Two Bubbles in the LMC

Date:	26.3.1997	Time:	2:29 UT	Exposure:	60s
Field of View:	4' x 4'	Receiver:	512² CCD	Filter:	H-alpha
Instrument:	Dutch 0.9m	Observatory:	La Silla	Observer:	S. Kohle

The image above shows two bubbles in the Large Magellanic Cloud in the light of the ionized hydrogen. The bright bubble (DEM 180) has a diameter of about 15 lightyears, the fainter one a diameter of about 75 lightyears. Both bubbles have hot central stars which have a typical lifetime of a few 10⁶ years. These HII regions become visible after stars formed out of dense interstellar gas and the first "O"-star begins to ionize the surrounding hydrogen. Shock waves form and drive the surrounding gas outwards. They continue to expand but are only visible as long as the ionizing star(s) shine. The different diameters can thus be explained by different ages of the central stars. The fact that the smaller and younger bubble is located just at the edge of the older one may be explained by a model in which the expanding bubble triggered the collapse of a neighboring cloud leading to "sequential star formation".

Astronomical Institutes of the University of Bonn