Structure of the Universe

Question 1

What is a galaxy?

Answer 1

A cluster of stars and planets held together by gravity.

Question 2

What does our solar system consist of?

Answer 2

Everything that orbits the sun - planets, planetary satellites, asteroids and comets.

Question 3

How can we calculate distances and velocities in the solar system?

Answer 3

If we send out short pulses of radio waves, which will be reflected at some point, we can calculate the distance using the time it took for the radio wave to return, and the speed of the wave.
We can calculate the average velocity of an object relative to the Earth by doing this twice and calculating the distance the object has moved in that time.

Question 4

Define the AU.

Answer 4

The mean distance between the Earth and the Sun.

Question 5

Define the parsec.

Answer 5

One parsec is the distance away from Earth when the angle or parallax is one arcsecond. (1/3600 degrees).

Question 6

Define the light-year.

Answer 6

The distance that an electromagnetic wave can travel in a vacuum in one year.

Question 7

Give 1 light-year in SI units.

Answer 7

9.46x10^(15)m

Question 8

Give 1 parsec in SI units.

Answer 8

3.09x10^(16)m

Question 9

State Olber’s paradox.

Answer 9

If stars are spread randomly throughout an infinite universe then every possible line of sight must contain a star. This would make the whole night sky uniformly bright.

Question 10

State the cosmological principle.

Answer 10

On a large scale, the universe is homogenous and isotropic, so it doesn’t have a centre.

Question 11

State Hubble’s law.

Answer 11

The speed of recession of a galaxy is proportional to it’s distance from Earth.

Question 12

How do we change Hubble’s constant into SI units?

Answer 12

Change km to m, and Mpc into m, and these will cancel out to leave you with s-1.

Question 13

How can we calculate redshift or blueshift?

Answer 13

Calculating z using the formula in the book, where delta lambda is the difference between the observed wavelength and the emitted wavelength, lambda is the emitted wavelength, v is the velocity of the source in the direction of the observer, and c is the speed of light.

Question 14

Describe and interpret Hubble’s redshift observations.

Answer 14

He realised that galaxies apart from ones really close to us all showed redshift, so they were all moving away from us. Plotting velocity against distance tells us that the speed of galaxies depends on how far away they are. This suggests that the universe is expanding.

Question 15

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

Answer 15

It confirms the cosmological principle because it’s wavelength keeps increasing, showing redshift and therefore proving expansion. The radiation is largely isotropic and homogenous.

Question 16

Describe the formation of a star.

Answer 16

Clumps of dust and gas contract under the force of gravity.
The cloud fragments into different regions called prostars - they continue to contract and heat up.
Temperature reaches a few million degrees and hydrogen nuclei begin to fuse to form helium.
This releases energy and creates enough pressure to stop the gravitational collapse.

Question 17

Describe the Sun’s probable evolution.

Answer 17

• Core hydrogen burning occurs. This is fusion of hydrogen nuclei, of which the pressure balances out the gravitational force.
• When hydrogen in the core runs out, fusion stops and the outward pressure disappears, so the core contracts and heats up under the weight of the star.
• Shell hydrogen burning occurs, this is when hydrogen outside the core begins to fuse due to the heat provided by the contracting core.
• Core helium burning occurs, this is when contraction continues until the core is hot enough to allow helium to fuse into carbon and oxygen.
• This releases a huge amount of energy, pushing the outer layers of the star outwards, where they cool. The star is now a red giant.
• She’ll helium burning occurs - when helium in the core runs out, the carbon-oxygen core contracts and heats a shell around it so helium can fuse in this region.
• The core continues to contract under its own weight as it does not get hot enough for further fusion. Eventually,electrons exert enough pressure to stop further collapse.
• As the core contracts, the helium becomes unstable and pulses of the outer layer are ejected as planetary nebula.
• The star is now just the dense core, called a white dwarf. This will simply cool down and fade away.

Question 18

Describe what will happen to a star much more massive than our Sun will evolve.

Answer 18

It will evolve into a super red giant by faster fusion building up layers in an onion-like structure which can continue up to the fusion of iron. The star then explodes in a supernova, to form a neutron star or a black hole, depending on its mass.