12/19/2014 Stellar Evolution:

Aim: The Life Cycle of a Star

Do Now: Where do stars come from? What are they made of?

Notepack 23

What is a Star?

What is a Star?

• A star is a massive ball of gas that has

ignited due to tremendous heat and

pressure.

• Stars “burn” as the results of 2 Hydrogen

atoms fusing together to create a Helium

atom.

How do stars put hydrogen atoms together?

• Stars perform nuclear reaction to put hydrogens

together.

• There are 2 types of nuclear reactions:

– Fission reactions which splits atoms apart.

• This creates a lot of energy

• Makes harmful waste produces (radioactive)

• Used by nuclear power plants

– Fusion reactions which combines atoms to make new

ones.

• Creates enormous energy

• Needs a lot of energy to start it.

• Waste products are “clean”

• Used by stars.

Age of a Star

• Scientists can predict the age of a star based

on the ratio of Hydrogen and Helium it has.

• Stars that have a lot of hydrogen (majority)

is send to be a young star.

• Stars the have a lot of helium is said to be an

old star.

• Scientist use a spectrometer to measure

color emitted from the star to determine how

much hydrogen it has.

Types of Stars

• Due to the different ages of stars and the

amount of gas the star started with, we get

many different types of stars

Types of Stars

• There are many different types of stars – Hot Stars

• Blue

– Cold Stars • Red

– Large Stars • Supergiants

• Giants – These stars have a high luminosity – they are very bright

– Small Stars • Dwarfs

– These stars have a low luminosity – they are very dim

Luminosity

• Luminosity means the brightness of the star.

• The reason why some stars shine brighter than others is because these stars are creating enormous energy.

• The color of the star tells you it’s temperature.

Our Sun

• Description of our Sun

– Main Sequence

– Yellow

– About 6,000oC

• Our sun is an average star

– Not too big/Not too small

– Not too hot/Not too cold

– Not too bright/Not too dim

Medium Size Stars

• Take about 1 million years to form

• Spend about 10 billion years on the main-

sequence

• Similar to our sun

– Diameter: 1,391,400

– Mass: 1.99 x 1030 kg (1 solar mass)

Supergiant

• Largest and Hottest stars

• Form quickly (100,000 years)

• Die quickly (average 100 million years)

• Contain about 8 times (or more) the mass of the Sun

• They die in an explosion – Called a supernova

Red Dwarfs

• Smallest and coldest stars

• Take a long time to form (millions of years)

• Live for a very long time (trillions of

years?)

How Stars Form • Stars begin their lives as Nebula

• Nebula: A giant cloud of gas and dust in

space

• Typical Temperature: 100 K (-279°C)

COLD!!

Rosette Nebula

Red = Hydrogen, Green = Oxygen, Yellow = Sulfur

Eagle Nebula

Dark Nebula – there’s so much dust that it obscures

the background stars

Horsehead Nebula

North American Nebula

Pleides Star Cluster (7 sisters)

Bok Globule – Small dark clouds made of gas and dust

where new stars are forming

Collapse of a large cloud will ultimately result in a

star cluster

How Stars Form

• Every atom has gravity.

• Gravity is an attraction between atoms.

• As atoms get closer to each other the

attraction between them increase.

• Gravity increases as the atoms

combine. – The more atoms, the more

gravity.

• The more gravity there is – the more

atoms are pull to it.

How Stars Form

• Every atom in the nebula has gravity.

• As these atoms move close to each other,

they are attracted to each other.

• This attraction causes them to combine,

thereby increasing the gravity pull.

• Has the gravity pull increases, so is the

amount of atoms being pulled to the center.

• This action causes the nebula to be less like

a gas and more like a solid.

• 1st Stage of Life

– Ball of gas and dust that is pulled

together by gravity

– Nuclear fusion starts as gas cloud

becomes denser and hotter

• Nuclear Fusion hydrogen atoms fuse

into helium

– Creates the intense energy found in

stars

• Depending on how much gas was in

the nebula and how large in size the

nebula was will depend on how long

the star will stay in each stage of its

life.

– Stage 2 - Main Sequence

– Stage 3 - Giants

– Stage 4- Super Giants

– Stage 5 - White Dwarfs or Nova

• Main Sequence (the Sun)

–2nd stage

–Longest stage (usually)

–Hydrogen changes into helium

which creates enormous amounts

of energy

–The size of the star does not

change much

–The star is somewhat stable

• Giants and Supergiants

– 3rd and 4th stages

– Main Sequence star becomes a red giant

• Red giant star that expands and cools once is

loses all its hydrogen

– Center shrinks and atmosphere grows large and

cools

In the future, our Sun will become a Red Giant.

It will grow so large that it size will overlap the

Earth’s orbit, consuming the Earth.

• Possible 5th stage #1

• White Dwarf small hot star that is the leftover center of an older star

– Final stage

– Can shine for billions of years before they extinguish

• Possible 5th stage #2

• Red giant uses all of the hydrogen and collapse on itself.

• The collapsing star then explodes (a nova)

• The remnants of the star’s core will form either: – Black hole

– A neutron star

• Supernova gigantic explosion in

which a massive star collapses

– Occurs after a massive star uses up

its fuel source

• Neutron Star a star that has

collapsed to a point at which all

particles are neutrons

– A neutron star that spins and sends

out beams of radiation is called a

pulsar

• Age of stars

–Average stars become red

giants then white dwarfs

–More massive stars explode

into a variety of objects

• Hertzprung-Russell Diagram

–Shows the relationship between

a star’s surface temperature and

absolute magnitude

–Used to study the lives of stars

–Most stars lie along the main

sequence portion of the diagram