White Dwarf Stars in Open Cluster NGC 6791

In studying the dimmest burned-out stars in globular star cluster NGC 6791, NASA's Hubble Space Telescope has uncovered a paradox: three different populations of stars exist in an object where all the stars should have formed at the same time out of an interstellar cloud of gas and dust.

[Left] – This is a ground-based telescopic view of NGC 6791, located 13,300 light-years away in the constellation Lyra. The green inset box shows the view with Hubble's Advanced Camera for Surveys.

[Top right] – The full Hubble Advanced Camera for Surveys field is full of stars estimated to be 8 billion years old. Two background galaxies can be seen at upper left.

[Bottom right] – A blow up of view of a small region of the Advanced Camera for Surveys field reveals very faint white dwarfs. The blue circles identify hotter dwarfs that are 4 billion years old. The red circles identify cooler dwarfs that are 6 billion years old.

About the colours

The HST images (right) are a composite of several separate exposures made by the ACS instrument on the Hubble Space Telescope. Two filters were used to sample broad wavelength ranges in the visible and near infrafed. The color results from assigning different hues (colors) to each monochromatic image. In this case, the assigned colors are: Cyan: F606W (V) Orange: F814W (I)

About the data

HST Proposal: 9815 I. King (University of Washington), J. Anderson (STScI), G. Piotto (University of Padova), and L. Bedin (STScI); and 10471 : I. King (University of Washington), J. Anderson (STScI), G. Piotto (University of Padova), L. Bedin (STScI), S. Cassisi (Collurania Astronomical Observatory, Italy), and M. Salaris (Liverpool John Moores University).