Starspots seen on zeta Andromedae

A strange arrangement of dark blobs on a star 180 light-years away has been snapped by astronomers. 

These "starspots" appear similar to the sunspots that pop up on the Sun, but with a twist: rather than appearing around the equator, as we see on the Sun, those on zeta Andromedae – a star around 15 times the Sun's radius – are all over the place. 

Astronomers from Europe and the US, led by the University of Michigan's Rachael Roettenbacher, were able to pick out the starspots by gathering images from several different telescopes. The resulting images, the highest quality so far of a star other than our Sun, show ancient zeta Andromeda's internal processes to be more like those of a young star.

Most stars, such as our Sun, are massive, spinning balls of gas that throw off heat and light as elements fuse in their core. Charged particles swirl inside the star, creating a tangled mess of magnetic fields which occasionally burst out of the star's surface.

On the Sun, these appear as sunspots. And because blasts of magnetic field slow heat flow, those patches are cooler than their surrounds. 

At the start of each 11-year solar cycle, sunspots tend to appear just north and south of the Sun's equator. As the cycle wears on, though, they pop up closer and closer to the equator. Why? Astronomers don't know – but it must have something to do with the Sun's magnetic field cycle.

Do starspots behave the same way all over the Universe? Astronomers suspected not, but the answer was a long time coming. 

"While imaging sunspots was one of the first things that Galileo did when he started using the newly invented telescope, it has taken more than 400 years for us to make a powerful enough telescope that can image spots on stars beyond the Sun," says study co-author and University of Michigan astronomer John Monnier.

To directly image zeta Andromedae, six telescopes in the Centre for High Angular Resolution Astronomy (CHARA) array in California simultaneously peered at the star for a total of 25 days, snapping pictures of its surface. Light captured by the telescopes was combined to build a time-lapse of the star during one of its 18-day rotations.

"Now we can see that the spots aren’t restricted to forming only in symmetric bands around the equator as sunspots are," Roettenbacher says.

"We see the starspots in both hemispheres and at all different latitudes. This can’t be explained by extrapolating theories about the Sun’s magnetic field."

So why are zeta Andromedae's spots so odd?

It's a very different star to the Sun, says Heidi Korhonen, an astrophysicist at the University of Copenhagen in Denmark and co-author of the study. It's a binary, meaning it and a smaller as-yet-unseen object orbit each other. This gives them an energy boost and they rotate quickly – zeta Andromedae has been clocked spinning at 40 kilometres per second. The Sun, in contrast, rotates at a comparatively leisurely two kilometres per second.

Zeta Andromedae's rapid rotation creates a magnetic field stronger and more complex than the Sun's. It "resembles what you see at the stage where a new star is being created", Korhonen says, because baby stars spin quickly and contract as they suck in dust and gas around them.

So zeta Andromedae, which is nearing the end of its life, appears more youthful than the younger Sun. Probing it further with even more powerful telescope technology will provide insights in how the Sun developed and grew, Roettenbacher says.

"It's important to understand the Sun's history because that dictates the Earth's history – its formation and the development of life."

The work was published in Nature.