Notes 30.2 Stellar Evolution

Std 2d: Know stars differ in their life cycles and that may be used to collect data that reveal those differences

Std 2f: Know the evidence indicating that the color, brightness, and evolution of a star are determined by a balance between gravitational collapse and nuclear fusion

Objective 1: Describe how a protostar becomes a star

Objective 2: Describe the evolution of a star after its main-sequence stage

Stellar Evolution

• A typical star exists for billions of years. Astronomers study stars at different stages

Classifying Stars

• The Hertzsprung-Russell Diagram (H-R Diagram) - a graph that plots the surface temperature of stars against their luminosity – X- axis - the temperature of the star– Y-axis - the luminosity of the star• Most stars are known as main-sequence

stars (ex. Sun)

Star Formation

• Nebula - a cloud of gas and dust (star begins)1. An outside force (ex. Explosion of a nearby star)

causes the cloud to compress

2. The gravitational force causes particles to be pulled toward the area of increasing mass

3. Dense matter begin to build up with in the cloud

Protostars

• Gravity can cause these dense regions to spin greatly

• Protostar - flatten disk that has a central concentration of matter– Continues to contract and increase in temperature

for several million years

The Birth of Star

• Temperature continues to increase in a protostar to about 10,000,000 ⁰C. – Nuclear fusion begins

• Nuclear fusion releases enormous amounts of energy.

Objective 1: Describe how a protostar becomes a star

• What is the beginning of a protostar?– Nebula

• What causes the nebula to contract?– An outside force (star exploding)

• What is a flatten disk that has an area of concentrated matter?– Protostar

• What releases an enormous amount of energy?– Nuclear fusion

Draw this diagram

The Main-Sequence Stage

• 2nd stage (longest in the life of a star)• A star similar to the sun’s mass stays main-

sequence for about 10 billion years • More massive stars = 10 million years because

they fuse hydrogen rapidly • Scientists estimate that the sun is 5 billion

years old.

Leaving the Main-Sequence

• 3rd stage when almost all of the hydrogen atoms in the core have fused into helium atoms

• The helium core becomes hotter, it transfers energy into a thin shell of hydrogen surrounding the core

• The on-going fusion of hydrogen radiates energy outward, which causes the star to expand greatly

Giant Stars

• A star grows cooler as it expands. They begin to glow with a reddish color.

• Giants – a very large and bright star whose hot core has used most of its hydrogen– 10x or more larger than the sun

• Stars like the sun will become giants

Supergiants • 3rd stage -Stars that are much larger than the

sun become supergiants• Supergiants are often at least 100x larger than

the sun

The Final Stages of a Sunlike Star

• Fusion in the core will stop after the helium atoms have fused into carbon and oxygen

Planetary Nebula

• As the stars outer gases drift away, the remaining core heats these expanding gases

• Planetary nebula - A cloud of gas forms around a dying star

White Dwarfs

• Final stage - A small, hot, dim star that is the leftover of an old star is a White Dwarf

• Gravity causes the remaining matter in the star to collapse– Hot but dim – About the size of Earth –When a white dwarf no longer gives off light, it

becomes a black dwarf (do not exist yet)

Novas and Supernovas

• Nova – A star that suddenly becomes brighter– If a white dwarf revolves around a red giant, its

gravity may capture gases from the red giant – Nova - Pressure will begin to build on the surface of

the white dwarf and may cause large explosions– A white dwarf may become a nova several times

• Supernovas - a star that has such a tremendous explosion that blows itself apart – If a white dwarf accumulates too much mass it

may become a supernova

The Final Stages of Massive Stars

• Massive Stars become Supernovas as part of their life cycle – The collapse produces high pressure and

temperature causes nuclear fusion to begin again• Fusion continues until the core is almost all iron • The core begins to collapse by gravity• Energy released as the core collapses explodes

outward w/tremendous force

Neutron Stars

• Massive stars do not become white dwarfs

• After a Supernova explosion the core contracts and becomes a Neutron star

• Neutron star – a star that has collapsed under gravity to the point that the electrons and protons have smashed together forming neutrons

Pulsars

• Neutron stars emit a beam of radio waves that can be detected from Earth.

• Pulsar – a rapidly spinning neutron star that emits pulses of radio and optical energy

Black Holes

• The force of the contraction crushes the dense core of the star and leaves a Black Hole

• Black hole - an object so massive and dense that light cannot escape its gravity– Gravity is so great that light cannot escape– They are difficult to locate

Objective 2: Describe the evolution of a star after its main-sequence stage

• What are objects that are so massive and dense light cannot escape?– Black holes

• What are spinning stars that emit radio waves?– Pulsars

• What do Red Supergiants become?– Supernovas

• What type of star do pulsars form from?– Neutron stars