Contact binary stars orbit so closely that they share a single atmosphere, making them appear as a giant glowing peanut.

Contact binary stars are officially classified as W Ursae Majoris variables, named after the prototype star system discovered in 1903. These systems consist of two stars that have expanded or moved close enough to fill their Roche lobes—the regions where their gravity dominates. Once these lobes are filled, stellar material overflows and creates a common convective envelope surrounding both stellar cores.Research published in journals like Nature and The Astrophysical Journal shows that these stars typically have orbital periods ranging from only 0.2 to 1.0 days. Because they share a gaseous envelope, heat transfers efficiently between the two bodies. This process ensures that both stars maintain nearly the same surface temperature, despite often having significantly different masses and internal structures.The evolution of these systems is driven by angular momentum loss through magnetic braking and gravitational radiation. As they lose energy, the stars slowly spiral toward each other over millions of years. This process eventually leads to a stellar merger, resulting in a single, rapidly rotating star known as a Blue Straggler. Observations by the Kepler Space Telescope have provided critical data on thousands of these systems, helping astronomers understand the final stages of stellar binary evolution.