Astronomy Lecture Notes Week of Feb. 24
Astronomy Lecture Notes Week of Feb. 24 ASTRON 0089
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Date Created: 02/27/15
Interstellar medium consists of gas and dust Gas atoms and small molecules mostly hydrogen and helium cold less than 100K Dust sootsmoke larger clumps of particles 0 Absorbs light extinction and reddens light that gets through reddening o Reddening can interfere with blackbody temp measurement but spectral lines don t shift dust only changes color ISM gas is very diffuse Dust is many light years deep which is why we can t see through it Interstellar dust grains complex in shape 0 Grow linearly and become rodlike on small scales but become tangled and twisted in complex ways on larger scales How Nebulae Work Strong interaction bt nebula and stars within it photoevaporation Light from hot stars scatters to dust cloud which appears blue to you because of the re ected scatter light star would appear red Emission nebulae are made of hot thin gas which exhibits distinct emission lines Some emission lines come from forbidden transitions they re so rare that under standard lab conditions they re never seen Forbidden transition in doubly ionized oxygen 0111 is responsible for green in Orion nebula Dark Dust Clouds Average temperature of dark dust clouds is a few tens of Kelvins Absorb visible light top and emit at radio wavelengths bottom Emission os from the CO molecule Ex Ophiuchus dust cloud irregular shape with streamers o Horsehead Nebula famous 21Centimeter Radiation Interstellar gas emits lowenergy radiation due to a transition in the hydrogen atom The emitted photon has a wavelength of 21 cm which is in the radio portion of the electromagnetic spectrum Proton amp electron spin parallel to each other and then ip to a more stable state antiparallel spins emitting a photon which has that 21 cm wavelength Interstellar Molecules The densest gas clouds are also very cold around 20 K These clouds tend to contain molecules rather than atoms Transitions between rotation states faster to slow of a molecule emit radio frequency photons Radio waves aren t absorbed much so molecular gas clouds can be detected even though there may be other gas and dust clouds in the way These clouds are mostly molecular hydrogen which doesn t emit in the radio portion of the spectrum but other molecules do Molecular cloud complexes are enormous around 50 pc across and have enough material to make a million Suns 0 There are about 1000 known in the Milky Way Star Formation and Evolution The Formation of Stars Like the Sun Stage 1 Interstellar clouds start to contract triggered by shock or pressure wave from nearby star or supernova As it contracts cloud fragments into small pieces eventually forming many tens or hundreds of individual stars Stars form in clusters Stage 2 Individual cloud fragments begin to collapse Once density is high enough no further fragmentation Stage 3 Interior of the fragment has begun heating and is about 10000 K Stage 4 Core of cloud is now a protostar and makes its rst appearance on HR Diagram 0 It now has an L and T that are within range of this plot The dusty interstellar cloud condenses under selfgravity Gravitational energy is converted to heat and light energy 9 this is a protostar Since gravity is strongest at the center the center condenses faster and heats up faster Central density and pressure go up Energy source gravity CORE TEMP OF SUN 15 MILLION K Planetary formation has begun but the protostar is not in equilibrium all heating comes from the gravitational collapse Stage 5 protostar s luminosity decreases even as its temperature rises bc it s becoming more compact Stage 6 core reaches 10 million K and nuclear fusion begins 9 protostar has become a star Object that can produce its on thermonuclear fusion Stage 7 star continues to contract and increase in temperature until it s in equilibrium Star has reached the Main Sequence and will remain there as long as it has hydrogen to fuse The Zero Age Main Sequence ZAMS The star is in equilibrium The inward pull of gravity is balanced by the gas pressure T core 15 million K T surface 5800 K R l R Energy Source pp chain Observations of Cloud Fragments and Protostars Emission nebulae are heated by the formation of stars nearby Protostars have very strong winds jets which clear out an area around the star roughly the size of the solar system 9 these are T Tauri stars Proplyds protoplanetary disks Important stars don t move along the Main Sequence once they reach it they are in equilibrium and don t move until their fuel begins to run out When a star moves in the HR Diagram it means its L andor T change so that it now is at a different spot on the diagram Since L and T change smoothly it appears as if the star moves as it evolves Stars of other masses Stars less massive than the Sun have similar path shapes in the HR Diagram but they wind up in different places on the Main Sequence Pre Main Sequence evolution is Hayashi Track Some fragments are too small for fusion ever to begin they gradually cool off A protostar must have 008 M 80x the mass of Jupiter to become dense and hot enough that fusion can begin If the mass of the failed star is about 12 Jupiter masses or more it s luminous when rst formed and called a brown dwarf Star Clusters Because a single interstellar cloud can produce many stars of the same age and composition star clusters are an excellent way to study the effect of mass on stellar evolution The stars are also all at the same distance from us NGC 3603 newborn cluster of a hot young blue Type 0 and B stars an OB Association The Pleiades is a young star cluster an example of an open cluster 0 Most stars in this are on