NASA/ESA and Jesús Maíz Apellániz (Instituto de Astrofísica de Andalucía, Spain)

Explanation from the European Southern Observatory:

This image is a “close-up” view from the NASA/ESA Hubble Space Telescope of NGC 4755, or the Jewel Box cluster. Several very bright, pale blue supergiant stars, a solitary ruby-red supergiant and a variety of other brilliantly coloured stars are visible in the image, as well as many much fainter ones, often with intriguing colours. The huge variety in brightness exists because the brighter stars are 15 to 20 times the mass of the Sun, while the dim points are less than half the mass of the Sun. This is the first image of an open galactic cluster with imaging extending from the far ultraviolet to the near-infrared.

Did you catch that? The brighter stars in this photo are 15-20 times the mass of the Sun.