Lecture 14: The Birth of Stars and Life on the Main Sequence Flashcards
1
Q
Star Modelling: Theory
A
- Creating a theory to understand what is happening inside of stars is mathematically complex
- We assume models that work on the Sun will work everywhere, physics is the same
2
Q
Numerical models of Stars
A
- Assume that stars are spheres
- break up star into concentric shells
- Calculate density, temperature, and energy generated in each shell
- calculate change in pressure across shell
- calculate change in enclosed mass and change in outward movement of energy
3
Q
Evolution of Stars: Step 1; Star formation, from cloud to stellar object
A
- Clouds of gas are found everywhere around our solar neighbourhood (mixed with 1% dust)
- Gravitational contraction leads to the formation of stars
- Clouds fragment as they contract, each cloud can make many stars
- dust is an important component in star forming regions
4
Q
Dense interstellar clouds
A
Low density clouds are generally gravitationally stable
- Have lower mass and higher temperatures
Higher density clouds
- Have enough dust to block visible light from stars
- They are colder inside
- Also called molecular clouds
- we can see stars forming in interstellar clouds
5
Q
Why do Interstellar clouds collapse?
A
- Gravity and pressure become unbalanced
- A cloud will contract if gravity is greater than pressure
- Jeans Mass is the maximum mass that is stable
- More mass means more gravity and cloud collapses
- A dense cloud contracts and fragments to make stars
6
Q
How big are the stars?
A
- Typical temperatures in the interstellar medium are tens of degrees and typical densities of dense clouds are in the hundreds
7
Q
Main stages of star formation I
A
- Collapse begins
- Fragmentation, many dense cores formed in a single cloud, star clusters formed
- Core to protostar, gas now falling to central core, initially cools efficiently, then heats slowly
- collapses to disk, bipolar outflow forms along the disk rotation axis
- conservation of angular momentum
8
Q
Evolution of Stars: Step 2; from protostar to main sequence
A
- Protostar with luminosity dominated by accretion
- central star with accretion disk
- bipolar outflow decreases core rotation
- dust acts as shield and coolant
- Protostellar wind, remaining gas lost and accretion ends
- Pre-main sequence star, luminosity dominated by contraction
- becomes hot enough for fusion
- pressure balances gravity, contraction halted
9
Q
From initial collapse to main sequence
A
- Different stellar masses act differently
- timescale increases as mass increases
- higher masses react faster
10
Q
Star Formation Overview
A
- Timescale is more rapid for high mass stars
- Stellar mass range from 0.08-100 solar masses
- single stars are rare, usually binary or in clusters
- Far more low mass stars are produced than high mass stars
11
Q
Evolution of Stars: Step 3; Life on the Main Sequence
A
- Changes in core produce changes at the surface
- fusion gradually changes core chemical abundance
- rate of reaction of hydrogen into helium drops over time
- temperature increase and core radius increase to compensate
12
Q
CNO cyle
A
- Carbon-12 used as catalyst in Hydrogen Helium fusion
- produces Nitrogen 13 which decays to carbon 13
- this process continues back to nitrogen 13, then to oxygen 13, back to nitrogen 13, and back to carbon 12
- Produces more gamma rays which means more energy
13
Q
Life on the Main Sequence
A
- Fusion rate greater for massive stars
- Reaction rate is very temperature sensitive
- star has reached a balance between gravity and pressure
- Luminosity, Radius, and Temp are relatively stable and change only very slowly
14
Q
Convection
A
- Happens when the temp difference is too large for radiation to be effective
- mixes the gas in the star
