Module 5 - Detailed Sections of Astrophysics

Question 1

Nebula (process) (3)

Answer 1

• over time, dust and gas in a nebula will slowly come together to form denser and denser clumps of matter
• due to the gravitational attraction between individual atoms
• this inward movement of material is called gravitational collapse

Question 2

Protostar (process) (4)

Answer 2

• as more and more matter is pulled together, work is done on the particles of dust and gas by collisions
• leads to an increase in the particles’ kinetic energy
• leads to an increase in temperature (some denser areas of gas become hot enough to glow)
• this large core of material is called a protostar

Question 3

Main Sequence Star (process) (8)

Answer 3

• the protostar’s gravitational field continues to attract more and more matter until the temperature and pressure becomes much greater
• the hydrogen nuclei in the core then undergo nuclear fusion
• 4 protons –> 1 helium-4 nucleus, 2 neutrinos, 2 positrons, 2 gamma ray photons (enormous amounts of energy released)
• the momentum of photons released leads to an outwards acting force called radiation pressure
• there is also a force acting outwards due to - the pressure caused by the kinetic energy of the gas atoms
• in a star of stable size, the radiation pressure and gas pressure are in equilibrium with the gravitational force acting inwards
• the star is then a main sequence star, where it will remain for the majority of its life
• star is stable and continues to convert hydrogen into helium through fusion in its core

Question 4

Red Giant (process) (6)

Answer 4

• when most of hydrogen nuclei present in the core has been fused, fusion will slow and eventually stop
• the outwards radiation pressure decreases
• the forces are no longer in equilibrium
• the core collapses as it compresses under the weight of the star
• leads to an increase in temperature high enough for shell hydrogen burning and core helium burning (fusing)
• outer layers expand and cool, forming a red giant

Question 5

Planetary Nebula (process) (5)

Answer 5

• core helium burning releases massive amounts of energy causing the radiation pressure to increase, balancing the forces
• when helium in the core runs out, core contracts again
• this produced temperatures high enough to undergo shell helium burning
• the carbon-oxygen core is not hot enough to fuse heavier elements so star becomes unstable and collapses again
• outer layers of gas are ejected back into space forming a planetary nebula

Question 6

White Dwarf (process) (3)

Answer 6

• after a planetary nebula, a solid core is left behind
• core continues to collapse under its own mass until it can collapse no further
• left is a very hot, dense remnant of a low mass star (a white dwarf)

Question 7

Black Dwarf (process) (4)

Answer 7

• nuclear fusion stops since only heavy elements remain (e.g. C, O)
• star continues to radiate energy as the photons from past fusion reactions leak away
• star gradually cools down so its surface temperature is just a few K
• is then classed as a black dwarf

Question 8

Red Super Giant (process) (2)

Answer 8

• follows same process as red giants but are much brighter
• shell burning and core burning forms heavier and heavier elements, so that eventually an iron core is formed

Question 9

Supernova (process) (6)

Answer 9

• once iron core forms, it becomes unstable and begins to collapse as no more fusion reactions can occur
• during collapse, the immense gravitational pressure forces protons and electrons in the iron to combine to become neutrons
• this releases huge amounts of energy
• outer layers fall inwards and rebound off core causing shock waves
• shock waves cause an explosive blowing out of the outer shell (a supernova)
• this generates temperatures high enough to fuse heavy nuclei with neutrons to form all known elements beyond iron

Question 10

Neutron Star (process) (3)

Answer 10

• if star has a core with mass greater than Chandrasekhar limit,
• collapsed neutron core remains intact after supernova explosion (neutron star)
• neutron stars are very small and extremely dense

Question 11

Black Hole (process) (4)

Answer 11

• is neutron core mass is > 3MSun,
• the pressure on the core becomes so great the core collapses even further into a singularity
• gravitational forces are so strong, the escape velocity of the core is greater than speed of light
• photons (light) is unable to escape the black hole

Question 12

The Chandrasekhar Limit (5)

Answer 12

• the maximum mass of a stable white dwarf star
• 1.4MSun
• if the mass of a red super giant core is less than limit, star will remain a white dwarf
• if the mass of a red super giant core is more than limit, electron degeneracy pressure will not be sufficient to support the core
• it will undergo gravitational collapse leading to a supernova

Question 13

The Cosmological Principle

Answer 13

On a large scale the universe is uniform meaning it is:
- isotropic: the same in all directions to every observer
- homogeneous: of uniform density as long as a large enough volume is considered
- subject everywhere to the same physical laws and models that apply on Earth

Question 14

Evidence To Support the Big Bang Theory (2)

Answer 14

Hubble’s Law shows the universe is expanding
- if the universe is expanding, there must have been a singularity from where it started expanding
Microwave Background Radiation provides evidence the Universe has expanded from a single point and cooled significantly over time
- with Big Bang, no microwave background radiation
- if universe was younger than 13.7 billion years, temperature would be higher than 2.7K
- since temperature of radiation is mostly uniform, implies all objects in Universe are uniformly spread out

Question 15

The Evolution of the Universe (13)

Answer 15

• started as a singularity
• the Big Bang happened
• in <1s: the 4 fundamental forces originated
• in first 100s:
• • matter formed: all fundamental particles (quarks and leptons), photons
• • quarks formed (anti) protons and (anti) neutrons (1x10^-4s)
• • more matter present (which will leave a matter dominated universe)
• • matter and antimatter annihilate producing lots of gamma photons (cosmic background radiation)
• • protons cool enough to undergo nuclear fusion to form helium nuclei (no atoms yet)
• 300,000 years: electrons combine with protons and neutrons to form atoms
• 1 million years: galaxies begin to form
• 11 billion years: solar system and Sun are formed
• 13.7 billion years: life on Earth

Question 16

Evidence for Dark Matter (9)

Answer 16

• astronomers expect to observe the velocity of an object within a galaxy decrease as it moves away from galaxy’s centre
• since the gravitational field strength would weaken as it moves away
• observed in smaller mass systems
• not observed in larger mass systems (e.g. entire galaxies)
• shows mass is not concentrated at the centre of galaxies but is spread out
• however all observable mass is in the centre
• therefore there must be another type of matter that can’t be observed - dark matter
• predictions from Big Bang model of ratio of protons and neutrons formed in Early Universe agree very well with present H and He levels in the universe
• suggests dark matter not likely to be made up of ordinary matter

Question 17

Evidence for Dark Energy (4)

Answer 17

• used to try to explain the accelerating expansion
• astronomers found the acceleration of the universe was increasing and has been for the past 10 billion years
• therefore a material with a gravitational repulsive effect needs to be present
• would also need to exert a negative pressure