5.5: Astrophysics and Cosmology

Question 1

Describe what is meant by: a universe

Answer 1

Is everything that exists, e.g. stars, galaxies

Question 2

Describe what is meant by: the Milky Way

Answer 2

An example of of a galaxy, which are clusters of stars and planets held together by gravity

Question 3

Define: Astronomical unit(AU)

Answer 3

Mean distance between the centre of the Earth and the Sun,

1AU = APPROX 150 million km

Question 4

Define: Light year

Answer 4

Distance travelled by light in one year

1ly = APPROX 9.5x10^15m

Question 5

Define: Stellar Parallax

Answer 5

Apparent shifting in position of a star viewed against a background of distant stars when viewed from different positions of the earth
1pc =

Question 6

How are stars formed?

Answer 6

Due to a gravitational collapse, i.e. clouds of interstellar dust,gas are attracted together via gravitational attraction between individual atoms

Question 7

Define: Gravitational collapse

Answer 7

Inwards movement of materials in a star due to the gravitational force caused by its own mass

Question 8

What process begins in a protostar

Answer 8

Nuclear fusion , hydrogen nuclei fuse to from helium

Question 9

As a star’s temperature increases and its volume _________, the gas pressure _________

Answer 9

As a star’s temperature increases and its volume decreases, the gas pressure increases

Question 10

Describe 2 forms of pressure that prevent star at the beginning of its main sequence from collapsing due to gravity and explain what causes them

Answer 10

Radiation pressure and gas pressure counteract ‘g’ ∴ star doesn’t contract further

• rad due to momentum of photons released in fusion
• gas due to Ek of gas atoms acting outwards

Question 11

What causes a star to evolve from the main sequence to become a red giant

Answer 11

Hydrogen runs out in the core ∴ outwards pressure stops and core contracts heating up due to weight

Question 12

Define: A main sequence star

Answer 12

Star in the main part of life cycle, where it is fusing hydrogen to form helium in its core

Question 13

Define: Red giant

Answer 13

Later stage of a stars life where it has nearly exhausted the hydrogen and now is fusing helium nuclei into carbon and oxygen.
(Bigger in mass as surface layers have cooled and expanded)

Question 14

Define: White dwarf

Answer 14

End product of a low-mass star, where outer layers have dispersed into space. Very dense with a high surface temp and low luminosity

Question 15

Define: Planetary nebula

Answer 15

Glowing shell of ionised hydrogen and helium ejected from a red giant at the end of its life cycle

Question 16

Define: Electron degeneracy pressure

Answer 16

Pressure exerted by electrons to prevent the collapse of a low-mass star below the chandrasekhar limit
(Prevents White dwarf from collapsing)

Question 17

Define: Chandrasekhar limit

Answer 17

Maximum possible mass of a stable white dwarf star(1.4x the mass of the sun) anything above this will collapse into a neutron star or black hole

Question 18

Define: Red super giant

Answer 18

A star that has exhausted all the hydrogen in its core and has a mass much greater than the sun
(Fusion can involve heavier elements)

Question 19

What causes a supernova

Answer 19

A huge explosion caused when the core of a red super giant collapses

Question 20

Define: Neutron star

Answer 20

Remains of the core of a red super giant after it has has undergone a supernova explosion. Extremely dense and composed mostly of neutrons

Question 21

Define: Black hole

Answer 21

Core of a massive star that has collapsed to a point (called singularity), infinitely dense and very small, with a gravitational field so strong light cannot escape