physics unit 4 edexcel

2 topics i really dont understand - balmer lines and standard candles. an easy explanation please?

Balmer lines can be explained with a kind of 'goldilocks' scenario (though don't write this down in your exam!!!):

In a star's atmosphere, there exist Hydrogen atoms which fuel the star's fusion. As you know, a Hydrogen atom's atomic electron has several discrete energy levels in which it can exist, named n = 0 (ground state, unexcited), n = 1, n = 2 etc. As the numbers go higher the electron becomes more excited. We are interested in the n = 2 state when talking about Balmer lines. Now, you should know that the source of energy an electron is excited by doesn't matter so long as it is exactly equal to an energy gap between the current state and a higher energy level. This means the electrons can be excited into higher states by the temperature of the star's atmosphere. If the star is too cool (e.g: G, K, M class stars), very few electrons will become excited into the n = 2 state. Too hot (O, B, class stars), and the electron will be excited past the n = 2 state or even ionised completely. Just right, however (A class stars are the best for this) and there will be plenty of Hydrogen with electrons in the n = 2 state. Now, as you know, fusion in a star's core releases light. These photons of light may collide with the n = 2 electrons as they exit the star. Since the energy levels that the electron can exist in are finite and discrete, the electron will only become excited into an n > 2 state if the photon has energy equal to an energy gap between two energy levels of the electron. From $E = hf$, this results in the n = 2 electron only becoming excited by discrete frequencies of light. When the electron becomes excited, it then quickly de-excites back to ground state, releasing one or more photons (dependent of the de-excitation path) of total energy equal to that of the original photon. However, these photons are released in a random direction, so over many repeated excitations/de-excitations, the photons emitted by the electron spread in all directions. This means that the frequencies absorbed by the electron are re-radiated in all directions. This reduces the intensity of those frequencies to the observer, hence the appearance of absorption lines. That's the explanation from scratch, if I'm not mistaken. In the exam, you have to make sure you mention:
Hydrogen in the star's atmosphere
Electrons in n = 2 state
Absorb photons of discrete energies (and hence frequencies) equal to energy gap between discrete energy levels to excite into n > 2
Excited electrons de-excite, re-radiating the light in a random direction
This spreads out the absorbed frequency, reducing its intensity to an observer -> absorption lines observed.

Standard candles refer to type 1a supernovae, which ALWAYS reach a peak absolute magnitude of -19.3. This means you can use the equation:
$m-M = 5log(\frac{d}{10})$, with $M = -19.3$ and the apparent magnitude $m$ known from observation by telescopes to calculate the distance $d$ in parsecs. They are used in that way as a standard in calculating distances.

balmer lines explantion perfect, thanks! so what makes them so special? are they moving away or something, or changing brightness/

cheers for your help

Happy to help

What makes type 1a supernovae so special is that they always reach the same absolute magnitude, no matter where they are or their history; if they become a type 1a supernova, you know exactly how bright that star is so you can use its appearance to determine how far the star is from you.

but if its absolute magnitude is changing then how is that helpful? or is the point that the brightness doesnt change..

The absolute magnitude changes, but they all change in exactly the same way. The absolute magnitude increases, peaks at -19.3, then falls back off. If you observe the perceived brightness from Earth as this changes, then record the peak apparent magnitude, you know that for any type 1a, whatever peak you measure, the supernova's ABSOLUTE magnitude MUST have been -19.3. So in short, what's important is they all reach the same peak during the supernova.