Stars, Life and Light

S1-4-06b Explain the life cycle of a star.

KEY WORDSFusion Core NebulaMain sequence Red dwarf Neutron StarRed giant White dwarf SupernovaRed supergiant Black hole Galaxy

Our Sun• A small star• About 5 billion years old (should last 5 more) • Composed of 75% hydrogen and 25% helium • All elements in plasma phase

(hotter state than gas)

• Energy comes from a Nuclear Fusion Reaction

Nuclear Fusion ReactionFusion = join/merge• Joining two hydrogen to create helium • Releases large amount of energy

Fusion = join Fission = split

Sun: joining (fusion) things to release energyPower Plants: splitting (fission) things to release energy

There is obviously more to the Sun then “burning ball of gas”

Layers of the Sun:

Inside• Core – centre (site of nuclear fusion)

Outside• Photosphere – cooler surface (5500 oC

sunspots, surface storms – the part we “see”• Chromosphere – inner atmosphere

small solar explosions (flares), large (prominences)• Corona – outer atmosphere (1 million oC)

charged gas particles blow outward (solar winds)

“Life Cycle” of a Star:

Birth: Stars start out as NebulaeNebula – huge cloud of dust and gases

Youth – Middle Age:• Gravity collapses a nebula into star that begins to

release energy as a main sequence (MS) star

Scientists categorize MS stars according to mass: • Small - last longer (billions of years)• Large - brighter but short-lived (few million

yrs)

Actually nebulae captured by NASA space telescopes

Small MS star – Red Dwarf • Very cool temperatures of their outer gases

Death: • Forms a cool, dim star called a White Dwarf• Fades out until it no longer emits light energy

We can’t see most Red Dwarf stars –

their light is too weak

Medium MS star – like the Sun• Separated into small-med and large-med

Death: • Runs out of elements for fusion reaction• Collapses then swells to produce a Red Giant

• small-med eventually fades out as white dwarf• large-med explodes as a Supernova leaving behind a Neutron star

“Pulsar” neutron star

Large MS star• Extreme energy produced for a short period • Fuses heavier elements (iron) after lighter ones

Death: • Collapses then swells to a Red Supergiant • Gravity collapses the largest into a Black Hole

Stars are in balance: nuclear fusion explodes outwards, large gravitational force collapses inwards

When the reactor runs out of elements to fuse, gravity wins and the star collapses and (sometimes) explodes

Nothing can escape the gravitational pull of a black hole

Nebulae

main sequence (MS) star

Red Dwarf

White Dwarf

Red Giant

Supernova

Neutron star

Red Supergiant

Black Hole

Smallest Largest

Medium

Birth:

Death:

There is still more to this puzzle, but…this is close enough

WHAT DO YOU NOTICE?

Galaxies• Huge collections of gas, dust and billions of stars

and planets (collected by gravity)• Constantly in motion• Many shapes: spiral, elliptical, and irregular

Milky Way Galaxy• Contains 400 billion stars • Disc-shaped, with spiral arms – spiral galaxy We are on one of the arms• Has a thicker Central Bulge• Rotates around the bulge

New evidence suggests the “central bulge” is actually a SUPER-MASSIVE BLACK

HOLE!

CAN YOU ANSWER THESE QUESTIONS?

S1-4-06b:How do stars like the Sun make energy?

How do stars form and how do small, med and large stars eventually “die?”

KEY WORDSFusion Core NebulaMain sequence Red dwarf Neutron StarRed giant White dwarf SupernovaRed supergiant Black hole Galaxy