the Main Sequence 0 The brightest stars are the ones with luminosities over 1000 L 0 They are blueish and white B and A stars which we see Globular cluster absence of massive Main Sequence stars heavily populated Red Giant region Relatively old contain stars of all masses Cluster HR Diagrams give the age of the cluster Inhabit the halo of our galaxy Stellar Evolution Eventually as hydrogen in the core is consumed the star begins to leave the Main Sequence Its evolution from then on depends on the mass of the star 0 Lowmass stars go quietly o Highmass stars go outwith a bang Evolution of a Sunlike Star Even while on Main Sequence the composition of a star s core is changing Helium begins to accumulate over time due to Hydrogen fusion The sun is a little bigger today than when it rst formed Most of the core is Helium The core shrinks gets hotter amp denser the rate of Hydrogen fusion increases luminosity increases radius increases The Sun is now 40 more luminous and 6 larger As the fuel in the core is used up less radiation pressure can be produced and the core contracts remember constant tug of war bt gravity and radiation pressure When the fuel is used up nothing holds up the star so the core begins to collapse due to gravity Hydrogen in a shell around the core heats up and begins to fuse to form Helium It s now burning hotter than before and radiation pressure pushes the outer layers farther out Stage 9 The RedGiant Branch As the core shrinks it heats up the shell of Hydrogen outside it burns vigorously and generates much heat This makes the outer layers of the star expand and cool The sunlike star is now a red giant Despite its cooler temp its luminosity increases enormously due to its large size Stage 10 Helium Fusion Once the core temp has risen to 100000000 K the helium in the core starts to fuse through the triplealpha process 3 helium nuclei fuse to form carbon 0 Beryllium is produced very brie y 4He 4He 9 8Be energy 8Be 4He 9 12C energy The 8Be nucleus is highly unstable and will break up into 2 helium nuclei in about 10A12 seconds unless another helium nucleus fuses with it first This is why temperatures and densities are necessary The Helium Flash Electron Degeneracyz two electrons can t be in the same quantum state they can t be squeezed together beyond a certain point So as the core collapses due to gravity the electrons reach this point of degeneracy and hold up the star When helium starts to burn in the core the temperature rises but the gas can t expand to release pressure in response to the rise in temperature because electron degeneracy has made the core rigid So the temperature rises even faster it s like putting a tight lid on a pot of boiling water Helium begins to fuse extremely rapidly finally the temperature gets high enough to break the electron hold and let the core expand again Within hours the enormous energy output is over and the star once again reaches equilibrium Stage 11 Back to the giant branch As the helium in the core fuses to carbon the core becomes hotter and hotter and the helium burns faster and faster The star is now similar to its condition just as it left the Main Sequence except now there are two shells The star has become a red giant for the second time Such stars like the Sun never become hot enough for fusion past carbon to take place There is no more outward radiation pressure due to fusion being generated in the core so the core continues to contract The outer layers become unstable and are eventually ejected Over 1000 years the star loses 40 of its mass and exposes the 100000 K core Contraction of the core stops when electron degeneracy kicks in again the electrons hold up the star But this time there is no helium burning taking place that can increase the temperature and unlock the electrons The core is now called a white dwarf the star is dead It doesn t produce any energy anymore it cools off The Death of a LowMass Star Stage 12 The ejected envelope expands into interstellar space forming a planetary nebula why we see a ring The star now has two parts 0 A small very dense carbon core 0 An envelope about the size of our solar system planetary nebula As the dead core of the star cools the nebula continues to expand and dissipates into the surroundings Stages 13 amp 14 White and Black Dwarfs Once the nebula has gone the remaining core is extremely dense and hot but quite small It s luminous only due to its high temperature White dwarf shell burning dies out leaving behind a CO core that contracts due to the force of gravity 0 The contraction is stopped by electron degeneracy pressure 9 it s now a white dwarf o It weighs a ton a teaspoon Density 1 million gcmA3 R R earth T few 10000 K Energy source cooling off The Hubble Space Telescope has detected white dwarf stars in globular clusters As the white dwarf cools its size doesn t change signi cantly it simply gets dimmer and dimmer until it ceases to glow Stars of the same age but different mass can appear as the whole cluster ages After 10 million years the most massive stars have already left the Main Sequence While many haven t even reached it yet After 100 million years a distinct mainsequence turnoff begins to develop this shows the highestmass stars that are still on the Main Sequence After 1 billion years the mainsequence turnoff is much clearer After 10 billion years a number of features are evident o The redgiant subgiant asymptotic giant and horizontal branches are all clearly populated White dwarfs also appear indicating that solarmass stars are in their last phases Hyades Cluster young mainsequence turnoff indicates an age of 600 million years 47 Tucanae about 1012 billion years old
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