4. Hubble Light Echo

The astronomers who took this series of pictures were stunned when they first saw them. They still don't really know what was going on. The star, named V838 Monocerotis, suddenly grew as large as the orbit of Jupiter, but it did not erupt. The surface remained cool. This type of activity should come from a star that gets hotter.

Though the images show the scene as of 2002, the pictures were released early in 2003 (crunching telescopic observations often takes months).

The expanding cloud, interestingly, is not part of the mystery. It is just pretty. It's called a light echo. The star's light bounces off surrounding dust clouds and is reflected to Earth. The echo grows over time as the light races out to other layers of material cast into space long ago by one or more previous eruptions of the star.

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