Stars and Stellar Evolution

1. Stars and Stellar Evolution Unit 6: Astronomy

2. What are stars? • Stars = spheres of very hot gas • Nearest star to Earth is the sun • Constellations = group of stars named for a mythological characters • 88

3. Characteristics of Stars • Star color and temperature • Color can give us a clue to star’s temperature • Very hot (above 30,000 K) = blue • Cooler = red • In-between (5000-6000 K) = yellow

4. Characteristics of Stars • Binary stars and stellar mass • Binary star = stars that orbit each other (Pair) • Because of gravity • 50% of all stars • Can calculate mass of star • Equal mass --> center of mass halfway between stars • Size of orbits known --> masses can be calculated

5. Distances to Stars • Light-year = distance light travels in a year (9.5 trillion kilometers) • Parallax = slight shifting in the apparent position of a nearby star due to the orbital motion of the Earth • Photographs (comparisons) --> angle • Nearest = largest angles; distant = too small to measure • Only a few thousand stars are known

6. How bright is that star? -1.4 • Brightness = magnitude • Apparent magnitude = a star’s brightness as it appears from Earth • How big it is • How hot it is • How far away it is • Larger number = dimmer

7. How bright is that star? • Absolute magnitude = how bright a star actually is • Magnitude of star if I was a distance of 32.6 light-years • Ex: Sun = apparent magnitude: -26.7, absolute magnitude: 5 • More negative = brighter, more positive = dimmer

8. Hertzsprung-Russell Diagram • H-R diagram shows the relationship between the absolute magnitude and temperature of stars • Can also help us infer distance, life span mass • Stars are plotted according to their temperature and absolute magnitude • Interpret stellar evolution • Birth, age, death

9. H-R diagram • Bright stars are near the top and dimmer stars are near the bottom • About 90% are main-sequence stars • Hottest = brightest • Coolest = dimmest

10. H-R diagram • Brightness of main-sequence stars are related to mass • Hottest blue stars are 50 times more massive than the sun • Coolest red stars and only 1/10 as massive • Main-sequence stars appear in decreasing order • Hotter, more massive blue stars --> cooler, less massive red stars

11. H-R diagram Betelgeuse • Red giants • Above and to right of main-sequence stars • Size --> compare them with stars of known size that have same temperature • Supergiants = bigger • Ex: Betelgeuse

12. H-R diagram • White dwarfs • Lower-central part • Fainter than main-sequence stars of same temperature

13. Variable Stars • Stars might fluctuate in brightness • Cepheid variables = brighter and fainter in regular pattern • Nova = sudden brightening of a star • Outer layer ejected at high speed • Returns to original brightness • Binary systems

14. How does the H-R diagram predict stellar evolution? • Illustrate changes that take place in a star in its lifetime • Position on H-R diagram • Represents color and absolute magnitude at various stages of evolution

15. Stellar Evolution • How stars are born, age and die • Study stars of different ages

16. Life of a Star

17. Star Birth • Born in nebula = dark, cool, interstellar clouds of gas and dust • Milky Way = 92% hydrogen, 7% helium • Dense --> contracts --> gravity squeezes particles toward center --> energy converted into heat energy

18. Protostar Stage • Protostar = a developing star not yet hot enough to engage in nuclear fusion • Contraction continues --> collapse causes the core to heat much more intensely than the outer layer • When is a star born? • Core of protostar reaches about 10 million K --> nuclear fusion of hydrogen starts

19. Balanced Forces • Hydrogen fusion • Gases increase motion --> increase in outward gas pressure • Outward pressure from fusion balances inward force of gravity • Becomes main-sequence star (stable)

20. Main-sequence stage • Balanced between forces of gravity (trying to squeeze into smaller space) and gas pressure (trying to expand it) • Hydrogen fusion for few billion years • Hot, massive blue stars deplete fuel in only few million years • Least massive main sequence remain stable for hundreds of billions of years • Yellow star (sun) = 10 billion years • 90% of life as main-sequence star • Runs out of hydrogen fuel in core --> dies

21. Red Giant Stage • Zone of hydrogen fusion moves outward --> helium core • All hydrogen in core is used up (no fusion in core) --> still taking place in outer shell • Not enough pressure to support itself against force of gravity --> core contracts • Core gets hotter --> hydrogen fusion in outer shell increases --> expands outer layer --> giant body • Surface cools --> red • Core keeps heating up and converts helium to carbon to produce energy

22. Burnout and Death of Stars • Low-mass stars • 1/2 mass of sun • Consume fuel slowly --> main sequence for up to 100 billion years • Consume all their hydrogen --> collapse into white dwarfs • Eventually ends up as a black dwarf

23. Burnout and Death of Stars • Medium-mass stars • Similar to sun • Turn into red giants --> once fuel is gone, collapse as white dwarfs --> eventually black dwarf

24. Burnout and Death of Stars

25. Burnout and Death of Stars • Massive stars • Shorter life spans • End lives in supernovas = becomes 1 million times brighter (rare) • Consumes most of its fuel -> gas pressure does not balance gravitational pull --> collapses --> huge implosion --> shock wave moves out and destroys the star (outer shell blasted into space)

26. Formation and Destruction of Stars

27. Stellar Remnants • All stars collapse into one of the three: white dwarf, neutron star, or black hole • White dwarf = remains of low-mass and medium-mass stars • Extremely small with high densities • Surface becomes very hot • No energy --> becomes cooler and dimmer • Last stage of white dwarf = black dwarf (small, cold body) • Smallest = most massive • Collapse of larger stars • Largest = least massive • Collapse of less massive stars

28. Stellar Remnants • Neutron star = smaller and more massive than white dwarfs • Remnants of supernovas • Composed entirely of neutrons

29. Stellar Remnants • Supernovae = outer layer of star is ejected --> collapse into hot neutron star • Pulsar = emits short bursts of radio energy • Remains of supernova

30. Stellar Remnants • Black hole = a massive star that has collapsed to such a small volume that its gravity prevents the escape of everything • Cannot be seen • Evidence of matter being rapidly swept into an area • Animation

31. Where did the elements of the universe come from? • After the universe became cool enough for atoms to form, they began to clump together into clouds of gas • First stars made up of mostly hydrogen with a small amount of helium • Heavier elements like iron and silicon not yet made inside stars • More and more stars formed, became main-sequence, grew old, and died • More and more matter was fused into heavier elements and expelled back into interstellar space by supernovas and dying red giant stars • Eventually our sun and its planets formed from this interstellar gas and dust

32. Citations • TLC Elementary School: The Moon and Beyond. Discovery Channel School. 2004.unitedstreaming. • Science Investigations: Earth Science: Investigating Astronomy. Discovery Channel School. 2004. unitedstreaming.