The Hubble Space Telescope has been used to take a 1.5 billion pixel image of the Andromeda Galaxy.
You can see an overview of the image below, but that doesn't do the full image justice. Most monitors display less than two million pixels, so when you look at the whole image you are probably losing almost 99.9% of the resolution.
To take full advantage of the effort NASA and the European Space Agency have put into creating this image, you need to go to the Hubble site where it is possible to zoom into patches of the galaxy, seeing the extraordinary detail come to life every time you click. What may have seemed like a single star is revealed as a dense cluster or a giant surrounded by hundreds of others too faint to make out in the wider view.
Also known as Messier 31, or M31, the Andromeda Galaxy is the only member of the Local Group of galaxies that has more mass than our own Milky Way. More than 100 million of its estimated trillion stars are visible in this image, provided you go deep enough.
Most of Andromeda's stars are simply too faint to be seen at this distance, even with a telescope as powerful as Hubble. However, the image also doesn't take in the full width of this mighty galaxy. The part photographed is 40,000 light-years across, but represents only one side of the galaxy, since the left hand end only just captures the galactic center. The outermost regions have also been excluded with the galaxy's full diameter estimated at 3-5 times this size.
While the effort may launch a thousand screensavers and wall posters, it is more than good PR for the space program. The Panchromatic Hubble Andromeda Treasury (PHAT) project imaged Andromeda from near-ultraviolet wavelengths to the near-infrared, with both red and blue filters. This level of detail across such a range will be used to test theories about the structure of spiral galaxies, and interpret results for more distant spirals.