An Etymological Dictionary of Astronomy and Astrophysics
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A → cluster of galaxies at a → redshift of z = 0.296 undergoing a violent → merger process nearly in the → plane of the sky. Also known as 1E 0657-558. The head-on collision between the main cluster and a subcluster ramming with an apparent speed of about 4700 km s^-1 occurred about 150 x 10⁶ years ago. The two clusters are currently moving away from each other while the space between them is filled with a very hot gas (first observed in X-rays by → Chandra) resulting from the overheating due to the collision. The Bullet cluster has the highest X-ray luminosity and temperature of all known clusters. The X-ray gas of the bullet (amounting to 2 x 10¹³ solar masses) collides with the X-ray gas of the main cluster (10¹⁴ solar masses) and forms a well defined → supersonic (Mach 3) → bow shock. A significant offset between the distribution of X-ray emission and the mass distribution has been observed, and diversely interpreted.

The name Bullet refers to the smaller subcluster, that has created the bow shock; → cluster.