Lives of Stars

1. Lives of Stars Stars burn Hydrogen as fuel The more massive a star is the more blue it is Also the more massive a star is the shorter its lifetime because it must use much more fuel to maintain its size example our sun has a main sequence lifetime of 10 billion year, but a star of 30 Msun only has a lifetime of 1/30 that of our sun As stars reach end of lifetime they leave the main sequence and grow in size (some as much as 1000 x original size) (upper right) Eventually run out of fuel and die and become white dwarf, neutron stars (lower left)

2. Lives of Stars

3. Stars Luminosity – amount of power star radiates into space power= energy / time Ways to measure star’s distance: 1) Apparent Vs. Absolute Luminosity 2) Stellar parallax 3) Cepheid variables 3) Redshift (outside galaxy)

4. Apparent Vs Absolute Luminosity Q- why are stars dimmer then the sun ? A- farther away goes by inverse square law Relate to a sphere (see figure 15.1) If a star is 2 d it has ¼ Brightness Lsun = 3.8x 10^26 watts Betelgeuse is 38,000 Lsun Alpha Cent is 0.0006 Lsun May use a CCD camera to measure apparent brightness

5. Stellar parallax Parallax apparent change in the position of a nearby object relative to a distant object when the observer moves to a new position Measured in parsecs 1 parsec = 1/angle (arcsec) 1 pc= 3.09 ly nearest star to sun is about 1 pc 1000 pc farthest measure by spacecraft 8000 pc to galaxy center see link

6. Redshift Used to measure distance to stars outside our galaxy Uses Doppler Shift Due to Universal expansion (Big Bang) More last week of class

7. Binary Stars Most stars are in binary systems May use Newton’s Version of Kepler’s third law to measure mass Where P= Period and a=length of semi-major axis both orbit about common center of mass

8. Binary Stars

9. Types of Binary Stars Visual - Any two stars seen close to one another is a double star, the most famous being Mizar and Alcor in the Big Dipper

10. Types of Binary Systems Spectroscopic cannot be resolved as a visual binary even with telescopes of the highest existing resolving power. Uses Doppler shift to resolve system

11. Types of Binary Systems Eclipsing - orbital plane of the two stars lies so nearly in the line of sight of the observer that the components undergo mutual eclipses.

12. Cepheid Variable Stasrs Discovered in 1912 at Harvard by Henrietta Leavitt found that the brightness of these stars vary as a function of time Star is changing size There is a relation between the brightness of the star and its luminosity use to measure distances (more in chap 19)

13. Star Clusters Groups of stars 2 types open fairly modest (example Pleiades) closed more -densely packed Interesting for two reasons All stars in cluster are roughly same distance from Earth Stars formed around same time (within a few million years May look at stars which have left main sequence and use this to date the cluster Must use HR diagram

14. Star Stuff Types of stars Low Mass stars (less than 2 Msun) Intermediate Mass Stars (between 2 Msun and 8 Msun) High Mass Stars (greater than 8 Msun)

15. Star Birth Most stars begin life as part an interstellar cloud. Typically about 30 Kelvin also known as molecular clouds (allow Hydrogen atoms to form H2 molecules) This cloud begins to collapse and form a protostar (see fig 16-2)

17. Protostar The dust collapses into a disk due to conservation of angular momentum Angular momentum = m v r Think of an ice skater’s arms Thus the dust forms a flat disk as it shrinks

18. Protostar to New star 1) dust begins to form protostar fairly bright because even though it is cool, it has a large surface area 2) looses energy due to this radiation; begins to contract and temperature rises 3)When Temps reach a few million K, fusion begins at core 4) For a few million years, star continues to decrease in size thus raising the rate of fusion.

19. Stellar Birth Weights Stars must have masses between 0.5 Msun and 100 Msun. If too light not enough gravity to ignite fusion 0.08 - 2 Mjupiter called “brown dwarfs” first “brown dwarf” seen in 1995. If too heavy the fusion is too rapid and gravitation can’t control blows itself apart (Theory)

20. Low Mass stars masses less then 2 Msun. Live much longer then more massive stars good for life on surrounding planet (gives time for life to evolve) our star is relatively stable, some low mass stars rotate faster Thus have higher magnetic fields give rise to violent flares Proxima Centauri is a flare star (closest star to sun)

21. End of Medium to Low Mass stars As the star exhausts its fuel of hydrogen, the core begins to collapse due to no outward pressure to resist the pull of gravity But the outer surface of the star grows Now termed a “Red Giant”.

22. End of Medium to Low Mass stars Seems paradoxical: why does core shrink but star grows Answer: Hydrogen shell burning The inner core is now helium (result from H burning) around this core is hydrogen This hydrogen shell is at such a high pressure that it burns faster then before results in a large outward pressure which forces the star to expand

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24. End of Medium to Low Mass stars Cycle ends when temperature at core reach 100 million K. At this temperature Helium begins to fuse together and form Carbon 12 called “triple alpha process” Dumps even more energy into star, “helium flash” Causes core to expand thus lowing temperature, then reducing rate of fusion star begins to contract

25. Death of Medium to Low Mass stars Star may pulsate burning He for a while, but it can’t maintain this Star has one last gasp and throws out outer layers called planetary nebula then forms a white dwarf.

26. Life of High Mass stars Have much smaller lifetimes then lower mass star Due to fact that they are higher temperature and thus burn hydrogen much faster In fact, the stars are so hot that heavier elements fuse inside of them CNO cycle - a more complex version of the proton-proton chain that produces Carbon, Nitrogen, and Oxygen.

28. End of High Mass stars After the core is exhausted it becomes Helium But now not only does the He burn but heavier elements are created These heavier elements also begin to fuse together forming still heavier elements. Each time a new element is created in the core the core must shrink until it has enough pressure (temperature) to fuse next element

29. End of the line for High Mass stars

30. End of the line for High Mass stars Process continues until one has a iron core (see fig 16.16) Ends at iron because it is no longer energetically favorable to form elements higher then Iron In other words it requires more energy to fuse iron and get a heavier nuclei then you get out

31. Supernova Now we have an iron core where the pressure is increasing There is a limit to how much we can press this core (degeneracy pressure) In a fraction of a second the core is compressed from size of sun to that earth This compression releases HUGE AMOUNTS OF ENERGY Causes an explosion - Supernova Work still goes on to model what exactly happens, but star will form either a neutron star or a black hole.

33. Hubble picture of Crab Nebula

34. Supernove 1987 A

35. Origin of elements Can’t see inside a star, so why do we think heavy elements are created inside stars Look at gas in Nebula very old star have only 0.1% heavy elements younger stars have 2-3% heavy elements this shows that the younger stars have gained elements which were created in previous supernova

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