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Lecture 10 6/20/07 Astro 1001 Basics • We can’t observe any star going through multiple stages of their lifetime – Can observe multiple stars in different phases of their lifetime • Two to Three new stars formed a year • Gas and dust in between the stars is called the Interstellar Medium The ISM • Can use spectroscopy to determine the composition of the ISM – 70% hydrogen, 28% helium, 2% other stuff • Density and temperature of gas varies greatly • Stars form in coldest, densest clouds of molecular gas • Interstellar dust is also an important part of the ISM Why Do Stars Form? • Gravity causes clouds to contract – Continues until the central object becomes hot enough to do fusion on its own • Star formation doesn’t happen everywhere because gas pressure can prevent gravity from collapsing the cloud – Called thermal pressure • Exploding star might help trigger the collapse of the cloud Clusters and Stars • Most stars are born in clusters of thousands of stars – Average cloud mass is 1000x that of the Sun • Several additional sources prevent gravity from going nuts – Magnetic fields – Turbulent motion Group Work • The mathematical insight on page 532-533 shows how the minimum mass of a star forming cloud varies with density. Following these examples (especially the ones on page 533), figure out how dense the could would have to be to form a single, 1 solar mass star. What does this say about why stars usually form in clusters? Fragmentation • Collapse of a cloud results in many smaller stars instead of one huge star – Clouds are turbulent and lumpy – Small clumps will individually collapse • Isolated stars can also form – This process has been observed – Not fully understood The First Generation of Stars • Astronomers call elements other than hydrogen and helium “metals” – Metallicity is a measure of how much of something is made out of metals • Original stars had essentially 0 metallicity – Were very large – Didn’t live long – Provided the metals for all prior generations of stars Stages of Star Birth • Protostar – Looks a lot like a real star – No nuclear fusion • Accretion – Matter is drawn onto the protostar by gravity Details of Star Formation • A protostellar disk also forms around the protostar • A protostellar wind forces particles off into space • Protostellar jets often form • Binary stars often formed The Genesis of Nuclear Fusion • Protostar gravitationally collapses – About half of the energy is trapped in the star – Raises temps from about a million degrees to about 10 million degrees – Might take millions of years to do Degeneracy Pressure • Recall that the Exclusion Principle doesn’t allow particles to be packed too close together • In order for stars below about .08 solar masses, you would need to violate the Exclusion Principle in order to reach necessary densities Brown Dwarfs • Brown Dwarfs are on the dividing line between planets and stars • Would have been stars, but degeneracy pressure halted their collapse • Very dim – Shine only due to gradual cooling of their interior The Biggest Stars • As stars get larger, they create more and more pressure – Very large stars create primarily Radiation Pressure • Radiation pressure would blow apart a star if it was much over 150x the mass of the Sun Initial Mass of Stars • Small stars are much more likely than huge stars • Most stars are less massive than the Sun Quiz Review Mass and Fusion • Large stars have much more gravity that has to be balanced by more pressure – Hence they have a greater rate of fusion and much greater luminosities • When a star runs out of hydrogen, it has to do something new – Might fuse heavier elements – Might collapse and die Types of Stars • Low mass stars – Less than 2 solar masses – Most common type • Intermediate mass stars – Between 2 and 8 solar masses – Won’t talk much about these • High mass stars – Greater than 8 solar masses – Rare – Have a very great effect on their surroundings Low Mass Star Basics • Spend about 10 billion years turning hydrogen into helium via the proton-proton chain • Size of convective zone varies with mass – Low mass stars can be almost entirely convective zones – High mass stars have no convective zones, but a convective core – No convective zone means that the star can be a very violent flare star Red Giant Stage • Core can no longer support itself and shrinks • Outer layers (called the envelope) still has hydrogen, which will start to burn • Eventually the core will get hot enough to burn helium – Helium fusion stars off violently with the helium flash – Is now on the Horizontal Branch The Death of the Sun • Through winds, the Sun will eject its outer layers – The Core will be exposed and is now a White Dwarf – The WD will light up the gas around it – Forms a Planetary Nebula Massive Stars • Early life similar to that of low mass stars, but faster • Use the CNO Cycle to generate energy – End result is to turn 4 hydrogen atoms into 1 helium atom • High mass stars go through similar stages when they run out of hydrogen fuel at first Heavy Elements • Massive stars can get so hot in their core that they can fuse carbon (and maybe other elements) – Can produce oxygen, neon, magnesium up to iron • Iron can’t be fused and give you energy • This picture is confirmed by observations – Young stars have higher metallicities – Even numbered elements much more common than odd numbered elements Death of a High Mass Star • Electrons combine with protons, so the pressure instantly vanishes • Star collapses, releasing tremendous amounts of energy • Star explodes in a supernova – Might form a neutron star held together by neutron degeneracy pressure – Might be so massive that it forms a black hole Supernova Observations • Supernovae are so bright that they can appear from nowhere (or even shine during the day) • A supernova helped prove that Kepler was right about the heavens being able to change • In 1987, modern science got its first look at a supernova