Space may seem empty, but actually it is filled with thinly spread gas and dust. This gas and dust is called interstellar medium. The atoms of gas are mostly hydrogen (H2), and the gas atoms are typically about a centimeter apart. The dust is mostly microscopic grains and comprises only a few percent of the matter between stars. The dust is mostly carbon and silicon. In some places, this interstellar medium is collected into a big cloud of dust and gas known as a nebula. This is the birthplace of stars because the gas and dust is what makes up a star. In fact, our sun was probably born in a nebula nearly 5 billion years ago.