4. Physics only Astro - HR diagrams, Big Bang, Red Shift

Question 1

What is apparent magnitude?

Answer 1

Brightness of a star

• depends on : luminosity (how much energy it radiates) + distance from Earth
• very bright = small/negative apparent magnitudes
• very dim = high apparent magnitudes

Question 2

What is absolute magnitude?

Answer 2

• allows us to compare brightness of stars -> measure of how bright they would appear if all same distance from earth (only depends on luminosity)
• again, lowest magnitude highest brightness

Question 3

Why do we see fewer stars in top left and bottom right of HR diagram?

Answer 3

1. Top left = large mass main sequence stars -> short lifetime -> evolved into red giants -> so few here at one time
2. Bottom right low mass stars , lower luminosity, very faint + less detected

Question 4

How will the absolute magnitude of a star alter when it becomes a red giant?

Answer 4

• decrease

- larger radius + larger sa so will transfer more energy

Question 5

How can you deduce the relative ages of galaxies from HR diagrams?

Answer 5

• HR diagram showing only ms stars indicates younger galaxy than one showing less main sequence stars, and more red giant etc stars
• enough time has passed for a ms to turn into an rg or a wd

Question 6

Describe the past evolution of the universe

Answer 6

• Universe started 13.8 billion years ago with Big Bang
• universe expanded from sate of high density + temp
• as universe expanded universe cooled + decreased in density
• as it cooled atoms, molecules, stars + galaxies formed

Question 7

What is CMBR radiation and how does it support the Big Bang theory?

Answer 7

• cosmic microwave background radiation emitted during Big Bang
• emitted as gamma radiation
• wavelength stretched because universe is expanding
• microwave radiation present everywhere leftover from Big Bang, which implies all parts of the universe were in contact a long time ago

Question 8

If a galaxy is moving towards us, how would you expect the frequency of light to change?

Answer 8

• frequency will increase (blue shift)

Question 9

If a a galaxy is moving away from us, how would you expect the frequency of light to change?

Answer 9

Frequency will decrease (red shift)

Question 10

How does red shift of galaxies support the Big Bang theory?

Answer 10

• the further the galaxy is from earth, the greater the red shift
• the greater the red shift, the faster the galaxy is moving away
• speed of galaxies increases with increased distance
• relationship between speed and distance implies expansion from a single point or since Big Bang

Question 11

What is the Doppler effect?

Answer 11

• when moving away from observer, apparent wavelength increases, decrease in frequency
• when moving towards observer, wavelength decreases, frequency increases
• wave speed is constant

Question 12

Do furthest galaxies move away from us faster?

Answer 12

Yes, they have more red shift