Low Mass Stars Flashcards
What are low mass stars?
Stars like the sun (<2 solar masses)
Why are 90% of stars low mass?
More likely to form than high mass and they live longer
What makes low mass stars active?
Convection can cause coronal mass ejections and flares which can affect the earth
What are the most dramatically active stars?
M dwarfs
Why are M dwarfs so active?
•very deep convection zones
•fast rotators
•for short periods may produce more energy in X-rays than in visible and infrared
When do low mass stars reach main sequence?
After 50-150 million years
What keeps fusion rate and overall luminosity steady in low mass stars?
Self regulation
What is inward gravitational force balanced by in low mass stars?
Outward pressure (energy generated in core released as stellar luminosity)
How long can very low mass stars (<0.4Ms) stay in hydrogen burning phase?why?
Almost a trillion years since helium will not build up in core
What happens after 10 billion years when (>0.4Ms) star exhausts all the hydrogen in its core?
Core collapses
Gravity shrinks both helium core and the surrounding shell of hydrogen. Hydrogen shell becomes hot enough to ignite fusion
What happens when shell burining is unregulated?
Stellar envelope expands
What happens to newly produced helium during shell burning?
•Helium Continually added to core
•core mass increases, amplifying its gravitational pull and shrinking hydrogen shell further
•fusion rate in shell continues to rise
•finally radius is 100 times initial
•final luminosity 1000 times suns
When does process of shell burning halt?
After 1 billion years when core temperature reaches 100 million K causing helium to burn
What happens to helium during helium burning?
Triple alpha reaction. 3 He combine to form one carbon nucleus
What is core supported by during helium burning?
Degeneracy pressure
What are stages of helium burning?
•burning heat’s core rapidly without causing it to expand
•rising temps cause rapid increase in helium fusion rate(helium flash)
•temp rises so much that thermal pressure becomes important and core begins to expand and starts to regulate helium fusion rate
What happens as temp decreases?
Total energy production decreases
•outer stellar layers begin to contract
•size and luminosity decline
•core burning means that a helium burning star has balance similar to main sequence
How does helium fusion compare to nuclear?
Very temperature sensitive and star is far less stable
What is Heisenberg uncertainty principle?
We cannot confine electrons infinitely since this would imply infinite momentum
What is Pauli exclusion principle?
No more than 2 electrons can have the same energy and position
What happens when helium core is exhausted?
Star will begin to expand again with double shell burning. Helium shell burns around carbon core and hydrogen shell burns around helium shell
How long does double shell burining last?
Only a few million years
Why can carbon fusion not occur in core of a low mass star?
Only possible above 600 million K
How are outer layers of stars ejected?
During thermal pulses that occur due to temperature sensitivity of He fusion
How does temperature change during death if low mass star?
Goes from 4000K (envelope) to 120000K (bare core)
How is a planetary nebula formed?
Uv photons from core ionise the ejected outer layers
When will planetary nebula disappear?
In a few million years
What happens to hot carbon cores >100000K?
Remain as white dwarfs which then cool to black dwarfs within next trillion years
When will sun enter final stages of its life?
In about 5 billion years
What will happen when sun dies?
•expand to almost orbit of earth
•solar prominences and flares may engulf earth
•planetary nebula will expand beyond the orbit of Pluto
