Unit 1 - Observational Astronomy

Question 1

What is an astronomical unit?

Answer 1

the distance between the earth and the sun

Question 2

How far is 1 parsec?

Answer 2

3.26 light years

Question 3

What are the 2 perpendicular components of light (and all EM radiation)?

Answer 3

an oscillating electric field
an oscillating magnetic field

Question 4

What wavelengths of radiation are absorbed and scattered the most by clouds / dust?

Answer 4

gamma rays and x-rays
shorter wavelengths scattered the most

Question 5

What is a black body?

Answer 5

an astronomical body that when cool absorbs all the incoming radiation of all wavelengths and when hot emits radiation with 100% efficiency

Question 6

What is the relationship between the peak wavelength (colour) emitted and temperature of the black body?

Answer 6

as peak wavelength increases temperature decreases (inversely proportional)

Question 7

What is the relationship between power of radiation emitted and temperature?

Answer 7

power is directly proportional to T^4

Question 8

Why are planets not black bodies?

Answer 8

they generate little radiative energy and instead reflect sunlight so the observed colour of a planet doesn’t give a true picture of temperature or power output

Question 9

Where are electrons found in the atom?

Answer 9

energy levels
the energy level closest to the nucleus is known as the ground state and has the smallest energy value

Question 10

What happens when electrons absorb a photon of light?

Answer 10

they ‘jump’ to a higher energy level as long as the photon corresponds to a specific energy gap

Question 11

What happens when an electron emits a photon of light?

Answer 11

it drops down an energy level and releases a photon of light corresponding to the specific energy gap (colour)

Question 12

What do the emission and absorption spectrum allow us to identify?

Answer 12

identify the elements present within a star or galaxy since each element has a unique spectrum which can be matched up

Question 13

What is the Doppler Effect?

Answer 13

when radiation emitted by astronomical bodies is effected by the relative speed at which the object travels

Question 14

Explain the Doppler Effect

Answer 14

a source approaching the observer will have a shorter wavelength and higher frequency
a source moving away from the observer will have a longer wavelength and lower frequency

Question 15

What significance is the + / - before the v in the Doppler Effect equation?

Answer 15

+v means distance is increasing, source is moving away
-v means distance is decreasing, source is moving closer

Question 16

What is redshift?

Answer 16

a shift in the spectral lines when all lines move to the right (red end of the spectrum) seen for objects moving away from the source

Question 17

What are the 3 main functions of telescopes?

Answer 17

1) maximise image brightness (light gathering)
2) resolve images that are separated by small angles
3) magnify distant objects

Question 18

What is the relationship between aperture and light gathered?

Answer 18

the larger the aperture the more light is gathered

Question 19

What else does the aperture size of a telescope determine?

Answer 19

the resolving power of the telescope
larger aperture = higher resolving power = smaller angle resolution

Question 20

What does light diffracted by a circular aperture produce?

Answer 20

a bright disk (Airy disk) with circular fringes

Question 21

What does Rayleigh’s Criterion state?

Answer 21

2 sources of light are said to be resolved if their Airy disks can be distinguished from each other

Question 22

What are the 2 different types of telescopes?

Answer 22

refractor telescopes - lenses
reflector telescopes - mirrors

Question 23

What is chromatic aberration in refractor telescopes?

Answer 23

lenses refract different wavelengths by different amounts, the focal length increases with wavelength, the lens therefore focuses the light of different wavelengths in different positions

Question 24

What is spherical aberration in reflector telescopes?

Answer 24

when you move away from the centre of the mirror the image becomes blurred because the focal length of the mirror decreases

Question 25

How can spherical aberration be fixed?

Answer 25

by using a parabolic mirror since it focuses every ray to the same point

Question 26

What is astronomical seeing?

Answer 26

the blurring and twinkling of astronomical objects caused by turbulence in the atmosphere

Question 27

What does astronomical seeing cause?

Answer 27

1) point sources to break up into speckles
2) long exposure images of these patterns blur into a disk
3) the brightness of stars to fluctuate known as scintillation or twinkling

Question 28

How can astronomical seeing be fixed?

Answer 28

by deforming the telescope optics, known as adaptive optics
light from a ‘beacon star’ or laser is analysed and the shape of the mirror deformation is calculated, the mirror is deformed by a series of pistons

Question 29

What is the altitude of a star?

Answer 29

it’s angular distance relative to the horizon

Question 30

What is the zenith?

Answer 30

the point on the sky directly above the observer

Question 31

What is the azimuth?

Answer 31

horizontal component of a stars position above us, partnered with altitude
measured relative to the observers meridian (line from N-S), always measured in a N-E direction

Question 32

What is declination?

Answer 32

analogous to latitude (+N and -S) and measured relative to the celestial equator

Question 33

What is right ascension?

Answer 33

the longitudinal component, partnered with declination
measured eastwards from the vernal equinox (zero point where the sun crosses the from the S-N hemisphere) along the equator
measured in time

Question 34

What do right ascension and declination define?

Answer 34

a star’s position on the celestial sphere
they are fixed points

Question 35

What is the relationship between the altitude of the celestial pole and the observer’s latitude?

Answer 35

in general altitude of celestial pole = observer’s latitude

Question 36

What is meant by upper culmination?

Answer 36

when a star is at the top of a circular orbit around celestial pole where altitude = max altitude

Question 37

What is meant by lower culmination?

Answer 37

when a star is at the bottom of a circular orbit around a celestial pole where altitude = min altitude

Question 38

What are circumpolar stars?

Answer 38

stars that do not rise or set

Question 39

What is sidereal time?

Answer 39

the time based on the movement of the Earth relative to the stars

Question 40

What is solar time?

Answer 40

the time based on the movement of the Earth relative to the Sun, based on the rising and setting of the sun

Question 41

How do sidereal and solar time compare?

Answer 41

there is 1 more sidereal day than solar days in a year, a solar day is 0.28% longer

Question 42

How is sidereal time measured?

Answer 42

by measuring the passage of the vernal equinox across the sky relative to the observers meridian

Question 43

In relation to time, what is right ascension?

Answer 43

the time between the vernal equinox crossing the observer’s meridian and the object of interest doing the same
also an angle but measured in units of time

Question 44

How do you locate a celestial object in the sky?

Answer 44

1) the observers position on Earth
2) the star’s coordinates (right ascension and declination)
3) local sidereal time

Question 45

How do you calculate the local hour time?

Answer 45

local sidereal time - right ascension