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!

Follow the rest of my #SummerofScience and me on Twitter @Sci_Sparks. Don’t forget to download my charity eBook, Science Bites: A Short Tour of the Universe HERE. 

The Andromeda Galaxy

This  is the Andromeda Galaxy, which I was lucky enough

 to take as part of the GCSE Astronomy that I do every

Wednesday evening at school

This is the Andromeda Galaxy (M31) which lies around 2.5 million light years away from Earth and it our closest spiral galaxy. Observations taken in 2006 by the Spitzer Space Telescope suggests that the Andromeda Galaxy contains around 1 trillion stars. The Andromeda Galaxy is estimated to be 7.1×10¹¹  solar masses. This means that the mass of the Andromeda Galaxy is 7.1×10¹¹ times heavier than the Sun. The mass of the Sun being approximately two nonillion kilograms. The value of this constant was found by Henry Cavendish in 1798 at the University of Cambridge. He did this with a torsion balance which is designed to measure very weak forces. The invention of the torsion balance is usually credited to Charles - Augustin de Coulomb who invented it in 1777.
 

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.