Star Light, Star Bright

3 Factors that determine the brightness of a star

Temperature

Hotter the star the brighter the star

Blue…..white……..yellow…….orange…….….red

HOTTEST----------------------------------------------COOLEST

Size

• Larger the star Brighter the Star

• Smaller the star Dimmer the Star

Distance

• Closer the star Brighter the star

• Further the star Dimmer the star

Brightness Key Terms

• Luminosity– How much light energy is coming from the surface

• Apparent Magnitude– How bright it appears to be due to distance

• Absolute Magnitude– How bright it is if its distance was a parsec or 33 light years

• Parallax– The shift of closer stars against a background of farther stars as

the Earth goes around the Sun

Luminosity

• Definition- actual or true brightness of a star

• Total amount of energy given off

• Dependent on 2 things:Temperature

6000K 10,000K

More luminous since more surface area

Size

6000K6000K

More luminous since more energy

Apparent Magnitude• Definition- how bright a star appears when seen from

earth

• System developed where a number is assigned to a star based on brightness– Scale is basically based on powers of 10 difference– A +2 is 10 times as bright as a 3, +100 times as bright as a +4

• Smaller # Brighter the star Bigger # Dimmer the star

• Sun -28 , Full Moon -11, Polaris 7• Hubble can see +28

Absolute Magnitude• Definition- brightness of a star as if all

stars were seen from the same distance

Earth Earth

Sun- average star, Abs. Mag of +4.8 (less luminous, looks brighter because closer)

Rigel- orion, Abs, Mag. of -6.4 (more luminous, further away)

To Determine: apparent magnitude and distance to earth

Moving Stars?

• Parallax- the apparent change in position of a star due to the movement of observer

Finger: left/right

Life Cycle of a Star

Nebula ProtostarMain

Sequence

Star

Red giant

Red supergiant

Planetary

Nebula

White

Dwarf

Supernova Black Hole

Neutron Star

Life Cycle of Stars

1

2

Life Cycle Introduction

• http://www.youtube.com/watch?v=f_KLOFe2rDY

• http://www.youtube.com/watch?v=YU6X3SPZAJo

Step 1: Initiation

• Stars are born in nebulae (huge clouds of gas and dust)

• Nebula begin to condense when an outside force, such as shock wave, acts upon it

Step 2: Pre-star

• A protostar forms when a part of the nebula contracts, shrinks, and pressure and internal temperature increases

• Protostar begins to glow where nebula is contracting

Step 2 contd: A star is born

• As contraction, temperature, density, and pressure increase protostar gets larger and brighter

• Center becomes so hot fusion begins

• Once fusion begins, a star is born

• TRUE STAR

Step 3: Teenager star• Internal temperature hot

enough to start fusion at center said to be a main sequence star

• Sun is an example• Each protostar will turn

into one main sequence star

• 90% of stars are main sequence stars

• Vary in surface temperature and absolute magnitude

Fate determined by Size

• If a normal size star (Sun) follows path 1

• If star is a GIANT follows path 2

Path 1

Step 4: Middle-aged star

• Red giant- very bright, once an average star, but is now close to end of life

- Has expanded to many times its original size (heat causes it to expand)

- Hydrogen core has turned to helium and eventually to carbon

- Our sun will become a red giant star in about 5 billion years

Star like our sun begins to die

• Star begins to die when its core temperature rises to a point where fuel is used up

• A carbon-oxygen core forms • Eventually the gases at a

star’s surface begin to blow away in abrupt bursts

• Resulting glowing halo is called a planetary nebula

Death of a star like our sun

• atoms no longer fuse, fuel is used up

- Outer gases escape leaving the core which collapses and shrinks

- Heat still present but will continue to escape for about a billion of years

- White Dwarf- small, very dense, hot star at the end of its life, mostly carbon with nuclear cores depleted (about the size of earth but heavier)

Path 2

Step 4: Middle-aged massive star• Supergiant- largest

known type of star- can be as large as our

solar system- rare but exists- In a massive star,

hydrogen is fused more quickly and fusion continues until a iron nuclei is formed

Death of a Supergiant

• Elements are used up very quickly and eventually runs out of fuel

• Collapse of the core produces a shock wave that blasts the star’s outer layers into space producing a supernova

• Supernova- exploding star

Option A after a supernova

• After a massive star “goes supernova” it leaves behind its core (called a neutron star)

-Neutron Star- small, dense star made of neutrons

• When neutron star is first formed, it spins very rapidly and gives off radio waves

Option B after a supernova

• Black Hole- a star that collapses

• How do we know they exist? Pulls gases off nearby stars, as gas is pulled into this “nothing space” x-rays are emitted from the gas as molecules are pulled in

Cycle continues

• Star is born from great clouds of gas and dust

• Stars mature, grow old, and die

• As a star dies, it makes new clouds of dust gas and dust where new stars can begin to form

• More massive a star, the shorter its life

• http://www.youtube.com/watch?v=HfqcZdNnQ6s

• http://www.youtube.com/watch?v=hoLvOvGW3Tk&feature=related

Hertzsprung-Russell Diagram

• Graph that compares temperature and absolute magnitude (brightness)

The Hertzsprung-Russell Diagram (H-R diagram)

Cool and bright

Cool and dim

Hot and dim

Hot and bright

Sun

CoolHot

Dim

Bright

Cool and dim

Cool and bright

Hot and dim

Hot and bright

Main sequence

Giants

Supergiants

White Dwarfs