05 stellar evolution mc neely
TRANSCRIPT
Astronomy
Ch. 05: Stellar Evolution
Sirius is a main sequence star with a small, white
dwarf companion as displayed in this HST
photo
Stellar Evolution
The changes that take place in stars as they age
Life cycle of starsOver millions-billions of years
Birthplaces
Stars form out of gigantic interstellar clouds (nebulas)
Famous Orion Nebula located 1500 light-years away, a region of intense star formation
Orion Nebula, located in Orion’s Sword, appears as a greenish-cloud in telescopes
Orion’s Sword
A Star is Born
Protostar: Star in its earliest phase of evolution; Baby star
Proplyd: “Protoplanetary Disk”, another term for protostars and their nebular clumps
Protostars
Protostar can be surrounded by rotating disk that will form a solar system
Nuclear fusion when 10 million K internal temp
Bipolar jets, material erupting into space along the axes of rotation
Hydrostatic Equilibrium
Hydrostatic Equilibrium: Internal balanceGravity balances pressure of hot gases in star Holds star together
Stars spend their lives fighting the inward crush of gravity
3 Steps in Birth of a Star
1. Gravitational contraction within a cloud of gas and dust
2. Rise in interior temperature and pressure
3. Nuclear fusion begins once internal temperature reaches 10 million Kelvin
Protostar Diagram
This artist’s view of a protostar displays
bipolar jets
Beta Pictoris Circumstellar Disk; Orion Proplyd
Star Beta Pictoris is surrounded by a disk of gas and dust, the nebula from which the star formed
This HST image shows proplyds located in the Orion Nebula
Lifetimes
A function of a star’s mass and chemical composition
High mass stars evolve fastest, low mass stars evolve slowest
Stars move throughout the HR Diagram as they age; i.e., their temperatures and luminosities change over time
Main Sequence stars are “adults”
Life Cycles of Stars (HR Diagram)
Why Stars Shine
Fusion: 4 hydrogen nuclei are converted into 1 helium nuclei, excess mass is given off as energy (heat, light)
Energy released by fusion can be calculated using Einstein’s famous E=mc2
(E=energy, m=mass difference, c=speed of light)
Old Age of Stars
Main sequence stars shine until all available hydrogen has been converted into helium
Then the star begins to dieThe sun has been shining for about 5
billion years. It is middle-aged
Massive Stars
Very massive, hot, bright stars die fastest because they use up their hydrogen rapidly;
Massive stars spend only a few million years as main sequence stars. Ex: Rigel, hot, blue star in Orion
Least massive, cool, dim stars such as red dwarfs can last billions of years
Red Giants
Red giants are senior citizen starsAfter hydrogen fuel in core runs out, star
swells into a giantRed giants are cooler and redder, they
leave main sequence and enter upper right corner of HR Diagram
Examples include Antares and BetelgeuseOur sun in the future
5,000,000,000 AD
Talk about global warming!
Red Giant Stars are HUGE! Ex: Betelgeuse
Nucleosynthesis
The creation of elements in starsMain sequence hydrogen fusionHelium Fusion
When red giant stars achieve 100 million K internally, helium is converted into carbon (helium flash)
Red Giant Nucleosynthesis
Red giant stars form internal shells that produce progressively higher elements
Large red giants can create heavier elements such as oxygen, aluminum, and calcium
Stars can produce elements up to iron before exploding
Elements higher than iron are produced in the brief explosions of stars
Red Giant Nucleosynthesis
Each shell in the red giant produces progressively heavier elements with depth
Betelgeuse
http://malyszp.tripod.com/stars/betelgeuse.jpg
Beetle Juice (1989) was
inspired by the star in Orion
Variable Stars
Stars that change brightness in regular or irregular cycles
Pulsating Variable StarsMove back and forth between the main
sequence and red giant region of the HR diagram for unknown reasons
Such stars vary in light output, expand and contract
Ex: Cepheid variables
Cepheid Variables
Luminous, yellowBrightness varies from 1-70 daysFamous example, Delta CepheiPeriod-luminosity relationship, used to
calculate distances
Cepheids: Distance Markers
Period-Luminosity Relationship: For Cepheids, the longer the period of brightness change, the greater the luminosity
This relationship enables the calculation of absolute magnitude.
