Mass-loss from young red-giants in globular cluster NGC 104

This image shows the globular cluster NGC104 (also known as 47 Tuc), located at an estimated distance of around 15000 light years from us, visible from the southern hemisphere in the constellation of Tucana. The object was observed with the Near- and Mid-Infrared Camera (IRC) on board AKARI, at 3, 4, 7, 11, 15, and 24 micrometres. It shows evidence of high-rate mass-loss from young red-giant stars. This image is a three-colour composite of the target, observed at 3, 11, and 24 micrometres.

Globular clusters are dense spherical groups of about one million stars. NGC104 is thought to have formed about 11 thousand million years ago (approximately the age of the Milky Way). The massive stars in the cluster have long burnt out, and those that currently light the cluster are of mass similar to that of our Sun - or rather to what our Sun will evolve into approximately 6 thousand million years from now.

The stars of NGC 104 with similar mass to our Sun have already extinguished the hydrogen in their cores – the fuel required for nuclear fusion – and proceeded to later stages of stellar evolution to become red-giant stars. Sun-like stars undergo ‘mass-loss’ at this stage: they eject about 40 per cent of their mass into interstellar space becoming ‘white dwarf’ stars. The mass loss in the early red-giant phase, important for understanding the evolution of galaxies, had been predicted only in theory so far.

The bright red stars seen in the image are mostly in the final stages of the red-giant phase with mass loss rates as high as one millionth of their mass per year. Stars in the earlier red-giant phase are seen as slightly darker. The star marked in the bottom-left corner has a high rate of mass loss while still a young red-giant, confirming the presence of mass-loosing younger red-giants for the first time. The presence of other similar young red giants in the cluster without mass-loss implies that the phenomenon in young red-giants is not continuous but rather episodic in nature, occurring sporadically over relative short periods when put into a cosmological context. Further analysis and modelling will enable to the scientists to understand the future evolution of our Sun, and the effect on the Solar System.