20 000 light-years
The star-forming region NGC 3603 seen here in a Hubble Space Telescope image contains one of the most impressive massive young star clusters in the Milky Way. Bathed in gas and dust, the cluster formed in a huge rush of star formation thought to have occurred around a million years ago. The hot blue stars in the core are responsible for carving out a huge cavity in the gas seen to the right of the star cluster in NGC 3603's centre.
Credit: NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration.