Astronomy Part 1

Question 1

The principal contents of the universe.

Answer 1

Stars, galaxies, radiation, dust clouds, ‘dark’ matter.

Question 2

Describe the formation of a star, such as our Sun, from interstellar dust and gas.

Answer 2

• The interstellar gas cloud collapses under its own gravitational attraction.
• The loss in the potential energy due to gravitational collapse leads to an increase in the kinetic energy of the gas atoms and nuclei.
• The temperature of the gas cloud increases.
• As the temperature reaches about 10^8 K, fusion takes place between the hydrogen nuclei. (4)H → 4/2He + (2)e + (2)ν
• The energy from the fusion reactions further increases the temperature of the gas cloud. The gas cloud now has a high-temperature core that creates energy from fusion reactions. A star is born.

Question 3

Describe the Sun’s probable evolution into a red giant then a white dwarf.

Answer 3

• Core of star runs out of hydrogen.
• Outward radiation pressure cannot overcome gravity and the core of the star collapses and heats up until fission of hydrogen can occur in a shell around the core.
• The outer layers of the star expand and cool to form a red giant.
• Helium can fuse to form Carbon when temperatures get hot enough. When helium runs out, the star loses it’s outer layers in an expanding shell of gas called a planetary nebula.
• Core of the star collapses as fusion stops and becomes a cooling white dwarf star. Can’t collapse more due to electron degeneracy pressure (Fermi pressure).

Question 4

Describe how a star much more massive than our Sun will evolve into a super red giant and then either a neutron star or black hole.

Answer 4

• Core of star runs out of hydrogen.
• Outward radiation pressure cannot overcome gravity and the core of the star collapses and heats up until fission of hydrogen can occur in a shell around the core.
• The outer layers of the star expand and cool to form a red giant.
• Hotter temperatures allow fusion reactions to create elements up to iron, where fusion stops.
• Gravitational force is no longer balanced by radiation pressure. The star collapses and explodes in a supernova explosion.
• If remnant less than 3 solar masses then remnant is a black hole. If greater than 3 solar masses then remnant is a black hole.

Question 5

Interpret Olbers’ paradox to explain why it suggests that the model of an infinite, static universe is incorrect.

Answer 5

• According to the paradox, the night sky should be bright because of light received from stars from all directions.
• This was based on the ideas that intensity of light from stars decreases as 1/(distance)^2 and the number of stars increases as (distance)^2
• The two scaling factors cancel out and therefore the night sky should be bright from the light from the infinite number of stars.

Question 6

Describe and interpret Hubble’s redshift observations

Answer 6

For galaxies moving away from us, the wavelengths of light were stretched and shifted towards the red side of the spectrum.

Question 7

Describe and explain the significance of the 3K microwave background radiation

Answer 7

• This is electromagnetic radiation in the microwave region of the spectrum coming from all directions in the universe.
• The radiation corresponds to the universe being at a temperature of about 3K.
• This radiation is nearly isotropic/has the same intensity in all directions.

Question 8

Describe qualitatively the evolution of universe 10-43 s after the big bang to the present

Answer 8

• Very soon after the Big Bang, all matter is in the form of a quark and lepton ‘soup.’
• The quarks then combine to form hadrons (protons, neutrons etc)
• Primordial helium is created by fusion reactions.
• Atoms form as electrons and nuclei combine.
• Matter lumps together to form stars and galaxies.
• The universe cools as it expands.
• At present, the temperature of the universe is about 2.7K.