Our Milky Way Galaxy, which is approximately 100,000 light-years in diameter, may be getting even bigger, according to a study by the Instituto de Astrofísica de Canarias and the Universidad de La Laguna.
This artist’s impression shows how the Milky Way Galaxy would look seen from almost edge on and from a very different perspective than we get from the Earth. The central bulge shows up as a peanut-shaped glowing ball of stars and the spiral arms and their associated dust clouds form a narrow band. Image credit: ESO / NASA / JPL-Caltech / M. Kornmesser / R. Hurt.
The Solar System is located in one of the arms in the disk of a barred spiral galaxy we call the Milky Way.
The Galaxy consists of several hundred billion stars, with huge amounts of gas and dust, all intermingled and interacting through the force of gravity.
The nature of this interaction determines the shape of a galaxy, which may be spiral, elliptical or irregular.
As a barred spiral galaxy, the Milky Way consists of a disk in which stars, dust, and gas lie mostly in a flat plane, with arms stretching out from a central bar.
In the Milky Way’s disk there are stars of many different ages.
Massive, hot, blue stars are very luminous and have a relatively short lifespan of millions of years, whereas lower mass stars eventually end up redder and much fainter and may live for hundreds of billions of years.
The younger short-lived stars are found in the disk of the Galaxy, where new stars continue to form, whereas older stars dominate in the bulge around the Galactic center and in the halo that surrounds the disk.
Some star-forming regions are found at the outer edge of the disk, and models of galaxy formation predict that the new stars will slowly increase the size of the Galaxy they reside in.
One problem in establishing the shape of the Milky Way is that we live inside it, so astronomers look at similar galaxies elsewhere as analogues for our own.
Astronomer Cristina Martínez-Lombilla and co-authors set out to establish whether Milky Way-like galaxies are really getting bigger, and if so what this means for our own Galaxy.
They used the ground-based SDSS telescope for optical data, and the two space telescopes GALEX and Spitzer for near-UV and near-IR data respectively, to look in detail at the colors and the motions of the stars at the end of the disk found in the other galaxies.
They measured the light in these regions, predominantly originating from young blue stars, and measured their vertical movement (up and down from the disk) of the stars 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 m/sec.
“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,” Martínez-Lombilla said.
“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 neighboring Andromeda Galaxy in about 4 billion years, and the shape of both will then change radically as they merge.”
Martínez-Lombilla and colleagues presented their results yesterday at the European Week of Astronomy and Space Science (EWASS) in Liverpool, UK.
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Cristina Martínez-Lombilla et al. Measuring disc growth in Milky Way-like galaxies. EWASS 2018, abstract # 982
