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 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