How do we know what the Milky Way looks like?

You're right, it's really difficult to deduce the shape of the Milky Way, the main reason of course being that we're in it! On the other hand, astronomers can map the distribution of the stars and motion of them really easily. This research indicates that we're orbiting a 'central bulge' , this means the planets and stars are distributed along a thin plane. The most convincing evidence for the structure of the Milky Way comes from the studies of gas clouds - here they measure the velocities and positions of these clouds. So although we know we have spiral clouds - we have no idea how many spiral arms there are!

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Could a planet exist without a Sun?

Yes planets can definitely be "free floating" without a star. These type of planets may have been ejected from their first, original star system. Scientists often refer to this as "rouge planets" or "normad" planets. Another way they can form is by the condensation of material in much the same way as stars themselves. These are normally called "sub-brown dwarfs".  The closest rogue planet to be discovered yet is 'S Ori 52' a member of the Orion star system and is around 80 light years away. Some recent studies have suggested that there might be more free floating planets in the Milky Way than stars!


An example of a rogue planet

Why do galaxies collide?

Well, they collide due to their 'mutual gravitational attraction' in other words they collide because gravity draws them together. However in general galaxies are moving away from each other at an ever increasing rate, basically because of the Universe expanding at an ever increasing rate. This can be modeled simply by drawing stars or dots (imagine these as galaxies) onto a deflated balloon then blowing up the balloon. An idea that has sprung from this is the Red Shift. This is where astronomers have found that the further away a star is the more it's light has been red shifted. So this tells us that distant galaxies are moving away from us and that the further away a galaxy is the faster away its moving away from us.

The Red Shift is an example of the Doppler effect, named after the Austrian physicist Christian Doppler who proposed this theory in 1842 in Prague. You may have noticed that as a police car comes towards you the siren becomes high pitched and low pitched as the car drives away. This effect is where there is a change in frequency and wavelength. So when a source moves towards an observer the observed wavelength decreases and frequency increases and vice versa. In terms of Astronomy the results of the Doppler effect are used to identify if a nearby star is what astronomers call a binary star. This is two stars orbiting around a 'common center of mass' which is where the relative position of the distributed mass adds to zero. The brighter star is called the 'primary star' and the other the 'secondary star'

In our own 'Local Group' of galaxies for example some of the galaxies are moving away from us and some are moving towards us. Take our nearest galaxy the Andromeda galaxy for example which is currently moving towards the Milky Way at 402, 336 km/h and is due to hit us in 4 billion years.