This guide will discuss the life cycle of a star

(stellar evolution). Throughout the guide,

pay attention to highlighted words, they

may be on the quiz

Learning EnvironmentStudents will only need a computer lab or

home desktop/laptop to complete this study guide and quiz

Grades 9-12For individual learning

Target Audience

ObjectivesStudents will learn about the life and

death of stars, and upon completion of this study guide, be able to identify layers of a star and

other material contained in this study guide with 80% accuracy

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Astronomy:

The Life Cycle of Star

Main MenuTable of Contents

•Study Guide

•Quiz

•Keywords

Main Menu

Table of Contents•Birth of a Star•Main Sequence Stage•Types of Stars•Zones of a Star

What is Stellar Evolution?

• Stellar Evolution is the life cycle of a star and the processes that it will undergo throughout its life

What is a Star?• Stars form from dense molecular clouds, also

known as GMCs (Gaseous Molecular Clouds)• The cloud becomes so large, the force of

gravity is extremely intense• The GMC collapses in on itself with a massive

amount of pressure (this is known as gravitational collapse)

Gravitational Collapse?• The collapsing gas implodes with so much

pressure, it creates a massive amount of heat and energy (similar to more friction = more heat)

• The core of the GMC is so hot and dense, gravity takes over and it begins to spin

• This is called a protostar (like a baby star)

• The faster the protostar spins, the hotter it becomes• Once the internal temperature reaches

10 million Kelvins, the proton-to-proton chain reaction begins• At this point, the hydrogen (H) molecules

in the core begin to fuse with the helium (He) molecules

• If the protostar‘s temperature never begins the proton-proton reaction, it become what is know as a brown dwarf • Essentially this is known as a dead star • Over hundreds of thousands of years it

will slowly cool and become dimmer and dimmer

What is Next?• Once the star’s internal pressure pushing outwards

equals the force of gravity pushing inwards, the star reaches the stage known as hydrostatic equilibrium

• The outward pressure is created by more and more molecules fusing, creating more heat

• Example: a balloon in a cold room versus a balloon in a hot room (hot one expands more, creating more outward pressure)

• The star then enters the Main Sequence stage of its life

Main Sequence Stage• Depending on the mass of the star, it can

remain a Main Sequence (M.S.) star for an extremely long time• Larger stars burn up their “fuel” faster,

thus remain a main sequence star for shorter amounts of time

This the Main Sequence Chart (also known as the Hertzsprung-Russell diagram)

•M.S. stars are placed on the Hertzsprung-Russell Diagram according to luminosity and temperature• The brighter the star, the hotter it

is, less heat = less luminous

• Depending on the size of the star, it may go through its life cycle and end up as many different things• Smaller stars become white dwarfs

eventually• Larger stars can become supernovas,

black holes, or neutron stars

Choose one to explore the life cycle of that type of star

Low-Mass Star

Mid-Size Star

High-Mass Star

Low Mass Star• Stars smaller than .05 solar masses will die

when they have used up their supply of hydrogen molecules

• Not all low mass stars become large enough to reach the main sequence stage

• When they use all the hydrogen, they collapse in again and create a white dwarf

A White Dwarf Star

Mid Size Stars• Range from 1.4 solar masses to 10 solar masses• Depending on the size of the star, it will live

longer• Mid size stars, like our sun, can survive for

millions of years• As a star reaches its end, the core heats rapidly

and the outer layers expand away from the core (in effect the star begins to swell)

• The hotter the internal temperature gets, the more force the outer layer are pushed outward with (the bigger the star the more force the layer are pushed out with)

• The further the layers get from the core, the cooler they become

• Once expansion of the cooling outer layers stops, they become a reddish color

• A star in this stage is known as a red giant

• The farther the layers get from the core, the less gravity they have holding them to the star• Eventually they will get pulled off of the

star and be dispersed back into space • This forms planetary nebulae• Eventually, the core implodes and forms

a white dwarf

Red GiantPlanetary Nebulae

Massive Stars• Use the CNO cycle instead of the proton-proton chain to

maintain equilibrium • Range from 15 solar masses to 115 solar masses (most

massive, like VY Canis Major)• They are so large, they are not usually main sequence stars• Smaller massive stars outer layer expand and cool just like

some main sequence stars• Even though these stars are cooler than some smaller

stars, they are brighter (more heat = brighter star)

Sounds like a Contradiction…• It seems incorrect that if heat creates a brighter star,

how can a cooler star be brighter?• Example:– Two side by side 100 watt light bulbs compared to

