The Scale of the Universe Three ways to see a nebula. Each is studied so that the physical processes at work in the nebula can be understood. The visible-light photo shows glowing, electrically excited gas, heated by radiation from adjacent hot stars, themselves not clearly distinguished.
Dark lanes are clouds of interstellar dust that hide the bright nebula beyond.
To discuss the universe, we must abandon our ordinary frames of reference and think along almost unimaginable scales of distance and energy, and we must consider unusual, even fantastic, states of matter. True, our own horizons have expanded through space exploration. Humans have gone higher and faster than ever before. But our experience is still drastically limited by comparison to the immensities of structures in the universe. This velocity is a fundamental constant of the universe, and it provides a standard forjudging our progress into space.