Giant stars are sometimes mysteriously present in places they shouldn’t be, far from the galactic plane – and astronomers have discovered why.

The explanation reminds us stellar evolution can be far more complex than we usually acknowledge.

Stars – particularly large ones – form near the galactic bulge or plane. Gas is seldom dense enough on the outskirts of the galaxy to condense and make a star, let alone a really big one.

Stars born near the plane sometimes voyage far above or below it – but a few of these pose a mystery, getting further from their likely place of birth than they should have been able to travel in their short lifespan.

A paper in The Astronomical Journal uses the example of the giant HD93521 to offer an answer to the discrepancy between these stars’ apparent ages and their calculated travel time.

"Astronomers are finding massive stars far away from their place of origin, so far, in fact, that it takes longer than the star's lifetime to get there," said study author Professor Douglas Gies of Georgia State in a statement. 

The more mass a star has, the brighter it shines and the faster it consumes its material. The relationship between mass and lifespan is almost an inverse cube law; a star with a mass 10 times that of the Sun will have a lifespan nearly a thousand times shorter.

HD3521 is an impressive 17 times the mass of the Sun. Its entire lifespan should be around 20 million years. With plenty of its original helium unconverted, it appears to be only around 5 million years old, give or take two million – which makes it very puzzling that it sits 3,600 light years above the galactic plane. Based on its current motion, it would have taken it 39 million years (plus or minus three million) to get there, even if it started immediately after formation.

Cases like this leave astronomers with three possible explanations: the star is much older than it looks, it somehow formed already far from the galactic plane, or it was initially moving with staggering speed and something slowed it down.

The absence of neighbors to provide a braking force largely rules out the last of these.

The study authors explain HD3521 as the recent merger of two stars. The same mass distributed over two or more stars will fuse more slowly. If the material that now makes up HD3521 was previously divided almost evenly, each component would have taken around 40 million years to have reached the current stage of development.

The twin stars would need to have travelled together from the galactic plane, starting soon after their formation – but an early beginning to their trajectory is unsurprising. Massive stars are usually born in clusters where powerful forces can fling them outwards; either a gravitational slingshot from an even larger pair or the blast of a nearby supernova.

It’s not common for stars to merge, but it does happen when their original orbit is very tight. The tell-tale sign of a past merger is rapid rotation – the angular momentum of their mutual orbit has to go somewhere, and it ends up spinning the merged star very fast.

HD93521’s rotational speed is hard to measure, but appears to be among the fastest we have seen.

Co-author graduate student Peter Wysocki is seeking examples to show HD3521 is not unique. He recently submitted a paper reporting one member of the massive runaway pair IT Librae is stealing material from the other prior to full merger. The process is rejuvenating the higher mass star, making it appear younger than its true age, another HD93521 in the making.