There has been some speculation among astronomers that as Comet ISON approaches the Sun, it may break up into pieces. ISON, the type of comet known as a "sungrazer," will pass extremely close to our star. Current projections of the comet's path bring it within 700,000 miles (1.2 million kilometers) of the Sun.
Many people think that the Sun's heat would be the source of a break-up, but that's not the case. The powerful gravity of the Sun, acting on the comet, could tear it apart. Whether that happens or not depends on the comet's strength and composition.
If ISON is strong enough — meaning it is pretty tightly packed — it may hold together as it passes, but if it is a loose collection of rocks, ice and dust, it could be easily pulled apart.