18 Stellar Evolution and Cosmology

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

Which of the following is NOT evidence that supports the Big Bang Theory

A. Variation in gravitational field strength

B. The red-shift observed in light reaching Earth from distant galaxies.

C. Cosmic microwaves background radiation.

Answer 3

A. Variation in gravitational field strength

Question 4

Describe the absolute magnitude scale.

Answer 4

-10 0 +10

Question 5

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 5

Large mass star

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

Question 6

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 6

Very large mass star

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

Question 7

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

Answer 7

Bottom right

Question 8

Which of the following is not formed after a supernova?

A. a neutron star

B. a black hole

C. a red super giant

Answer 8

C. a red super giant.

Question 9

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 9

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 10

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 10

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 11

In general, the further away a galaxy is….

A. the smaller the observed increase in the wavelength of its light.

B. the slower it is moving away from us.

C. the bigger the observed increase in wavelength of its light.

Answer 11

C. the bigger the observed increase in wavelength of its light.

Question 12

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

Answer 12

highest (blue)

Question 13

Below is a description of how the universe started…

1) initially, all the matter in the universe occupied a single point
2) This tiny space was very dense and very hot

3) This single spot then exploded

4) Space started expanding, and the expansion is still going.

What is stage 3 called?

Answer 13

The Big Bang

Question 14

Locate the white dwarf in the HR diagram below.

Answer 14

D

white dwarf stars are dim and cool

Question 15

What is cosmic microwave background (CMB) radiation believed to be?

A. Energy from exploding supernovae.

B. Radiation given out by man-made objects in space, such as satellites and telescopes.

C. Leftover energy from the Big Bang explosion.

Answer 15

C. Leftover energy from the Big Bang explosion.

Question 16

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 16

red giant (small mass star)

red supergiant (large mass star)

Question 17

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

Answer 17

Bottom left

Question 18

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

Answer 18

lowest (orange and red)

Question 19

Light from galaxies is red-shifted. What do measurements of the red-shift tell us?

A. All galaxies are moving away from us at the same speed

B. More distant galaxies are moving away faster than closer galaxies

C. Closer galaxies are moving away faster than distant galaxies.

Answer 19

B. More distant galaxies are moving away faster than closer galaxies

Question 20

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

Answer 20

surface temperature

Question 21

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 21

Hydrogen nuclei undergo nuclear fusion to form helium nuclei.

This gives out huge amounts of energy!!

Question 22

What is CMB or cosmic microwave background radiation?

Answer 22

It is the left over energy of the Big Bang.

Question 23

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 23

B. stars with a larger mass than the Sun

Question 24

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

Answer 24

a fixed distance from the Earth.

Question 25

True or False ' A star that emits blue light will have a higher surface temperature than a star that emits red light'

Answer 25

True

Question 26

True or false 'Our Sun is a yellow star and has a core temperature of around 5000K'

Answer 26

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 27

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 27

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 28

Wavelength of light is commonly stated in nanometers. What does the prefix nano represent? e.g. 450 nm

Answer 28

450 nm = 450 x 10^-9 m

Question 29

You need to be able to make calculations involving red-shift. How is the amount the light from a galaxy is red-shifted (**change in wavelength**) related to the velocity of the galaxy?

Answer 29

This equation can be rearranged to find any of the variables.

Question 30

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

Answer 30

Top right

Question 31

According to the Big Bang theory, when the universe began it was... ## Footnote A. small, dense and hot. B. identical to the current universe C. much larger and cooler than the current universe.

Answer 31

A. small, dense and hot.

Question 32

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

Answer 32

C- red super giants - brighter than.... B- red giants

Question 33

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

Answer 33

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

Question 34

Explain how red-shift provides evidence that the universe is expanding

Answer 34

- red-shift suggests that the galaxy is _moving away from us_ - red-shift measurements indicate that distant galaxies are _moving away from us in all directions_ - _more distant galaxies have greater red-shifts than closer galaxies_. indicating that they are _moving away faster_ than closer galaxies - this indicates that they _whole universe is expanding._

Question 35

Different elements (such as hydrogen) absorb different frequencies of light. When light, formed at the centre of star, passes through an outer atmosphere of hydrogen, a pattern of dark lines is produced. These lines are found at exactly the same position that the hydrogen absorb. **What happens to these lines if the galaxy or star producing the light are _moving away from us?_**

Answer 35

All the lines are shifted (the same amount) towards the red end of the spectrum - they are red-shifted.

Question 36

An observer measures the frequency of a passing ambulance's siren with an oscilloscope. After the ambulance passes the observer, a decrease in the siren's frequency is recorded. Name the effect that causes this change in frequency and explain why it happens.

Answer 36

Name of the effect: **_Doppler effect_** Explanation: The **relative motion** of the ambulance away from the observer **stretches the wavelength** of the sound waves, which **lowers their frequency.**

Question 37

Different elements (such as hydrogen) absorb different frequencies of light. When light, formed at the centre of star, passes through an outer atmosphere of hydrogen, a pattern of dark lines is produced. These lines are found at exactly the same position that the hydrogen absorb. **What happens to these lines if the galaxy or star producing the light are _moving towards from us?_**

Answer 37

All the lines are shifted (the same amount) towards the blue end of the spectrum - they are blue-shifted.

Question 38

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

Answer 38

red

Question 39

True or False The life cycle of a star depends on its mass

Answer 39

True Very large mass stars end their lives as a black hole, Low mass stars end their lives as a white dwarf

Question 40

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

Answer 40

Top left

Question 41

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

Answer 41

blue

Question 42

Why is the intense energy/light from the Big Bang that we now see all around us (Cosmic Microwave Radiation) in the microwave region of the spectrum?

Answer 42

After the Big Bang the energy was released in all direction and has _cooled._ The _expansion of the universe_ since then means that this energy (light) has been red-shifted so far down the red end of the spectrum that it is now in the **microwave region.**

Question 43

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

Answer 43

nebula

Question 44

Locate the main sequence in the HR diagram below.

Answer 44

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 45

Some of out local galaxies are blue-shifted. What does this mean?

Answer 45

These local galaxies have a relative motion which is towards us

Question 46

How is an expanding balloon with galaxies drawn on it a good analogy for what is happening to our universe? Why is it a bad analogy?

Answer 46

Good analogy- 1) As you blow up the balloon, it stretches and the galaxies move further away from each other 2) the balloon represents the universe, as time goes on, space stretches and expands, moving the galaxies away from each other Bad analogy- 1) the balloon only stretches so far 2) there would be galaxies inside the balloon as well

Question 47

Which process releases energy in stars? A. Combustion B. Nuclear fission C. Nuclear fusion

Answer 47

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