Scientists Build a Detailed Image of U Mon Binary

Astronomers have painted their best picture yet of an RV Tauri variable, a rare type of stellar binary where two stars – one approaching the end of its life – orbit within a sprawling disk of dust. Their 130-year dataset spans the widest range of light yet collected for one of these systems, from radio to X-rays. The system, U Monocerotis or U Mon for short, lies around 3,600 light-years away in the constellation Monoceros. Its two stars circle each other about every six and a half years on an orbit tipped about 75 degrees from our perspective. The primary star, an elderly yellow supergiant, has around twice the Sun’s mass but has billowed to 100 times the Sun’s size. A tug of war between pressure and temperature in its atmosphere causes it to regularly expand and contract, and these pulsations create predictable brightness changes with alternating deep and shallow dips in light – a hallmark of RV Tauri systems. Scientists know less about the companion star, but they think it’s of similar mass and much youn