I got 2 problems really need help, coz test tomorrow?

Question

1) If nothing continues to heat a gas, all the electrons in the atoms of the gas eventually will end up in the lowest energy level (the ground state). Use this fact to explain why we should not expect to see an emission line spectrum from a cold cloud of gas.

2)The spectrum of light that we see from the star Alpha Centauri shows a blue shift, if you lived on a planet in the Alpha Centauri system, would you discover our Sun’s spectrum to show a redshift, a blue shift, or on Doppler shift at all? Please Explain the answer.

Answer 1

1) Emission is caused by electrons falling to a lower energy state. If they are all already at the lowest state, they can't fall and emit light.

2) You would see the same blue shift looking at the Sun from Alpha Centauri as you see looking at Alpha Centauri from the Sun.

(So cosmo is exactly correct.)

Answer 2

An emission line occurs when an electron transitions from one energy state to a lower energy state. If the gas is cold enough, essentially all the electrons will be in the lowest state, and therefore will be unable to transition to a lower one.

The blue shift of Alpha Centauri means that they are moving towards us. From their point of view, we are moving towards them. Therefore, when they look at us they see us blueshifted.

Answer 3

I suspect that, to get an emission from a molecule, it needs to jump from a 'high' quantum state to a 'lower' state associated with less energy than the high state. If the 'high' states aren't populated then that can't happen.

To have an overall blue shift it has to be moving closer to us (not necessarilly directly).
If you were stood on a planet around alpha centauri would you think we were moving further away?

Answer 4

If the gas electrons are all at ground state it won't be emitting photons. You can't get spectral lines when there's no light to get them from.

Blue shift is caused by objects moving toward each other.