Some of the twinkling stars that spangle Earth’s skies are relics from the earliest beginnings of the Milky Way galaxy, astronomers have discovered. Formed within a few billion years of the big bang, these stars populated the burgeoning stellar conglomerate that, over eons, would grow and sculpt itself into the spiral galaxy we live in today.
“They are as old as the oldest stars in the universe,” says Carme Gallart of the Institute of Astrophysics of the Canary Islands, who reports the findings today in the journal Nature Astronomy. The work marks the first time astronomers have pinned precise ages onto these ancient stars, which are a whopping 10 to 13 billion years old.
“Identifying the oldest populations of stars that actually formed inthe Milky Way is quite interesting, because they give us a kind of window into our galaxy's past,” says Chris Hayes of the University of Virginia, who was not involved in the study.
“These earliest populations of stars should exist, but now that they have been identified, they provide a powerful tool to piece together our galaxy's history.”
The galaxy’s history is written into the ages, compositions, and placements of these aging stars, meaning that they act a bit like archaeological clues. For instance, in the process of examining these ancient stars, the team uncovered evidence for a massive galactic collision early in our galaxy’s lifetime.
Roughly 10 billion years ago, the primordial Milky Way and a smaller galaxy, called Gaia-Enceladus, slammed into each other. Today, a distinct population of blue stars comprises the scattered, glittering remains of the vanished, smaller body.
Gallart and her team made these discoveries using data primarily gathered by the European Space Agency’s Gaia satellite. From its position in space, Gaia studies the nearest and brightest billion or so stars, recording highly detailed information about their motions and locations.
Using the precise distances delivered by Gaia for roughly half a million nearby stars—those that are within about 6,500 light-years of Earth—Gallart and her team could determine the exact luminosities and colors of those stars. From there, the team calculated the stars’ ages, and in the resulting data swarm, several compelling patterns emerged.
Picture a pizza inside of a balloon. The pizza would be the Milky Way disk; the air and dust inside the balloon would be the stars in the halo.
Put simply, the team uncovered evidence that two populations of stars are identical in age, with each being no younger than 10 billion years old. One group is redder, and the other is bluer, and both primarily live in the Milky Way’s halo, a spherical region that encompasses the entire galaxy.
“The halo surrounds us, is everywhere,” Gallart says. “Picture a pizza inside of a balloon. The pizza would be the Milky Way disk; the air and dust inside the balloon would be the stars in the halo.”
How these long-lived stars ended up like that is a story that unfolded over billions of years.
The older, redder stars started forming during the first billion or so years of the universe’s existence. Fashioned from gas, dust, and metals that had been heaved into the cosmos by an even more primordial stellar population, these stars stuck together and forged a primitive Milky Way.
For about three billion years, that proto-Milky Way slowly and quietly made more suns, grabbing onto gas and igniting infant nuclear furnaces. Then, about 10 billion years ago, that growing galaxy ran into a smaller neighbor. Containing maybe 30 percent as many stars as the Milky Way, the dwarf galaxy ended up being consumed by its larger opponent.
Today, the lost galaxy’s blue stars are scattered throughout the Milky Way's halo, but they carry a distinct chemical signature, a fingerprint that tells astronomers they formed in a different region of space seeded with different amounts of metals.
The blue stars also move differently, an observation that led the University of Groningen’s Amina Helmi and her colleagues to tell a similar story about the merger last year, and bestow the name Gaia-Enceladus upon the now-vanished galaxy.
Nitya Kallivayalil, an astronomer at the University of Virginia who studies the Milky Way’s interactions, says the work the team has done provides important independent evidence for that merger with Gaia-Enceladus, a collision that forever changed our galaxy’s shape.
The merger flung those ancient, red stars from the galactic disk into the halo, where they still exist today; we can even see some of these stellar elders from Earth with a backyard telescope. As well, the collision compressed and poured gas into the freshly merged galaxy, temporarily fueling much more rapid star formation within the galaxy’s disk.
Whether this was the first major collision the galaxy experienced is still unanswered, but it’s clear the event was substantial.
“This would be the most massive merger the Milky Way has experienced—and if not the earliest one, one of the earliest ones,” Gallart says. “We could produce an actual picture of this because we know the ages when all of this happened, and then can imagine the ordering.”