# Structure of the Universe Flashcards

What is a galaxy?

A cluster of stars and planets held together by gravity.

What does our solar system consist of?

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

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

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.

Define the AU.

The mean distance between the Earth and the Sun.

Define the parsec.

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

Define the light-year.

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

Give 1 light-year in SI units.

9.46x10^(15)m

Give 1 parsec in SI units.

3.09x10^(16)m

State Olber’s paradox.

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.

State the cosmological principle.

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

State Hubble’s law.

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

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

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

How can we calculate redshift or blueshift?

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.

Describe and interpret Hubble’s redshift observations.

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.

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

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