Imagine the 100 billion stars of the Milky Way as a flat, calm lake of water. Now, imagine someone throwing A stone the size of 400 million suns in this water.
Calm shattered. Wave after wave of energy ripples across the galactic surface, rippling and bouncing its stars in a chaotic dance that takes centuries to settle down.
Astronomers think this may have already happened — not just once, but many times over the past billion years.
In new research published September 15 in the Monthly Notices of the Royal Astronomical Society, researchers explain how a small nearby galaxy – the Sagittarius dwarf galaxy – appears to have crashed through the Milky Way on at least two separate occasions, causing all the stars around The galaxy mysteriously swings at different speeds.
Using data from the European Space Agency’s Gaia space observatory, the researchers compared the motions of more than 20 million stars located throughout the Milky Way, but especially in the outer regions of the galactic disk. The data revealed a mysterious ripple, or vibration, that appears to shake stars across the galaxy.
“We can see that these stars are oscillating and moving up and down at different speeds,” study author Paul Macmillan, an astronomer at Lund University in Sweden, said in a translated statement.
Through a process the researchers likened to “galactic seismology,” the team designed a wave model that could explain the strange ripple effect that regulates the stars of the Milky Way. They concluded that the ripples were likely released hundreds of millions of years ago, when the Sagittarius dwarf galaxy last passed through our galaxy — “like a stone falling into a pond,” MacMillan said. It’s possible, the researchers added, that a second collision, even before that, occurred between the two galaxies.
Previous studies (opens in new tab) have suggested that an ancient collision with Sagittarius may have caused ripples in the galactic center, but this new research is the first to show that these ripples extend to the edge of the galactic disk, causing stars to vibrate with every step. The researchers write that this new research should help piece together the long and violent history of our galaxy and its smaller neighbour.
Today, the Sagittarius dwarf galaxy is estimated to have about 400 times the mass of Earth’s sun – a…a grain compared to the Milky Way’s estimated mass of 1.5 trillion suns. Scientists believe that Sagittarius was much larger, but it has lost up to 20% of its mass in our galaxy after repeated collisions over the past billion years.
It’s possible that these collisions changed the shape and size of our galaxy as well. A 2011 study suggested that the spiral arm of the Milky Way is the result of two collisions with the Sagittarius dwarf galaxy. Another study of Gaia data released in 2020 showed that cosmic collisions between our galaxy and Sagittarius caused new stars to explode in the Milky Way every time the two galaxies meet.
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