Module 5 - Astrophysics (Massive Stars)

Question 1

What is meant by a massive star

Answer 1

A star with a mass at least 10x the mass of our sun

Question 2

Why do massive stars run through hydrogen fusion faster than main sequence stars

Answer 2

• They have a greater rate of fusion as their is higher pressures and temperatures

Question 3

What happens when hydrogen runs out to form a Red Supergiant

Answer 3

• Radiation pressure falls causing gravity to collapse the core.
• Collapse raises core temperature and pressure, allowing fusion of the next heavier element.
• Released energy expands the surrounding shell, allowing fusion of the element the core fused previously.
• This is repeated for heavier elements up until iron

Question 4

Describe the pattern for the elements mass to the proximity to the core

Answer 4

• heavier elements at the core

Question 5

Describe the change in KE and Temperature of a high mass core and low mass core when collapsing

Answer 5

The kinetic energy and the temperature of the core increases more for high mass cores

Question 6

What is Pauli’s exclusion principle

Answer 6

Electrons cannot occupy the same energy state (position)

Question 7

What is Chandrasekhars Limit and its quuantity

Answer 7

1.44 solar masses - The mass of a core beyond which its gravity overcomes the electron degeneracy pressure

Question 8

Describe how a Red Supergiant forms a supernova when it has an iron core

Answer 8

• Iron cannot be fused so radiation pressure drops
• Gravity pulls in all the stars shells which rebounds and implode on the solid iron core, which ejects all the material into space
• This explosion is a type 2 supernova which leaves behind a solid iron core

Question 9

Describe the formation of a neutron star from a star core

Answer 9

• If the star core has a mass greater than chandrasekhars limit but less than 3 solar masses
• The gravity will be greater than the electron degeneracy pressure, causing the core to collapse on itself
• The electrons fuse with the protons, to form neutrons, forming a neutron star

Question 10

Describe the formation of a black hole from a star core

Answer 10

• If the star core has a mass greater than 3 solar masses
• The gravity will be so large that it is not only greater than the electron degeneracy pressure but it compresses the core to form a black hole with a gravitational field with an escape velocity greater than the speed of light.

Question 11

Features of black holes

Answer 11

• Light cannot escape
• Thought to be at the center of galaxies

Question 12

In what stage of a star are elements heavier than iron formed

Answer 12

• In the supernova explosion

Question 13

What is meant by a red super giant

Answer 13

A massive star in the last stages of its life, with fusion of heavy elements heavier near the star’s core and lighter elements closer to the stars shell

Question 14

what is meant by a supernovae

Answer 14

An implosion of a red supergiant following the formation of iron in its core. The outer layers bounce off the inner core and are ejected into space, leaving an inert remnant core.

Question 15

What is meant by a neutron star

Answer 15

A star made entirely neutrons after a supernova core collapses and electrons fuse with protons to form neutrons

Question 16

What are the factors that show the surface temperature of a star and explain the trend

Answer 16

• Colour of the star
  The bluer the star the hotter
• Size of the star
  The bigger the star the further the surface is from the core meaning more cooling occurs

Question 17

When is the only time the size of a star affects its surface temperature and why

Answer 17

A larger star only means a lower surface temperature when comparing stars at different evolution stages. Stars in the same stage are too similar in size for the temperature change to be noticeable

Question 18

What are the factors that show the luminosity of a star and explain the trend

Answer 18

• The size of the star
  The bigger the star the greater its surface area and the light being transmitted

Question 19

Describe the axes of the HR diagram

Answer 19

X –> Temperature in K, from high to low, logarithmic scale

Y –> Luminosity, logarithmic 10 scale with 1 at the middle

Question 20

Describe the shape of a HR diagram, and what stars go where

Answer 20

Slanted F, with low mass stars on the bottom and high mass star at the top, with a dot at the bottom left for white dwarfs

Question 21

Describe the journey of a low mass star on a HR diagram

Answer 21

Starts at the bottom of the F - hotter, larger main-sequence stars are brighter, moving diagonally up-left.

At the red giant stage, expansion increases luminosity and lowers surface temperature, shifting up-right.

As a white dwarf, only the hot, dim core remains hotter than before but much less luminous, moving it down and right

Question 22

Describe the journey of a massive star on the HR diagram

Answer 22

Larger main sequenced massive star means hotter and brghter so starts higher on the F

At the red supergiant stage, expansion increases luminosity and lowers surface temperature, shifting up-right.

Question 23

Why is a supernovae not on the HR diagram

Answer 23

It is too brightW