18 Stellar Evolution and Cosmology Flashcards
Why is it difficult to classifying stars by their brightness?
The brightness they appear from Earth also depends on their distance from the Earth.
The closer the star, the brighter it appears.
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.
The main sequence
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.
A. Variation in gravitational field strength
Describe the absolute magnitude scale.
-10 0 +10
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?
Large mass star
Nebula –> Protostar –> Main sequence star –> Red super giant –> Supernova –> Neutron star
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?
Very large mass star
Nebula –> Protostar –> Main sequence star –> Red super giant –> Supernova –> Black hole
Where are the cool and dim stars found on the HR diagram?
Bottom right
Which of the following is not formed after a supernova?
A. a neutron star
B. a black hole
C. a red super giant
C. a red super giant.
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
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)
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
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.
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.
C. the bigger the observed increase in wavelength of its light.
A hot star will emit most its visible light at the _____________ frequencies of visible light.
highest (blue)
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?
The Big Bang
Locate the white dwarf in the HR diagram below.
D
white dwarf stars are dim and cool
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.
C. Leftover energy from the Big Bang explosion.
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?
red giant (small mass star)
red supergiant (large mass star)
Where are the hot and dim stars found on the HR diagram?
Bottom left
A cool star will emit most its visible light at the _____________ frequencies of visible light.
lowest (orange and red)
What is CMB or cosmic microwave background radiation?
It is the left over energy of the Big Bang.
True or False
’ A star that emits blue light will have a higher surface temperature than a star that emits red light’
True
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?
This equation can be rearranged to find any of the variables.
Where are the cool and bright stars found on the HR diagram?
Top right
Locate the red giants and red super giants in the HR diagram below.
C- red super giants - brighter than….
B- red giants
Explain how red-shift provides evidence that the universe is expanding
- 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.
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?
All the lines are shifted (the same amount) towards the red end of the spectrum - they are red-shifted.
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.
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.
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?
All the lines are shifted (the same amount) towards the blue end of the spectrum - they are blue-shifted.
Where are the hot and bright stars found on the HR diagram?
Top left
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?
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.
Stars initially form from a cloud of dust and gas called a….
nebula
Locate the main sequence in the HR diagram below.
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.*
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?
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
