The Birth of Stars Astronomy Midterm

Molecular Clouds

Where stars form because they are the coldest and most dense

• Full of molecules, can’t see through the dust bc the dust and gas are so dense

• Gravity is strong, pressure is weak bc temp is low, so they collapse

• Dark because they are full of interstellar dust

  • what keeps them cold

Why do stars form only in the densest ISM?

• To collapse, GRAVITY must be stronger than the gas PRESSURE

  • the sun doesn’t collapse bc the closer you get to the center, the stronger the pressure is (hydrostatic equilibrium)

• Temp lower = pressure lower = collapse = stars

• Need high density for gravity to start to win

Dense parts of the interstellar medium are hidden by what?

Dust!

• Red light gets through, Infrared makes it through even easier

  • Objects behind fust will appear DIMMER (darker) & REDDER

Infrared telescopes

Can see where stars are being born

• dust will be transparent

Protostar

When a molecular cloud (close, dense) is collapsing, the center becomes hotter and denser, eventually becoming opaque with photosphere

• Heats up as it collapses, doesn’t obtain its energy from fusion

  • can only stay hot by collapsing further, so it’s not in hydrostatic equilibrium

• Large AND luminous

When do we consider a star to be born?

Once nuclear fusion begins

How does star formation begin

Collapse of a cloud

• Gravity has to be winning over pressure

  • once it starts winning, it doesn’t stop

• The densest parts of the clouds breaks into pieces to form a star

Agular momentum

measures the amount of spinning in a system

• If you start spinning it is hard to stop spinning

• Has to stay the same, if it gets smaller it has to keep spinning but FASTER

• Can only change it if you come from above

  • from the top you see it spinning, from the side you can see it flattening

Growth of protostar

Temperature increases so that pressure can balance gravity

• But luminosity decreases bc it is becoming much smaller

How does protostar generate energy

from collapsing

How does a star generate energy

from fusion starting

• As the protostar shrinks, the core gets hotter and then the protons will repel to each other to make fusion

  • fuses hydrogen into helium

• need high pressure and density