Stars and SunsStars and Suns

Everything you need to know about them and Everything you need to know about them and their livestheir lives

Star VocabularyStar Vocabulary

Before we get into a stars life, it helps to Before we get into a stars life, it helps to understand the terms that will be used.understand the terms that will be used.

A A ProtostarProtostar is a star that is forming and has is a star that is forming and has not yet reached the temperature necessary to not yet reached the temperature necessary to “ignite” the star.“ignite” the star.

When we say that a star “ignites” or “burns”, When we say that a star “ignites” or “burns”, we are not talking about the same type of we are not talking about the same type of burning that occurs with fire.burning that occurs with fire.

We are talking about We are talking about FusionFusion which is an which is an entirely different processentirely different process

““Main Sequence”Main Sequence” The The most important vocabulary wordmost important vocabulary word

(besides fusion) when talking about stars (besides fusion) when talking about stars is is “Main Sequence”“Main Sequence”

In human terms, the “main sequence” is In human terms, the “main sequence” is equivalent to adulthood, when a star reaches equivalent to adulthood, when a star reaches “maturity”“maturity”

There is an important relationship between There is an important relationship between mass, temperature, brightness, and color for mass, temperature, brightness, and color for stars that have reached their main sequence. stars that have reached their main sequence. This relationship is demonstrated in the This relationship is demonstrated in the Hertzsprung-Russell diagram:Hertzsprung-Russell diagram:

The Hertzsprung-Russell DiagramThe Hertzsprung-Russell Diagram

Star ClassificationStar Classification

Another thing that will be important to Another thing that will be important to understand before we talk about stars is understand before we talk about stars is their classification system.their classification system.

Stars are usually classified by the Stars are usually classified by the temperature and color they are when they temperature and color they are when they are are in their Main Sequencein their Main Sequence

Notice the classification letters at the Notice the classification letters at the bottom of the Hertzsprung-Russell bottom of the Hertzsprung-Russell diagramdiagram

Star Classification in the Star Classification in the Hertzsprung-Russell DiagramHertzsprung-Russell Diagram

=>=> <=<=

Remembering Star ClassesRemembering Star Classes

It is easy to remember the star classes: O, It is easy to remember the star classes: O, B, A, F, G, K, and M in order by B, A, F, G, K, and M in order by remembering the following phrase:remembering the following phrase:

““Oh Be A Fine Girl… Kiss Me”Oh Be A Fine Girl… Kiss Me”

Star ClassesStar Classes

WordWord ClassClass MassMassIn sunsIn suns

Temperature Temperature x 1000x 1000

ColorColor DeathDeath

OhOh OO 20-6020-60 28 – 5028 – 50 BlueBlue NovaNova

BeBe BB 3-183-18 10 – 2810 – 28 BlueBlue NovaNova

AA AA 2-32-3 7.5 – 107.5 – 10 Light BlueLight Blue DwarfDwarf

FineFine FF 1.1-21.1-2 6 – 7.56 – 7.5 WhiteWhite DwarfDwarf

GirlGirl GG .85-1.1.85-1.1 4.9 – 64.9 – 6 YellowYellow DwarfDwarf

KissKiss KK .3-.85.3-.85 3.5 – 4.93.5 – 4.9 OrangeOrange DwarfDwarf

MeMe MM < .3< .3 2 – 3.52 – 3.5 RedRed DwarfDwarf

A Star’s LifeA Star’s Life

Formation of a ProtostarFormation of a Protostar Almost all stars Almost all stars

originate from Nebulasoriginate from Nebulas Stars are formed from Stars are formed from

Hydrogen gas (and Hydrogen gas (and minute amounts of minute amounts of other gases) that exist other gases) that exist in space, usually in the in space, usually in the form of nebulas.form of nebulas.

Gravity is the force that pulls the hydrogen Gravity is the force that pulls the hydrogen and other elements togetherand other elements together

Before star’s start to “burn” they are called Before star’s start to “burn” they are called protostarsprotostars

Formation of a Protostar Continued…Formation of a Protostar Continued…

Stars that form in regions in space where there is lots Stars that form in regions in space where there is lots of gas [probably] form more quickly and become of gas [probably] form more quickly and become more massivemore massive

As gas falls into the protostar’s gravity well, the As gas falls into the protostar’s gravity well, the pressure causes the gases in the center to heat more pressure causes the gases in the center to heat more and more.and more.

The hydrogen in the star will continue to collapse The hydrogen in the star will continue to collapse inward until the star reaches approximately 8 million inward until the star reaches approximately 8 million degrees, and fusion (of hydrogen) begins.degrees, and fusion (of hydrogen) begins.

FusionFusion

Fusion is a nuclear reaction that occurs Fusion is a nuclear reaction that occurs when two lighter atomic nuclei are forced when two lighter atomic nuclei are forced together, making a heavier nucleustogether, making a heavier nucleus

Fusion is the process that drives and Fusion is the process that drives and defines a stardefines a star

Fusion liberates the “strong” force, Fusion liberates the “strong” force, releasing an immense amount of energy in releasing an immense amount of energy in the form of heat and light.the form of heat and light.

