See What Happens When Stars Collide

In stellar astronomy, sometimes 1 + 1 = 1.

Astronomers haven’t lost their grip on math. The latest evidence for this calculus comes from the nebula NGC 6164/6165. At its heart lies the binary HD 148937, which comprises a pair of hot, massive O-type stars.

But not too long ago, there were likely three stars.

Reporting in the April 12th Science, Abigail Frost (KU Leuven, Belgium, and European Southern Observatory, Chile) and an international team of astronomers have found compelling evidence that the more massive star of the pair formed when two earlier stars merged.

Many stars form in binaries or higher-order multiple systems. In fact, nearly all of the most massive stars have at least one companion.

Under the right circumstances, these stars can actually spiral in and merge with each other in a violent splash. When they do, they throw off gas that cools and creates dust, temporarily shrouding the new star. This event is called a red nova (red because of the dust’s effect).

Astronomers have found a handful of examples of stellar mergers, either caught in the act or as suspicious-looking stars that have all the hallmarks of having been created in a crash.

The primary star of HD 148937 is one of the latter.

Frost’s team combined ground- and space-based data, including observations taken over a period of nine years with the Very Large Telescope Interferometer in Chile, to confirm that the system’s primary is a massive O star with a strong magnetic field.

Massive stars like these aren’t supposed to have strong surface magnetic fields, because they don’t have the same churning in their interiors that sustains magnetic fields in stars like the Sun. Yet approximately 7% of massive stars have observable fields.

A stellar merger, on the other hand, would stir up the inside of a newly created star, creating a strong magnetic field. The new star would spin abnormally fast, too, and it would look “too young” for its mass — and both of these are properties of HD 148937’s primary. More massive stars evolve faster, and stars born as binaries form together at the same time. Yet despite being heftier than its companion, the primary looks at least 1.4 million years younger.

Based on the nebula’s expansion speed, the team suggests the merger happened several thousand years ago.

Notably, about 8% of O stars are predicted to experience a merger — similar to the fraction of them found with mysterious magnetic fields.

You can read more about what happens when stars merge in Sky & Telescope’s June 2024 issue.