Stars

1. Measuring/Distances to stars2. Stars aren’t always “there”. They live,

they die, we will learn that cycle.3. Properties of stars: color, brightness,

temperatures, fuels, formations. . .4. Hertzsprung-Russell Diagram

Measuring and Distances

Primary distance = light year– 9.5 X 1012 km (9.5 trillion km)– A.k.a. the distance light travels in

one year.Fun fact: Position of close stars is determined by

parallax

– Geometry! Want to be an astronomer? Study MATH!

Fun fact: Mass determined by using binary stars.

Properties of Stars

Temperature and Color are related– Not all stars are created equal!– Very hot stars appear blue (30,000K +).– Cool stars appear red (3,000 K).– (This is backwards from the bathroom!)– Medium stars (like the sun) appear

yellow (5,000-6,000K).

Brightness or Apparent Magnitude

How bright a star appears to us is dependant upon three factors:1. Size of the star2. Temperature of the star3. Distance from Earth

Absolute Magnitude

Absolute magnitude is how bright a star REALLY IS– A star may appear much dimmer than

another simply because of its distance.

– Scientists “standardize” the stars to figure out the absolute magnitude

– (More “negative” # = brighter)– (More “positive” # = dimmer)

Hertzsprung-Russell Diagram

• Diagram that shows the relationship between temperature of stars (x-axis) and the absolute magnitude (y-axis)

• Hotter (blue) and brighter are upper left.• Colder (red) and dimmer are lower right.• 90% of stars are main sequence stars.

– Prime of their life, livin’ the dream!•Nuclear fusion makes them “shine”

– Not on main sequence means it is dying!• Ex. Red giants/supergiants, white dwarfs

H-R Diagram

Star life cycle

First comes love. . .• Nebula-clouds of dust and gases• Very spread out, may contain lots of

matter/mass!• Remember, Mass = Gravity• If there is enough gravity, then matter

will contract and the star story begins.

Nebula

Then comes marriage. . .

As the nebula contracts, gravitational energy is converted into heat energy.

After enough contraction and heat (million years), a Protostar is made.– Protostar is not a true star, YET. . .– Can’t be a star, because the only

energy it is giving off is heat due to gravity. (Not making its own energy.)YIPEE!

Then comes the baby in the baby carriage!

• When there is enough heat generated (10 million K) the real star is “born”.

• Pressure (gravity) within this environment causes nuclear fusion to take place.– Simplified: Hydrogen atoms (1

proton each) crash together to make Helium (2 protons each) and releasing enormous amounts of energy.

Main sequence

• Majority of the star’s “life” is here.• Balance between gravity pulling inward and

outward pressure from heat/gas of nuclear reaction.

• Hotter/brighter the star = shorter life• Smaller/cooler stars = longer life• Yellow stars = average size, temp, and

length of life• If our sun was a storybook character, we got

Goldilocks! Everything is “just right”.

Star “death”

• A.K.A. Falling off the main sequence.• When the balance between gravity and

outward pressure cannot be maintained.• Easy fuel is gone! Star has run out of

Hydrogen, going through its Helium (He + He +He = C), making heavier/more massive elements

• C + C +. . .= Fe, etc.• Gravity has the upper hand. The core of the

star is contracting, outer layer is expanding.

Burnout path(a.k.a. choose your own adventure!)

• Path the star takes is determined by mass– Low mass stars (less than 1/2 mass

of sun) go from Main Sequence directly to White dwarf.

– Medium mass stars (sun sized) become Red Giants.

– Massive stars (3X mass of sun) become Red Supergiants.

Red Giants/Supergiants

• Fusion in the core is Helium to Carbon.• Core is contracting (due to mass & gravity).• Outer layers expanding and cooling.

– Explains the red color!• Still a fight between gravity inward and

pressure outward.• Red giants = give off outer layer “gently”

to form a planetary nebula.• Red supergiants = “give” off outer layer in

an implosion called a supernova.

Red Giants/Super Giants

Supernova

The end!

Medium mass main sequence Red Giant White Dwarf Black Dwarf

White Dwarf- Red giant core has collapsed to about the size(volume) of Earth! – Frame of reference: Started with a star that

1 million Earth’s could fit into!– Giving off leftover heat, and heat from

molecular particles– Q: How does the sun compare to a white

dwarf star?Black Dwarf-what’s left after all of the heat is

gone. Lots of pressure, lots of Carbon, infer diamonds?

The end! (again)

Massive stars Red supergiant supernova Neutron star or Black hole

Neutron Star- smaller & more massive than White dwarves, so much gravity that electrons combine with protons to make neutrons. (Size of a pea = 100 million tons), – strong magnetic fields + rapid rotation

= Pulsars

Neutron Star

Black Holes

Massive stars Red supergiant supernova Neutron star or Black hole

Black holes-smaller and more massive than Neutron Stars– Therefore, have much more GRAVITY– Centered on a point, so much that nothing

can escape.– Thought to give off energy (X-rays) as

matter is “eaten”.

Black Holes

Above: Hubble image of dust cloud around a black hole. Right: artist renditions.

Life cycle summary