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