New data from the European Space Agency explains why some stars are going the wrong way, galactically speaking.

Credit: Getty Images

The Milky Way has a dark secret.

According to new data beamed in from European research spacecraft,
our Milky Way galaxy encountered and devoured a second nearby
galaxy around 10 billion years ago.

More precisely, the Milky Way collided with the second galaxy,
absorbing many of its stars and spiraling out a chaotic tangle of
stellar matter — birthing new stars, altering the orbits of
others, and sending some in the opposite direction of the Milky
Way’s own rotation.

That last bit provided the first clue to astronomers, who have long
observed that some stars in our home galaxy are essentially going
the wrong way, moving against the trailing arms of the galaxy.
Other stars have been observed to spin in strange clusters,
bolstering the theory that the Milky Way is actually the result of
a number of previous mergers and meals.

New research published
in the journal Nature confirms that a good number of these rogue
stars have a common origin — a deceased galaxy dubbed
Gaia-Enceladus. It also suggests that our galaxy was largely formed
by a few big collisions, rather than a lot of smaller ones, which
helps answer a question that has occupied astronomers for

Credit: ESA

The discovery was made by an international team of researchers led
by Amina Helmi,
an astronomer with the University of Groningen in the

Helmi has spent most of her career studying the Milky Way for
“fossil stars” that provide clues to the ultimate origin of our
galaxy. She has pioneered a specific approach to the mystery by
combining analysis of the stars’ placement, trajectory, and
chemical compositions.

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The new research provides hard evidence for previous theories
concerning the ancient galactic collisions. That evidence comes
largely from data sent by the Gaia satellite mission, a major
initiative from the European Space Agency.

The Gaia satellite, launched in 2013, recently beamed back a
colossal chunk of new data detailing the position, brightness, and
motion of more than 1.3 billion stars in the Milky Way. Helmi and
her team have been crunching the numbers since April.


In materials issued with the new research, Helmi said that the
chemical signature of many wrong-way stars was demonstrably
different than “native” Milky Way stars.

“And they are a fairly homogeneous group, which indicates they
share a common origin,” she said. “The youngest stars from
Gaia-Enceladus are actually younger than the native Milky Way stars
in what is now the thick disk region. This means that the
progenitor of this thick disk was already present when the fusion
happened, and Gaia-Enceladus, because of its large size, shook it
and puffed it up.”

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Size is a tricky concept when working on the galactic scale, but
Helmi’s team estimate that Gaia-Enceladus, the dearly departed
galaxy, was slightly more massive than the Small Magellanic
(SMG), a dwarf galaxy with a total solar mass about one
percent of the Milky Way. (By the way, the Milky Way is slowly
eating the SMG, too.)

An interesting final note: When Helmi finally pieced together all
the data from the Gaia info dump, she discovered that the numbers
looked awfully familiar. They reminded Helmi of simulations
performed by a former Ph.D. student more than ten years ago. The
student’s simulations of the merging of a large disc-shaped
galaxy with the young Milky Way produced results that were totally
in line with the Gaia data.

“It was amazing to look at the new Gaia data,” Helmi said,
“and realize that I had seen it before.”

Source: FS – All – Science – News 2
New data from the European Space Agency explains why some stars are going the wrong way, galactically speaking.