Fusion of HydrogenFusion of Hydrogen

The fusion of hydrogen The fusion of hydrogen is the most common is the most common form of fusion.form of fusion.

In a star, the overall In a star, the overall process can involve process can involve ordinary Hydrogen, but ordinary Hydrogen, but in a laboratory this is in a laboratory this is the reaction that the reaction that occurs: occurs:

33H + H + 22H => H => 44He + He + 11nn

Hydrogen-3 plus Hydrogen-3 plus Hydrogen-2 makes Hydrogen-2 makes Helium-4 (and a Helium-4 (and a neutron)neutron)

The Birth of a StarThe Birth of a Star

As fusion of As fusion of hydrogen hydrogen begins, a true begins, a true star is bornstar is born

At this point, At this point, most stars enter most stars enter what is called what is called their “Main their “Main sequence”sequence”

The Battle WithinThe Battle Within

Keep in mind that inside a star there is a Keep in mind that inside a star there is a constant battle between gravity (pulling constant battle between gravity (pulling inward) and fusion energy (pushing inward) and fusion energy (pushing outward)outward)

At this point, most At this point, most stars enter what is stars enter what is called their “Main called their “Main sequence”sequence”

The Main SequenceThe Main Sequence

During the Main sequence, During the Main sequence, the push/pull battle is in the push/pull battle is in equilibrium and the star is equilibrium and the star is stable for most of its life.stable for most of its life.

Most stars will stay in the Most stars will stay in the main sequence for at least main sequence for at least 80% of their life time.80% of their life time.

During this period, planets will finish forming, During this period, planets will finish forming, mature and life will flourish (if the star is mature and life will flourish (if the star is capable of supporting life).capable of supporting life).

Life of a Typical StarLife of a Typical Star

When a Star Gets “Old”When a Star Gets “Old” By the time a Star has used up its hydrogen By the time a Star has used up its hydrogen

fuel in its core, most stars will start to collapse fuel in its core, most stars will start to collapse as outward pressure due to fusion is overcome as outward pressure due to fusion is overcome by gravity. by gravity.

This brings some hydrogen into its core from This brings some hydrogen into its core from the outside layers, allowing fusion to continue the outside layers, allowing fusion to continue while at the same time causing extra heat as while at the same time causing extra heat as the star compresses.the star compresses.

The extra heat makes the star hot enough to The extra heat makes the star hot enough to start burning Heliumstart burning Helium

Remember that “burning” refers to fusion.Remember that “burning” refers to fusion.

When a Star Gets “Old” ContinuedWhen a Star Gets “Old” Continued

As helium starts to burn, the star will become As helium starts to burn, the star will become significantly hotter and as a result, the star starts to significantly hotter and as a result, the star starts to change, expanding as the pressure from the new heat change, expanding as the pressure from the new heat pushes outward. Usually it will grow in size becoming pushes outward. Usually it will grow in size becoming a giant.a giant.

Stars that are at least 3 times more massive than the Stars that are at least 3 times more massive than the sun may become even larger stars called supergiants.sun may become even larger stars called supergiants.

At this point, a star leaves the main sequence, At this point, a star leaves the main sequence, becoming “old.”becoming “old.”

What happens after that depends on the type of star.What happens after that depends on the type of star.

Red Giants to White DwarfsRed Giants to White Dwarfs

As most stars leave the main sequence, they turn into As most stars leave the main sequence, they turn into Red Giants. Red Giants.

Star’s like our sun will grow to a size big enough that Star’s like our sun will grow to a size big enough that Earth itself will be part of the star.Earth itself will be part of the star.

Planets that are not swallowed by the sun now get Planets that are not swallowed by the sun now get blown apart by intense solar winds as the sun expands.blown apart by intense solar winds as the sun expands.

As Helium burns, the core of the star grows hot enough As Helium burns, the core of the star grows hot enough to form Carbon-12 and Oxygen-16to form Carbon-12 and Oxygen-16

When helium starts to run out, most stars will NOT be When helium starts to run out, most stars will NOT be hot enough to burn Carbon or Oxygen and so the star hot enough to burn Carbon or Oxygen and so the star will stop burning and start to shrink.will stop burning and start to shrink.

At this point, the star starts it’s slow “death”At this point, the star starts it’s slow “death” It will shrink to a size that roughly equals the size of It will shrink to a size that roughly equals the size of

Earth. Earth. Because the star’s heat becomes compacted, it will heat Because the star’s heat becomes compacted, it will heat

up again to a white colorup again to a white color This is what is known as a “White Dwarf”This is what is known as a “White Dwarf” For lighter stars, Carbon and Oxygen may start to burn For lighter stars, Carbon and Oxygen may start to burn

but it doesn’t last long since there isn’t enough to sustain but it doesn’t last long since there isn’t enough to sustain the next fusion cycle for an extended period of time.the next fusion cycle for an extended period of time.

Eventually, it will cool to a “Black Dwarf”Eventually, it will cool to a “Black Dwarf”

Star Death (Classes A – M)Star Death (Classes A – M)

Supergiants: Fusion of Carbon Supergiants: Fusion of Carbon and Oxygen and Oxygen

For star classes O and B, there is another step before For star classes O and B, there is another step before death.death.

