Astro - Cosmology

Question 1

What is the Doppler effect ?

Answer 1

• the apparent change in the frequency of a wave caused by relative motion between the source of the wave and the observer

Question 2

What is red shift ?

Answer 2

When the light shifts to a longer wavelength ( moving away from the observer )

Question 3

What is blue shift ?

Answer 3

• the light shifts to a shorter wavelength ( moving towards the observer )

Question 4

How will the observed spectral lines change for a star moving away from the observer ?

Answer 4

• spectral lines will shift towards the red end

Question 5

What is Doppler shift ?

Answer 5

• the ratio of the change in frequency divided by the frequency
• the ratio of the change in wavelength to the original wavelength emitted by the radiation

Question 6

What does it mean if z is positive and negative ?

Answer 6

Positive ( red shift ) towards us

Negative ( blue shift ) away from us

Question 7

What evidence do we have for the big band / expansion of the universe ?

Answer 7

• the Doppler effect as most galaxies are mainly red shift ( indicating mowing away from each other

Question 8

What is meant by visual binaries ?

Answer 8

• the angular separation between the two stars is large enough , they can both be resolved using a telescope

Question 9

What is meant by spectroscopic binaries ?

Answer 9

• when two stars can not be resolved due to the merger of the two spectral lines into one line

Question 10

What are eclipsing binaries ?

Answer 10

• binary star systems whose orbits lie in the same plane as the line of sight of the observer such that the stars undergo mutual eclipses

Question 11

What is hubble’s law used for ?

Answer 11

To estimate the age of the universe

Question 12

How would you use hubbles law to work out the age of the universe ?

Answer 12

• since v = h d and v = d/ t

So 1/ h = t

( with all distances converted into m )

Question 13

Approximate age of the universe ?

Answer 13

15 billion years

Question 14

What does hubbles law state ?

Answer 14

The greater the distance from the earth , the greater the absorbtion spectra from a galaxy is red shifted , the greater the recession velocity of a galaxy

Question 15

What does calculating the age of the universe using hubble’s law assume ?

Answer 15

• the universe has been expanding at a constant rate ( therefore age would be an overestimate)

Question 16

What is meant by the voids of the universe ?

Answer 16

Little matter in between galaxies where the scansion of the universe can take place

Question 17

What is the Big Bang theory ?

Answer 17

• the universe started off very hot and very dense and has been expanding ever since

Question 18

What evidence is there for the Big Bang ?

Answer 18

• red shift
• hubble’s law
• CMBR

Question 19

How is the red shift evidence for the Big Bang theory?

Answer 19

• spectral lines from galaxies all show red shift - this means they are all moving apart
• greater red shift means greater distance and faster recession velocity
• this suggests that the galaxy is moving away from us and the universe is expanding

Question 21

What are quasars ?

Answer 21

They are stellar objects which are strong radio wave emitters which are extremely distant

• a super massive black hole surrounded by a sea of dust and gas

Question 22

How were quasars detected ?

Answer 22

• detected due to their high power radio emissions which showed as a bright radio source

Question 23

What is an exoplanet ?

Answer 23

Is a planet which orbits a star outside our solar system

Question 24

What are the issues of detecting exoplanets via a telescope?

Answer 24

• the exoplanets orbiting a star which is much brighter than they are
• the coolants are drowned out from the bright light emitted from the star that they are orbiting
• the angular separation between the star and the exoplanet is much smaller than the minimum angular separation of our telescopes

Question 25

What are other methods to detect exo planets ?

Answer 25

- transit method and the radial velocity method

Question 26

Explain the radial velocity method ?

Answer 26

-

Question 27

Explain the radial velocity method ?

Answer 27

- when exoplanets orbit of star ( This causes a slight variation/wobble on the star along its axis ) - When the Star wobbles was towards us ( blue shifted EM spectrum is visible ) and similarly red shifted Em spectrum when it moved away. - This red shift and blue shift would be periodic as the star orbits - Smaller the planet smaller the shift so mass can be determined

Question 28

Issues with using the radio velocity method

Answer 28

- only noticeable when the exoplanet is close to the size of the star - movement of the star must be aligned with the observers line of sight, if planet orbits the star perpendicular to our line of sight no detectable shift

Question 29

What causes the slight variation/wobble on a sun along its axis?

Answer 29

When a planet orbits a star the star co orbit around a common centre

Question 30

Describe the transmit method

Answer 30

The transit method measures the change in apparent magnitude as an exoplanet transits in front of the star from the view of earth as it orbits the star

Question 31

How would I use the apparent magnitude against time graph to determine information about the planet ?

Answer 31

- the greater the planet , the larger the drop in apparent magnitude - the further away from the sun the longer the drop in apparent magnitude

Question 32

Why are three transits required to confirm the existence of an exoplanet ?

Answer 32

- as the intensity drop can be due to sun spots / variation in out put of stars itself or other stellar objects such as comets

Question 33

What are issues with the transit method ?

Answer 33

- a dip in the light curve can be caused by other effects - periodic dips can take a very long time to establish if it is a planet - the alignment must be correct for a planet and star to eclipse in the plane of the earth ( unlikely )