Free Astronomy Magazine March-April 2015

26 SPACE CHRONICLES T his artist’s impression and the side video show the central part of the planetary nebula Henize 2-428. The core of this unique object consists of two white dwarf stars, each with a mass a little less than that of the Sun. They are expected to slowly draw closer to each other and merge in around 700 million years. This event will create a daz- zling supernova of Type Ia and de- stroy both stars. [ESO/L. Calçada] discovered a close pair of white dwarf stars — tiny, extremely dense stellar remnants — that have a total mass of about 1.8 times that of the Sun. This is the most massive such pair yet found, and when these two stars merge in the future they will create a runaway thermonuclear explosion leading to a Type I a su- pernova. (Type I a supernovae occur when a white dwarf star acquires extra mass — either by accretion from a stellar com- panion or by merg- ing with another white dwarf. Once the mass exceeds the Chandrasekhar limit the star loses its ability to sup- port itself and starts to contract. This increases the temperature and a runaway nuclear reaction occurs and blows the star to pieces. The Chandrasekhar limit is the greatest mass that a white dwarf star can have and sup- port itself against gravitational col- lapse. It has a value of about 1.4