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Stars and Stellar Evolution
Unit 6: Astronomy
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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
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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 ( K) = yellow
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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
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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
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How bright is that star? Brightness = magnitude
-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
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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
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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
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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
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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
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H-R diagram Red giants Supergiants = bigger
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
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H-R diagram White dwarfs Lower-central part
Fainter than main-sequence stars of same temperature
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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
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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
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Stellar Evolution How stars are born, age and die
Study stars of different ages
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Life of a Star
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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
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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
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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)
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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
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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
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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
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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
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Burnout and Death of Stars
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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)
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Formation and Destruction of Stars
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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
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Stellar Remnants Neutron star = smaller and more massive than white dwarfs Remnants of supernovas Composed entirely of neutrons
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Stellar Remnants Supernovae = outer layer of star is ejected --> collapse into hot neutron star Pulsar = emits short bursts of radio energy Remains of supernova
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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
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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
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Citations TLC Elementary School: The Moon and Beyond. Discovery Channel School unitedstreaming. Science Investigations: Earth Science: Investigating Astronomy. Discovery Channel School unitedstreaming.
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