You're looking at the biggest and sharpest image ever taken of the Andromeda Galaxy. Though it only includes one third of the massive celestial object, the image contains a jaw-dropping 1.5 billion pixels.
At that resolution, it would require more than 600 high-definition television screens to display the entire image. The full size original image is a whopping 4.3 GB. More reasonable, you can download the 5.3 MB JPEG or 138.8 KB JPEG.
The Hubble Space Telescope site explains more:
The large groups of blue stars in the galaxy indicate the locations of star clusters and star-forming regions in the spiral arms, whilst the dark silhouettes of obscured regions trace out complex dust structures. Underlying the entire galaxy is a smooth distribution of cooler red stars that trace Andromeda's evolution over billions of years.
The Andromeda Galaxy is a large spiral galaxy — a galaxy type home to the majority of the stars in the Universe — and this detailed view, which captures over 100 million stars, represents a new benchmark for precision studies of this galaxy type. The clarity of these observations will help astronomers to interpret the light from the many galaxies that have a similar structure but lie much further away.
Because the Andromeda Galaxy is only 2.5 million light-years from Earth it is a much bigger target on the sky than the galaxies Hubble routinely photographs that are billions of light-years away. In fact its full diameter on the night sky is six times that of the full Moon. To capture the large portion of the galaxy seen here — over 40 000 light-years across — Hubble took 411 images which have been assembled into a mosaic image.
Because of the enormous size of the image, Hubble has provided this zoom tool. Alternately, you can watch this video:
As an aside — and just to give you an idea of how close the Andromeda Galaxy is to Earth — this is what it would actually look like if it were considerably brighter.
[ Hubble ]
Image credit: NASA, ESA, J. Dalcanton (University of Washington, USA), B. F. Williams (University of Washington, USA), L. C. Johnson (University of Washington, USA), the PHAT team, and R. Gendler.