10 side by side 75 watt bulbs– Less powerful bulbs, but based on pure wattage the

10 bulbs are brighter–Massive stars are cooler, but they have so much

surface mass creating the light, they are brighter

• Massive star end their lives in one of two ways: neutron stars or black holes

• The star is so large, when it dies the sheer mass collapsing in on itself can turn the protons inside the star into neutrons, forming a star made completely of neutrons

• It the star is a large massive star, it will explode, and then collapse in back on itself with so much force and pressure that gravity will never take hold, and the star will continue to collapse forever, creating a black hole

Nuetron Stars

• Neutron stars are extremely small (in star terms)• They are usually about 10km across, the size of a

small city• They are super dense and spin at about 600

revolutions per second

Black Holes• 2-3 solar masses in size• Black holes can’t actually be seen through a regular

telescope• Scientists can only detect them in one of 2 ways: super

nova remnant or a black hole “sucking up” matter from a nearby star or space gas

• Before a black hole is created, it is preceded by a supernova (major expansion just before the implosion, like a “cosmic bomb”) that disperses the stars outer layers (cosmic matter) back into the universe

A black hole “sucking up” cosmic matter from a nearby star

Zones of a Star

1. Corona– atmosphere of a star”

2. Chromoshpere– Visible during a total eclipse of a star

3. Photoshpere– About 10,000 degrees Kelvin– The visible layer that we “see”

4. Convection Zone– Layer just below the surface where all the stars material is in

constant motion via convection

5. Radiation Zone– Extremely high temperatures allow gas to be copleteley

ionized

6. Core– Almost 17 million degrees Kelvin– center of the star”

An Acronym to Remember• When you are trying to remember the

different zone of a star, try to remember these two words:• Co/Ch/pho Con/ra/Core = Corona,

chromoshpere, photoshpere, Convection Zone, Radiation Zone, Core

QuizTable of Contents

KeywordsHome

Keywords• Protostar• Proton-Proton Chain• Main Sequence Star• Luminosity• Solar Masses• White Dwarf• CNO Cycle

Begin

Protostar• When a GMC collapses in on itself,

then stabilizes through hydrostatic equilibrium. • This is how a star is born, so you

could say a protostar creates a baby star

Proton-Proton Chain• Process of fusing hydrogen to helium

atoms at the core of a star• Occurs in low to mid-mass stars• What causes the stars collapse to stop

and allow it to become balanced

Main Sequence Star• A graph (also known as the Hertzsprung-

Russell Diagram) that plots a star based on color and brightness• Where a star is placed on this graph can

tell you how long it will “live” and what it looks like to the naked eye

Luminosity• Candela/ per Square Meter• How bright the star actually is, not

how bright it appears to be to the naked eye

Solar Masses• A unit used to describe stars size• Based on the size of our sun (1 sun =

1 solar mass)• Example: A star of 1.9 solar masses

is 1.9 times bigger than the sun

White Dwarf• Final evolutionary stage of a star that did

not have enough mass to become a supernovae or neutron star• White dwarfs are so dense they are

about the mass of the sun crammed into a star remnant the size of the Earth!!!

CNO Cycle• Carbon-Nitrogen-Oxygen cycle• How massive stars obtain hydrostatic

equilibrium• Usually occurs in stars above 3.3 solar

masses

Quiz Time!!!You will now be quizzed over some of the

keywords and material in the study guide. Hope you were paying attention!!

(click on the letter of the answer, not the words)

Begin

1. What is Stellar Evolution?a. Life of a star and the processes

that it undergoes.b. Evolution from birth to death of

the universe.c. How a star is created.d. The age of a star.

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2. True or False?Stars form from the

expansion of a cloud of space dust

True False

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3. A More massive star:a. Live longer than low-mass stars.b. Die and Create a black hole or neutron star.c. Have shorter lives than main sequence stars.d. A and Ce. B and C

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4. The order of layers of a star from outermost to innermost is:

a. Convection, Corona, Core, Photoshpereb. Photoshpere, Chromoshpere, Corona, Corec. Corona, Chromoshpere, Photoshpere, Convection, Radiation, Cored. Convection, Radiation, Photoshpere, Chromoshpere, Corona, Core

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5. Low-mass stars that “die” before they begin the proton-proton chain

become:a. Supergiantsb. White Dwarfsc. Brown Dwarfsd. Neutron Stars

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