The helium burning phase will add energy to its already The helium burning phase will add energy to its already intense heat.intense heat.

At a certain point, the mass of the star (pressure due to At a certain point, the mass of the star (pressure due to gravity) will cause the core to collapse again, heating gravity) will cause the core to collapse again, heating the star furtherthe star further

It doesn’t take very long for massive stars for helium It doesn’t take very long for massive stars for helium burning to reach over 15 million Kelvins, and so fusion burning to reach over 15 million Kelvins, and so fusion of carbon and (and oxygen) begins. This time, there is of carbon and (and oxygen) begins. This time, there is enough to keep the star burning.enough to keep the star burning.

Nova (Star Death for some Class B stars)Nova (Star Death for some Class B stars)

The fusion of carbon can produce star The fusion of carbon can produce star instability.instability.

In stars with masses between 3 and 8 In stars with masses between 3 and 8 solar masses (smaller Class B only), the solar masses (smaller Class B only), the outer layers get ejected in a “small” outer layers get ejected in a “small” explosion that leaves behind a white explosion that leaves behind a white dwarf.dwarf.

This dwarf can collapse further into a This dwarf can collapse further into a neutron star if it has sufficient massneutron star if it has sufficient mass

SupernovaSupernova

In stars that are more massive (than 8 solar In stars that are more massive (than 8 solar masses), eventually neon fusion (burning) masses), eventually neon fusion (burning) begins as the star runs out of oxygen and begins as the star runs out of oxygen and carbon.carbon.

At this point, the star burns too quickly for the At this point, the star burns too quickly for the outer layers to respond to changes in the star outer layers to respond to changes in the star and the star explodes, creating a supernova. and the star explodes, creating a supernova.

Supernovas can be brighter than galaxies and Supernovas can be brighter than galaxies and produce some of the most exotic stars in produce some of the most exotic stars in existence: neutron stars and black holes.existence: neutron stars and black holes.

Nebulas resulting from NovasNebulas resulting from Novas

Gases that are Gases that are ejected during ejected during nova’s and nova’s and supernovas supernovas create large create large clouds of clouds of interstellar interstellar gases known gases known as nebulasas nebulas

Neutron StarsNeutron Stars(Death of B and most O class stars)(Death of B and most O class stars)

Supernovas from B and Supernovas from B and many O class stars leave many O class stars leave behind a neutron starbehind a neutron star

The explosion The explosion compresses the stars compresses the stars core to a point where core to a point where normal matter decays into normal matter decays into neutronsneutrons

Neutron stars are not dense enough to become Neutron stars are not dense enough to become black holesblack holes

Neutron stars are so dense that a teaspoon weighs Neutron stars are so dense that a teaspoon weighs 100 million metric tons100 million metric tons

Black Holes (Ultimate Death!)Black Holes (Ultimate Death!)

The most massive O stars are so massive The most massive O stars are so massive that even the density of a neutron star that even the density of a neutron star cannot keep its core from collapsingcannot keep its core from collapsing

At this point, the star collapses into a size At this point, the star collapses into a size so small (imagine our sun being squeezed so small (imagine our sun being squeezed into a ball 2.5 miles across) and so dense, into a ball 2.5 miles across) and so dense, that not even light can escape its surface.that not even light can escape its surface.

It becomes a black hole, a star that is so It becomes a black hole, a star that is so incredibly dense, that scientists can only incredibly dense, that scientists can only guess what happens inside them.guess what happens inside them.

Other Life in the Universe?Other Life in the Universe?It’s definitely a possibility!It’s definitely a possibility!

Think of the millions of galaxies, and the billions Think of the millions of galaxies, and the billions upon billions of stars… Most likely, life exists upon billions of stars… Most likely, life exists

somewhere somewhere out there in all of that space.out there in all of that space.

Stars that support lifeStars that support life

Most star types CANNOT support life however.Most star types CANNOT support life however. Classes O through A cannot support life because Classes O through A cannot support life because

they do not last long enough for life to evolve or they do not last long enough for life to evolve or flourish (and with O stars, planets do not mature to flourish (and with O stars, planets do not mature to a state where they could support life at all).a state where they could support life at all).

M class stars cannot support life for a few reasons M class stars cannot support life for a few reasons but the most important one is because a planet of but the most important one is because a planet of the correct temperature to support life will be too the correct temperature to support life will be too close to the star and become tidally locked so that close to the star and become tidally locked so that one side ALWAYS faces the sun.one side ALWAYS faces the sun.

Classes that might support lifeClasses that might support life

The F class star may be able to support life The F class star may be able to support life as long as the star does not die too quickly as long as the star does not die too quickly for life to evolve.for life to evolve.

All G class stars are capable of supporting All G class stars are capable of supporting life as long as terrestrial planets form in the life as long as terrestrial planets form in the habitable zone (region surrounding a star habitable zone (region surrounding a star capable of supporting life). . capable of supporting life). .

K class stars may support life as long as they K class stars may support life as long as they are warm enough so that the planet can form are warm enough so that the planet can form far enough away from the sun.far enough away from the sun.