Astrophysics

Question 1

Lens formula

Answer 1

1/u + 1/v = 1/f

u = distance of object from centre of lens
v = distance of image from centre of lens
f = focal length of a thin lens

If diverging: f and v is negative

Question 2

When is a virtual image produced

Answer 2

When the object is closer than the focus point

Question 3

AU

Answer 3

Mean orbital radius of the earth around the sun

Question 4

Parsec

Answer 4

Distance to a star which subtends an angle of 1 arcsecond to the line from the centre of the Earth to the centre of the Sun.

Question 5

1 arc second

Answer 5

4.85x10^-6 radians

Question 6

Parallax angle

Answer 6

Parsecs = 1/parallax angle in arc seconds

Parallax method acceptable up to 100pc

Question 7

What unit of distance is used for the absolute magnitude equation

Answer 7

m - M = 5log(d/10) uses parsecs

Question 8

Where is our sun in the Hertzsprung-Russel diagram

Answer 8

Absolute magnitude 5
Temperature just over 5000 / right side of class G

Question 9

Visible spectral classes

Answer 9

O: 50-25 Strong: He+, He. Weak: H
B: 25-11 Strong: He, H
A: 11-7.5 Strongest H lines weak: metal ion
F: 7.5-6 Strong: metal ions
G: 6-5 metal ion and metal atom
K: 5-3.5 Mostly neutral metal atoms
M: less than 3.5 Neutral atoms and compounds like TiO

Question 10

Luminosity and intensity

Answer 10

Luminosity= power output of a star
Intensity = power of radiation/area

Question 11

Aperture

Answer 11

Diameter of objective lens / mirror

Question 12

Difference and similarities in telescopes that pick up different wavelengths

Answer 12

Optical telescopes are similar to infrared UV and radio, however instead of an eyepiece lens a CCD is used and an Ariel for radio. These 3 telescopes a cassegrain reflecting telescope is often used.
X-ray telescopes are usually in space due to the atmosphere preventing the majority of X-radiation reaching the Earths surface. They are very penetrating and not easily reflected off metal surfaces , some reflected and some transmitted. Iridium is used and they are reflected off a series of mirrors at a very shallow angle and focus some 10m away. Due to their short wavelength the telescopes can have a small diameter and still produce well resolved images.

Question 13

Infrared and UV telescope

Answer 13

Due to atmosphere UV telescopes usually orbit around Earth, some infrared penetrates through, so possible to have on top of mountains, others orbit the Earth to detect all infrared.
Due to being on either end of the wavelength spectrum their collecting powers are similar to optical as diameter similar, however due to UV’s smaller wavelength it has a better resolving power at same diameter. This is opposite for infrared.

Question 14

Radio telescope

Answer 14

Their mirrors/dishes are very large due to larger wavelengths, meaning collecting power is very high.
They are built so big due to ∅ ≈ lambda/diameter and therefore able to resolve to close radio sources

Question 15

Compare reflecting Vs refracting telescope

Answer 15

Reflecting pros:
•tend to be cheaper
•Easier to make concave mirror of large diameter than lens
•No chromatic abberation
•Easier to reduce spherical abberation
•Possible to make them with larger diameters as a lens with diameter of over 1m begins to sag under its own weight.

Refracting pros:
•Less maintenance is required, mirror in reflecting is exposed to air and therefore may need recoating..
•Secondary mirror in refracting may block light

Question 16

Construction of ray diagram

Answer 16

1) ray parallel to the principal axis is refracted so that it passes through the focal point
2) a ray that passes through the optical centre of the lens is undeviated
3) a ray that passes through the focal point is refracted so it travels parallel to the principal axis

Question 17

When comparing telescopes mention…

Answer 17

Magnifying power M = fo/fe = beta/alpha
Resolving power = ∅ = lambda/d
Collecting power = D1^2/D2^2

Question 18

Rayleigh Criterion

Answer 18

The minimum subtended angle between two objects
whose (images) can be resolved. ✔1
(Minimum angle is when) the central maximum of (the diffraction pattern of light
from) one object coincides with the first minimum of (the diffraction pattern) of
the second object. ✔2

Question 19

What to compare when comparing stars

Answer 19

Colour (temp)
Brightness (magnitude)
Power

Question 20

What happens at less than and more than 1.4 solar masses of a red super giant
What if the supernova mass is less than or more than 3 solar masses

Answer 20

White dwarf
Supernova
Neutron star
Black hole

Question 21

Electron degeneracy pressure

Answer 21

When the core has shrunk to about Earth-size electrons exert enough pressure to stop it collapsing
(White dwarfs)

Question 22

Absolute magnitude of a supernova at its peak

Answer 22

-19.3 +/- 0.3

Question 23

When mentioning balmer lines remember to say whether weak or strong

Question 24

The defining characteristics of a supernova

Answer 24

Rapid, massive increase in brightness

Question 25

What type of supernova has hydrogen balmer lines

Answer 25

Type 2 supernova

Question 26

How is a type 1a (a subset of type 1) supernova formed

Answer 26

When a white dwarf core absorbs matter from a nearby binary partner

Question 27

Pulsing neutron stars are called…

Answer 27

Pulsars due to them spinning fast and emitting radio waves

Question 28

Doppler effect definition

Answer 28

The apparent change in the frequency/wavelength of a wave due to the relative motion of the source and the absorber

Question 29

Exoplanet def

Answer 29

A planet found outside our Solar System, in orbit around another star

Question 30

Why is it hard to find exoplanets?

Answer 30

Light from the host star is much brighter than the reflected light from the planet
They subtend extremely small angles compared to the resolution of telescopes

Question 31

Radial velocity method/Doppler shift effect

Answer 31

As a planet orbits its host star, they both orbit around a common centre of mass
During the orbit, the star will move slightly towards, or away from the Earth as the planet moves to different positions in the orbit
The line spectrum of the star will showblueshiftwhen it moves towards the Earth, thenredshiftwhen it moves away

Question 32

How to determine the orbital period of an exoplanet using the radial velocity method

Answer 32

This causes very small, but measurable, periodic shifts in the wavelength of the light received from the star
The time period of the planet’s orbit is equal to the time period of the Doppler shift

Question 33

Light Curve using the Transit Method

Answer 33

The dip in brightness can be used to determine the size of the planet
The duration of the dip can be used to determine the orbital period of the planet

Question 34

Transit method limitations

Answer 34

The accuracy can be reduced if the Earth, planet and star are not aligned in the same plane
Only planets with a short orbital period can be detected

Question 35

Radial velocity limitation

Answer 35

Low-mass, or Earth-like, planets do not cause as much ‘wobble’ as high-mass planets since they have a greater gravitational pull on the star

Question 36

Quasar

Answer 36

The quasar features a black hole surrounded by an accretion disk and emits jets of radiation
As matter falls into the black hole,jetsof radiation are emitted from the poles
The equivalent of 100 solar masses of matter can fall into a quasar each year
The gravitational potential energy of infalling matter is transferred to electromagnetic radiation
Now it is known that quasars are strong emitters ofallwavelengths, not just radio waves

Question 37

Quasar characteristics

Answer 37

Extremely luminous star-like sources of radiation with very high redshifts
Quasars are thought to be some of the most disttant measurable objects in the known universe
This is evidenced by the extremely large redshifts they show