Have spiral arms, lots of new stars are forming in the spiral arm. The biggest nearby galaxy to us is called andromeda. Compact galaxy groups are very active (hickson compact group) More than 50 galaxies are called cluster, in their center there is a dominant bright light. Cd or super giant elliptical galaxies are giant galaxies. Gravitational lensing: when an object between the light source and the observer bends the light from the light source making it seem like the object is in another direction. Galaxy cluster: 100 to 1000, billion of light years in size weighs a trillion times the sun. Star cluster: 50 light years across, weighs a million times the sun. The time it takes for light to reach us. When a star explodes we will see it 100 years later. This means we are seeing the galaxy as they were years ago. In our own galaxy the lookback time can be 1000 of years.