Lecture 12 Flashcards
What are star forming clouds?
Stars form in dark clouds of dusty gas in interstellar space
What is the gas between the stars
Interstellar medium
How can you determine the composition of clouds
From its absorption lines in the spectra of stars
Composition of clouds
70% H, 28% He and 2% heavier elements in our region of milky way
What are molecular clouds
Most of the matter in star forming clouds is in the form of molecules (H2, CO, etc.)
These molecular clouds have a tempertaure of 10-30K and a desity of about 300 molecules per cubic centimeter
Gravity versus pressure
Gravity can create stars only if it can overcome the force of thermal pressure in a cloud
Emmision lines from molecules in a cloud can prevent a pressure buildup by concerting thermal energy into infrared and radio photons that escape the clpud
Mass of a star forming cloud
A typical molecular cloud (T- 30K, n- 300 particles/cm3) must contain at least a few hundred solar masses for gravity to overcome pressure
The first stars
Elelments like carbon and oxygen had not yet been made when the first stars formed
Without CO molecules to provide cooling, the clouds that formed the first stars had to be considerably warmer than todays molecular clouds
First Stars Versus Stars today?
First stars must be more massive than most of todays stars for gravity to overcome poressure
How do stars stay cool
Because of emmision by carbon monoxide
Contraction in stars
As contraction packs the molecules and dust particles of a cloud fragment closer together, it becomes harder for infared and radio photons to escape
Thermal energy then begins to build up inside, increasing the internal pressure
What is a protostar
Contraction slows down, and the center of the cloud fragment becomes a protostar
From protostar to main sequence
A protostar looks starlike after the surrounding gas is blown away, but its thermal energy comes from gravitational contraction, not fusion
Contraction must continue until the core becomes hot enough for nuclear fusion
Contraction stops when the energy released by core fusion balances energy radiated from the surface - the star is now a main sequence star
Birth Stages on a Life track
Assembly of a protostar
Convective Contraction
Radiative Contraction
Self Sustaining fusion
Assembly of a protostar
Luminosity and temperature grow as matter collects into a protostar
