Dec. 19 – Stars

Stars

Learning Goals: Understand that stars come in different types.

Success Criteria: You can describe the life cycle of different stars.

Stars come in many different sizes, colours and temperatures.  Here are a few ways we can tell the difference between stars.

Luminosity

• Total amount of energy produced by a star per second.
• The sun has luminosity of 1.

Apparent Magnitude 

• A star's brightness as seen from Earth.
• The sun = -27 apparent magnitude
• Full Moon = -13
• Brightest Stars = 1
• dimmest stars = 6

Absolute Magnitude

• The magnitude we would see if we were 33 ly away.

Colour

• A star's colour is related to temperature.
• Blue - hotter
• Yellow - medium
• Red - cooler

Spectrum 

• The rainbow of light emitted by a star.  
• Missing lines in the spectrum tell you what the star is made of.

Each element emits a different spectrum.  The missing colours (dark bands) line up with hydrogen, so the star definitely has hydrogen in it.

Mass

• The sun is 2 e 30 kg = 1 solar mass
• Most stars range from 0.1 to 120 solar masses.

The life cycle of stars

Nebula - a massive cloud of gas and dust.

Protostar - dust from a nebula collapses under gravity.

After millions of years, a protostar collapses further until fusion happens, 15 million °C.

The type of star depends on mass.

Hertzsprung-Russell Diagram

 - hot, luminous stars are more massive.

 - most stars fall on "Main Sequence"

Small stars:

After 10 billion years, burn off hydrogen and expand to become red giants.

hydrogen --> helium --> other elements (lithium, carbon, etc)

Eventually stops fusion and a white dwarf is left over.  Cool down and fade out.

Bigger Stars:

Shorter lives, burn faster.

Can fuse heavier elements

carbon --> iron

After fusion, gases collapse, then bounce back and explode into a super nova.

The Crab Nebula is the left over from a super nova.

Heavier elements are fused.

After super nova you can have one of two things left over...

Neutron Star - made of pure neutrons.

Super dense, gravity 300 000 times that of earth.

This is what Neutron Star would look like over Vancouver.  Much smaller than the Earth, but hundreds of thousands of times more massive.

Black hole - even more dense than neutron stars, gravity so strong that light cannot escape.

An artists drawing of a star being torn apart by a black hole.

The life cycle of different stars.

Homework

• P. 351 # 1, 2, 6, 7, 9
• P. 369 # 1-3, 6, 7, 8, 11