A molecular cloud is very cold, only a few degrees above absolute zero which is the lowest temperature possible (also called 0°K). But, when gas and dust start to collapse in a region within the molecular cloud, it slowly heats up. This is a consequence of a law of physics, which tells us that, when matter is squeezed together, the density of the matter will increase, and the matter will start to heat up. The outer edge of a collapsing region will have a temperature of around 10° above absolute zero (also called 10°K), and the inner region will slowly heat up to around 300°K, which is around room temperature.
When the collapsing region has reached a size of nearly 10,000 AU, it is called a pre-stellar core and is officially a star in-the-making. "Stellar" means star, so pre-stellar means, "before becoming a star." The word core refers to the gas and dust, which are now so dense that the term core is more precise than region or cloud. Also, this pre-stellar core will later become the interior core of the star.

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