Alien stars found in our Milky Way

Gas clouds in red hues in a very dense star field with some red stars.
Infrared image of stars at the center of our Milky Way galaxy, via the Spitzer Space Telescope. Observing in infrared makes it possible to peer behind the gas clouds that otherwise cover the central region of the galaxy. There are around 10 million stars within just 3.3 light-years of the galactic center. These are dominated by red giants, the same kind of old stars found to be from another galaxy in this study. Image via NASA/ JPL-Caltech/ S. Stolovy (SSC/Caltech).

Astronomers used a new technique – asteroseismology combined with spectroscopy – to pinpoint the ages of a sample of around 100 old red giant stars in the Milky Way. They were able to reach a much higher accuracy of the stars’ ages, they said in a statement on May 17, 2021. And they also found that a number of those red giant stars did not originate in the Milky Way! They are instead alien stars, which came here from another galaxy. Their original home in space was Gaia Enceladus (also known as the Gaia Sausage), a dwarf galaxy that collided and merged with our Milky way galaxy about 10 billion years ago.

This new research was published on May 17, 2021, in the peer-reviewed journal Nature Astronomy.

What do alien stars tell us?

So the idea here is that the Milky Way galaxy had already started forming many of its stars before a dwarf galaxy came by and merged with our galaxy, bringing its own stars with it. This event took place around 8-11 billion years ago. In contrast, the age of the Milky Way is about 13.6 billion years, give or take a few.

This merger, then, happened early in our galaxy’s history.

The dwarf galaxy – or the remnants of it – go today under the name Gaia Enceladus or the Gaia Sausage, because of the highly elongated shape it forms – like a sausage – as seen from data from the Gaia mission. In Greek mythology, Enceladus was the offspring of the goddess Gaia. It is also, incidentally, the name for one of Saturn’s moons.

In this new research, the astronomers were able to identify stars that are remnants of the merger. These stars provide a way of looking back to the distant past, when the merger took place. Josefina Montalbán at the University of Birmingham is the lead author on the paper. She said:

The chemical composition, location and motion of the stars we can observe today in the Milky Way contain precious information about their origin. As we increase our knowledge of how and when these stars were formed, we can start to better understand how the merger of Gaia-Enceladus with the Milky Way affected the evolution of our galaxy.

Milky Way galaxy, surrounded by many mostly radial short yellow lines, making the image look like an eye.
Artist’s concept of the stars from dwarf galaxy Gaia Enceladus, which merged with the Milky Way some 10 billion years ago. The Milky Way is in the center of the illustration, shown from above, and the Gaia Enceladus stars – debris of the dwarf galaxy – are represented by little arrows – vectors – that show their position and the direction in which they move. The data are from a computer simulation. Image via ESA.

How did astronomers find the stars?

These astronomers had targeted a sample of 100 old stars observed with the Kepler mission. These are red giant stars, at the end of their lives.

The team used data from three Milky Way research instruments to measure the stars’ ages, all with the task of mapping and analyzing Milky Way stars. One instrument was Kepler, as mentioned previously. The other two were the Gaia satellite and APOGEE.

With data from these instruments, the astronomers used the technique of asteroseismology that studies how stars oscillate. That is, the technique measures regular variations within the star. Asteroseismology is similar to helioseismology, the study of oscillations in the sun. Learning how a star oscillates lets astronomers gain info about a star’s size and internal structure, which, in turn, will let them estimate the star’s age.

Team member Mathieu Vrard at Ohio State’s Department of Astronomy, said:

[It] allows us to get very precise ages for the stars, which are important in determining the chronology of when events happened in the early Milky Way.

In addition, the astronomers also used spectroscopy – the study of the stellar spectrum – to learn the chemical composition of the stars. This also helps with age determination, and in combination, the methods let the astronomers determine the ages to an unprecedented precision.

The astronomers noticed that a number of them were of the same age, and that this age was a bit younger than most of the stars that we know started their lives in the Milky Way.

Team member Andrea Miglio at the University of Bologna added:

We have shown the huge potential of asteroseismology in combination with spectroscopy to deliver precise, accurate relative ages for individual, very old, stars. Taken together, these measurements contribute to sharpen our view on the early years of our galaxy and promise a bright future for [Milky Way] archeoastronomy.

Now the researchers want to apply their approach to larger samples of stars to get a better view of the Milky Way’s formation history and evolution.

Lightly smiling woman with short hair.
Josefina Montalbán at the University of Birmingham is the lead author on the research that measured the ages of Milky Way stars to a high accuracy using a new method, and thereby identifying stars that did not form in our galaxy but came with another. Image via J. Montalbán/

Bottom line: Astronomers used a new technique – asteroseismology in combination with spectroscopy – to precisely measure the age of stars, and found that a number of the red giants in their sample were not originally formed in our Milky Way galaxy, but came here as the result of a merger with dwarf galaxy Gaia Enceladus in the early history of the Milky Way.

Source: Chronologically dating the early assembly of the Milky Way

Via the University of Birmingham

Via Ohio State University

May 25, 2021

Like what you read?
Subscribe and receive daily news delivered to your inbox.

Your email address will only be used for EarthSky content. Privacy Policy
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.

More from 

Theresa Wiegert

View All