Low Mass Stars

Question 1

What are low mass stars?

Answer 1

Stars like the sun (<2 solar masses)

Question 2

Why are 90% of stars low mass?

Answer 2

More likely to form than high mass and they live longer

Question 3

What makes low mass stars active?

Answer 3

Convection can cause coronal mass ejections and flares which can affect the earth

Question 4

What are the most dramatically active stars?

Answer 4

M dwarfs

Question 5

Why are M dwarfs so active?

Answer 5

•very deep convection zones
•fast rotators
•for short periods may produce more energy in X-rays than in visible and infrared

Question 6

When do low mass stars reach main sequence?

Answer 6

After 50-150 million years

Question 7

What keeps fusion rate and overall luminosity steady in low mass stars?

Answer 7

Self regulation

Question 8

What is inward gravitational force balanced by in low mass stars?

Answer 8

Outward pressure (energy generated in core released as stellar luminosity)

Question 9

How long can very low mass stars (<0.4Ms) stay in hydrogen burning phase?why?

Answer 9

Almost a trillion years since helium will not build up in core

Question 10

What happens after 10 billion years when (>0.4Ms) star exhausts all the hydrogen in its core?

Answer 10

Core collapses
Gravity shrinks both helium core and the surrounding shell of hydrogen. Hydrogen shell becomes hot enough to ignite fusion

Question 11

What happens when shell burining is unregulated?

Answer 11

Stellar envelope expands

Question 12

What happens to newly produced helium during shell burning?

Answer 12

•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

Question 13

When does process of shell burning halt?

Answer 13

After 1 billion years when core temperature reaches 100 million K causing helium to burn

Question 14

What happens to helium during helium burning?

Answer 14

Triple alpha reaction. 3 He combine to form one carbon nucleus

Question 15

What is core supported by during helium burning?

Answer 15

Degeneracy pressure

Question 16

What are stages of helium burning?

Answer 16

•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

Question 17

What happens as temp decreases?

Answer 17

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

Question 18

How does helium fusion compare to nuclear?

Answer 18

Very temperature sensitive and star is far less stable

Question 19

What is Heisenberg uncertainty principle?

Answer 19

We cannot confine electrons infinitely since this would imply infinite momentum

Question 20

What is Pauli exclusion principle?

Answer 20

No more than 2 electrons can have the same energy and position

Question 21

What happens when helium core is exhausted?

Answer 21

Star will begin to expand again with double shell burning. Helium shell burns around carbon core and hydrogen shell burns around helium shell

Question 22

How long does double shell burining last?

Answer 22

Only a few million years

Question 23

Why can carbon fusion not occur in core of a low mass star?

Answer 23

Only possible above 600 million K

Question 24

How are outer layers of stars ejected?

Answer 24

During thermal pulses that occur due to temperature sensitivity of He fusion

Question 25

How does temperature change during death if low mass star?

Answer 25

Goes from 4000K (envelope) to 120000K (bare core)

Question 26

How is a planetary nebula formed?

Answer 26

Uv photons from core ionise the ejected outer layers

Question 27

When will planetary nebula disappear?

Answer 27

In a few million years

Question 28

What happens to hot carbon cores >100000K?

Answer 28

Remain as white dwarfs which then cool to black dwarfs within next trillion years

Question 29

When will sun enter final stages of its life?

Answer 29

In about 5 billion years

Question 30

What will happen when sun dies?

Answer 30

•expand to almost orbit of earth
•solar prominences and flares may engulf earth
•planetary nebula will expand beyond the orbit of Pluto