Astrophysics

Question 1

For a single lense to make an image appear to be diminished, where should the object be placed?

Answer 1

Beyond 2f

Question 2

For a single lense to make an image appear the same size, where should the object be placed?

Answer 2

At 2f

Question 3

The a single lens to make an image appear magnified, where should the object be placed?

Answer 3

Between f and 2f

Question 4

For a single lens to make an image appear as a virtual object, where should the object be placed?

Answer 4

Below f

Question 5

How does a CCD work?

Answer 5

1- a CCD is a silicon chip, divided into picture elements (pixels)
2- each pixel is associated with a potential well in the silicon
3- photons are incident on the CCD, and the photons within the semi-conductor cause the electrons to be released
4- the number of electrons liberated, is proportional to the intensity of the light
5- the electrons are trapped in the potential wells, and an electron pattern is built up that is identical to the image formed on the CCD
6- when the exposure is complete, the charged is processed to form an image

Question 6

What is meant by quantum efficiency, and what are the values for a CCD vs the human eye?

Answer 6

Number of photons detected/ number of photons incident
CCD: 80% or more
Human eye: 1%

Question 7

What is chromatic abrasion, and which type of telescope does it effect?

Answer 7

A colour distortion that creates an outline of unwanted colour along the edges of objects in photos, that effects refracting telescopes. (Blue first then red)

Question 8

What is spherical abrasion, and what type of telescope does it effect?

Answer 8

It occurs when the mirror isn’t a perfect parabola, and it effects both types of telescope

Question 9

What is the Rayleigh criterion?

Answer 9

2 objects will just be resolved when the first minimum in the diffraction pattern of one image coincides with the central maximum of the other

Question 10

How can you improve resolving power?

Answer 10

Increase the size of the dish, or look at smaller wavelengths

Question 11

State 3 optical advantages of reflecting telescopes over refracting telescopes

Answer 11

• mirrors don’t refract light, so they don’t suffer chromatic abrasion
• mirrors can be larger, and therefore have greater resolving power
• mirrors can be larger, and therefore have smaller minimum angular resolution

Question 12

What is the maximum size of imperfections in a telescope dish can be?

Answer 12

Wavelength/ 20

Question 13

What is 1 parsec?

Answer 13

The distance at which 1 AU subs tends an angle of 1 arc second

Question 14

What is 1 arc second in degrees?

Answer 14

1/3600

Question 15

What is meant by apparent magnitude?

Answer 15

The brightness of a star as seen from earth

Question 16

What is meant by absolute magnitude?

Answer 16

The brightness of a star as seen from 10 pc away

Question 17

What is a standard candle?

Answer 17

Objects who’s absolute magnitude is known

Question 18

What is a black body?

Answer 18

And object that absorbed and emits all wavelengths of light

Question 19

What are hydrogen balmer lines?

Answer 19

Spectral lines corresponding to the visible wavelengths absorbed when electrons move from n=2 to higher energy levels

Question 20

Give a brief explanation of the key stages of the evolution of a sun like star

Answer 20

• dust and gas is drawn together via gravity to form a protostar
• protostars continue to contract and heat up until the core is hot enough to fuse hydrogen into helium (core hydrogen burning)
• hydrogen in the core runs out, core contracts and heats up. Outer layers expand and cool, star becomes red giant
• shell starts fusing hydrogen into helium (shell hydrogen burning)
  Hydrogen runs out, the core contracts and heats up. Helium now fuses into carbon and oxygen (core helium burning)
• helium in the core runs out, core contracts and heats up
• shell starts fusing helium (shell helium burning)
• for a sun like star, once fuel has run out, it contracts into a hot dense core, throwing off outer layers
• it has become a white dwarf

Question 21

For what mass of a star will the core contract to a white dwarf/ neutron star/ black hole?

Answer 21

1-1.4 solar masses: white dwarf
1.4-3 solar masses: neutron star
>3 solar masses: black hole

Question 22

What is a supernova?

Answer 22

A star whose luminosity increases enormously due to it exploding

Question 23

What is a neutron star?

Answer 23

A star with the density of nuclear matter

Question 24

What is a black hole?

Answer 24

An object with an escape velocity greater than the speed of light

Question 25

What is the schwarzchild radius?

Answer 25

The point at which the escape velocity is equal to the speed of light

Question 26

What is the Doppler effect?

Answer 26

The apparent change in frequency of a wave as a result of relative motion between the source and the observer

Question 27

What is the cosmological principle?

Answer 27

On a large scale, the universe is homogenous and isotropic

Question 28

What are the 3 pieces of evidence for the Big Bang?

Answer 28

1- red shift of galaxies
2- cosmic microwave background radiation
3- relative abundance of helium and hydrogen