Anyone who has ever seen a picture of Earth taken from deep space can be forgiven for thinking of these two words: “splendid isolation.” Surrounded by millions of miles of uninterrupted black, the fragile blue globe seems profoundly alone, disconnected from anything else.

Nothing could be further from the truth: Earth is profoundly connected to our star.

The bright blue disk is just the most obvious evidence. A non-stop flood of sunlight warms the planet, simultaneously allowing us to live and to see. Invisible connections are equally profound. Solar radiation puffs up our atmosphere, altering its structure and chemistry. Solar winds buffet our magnetosphere, lighting up polar skies with curtains of light, and driving currents of electricity through the soil below. Solar magnetism deflects cosmic rays, moderating the effect of the Galaxy on our tiny home in space.

Years ago, the study of the Sun–Earth connection was edgy stuff. Big Thinkers held the planet and the star to be a system. From this synthesis emerged many new ideas and a new discipline called “heliophysics.”

Now we know that they weren't thinking big enough. Like Earth, every world in the solar system is connected to its star. From the surface chemistry of Mercury, to the tattered atmosphere of Mars, to the flowing ices of Pluto, the fingerprints of solar activity may be found in all corners of the heliosphere.

In pop culture, people trace the “seven degrees of separation” between themselves and actor Kevin Bacon. Earth is connected much more closely to alien worlds. The central role of the Sun puts us just one degree of separation away from scores of planets, dwarf planets, moons, asteroids, and comets throughout the solar system. This proximity tells us something important: what we learn about those strange places, we also learn about ourselves.

The connectedness of things is the subject of this book: Active Stars, their Astropheres, and Impacts on Planetary Environments. In 13 graduate-level chapters, experts lay out new ideas about how stars carve out a place in the galaxy to shape their own solar systems. The chapters touch on subjects ranging from magnetic reconnection and magnetohydrodynamics to climate and aeronomy. It may be one of the most interdisciplinary textbooks ever written – at least in the physical sciences.