What happens when two stars spiral into each other?

In 2008, astronomers watched two stars spiral together until they smashed into one giant object.

This event, named V1309 Sco, solved a long-standing cosmic puzzle. For years, sky surveys picked up unusual flashes termed red novae. They shine brighter than ordinary novae but dimmer than massive supernovae. What makes them odd is their colour. Ordinary classical novae heat up to over 100,000 kelvin, but red novae rapidly cool down to just 2,000 kelvin. Instead of scorching space, they wrap themselves in cool clouds of cosmic dust.

The inward spiral

To understand why this happens, look at how paired stars behave. Two stars orbiting each other can become unstable through Roche lobe overflow, which happens when one star swells past its gravitational border. Gas spills out through escape points into open space. Another trigger is the Darwin instability, where tidal forces make the stars lose control of their spin and race inward.

Eventually, the smaller companion star plunges straight into the larger star's outer layers. Think of a spacecraft re-entering Earth's atmosphere. As the craft hits thick air, extreme drag slows it down and converts its high speed into glowing heat. In space, this drag is dynamical friction, the gravitational pull from surrounding gas. It drags the inner star inward, trading orbital energy for raw heat and light.

Cooling in the blast

Both stars now share a single gaseous shell in a phase called common-envelope evolution. The intense friction inflates this massive envelope and hurls gas outwards at hundreds of kilometres per second. That sounds fast, but it is slow compared to a typical supernova blast.

Because the blast is powered by moving mass rather than runaway nuclear burning, the ejected material behaves differently. As the gas cloud expands outwards, it cools quickly. It drops to low temperatures, allowing gas to condense into solid dust particles at 10 to 1,000 kelvin.

Astronomers categorise these events by how their light fades over weeks to hundreds of days. Some enter a flat plateau lasting up to 100 days. Others, known as risers, dim and then brighten a second time, dragging the total show out past 400 days.

What we still don't know

Astronomers have recorded only about 29 red novae across the entire sky. Measuring exact distances to these bursts inside our own galaxy is difficult, making their total energy tricky to calculate. Precursor flickers have only been caught for the nearest events.

Big questions remain open. We do not yet know if every single event ends in a complete stellar merger. Some stars might graze past each other, toss off their gas envelope, and survive as very tight pairs. Researchers also still debate whether fast stellar crashes inside crowded star clusters look the same as these slow, winding mergers.