Basics of Star Formation: Observations, Interstellar Medium, and Formation Mechanisms

1. Lecture 10 6/20/07 Astro 1001

2. 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

3. 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

4. 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

5. 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

6. 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?

7. 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

8. 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

9. Stages of Star Birth • Protostar • Looks a lot like a real star • No nuclear fusion • Accretion • Matter is drawn onto the protostar by gravity

10. 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

11. 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

12. 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

13. 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

14. 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

15. Initial Mass of Stars • Small stars are much more likely than huge stars • Most stars are less massive than the Sun

16. Quiz Review

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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