gs that makes astronomy so interesting and appealing is how visual it is. Looking up at the night sky we instinctively want to connect the dots between the stars to draw swans and bears and teapots. But it was the introduction of the telescope as a means of studying the heavens 400 years ago by Galileo that allowed us to realize there was more to those points of light than dots in the sky. Bigger and bigger telescopes allowed us to see more detail of these heavenly objects, but we had the need to capture those images. Sketches show that astronomers such as Lord Rosse did a good job recording what they saw, but the invention of 
photography allowed us to record images exactly, with the added advantage that one could now hold the shutter open, integrate for a longer time to catch more light and probe deeper than would ever be possible by eye alone.Photography by means of photographic plates has become obsolete since the invention of CCDs. Awarded half of this year's nobel prize in physics, the introduction of CCDs has revolutionized the field of astronomy. By translating our images into a digital format, we can not only more accurately quantify the errors in our measurement, but make more sensitive images in places difficult to retrieve film from (ie. outer space). Digital images have the added advantage of making it easy share information not just among scientists, but with the public as well. Anyone (yes, even you!) can go and directly download real Hubble telescope data.
Below are two of my favorite images which are mind-blowing, not just for their beauty, but for the scientific understanding that they allow us.
This image was presented at a talk by Andrea Ghez that ultimately convinced me that I wanted to study astronomy. What you are seeing is a very high resolution image of the center of our galaxy, and each of those moving objects are stars which are orbiting around a central object (marked by the star symbol) which is not detected optically. The paths these stars follow have been traced out over the 15 years this field has been observed, with some prediction at the end of where we expect them to go in the next year. The reason their future paths can be predicted so accurately is because they are moving on Keplerian orbits, controlled simply by gravitational forces, just like the planets in our solar system.
Not only is this evidence that gravity acts as we expect it to throughout the universe, but a calculation of the central mass needed to support these rapid orbits shows that there must be a mass approximately a million times the mass of our sun invisibly located at the focus of all these orbits! The detection of such a large mass in such a small space (the closest star comes within 90 AU of the object) provides the strongest observational evidence available of the existence of a supermassive black hole at the center of our galaxy. This image is so simple, tracing the motion of stars in the sky, yet it teaches us something fundamental about one of the most exotic types of objects in the universe.
There are many questions remaining in astronomy today that we are on the verge of answering. One of the biggest concerns the nature of dark matter. Cosmologists think they understand fairly well how much stuff exists in the universe based on their understanding of what happened during the big bang. They have been able to accurately predict the primordial Hydrogen, Helium and Deuterium abundances, however when observers compare their predictions of how much stuff there should be and how much we actually see emitting light, we are only able to account for a small percentage of the matter in the universe. This missing matter, aptly dubbed 'dark matter', is also evidenced by examining the rotation of stars inside of galaxies. In all galaxies observed, their stars do not move at the speed we would expect from Keplerian orbits when we only consider the gravitational effect from the things we know exist (stars, gas and dust). Their orbits require an additional massive component to the galaxy which is not observed.
The most convincing evidence of the existence of dark matter comes from the image above of the Bullet Cluster. This system consists of two clusters of galaxies crashing into each other, populated by the hundreds of galaxies you see filling this picture. Like most astronomical images, it is not in true-to-life colors. In this case, pink represents the hot gas in the system, which actually accounts for most of the observable mass, and is much more massive than the stars. The interaction of these two clusters is evident from the bullet shaped bow shock that one cluster creates as it passes through the other at high speed. The blue represents the location of the bulk of the actual mass, as determined from the gravitational lensing created when the mass of the cluster bends the light of background objects.
Immediately apparent is that these two concentrations of mass are not in the same place! Instead, the observable matter is significantly displaced from the dark matter, indicating that the majority of the mass in this system is entirely independent of the gas and stars. This directly refutes theories which had attempted to explain anomalies in galaxy rotation by making modifications to the theory of gravity. This evidence is entirely independent of gravity's effect on the dynamics of the system, and supports the idea that dark matter must be made up of some unknown particle. This picture alone gives a new understanding of the nature of dark matter in a single intuitive, accessible image.
There are thousands of amazing astronomical images, each of which reveals secrets about the nature of our universe. To enjoy more of these images and the science behind them, check out the site Astronomy Picture of the Day.
