14 Stellar Evolution

Question 1

Why is it difficult to classifying stars by their brightness?

Answer 1

The brightness they appear from Earth also depends on their distance from the Earth.

The closer the star, the brighter it appears.

Question 2

What is the name of the long stable period a star enters when the outward pressure caused by thermal expansion balances the force of gravity pulling everything inward.

Answer 2

The main sequence

Question 3

Describe the absolute magnitude scale.

Answer 3

-10 0 +10

Question 4

Stellar evolution for a small mass star is…

Nebula –> Protostar –> Main sequence star –> Red giant –> white dwarf

What evolutionary stages does a large mass star go through?

Answer 4

Large mass star

Nebula –> Protostar –> Main sequence star –> Red super giant –> Supernova –> Neutron star

Question 5

Stellar evolution for a low mass star is…

Nebula –> Protostar –> Main sequence star –> Red giant –> white dwarf

What evolutionary stages does a very large mass star go through?

Answer 5

Very large mass star

Nebula –> Protostar –> Main sequence star –> Red super giant –> Supernova –> Black hole

Question 6

Where are the cool and dim stars found on the HR diagram?

Answer 6

Bottom right

Question 7

Which of the following is not formed after a supernova?

A. a neutron star

B. a black hole

C. a red super giant

Answer 7

C. a red super giant.

Question 8

On the Hertzsprung-Russell diagram, white dwarfs group together because they have…

A ..negative absolute magnitudes and high temperatures

B .. negative absolute magnitudes and low temperatures

C .. positive absolute magnitudes and high temperature

Answer 8

C .. positive absolute magnitude and high temperatures

White dwarfs are dim and hot!

NOTE: Confusingly, the low absolute magnitude, the brighter the star ( -4 is brighter than +5)

Question 9

The absolute magnitude of a star is..

A. how bright the star would appear if it was at a standard distance from Earth

B. the brightest the star could appear to an observer

C. how bright the star appears from Earth

Answer 9

A. how bright the stat would appear if it was at a standadrd distance from Earth.

This allows us to compare the brightness of stars without worrying about their relative distances from Earth.

Question 10

A hot star will emit most its visible light at the _____________ frequencies of visible light.

Answer 10

highest (blue)

Question 11

Locate the white dwarf in the HR diagram below.

Answer 11

D

white dwarf stars are dim and cool

Question 12

Eventually a star runs out of hydrogen in it core. The force of gravity inward is larger than the pressure of thermal expansion outward.

The star is compressed, until it is dense and hot enough that the energy (and so the pressure) created makes the outer layers of the star expand.

What is the name of this stage?

Answer 12

red giant (small mass star)

red supergiant (large mass star)

Question 13

Where are the hot and dim stars found on the HR diagram?

Answer 13

Bottom left

Question 14

A cool star will emit most its visible light at the _____________ frequencies of visible light.

Answer 14

lowest (orange and red)

Question 15

The colour of a star is related to the star’s ____________ _____________

Answer 15

surface temperature

Question 16

The force of gravity pulls the dust and gas together to form a protostar. The temperature rises as the star gets denser and more particles collide with each other.

What occurs when the temperature gets hot enough (around 15 million K)

Answer 16

Hydrogen nuclei undergo nuclear fusion to form helium nuclei.

This gives out huge amounts of energy!!

Question 17

Which of the following will eventually become a supernova?

A stars with a similar mass to the Sun

B stars with a large mass than the Sun

C stars with a smaller mass than the Sun

Answer 17

B. stars with a larger mass than the Sun

Question 18

Absolute magnitude is a measure of how bright a given star would appear to be if it were…

Answer 18

a fixed distance from the Earth.

Question 19

True or False

’ A star that emits blue light will have a higher surface temperature than a star that emits red light’

Answer 19

True

Question 20

True or false

‘Our Sun is a yellow star and has a core temperature of around 5000K’

Answer 20

False- our Sun has a surface temperature of 5000K.

Its core temperature will be about 15 million K and therefore fusion can occur in its core.

Question 21

In a main sequence star, the outward pressure produced by nuclear fusion balances the force of gravity.

What causes this phase in the star’s life cycle to end?

Answer 21

The supply of hydrogen in the stars core, which is the fuel for nuclear fusion

Fusion ends when hydrogen runs out, meaning that the outward pressure no longer balances the gravitational force.

Question 22

Where are the cool and bright stars found on the HR diagram?

Answer 22

Top right

Question 23

Locate the red giants and red super giants in the HR diagram below.

Answer 23

C- red super giants - brighter than….

B- red giants

Question 24

How does the time a star spends on the main sequence vary with the mass of a star?

Answer 24

The more massive a star, the shorter the time it spends on the main sequence. (they burn hotter and faster!)

Question 25

The Sun is a yellow star. What colour would a lower surface temperature star be?

Answer 25

red

Question 26

True or False

The life cycle of a star depends on its mass

Answer 26

True

Very large mass stars end their lives as a black hole,

Low mass stars end their lives as a white dwarf

Question 27

Where are the hot and bright stars found on the HR diagram?

Answer 27

Top left

Question 28

The Sun is a yellow star. What colour would a higher surface temperature star be?

Answer 28

blue

Question 29

Stars initially form from a cloud of dust and gas called a….

Answer 29

nebula

Question 30

Locate the main sequence in the HR diagram below.

Answer 30

A

Stars spend most of their life time on the main sequnce.

• bright blue stars at the top left*
• dim red stars at the bottom right.*

Question 31

Which process releases energy in stars?

A. Combustion

B. Nuclear fission

C. Nuclear fusion

Answer 31

C. Nuclear Fusion

This occurs in the core of a star which is at more than 15 million Kelvin.