Astrophysics

Question 1

what is the universe?

Answer 1

a large collection of billions of galaxies

Question 2

what is a galaxy?

Answer 2

a large collection of billions of stars?

Question 3

what is the solar system?

Answer 3

a collection of planets orbiting a common star - our solar system is the Milky Way galaxy

Question 4

what is gravitational field strength?

Answer 4

the force per unit mass on a body in a gravitational field

Question 5

what is gravitational field strength measured in?

Answer 5

Newtons per kilogram (N/kg)

Question 6

what does gravitational field strength vary with?

Answer 6

the mass and size of the body and is therefore different on other planets and the moon compared to Earth

Question 7

gravitational force enables the various bodies to…

Answer 7

orbit around others

Question 8

how does the moon orbit planets?

Answer 8

the orbits are slightly elliptical with near constant orbital speed

Question 9

how do planets and comets orbit the Sun?

Answer 9

• the orbits of planets are slightly elliptical with near constant speed
• the orbits of comets of highly elliptical

Question 10

what are examples of how gravitational force allows various bodies to orbit the Earth?

Answer 10

• the moon orbits planets
• planets and comets orbit the Sun
• artificial satellites orbit the Earth

Question 11

the greater the orbital radius or the smaller the time period…

Answer 11

the greater the orbital speed

Question 12

why do comets have a greater speed nearer to the star?

Answer 12

because the ice inside them melts as they get closer (and warmer), causing their mass to decrease

Question 13

how do starts begin?

Answer 13

as a cloud of dust and gas called a nebula

Question 14

how does nuclear fusion occur in stars?

Answer 14

• the particles experience a weak attraction towards each other due to gravity and begin to clump together
• they continue to clump together until the pressure and temperature is great enough for nuclear fusion to occur
• hydrogen nuclei fuse together to form helium nuclei which releases a large amount of energy and causes a great outwards pressure
• this outwards pressure balances with the inwards pressure due to gravity and the star is now stable and called a main sequence star
• eventually the hydrogen in the star is used up - there is no longer enough outward pressure from nuclear fusion and it collapses under its own gravitational attraction, becoming unstable

Question 15

if a star has similar mass to the Sun…

Answer 15

it expands massively and becomes a red giant, then it becomes a white dwarf, and finally cools into a black dwarf

Question 16

if the star has a mass larger than the Sun…

Answer 16

• it expands and becomes a red super giant, before exploding in a supernova
• what remains is either a neutron star, or if it was exceptionally massive, a black hole

Question 17

how can stars be classified?

Answer 17

according to their colour - the colour of a star is related to its surface temperature, with hotter stars being bluer and cooler stars being redder

Question 18

what does the brightness of a star depend on?

Answer 18

where it is measured - at a standard distance it can be represented using absolute or apparent magnitude

Question 19

what is a stars absolute magnitude?

Answer 19

equal to the apparent magnitude that the star would have if it were viewed from exactly 10 parsecs (32.6) light years away

Question 20

what is a stars apparent magnitude?

Answer 20

how bright it appears at a particular point in space

Question 21

what does a HR diagram show?

Answer 21

the relationship between a stars surface temperature and brightness

Question 22

where are the main components on a HR diagram?

Answer 22

supergiants = top right
red giants = below supergians
the main sequence = a diagonal strip from top left to bottom right
white dwarfs = bottom left

note - other stages don’t show on the HR diagram because a star is only in them for a short period of time

Question 23

what does the Big Bang theory state?

Answer 23

that the universe expanded outwards from a single point

Question 24

what are two examples of evidence for the Big Bang theory?

Answer 24

• red shift
• cosmic microwave background radiation (CMBR)

Question 25

what is red shift?

Answer 25

- the red shift of light from galaxies shows that they are all moving away from us and that those which are furthest away are moving the fastest, suggesting that the universe was formed from an explosion at a single point, evidence for the Big Bang - this is the principle of the Doppler effect

Question 26

what is cosmic microwave background radiation?

Answer 26

- just after the Big Bang, lots of short wavelength radiation should have been released - this radiation, as the universe expanded over time, would have been stretched to become microwaves - the fact that there is CMBR present wherever you point a telescope in the sky provides evidence for the Big Bang

Question 27

why is the Big Bang theory the most accepted model?

Answer 27

it is the only theory currently which accounts for all the experimental evidence

Question 28

what is the Doppler effect?

Answer 28

if a wave source is moving relative to an observer, there will be a change in the observed frequency and wavelength - for example, when the siren of an ambulance is high-pitched as it approaches you and low-pitched when it goes away

Question 29

what is the Doppler shift responsible for?

Answer 29

the red-shift of light from galaxies which are moving away from Earth, which is when the wavelength of the light coming from them increases - the faster it is moving, the more its light is red-shifted