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Five fascinating Gaia revelations about the Milky Way

on 07 October 2020

With its two releases of data in September 2016 and April 2018, Gaia has truly revolutionised the study of the Milky Way. It ushered in the golden age of galactic archaeology, a discipline that searches for evidence of past galactic events in the characteristics and behaviour of stars and stellar populations that we see today. Gaia does not only reveal details of the Galaxy’s structure. The mission creates an awe-inspiring astronomical movie reconstructing the Milky Way’s evolution to the past and future over billions of years.

Here are the five interesting discoveries that Gaia has made about the galaxy that we are part of.

1. The Milky Way is a galactic cannibal

Astronomers have suspected since the 1990s that the Milky Way was born out of collisions between smaller galaxies that had taken place over the billions of years of its history. Earth-based telescopes, such as the Sloan Digital Sky Survey, provided first hints of the galaxy’s violent past, but it wasn’t until Gaia that astronomers could really deconstruct the processes that led to the creation of the Universe that surrounds us.

In 2018, a team from the University of Groningen, the Netherlands, led by galactic archaeology expert Amina Helmi, discovered that a group of 30 000 stars moves in a synchronised way through the neighbourhood of the Sun in the opposite direction to the rest of their sample of seven million stars.

Further analysis confirmed that the stars, which now form part of the Milky Way’s so-called inner halo and the outer layer of the galactic disc, must have originated from another galaxy. Gaia-Enceladus smashed into and was gradually devoured by the Milky Way, which was at that time only four times bigger than Gaia-Enceladus. The collision, therefore, must have profoundly shaken the Milky Way.

2. Stars form in the Milky Way because of a galactic crash

Gaia has also shed light on the interactions with the dwarf galaxy Sagittarius, which has been orbiting around the Milky Way’s core for billions of years.

Discovered in the 1990s, Sagittarius contains only a few tens of millions of stars (compared to the Milky Way’s hundreds of billions), which makes it 10 000 times less massive than the Milky Way. As the Milky Way’s gravity pulled Sagittarius closer, the smaller galaxy started smashing through the Milky Way’s disc. That happened at least three times in the past: some five or six billion years ago, two billion years ago, and one billion years ago. With each collision, the Milky Way stripped stars from Sagittarius, leaving the dwarf galaxy smaller and smaller. Eventually, Sagittarius will be completely devoured by the Milky Way.

2020 10 Gaia Revelations Image B

Image credit: ESA

3. The Milky Way’s arms and disc are alive

Before Gaia, astronomers were limited in their attempts to study the structure of the Milky Way. They knew that the Milky Way was a so-called spiral galaxy – a pancake-like disc of stars with a pattern of spiral arms rotating around a much denser core.

The spiral arms are areas of densely packed gas and stellar matter that appear, when observed in other galaxies, in bluer shades than the rest of the disc, which indicates the hotter temperature of the stars within them. Since hot stars are massive stars and since massive stars are young stars, astronomers can tell that spiral arms are areas where stars are being formed.

2020 10 Gaia Revelations Image C

Image credit: ESA

4. The Milky Way strips and catches stars from other galaxies and clusters

The Milky Way is also constantly stripping stars from dwarf galaxies and stellar clusters with which it interacts. Based on Gaia data, astronomers have identified streams of stars ripped from these bodies that frequently stretch over distances of thousands of light-years, covering a large portion of the sky above our heads.

Data about such streams can help astronomers assess the gravitational force and therefore the mass distribution of the Milky Way and hence reveal how galaxies acquire stars.

Gaia has also found stars in the Milky Way’s disc that are travelling at such high speeds that they might be able to escape the galaxy’s gravitational pull or might have been expelled from other galaxies and subsequently captured by the Milky Way.

2020 10 Gaia Revelations Image D

Image credit: ESA (artist’s impression and composition); Marchetti et al 2018 (star positions and trajectories); NASA/ESA/Hubble (background galaxies), CC BY-SA 3.0 IGO

5. The Sun is surfing a mysterious wave of gas in the Milky Way

In 2019, three scientists associated with the Radcliffe Institute for Advanced Studies of Harvard University discovered that interstellar gas clouds in the Sun’s galactic neighbourhood form a 9000 light-years long wave that undulates about 500 light-years above and below the galactic disc. The 400 light-years wide wave is part of what astronomers describe as the Local Arm, a small spiral arm of the Milky Way close to the Sun.

Interstellar gas clouds interest researchers because they give birth to stars when they collapse. Prior to the 2019 discovery, it was believed that such clouds in the solar neighbourhood are concentrated in the so-called Gould Belt, a ring of young stars, gas, and dust which arches above and below the galactic plane.

2020 10 Gaia Revelations Image E

Image credit: ESA

Main image: ESA