Compare absolute to apparent magnitude to estimate distance
Good to about 10 million light-years (closest galaxies)
Delta Cephei Light Curve
Delta Cephei has a roughly 5-day cycle of brightness
Delta Cephei Star Map
Delta Cephei Delta is a naked eye star in
Cepheus
RR Lyrae Variables
Named for star RR in LyraRR Lyrae stars are pulsating blue-white
giants with periods less than 1 dayDistance markers out to 600,000 ly
Long Period or Mira Variables
Mira in Cetus, pulsating red giantsPeriods between 80-100 days from dim to
brightMira means the “Wonderful” star,
proclaimed after its recognition in 1638Mira first variable star discoveredMira brightest every 333 days
The Wonderful Star
Mira Light Curves
•The diagram shows the changing brightness cycle of Mira
•Each strip represents 15 years, and each dot represents a magnitude estimate
•Most of these estimates were made by amateur astronomers who do this work as part of their hobby
Mira-Feb 2007
•In late winter, Cetus and Mira appear to be setting in the west after sunset
•This photo was taken in Stuttgart, Germany
Death of Stars
Depends on massSmall stars, up to 1.4 times the sun’s
mass, go to planetary nebula stage, fade away into dwarf stars
Larger stars (8 times the sun’s mass) explode
Planetary Nebulas
Type of nebula ejected by dying starsSize 0.5-1 ly in diameterLeaves behind a white dwarf star in centerFamous examples: M57, the Ring Nebula
in Lyra; NGC6543, Cat’s Eye in Draco
Ring Nebula
M57 Ring Nebula: HST
Note the central star, a white dwarf
Cat’s Eye: Amateur & HST
•The Cat’s Eye Nebula in Draco
•Planetary nebulas can reveal bizarre and complex shapes
White Dwarfs
Remains after planetary nebula stageStar can no longer resist inward pull of gravity,
squeezes down into an object about the size of the earth
Very dense, you would weigh 35,000 times greater if you could somehow stand on a white dwarf
A teaspoon of white dwarf matter would weigh over a ton
Can brighten suddenly as “novas”
Ziggy
Black Dwarfs
Gradually, the white dwarf cools, turns dull red, and shines its last energy into space
White dwarf becomes a black dwarf, corpse of a star
Our sun’s ignominious end
Life Stages of a Sun-Like Star
1. Protostar, gravitational contraction of gas and dust2. Stable, main sequence star shining by hydrogen fusion3. Evolution to red giant when helium core forms4. Red giant, shining by helium fusion5. Variable star, formation of carbon core6. Planetary nebula, outer atmosphere of star ejected into
space7. White dwarf, mass packed into a star about the size of
the earth8. Dead corpse, black dwarf in space
Exploding Stars
Stars 8 or more times greater than our sun explode
Supernova: A gigantic stellar explosion (exploding star)
Core of star begins fusing elements up to iron
Star collapses and explodes violentlySupernovas can be seen in other galaxies,
sometimes even in small telescopes
Supernovas
100 billion times the sun’s luminosity for a brief moment
Brief instant fuse chemical elements higher than iron on the periodic table
M51 Supernova (SN2005cs)
Where’s the supernova?
A supernova appeared in M51, a bright galaxy in Canes Venatici, in 2005
This supernova was visible in large amateur telescopes
Historic Supernovae
1054, Crab Nebula1597, Tycho’s Star1604, Kepler’s StarSupernova 1987A
Tycho (top) and
Kepler
Supernova 1987A
•SN1987a appeared in the Large Magellanic Cloud, a small satellite galaxy of our Milky Way that is visible from the Southern Hemisphere
•The supernova was positioned near the Tarantula Nebula, the large red glow in left center of the image to the right
Below: Large Magellanic
Cloud; Right: March ’97 Time
1054 Supernova, Chaco Canyon, Crab Nebula
This rock art in New Mexico may depict the 1054 supernova
The Crab Nebula (M1) is the remnant of the 1054 supernova
M1 StarMap (Taurus)
The Crab Nebula is visible as a glowing patch of light in small telescopes, it is the first object in Messier’s list (M1) http://www.eurekalert.org/images/rele
ase_graphics/ESA112905_1.jpg
Ecliptic
Neutron Stars
From explosions of massive starsNeutron star, a type of star more massive
than the sun but squeezed into a ball 10 miles across
Incredibly dense
Pulsars
Pulsars are rotating neutron starsPulsars can send sharp, strong signals
towards earthOriginally thought to be alien signals
(LGM) when first discovered in the 1960’sPulses range from milliseconds-4 secondPulsar found at center of the Crab Nebula
Black Holes
Really massive stars can explode and collapse into black holes
Black holes are denser than neutron starsRepresent the mass of entire star shrunk
into zero-radius objectGravity is so immense, even light can’t
escape
Black Hole Terms
Event Horizon: Boundary of no return where no light or matter will escape
Singularity: Center of black hole, a point of infinite density where the pull of gravity is infinitely strong
Anatomy of a Black Hole
Simulated black hole, the intense gravity distorts the light of stars in the
background
Black Hole Candidates
Cygnus X-1, intense X-ray source located 8000 ly away in Cygnus
Believed to be an eclipsing binary star (two stars orbiting), period 5.6 days, with unseen companion
Massive black holes may exist at the center of the Milky Way and other galaxies
Cygnus X-1
•Cygnus X-1 is located in Cygnus or the Northern Cross
•It is not visible in a telescope, but you can identify its general area using a star map
Center of Milky Way: Sgr A
Sagittarius A is a radio source at the center of the Milky Way and likely marks the location of a black hole
Sgr A
Stellar Evolution Summary
Sun-like starsProtostarMain sequence star
(yellow star)Red giantPlanetary nebulaWhite dwarfBlack dwarf
Massive StarsProtostarMain sequence (blue
star)Red supergiantSupernovaNeutron star or black
hole (depending on mass)
Summary