulfur and finally to iron. Iron is the most stable form of nuclear matter and there is no energy to be gained by burning it to any heavier element.

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• Despite what you might think, space is not a perfect vacuum. The space between   the stars is filled with a tenuous range of material that provides the building   blocks of stars. This material is gas and dust and collectively is known as   the interstellar medium (ISM). The ISM gas is predominantly   hydrogen whilst the dust is about 1% by mass and includes carbon compounds and   silicates. Dust is responsible for the interstellar reddening and extinction   of starlight. The more of the ISM a star's light travels through on its way   to an observer on Earth the more it gets scattered and absorbed, decreasing   the star's apparent brightness and reddening its appearance.
• Stars form in regions of the ISM where there is sufficient material available.   These are the giant molecular clouds or GMCs
• Their main   constituent is molecular hydrogen gas but other molecules such as water, carbon   monoxide, CO, ammonia, NH₃ and methanol, CH₃OH are also   present.
• An emission nebula is a cloud of dust and gas that shines due to the emission   of its own light. Gas is these nebulae is hot enough to be ionised and the emitted   light is due to interactions between the free electrons and the ions and to   electron transitions within ions

• In astronomy, HI refers to neutral hydrogen whilst HII is ionised hydrogen.   HII regions generally display distinctive red and green colours.The red is caused   by hydrogen-alpha emission and forbidden transitions of singly-ionised nitrogen.   OIII, doubly-ionised oxygen ions in which both the outer electrons have been   lost, produces a green emission due to forbidden transitions. This process only   takes place if photons have high enough energy such as the UV photons emitted   from hot O and B stars within or close to the nebula.

   

   

   

  • This is highlighted because you may find it interesting but you do not need to memorize or report this material. However, you should see the correlation between the colour of light emitted and the temperature of the matter.
    Blue end of the spectrum is hotter.
    Red end of the spectrum is cooler.
• Reflection nebulae are much less dense than emission nebulae. They appear bluish   due to the dust grains preferentially scattering blue light from nearby hot   stars. Our sky appears blue for much the same reason, blue light from the Sun   is scattered in all directions
• Protostars form when sections of giant molecular clouds start to collapse.   Clouds are initially diffuse enough that they do not contract unless something   triggers an increase in the density of some regions within a cloud.
• • The expanding shock front from a nearby supernovae sweeps up the local   ISM and compresses it. As the front passes through a cloud it causes irregularities   and local regions of higher density that may then collapse.
     
  • Spiral galaxies rotate without their arms "winding up". This is   due to a spiral density wave that sweeps around the plane of the galaxy, compressing   the material in its path including clouds of dust and gas. This helps account   for the observed star formation in spiral arms.
   
  • Stars and clouds are not fixed in place, as they move they may pass near   another cloud, triggering gravitational collapse due to variations in   density.
   
  • Regions with a number of young, hot O and B-class stars (called OB Associations)   produce large amounts of visible and ultraviolet radiation that compress   the ISM due to radiation pressure.

  • You do not need to meorize or report why the material collapses. What is important is that the material does collect, collide, increase in temp and released infrared radiation leading to a protostar.

• The gravitational collapse of a GMC does not result in a single, massive star.   Instead the cloud tends to fragment into smaller denser regions that each collapse   to form star systems. Up to a few thousand stars may typically form in a collapsing   GMC resulting in an open (or galactic) cluster.

   

  The dense regions collapse due to gravitational attraction between the particles.   Individual gas or dust particles move in towards the centre of the collapsing   region, losing gravitational potential energy. As the total energy of the system   is conserved the loss of gravitational energy is balanced by an increase in   the kinetic energy of the particles. These particles then undergo more collisions   which in turn raises the temperature of the gas. At this stage further collapse   is only possible if the cloud can radiate away the thermal energy so that the   radiation pressure outwards remains lower than the inward gravitational pull.   This is achieved via convection cycling warm material upwards within the cloud,   making the collapsing cloud visible in the infrared region.

   

   

     

     

  Credit: NASA,   John Trauger (Jet Propulsion Laboratory) and James Westphal (California Institute   of Technology), Nolan Walborn (Space Telescope Science Institute) and Rodolfo   Barba' (La Plata Observatory)

     

  HST visible and infrared images of star forming region,   30 Doradus in the Large Magellanic Cloud. The arrows point to protostars that   are obscured in the visible but visible at infrared wavebands. More details   are available from the NASA press   release.

   

    

  Continued collapse leads to higher densities so that eventually the cloud becomes   opaque, trapping the thermal energy within the cloud. This then causes both   the temperature and pressure to rise rapidly - the collapsing cloud is now a   protostar

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•   Stellar Evolution 

    If the star is about the same mass as the Sun, it will turn into a white dwarf star.  If it is somewhat more massive, it may undergo a supernova explosion and leave  behind a neutron star. But  if the collapsing core of the star is very great—at least three times the mass of the Sun—nothing can stop the collapse. The  star implodes to form an infinite gravitational warp in space—a black hole.
   
     
    The brightest X-ray sources in our galaxy are the remnants of massive stars that  have undergone a catastrophic collapse—neutron stars and black holes. Other  powerful sources of X-rays are giant bubbles of hot gas produced by exploding  stars. White dwarf stars and the hot, rarified outer layers, or coronas, of normal stars are less intense X-ray  sources.