Is the Milky Way getting bigger? Spanish team proves it

The galaxy we inhabit, the Milky Way, may be getting even bigger, as researchers found some star-forming regions at the outer edge of the disc, which may indicate the models of galaxy formation with new stars may slowly increase the size of it.

One problem in establishing the shape of the Milky Way is becuase we live inside it, so astronomers should look at similar galaxies elsewhere to assess its size as an analogue. Martínez-Lombilla and her colleagues at the Instituto de Astrofísica de Canarias in Tenerife, Spain have set out to establish whether other spiral galaxies similar to the Milky Way really are getting bigger, and if so what this means for our own galaxy.

The team used the ground-based SDSS telescope for optical data, and the two space telescopes GALEX and Spitzer for near-UV and near-infrared data respectively, to look in detail at the colours and motions of the stars at the end of the disc in the other galaxies.

They measured the light in these regions, predominantly originating from young blue stars, and measured their vertical movement to work out how long it will take them to move away from their birthplaces, and how their host galaxies were growing in size.

Based on this, they calculate that galaxies like the Milky Way are growing at around 500 metres per second, fast enough to cover the distance from Liverpool to London in about 12 minutes.

Martínez-Lombilla said, "The Milky Way is pretty big already. But our work shows that at least the visible part of it is slowly increasing in size, as stars form on the galactic outskirts. It won’t be quick, but if you could travel forward in time and look at the galaxy in 3 billion years’ time it would be about 5% bigger than today."

This slow growth may be moot in the distant future. The Milky Way is predicted to collide with the neighbouring Andromeda Galaxy in about 4 billion years, and the shape of both will then change radically as they merge.

Martínez-Lombilla, a PhD candidate, will present the work of her team in a talk on Tuesday 3 April, 2018 at the European Week of Astronomy and Space Science in Liverpool.

​Distant star Icarus captured by the Hubble Space Telescope in galaxy. (NASA/ESA/P. Kelly)