This is the "young" open star cluster Messier 35 in the constellation Gemini. This cluster is about 150 million years old and located about 2500 light-years away. Most of the stars you see here are more massive than the sun and burning at very hot temperatures, which makes them blue-white in color. The red stars in the cluster are nearing the ends of their lives and have swelled into red giant stars. At the upper right of the picture you can see the edge of the open cluster NGC 2158, an older and more distant star cluster. This image is made from three colors taken at the Kitt Peak Mayall Telescope with the Mosaic camera. The design of the camera explains the linear gaps you can see traversing the cluster if you look closely at the picture. The full moon would just barely fit in this image.
The star cluster Messier 67 is an old (about 4 billion years) open star cluster located in our galaxy. The image above is a composite of short exposures taken with the MMT "Megacam" camera. The camera is a 268 megapixel camera, and covers an area about the size of the full moon. This image is cropped to show the center of the cluster, and is roughly 15 arcminutes on a side. The reddish stars are red giants, stars reaching the ends of their lives and swelling from about the size of the sun into monsters that would engulf the Earth.
This image of Messier 67 is a close-up of the northern portion of the cluster taken with the Keck Telescope. The bright stars are about 250 times fainter than the faintest objects visible to the unaided eye (12th magnitude), while the faintest objects visible in the large version of this image are about a million times fainter than the faintest stars visible to the unaided eye (about 26th magnitude). When all of our images are stacked together, the faintest visible stars are about another factor of 10 fainter yet (28.5 magnitude). Besides stars in our own galaxy, this image also shows very faint galaxies extending most of the way across the universe, including a quasar at a redshift of 3.0, or about 11 billion light-years away. The streaks extending off of the bright stars are called "bleed trails" and are due to overexposure of those stars.
This is a composite color image of the outer regions of the globular cluster NGC 6397 taken with the VLT. The total exposure time is 8 hours in the I band (near infrared) and 5.8 hours in the V band (green). The long trails from the brighter stars are due to saturation of the CCD camera. In other words, the stars are too bright!