Q: Is it possible to pass through a 4 million-degree cloud of interstellar gas without burning up?
A: Four million degrees indeed sounds like a burningly high temperature. However, for a body to actually burn requires not only a high enough temperature, but also a sufficiently dense gas cloud. Interstellar space is mostly empty, especially in hot gas clouds, more so than any vacuum produced on Earth. Such a cloud simply does not have the heat or energy density (a term that refers to the amount of energy available in the gas per unit volume) high enough to effectively burn a human body, let alone a spaceship. So the high temperature is not a concern for interstellar travel.
However, one does need to consider cosmic rays — high-energy subatomic charged particles — as well as X-rays and gamma rays (both highly energetic forms of light). These can be extremely penetrating and can damage, or microscopically burn, cells and electronics. On Earth, the atmosphere and magnetosphere largely protect us from cosmic rays. Some lethal radiation is still present, but it is emitted mostly from rocks and soils — no cause for concern, as long as the radiation dose is low. Here again, the density matters!
Professor of Astronomy,
University of Massachusetts, Amherst