Milky Way Merged With Another Galaxy Instead Of Forming Alone

  • Researchers examine data of 7 million stars obtain from Gaia space observatory.
  • They found 30,000 stars moving in odd-directions in our galaxy. 
  • These stars belong to a different galaxy named Gaia-Enceladus. 

The assembly of the Milky Way galaxy can be reconstructed using the chemistry and motions of individual stars. Chemo-dynamical examinations of the stellar halo near our Sun have shown the presence of several components like clumps and streams, as well as associations between the stars’ orbital parameters and chemical abundances.

Recently, a team of researchers at the University of Groningen, Netherlands, analyzed data of 7 million stars obtained from Gaia’s first 22 months of observation. Gaia is ESA’s space telescope designed for measuring the position, distance, movement, and brightness of stars with unprecedented precision.

After examining the full three-dimensional positions and velocities, researchers found an “unusual collection” of nearly 30,000 stars moving through our galaxy. At present, these stars are passing by our solar neighborhood.

The Odd-Moving Stars In Our Galaxy

This strange collection of stars is interspersed with other stars and can be observed across most of the sky. Their characteristics are quite different, for instance, they all travel along elongated trajectories in the opposite direction to the other billions of stars in the galaxy, including our Sun.

They can also be easily identified in Hertzsprung–Russell diagram — a scatter plot that compares the brightness and color of stars — suggesting that these stars belong to a different stellar population.


A major event in the Milky Way formation 10 billion years ago | Credit: ESA / Amina Helmi, Carine Babusiaux

The researchers wondered whether this strange collection of stars has something to do with the formation of our galaxy. The data has made it clear that odd-moving stars have all the properties of what one can expect from the remnant of a merger between two galaxies. 

This led researchers to conclude that these stars were once part of another galaxy that was engulfed by the Milky Way billions of years ago.

Reference: Nature | doi:10.1038/s41586-018-0625-x | ESA

The odd-moving stars are now mostly present in the inner stellar halo of the Milky Way – a diffuse region of ancient stars that surround the major bulk of the galaxy called central disc and bulge.

The galactic disc is made of two components: The first one is a thin disc that is several hundred light-years deep, containing bright stars in a spiral arm pattern. The other one is a thick disc that is several thousand light-years deep containing up to 20% of the galaxy’s star.

Computer Simulations

The team used computer simulations to better understand how these odd-moving stars are scattered in the Milky Way. The results show that the accreted galaxy could have disrupted pre-existing stars in the Milky Way, which later formed the thick disc.

The astronomers named this galaxy Gaia-Enceladus. In particular, they found 13 globular clusters (containing millions of stars) and hundreds of variable stars in the Milky Way that follow same trajectories as the stars from Gaia-Enceladus.

Further investigations unveiled that Gaia-Enceladus was about 10 times smaller than the Milky Way’s current size. The merger took place nearly 10 billion years ago when Milky Way itself wasn’t much bigger. The ratio between these two galaxies was something around 4:1.

Read: The Richest Star Catalogue Of 1.7 Billion Stars | Details Of Milky Way

It has been more than 26 years since the Gaia mission was first proposed. One of the major objectives of the mission was to study different stellar streams in our galaxy, and uncover its early history. The vision is paying off.

Written by
Varun Kumar

Varun Kumar is an experienced science and technology journalist interested in machines, AI, and space exploration. He received a Master's degree in computer science from Indraprastha University. To find out what his latest project is, feel free to directly email him at [email protected] 

View all articles
Leave a reply