The Milky Way galaxy holds the Sun and all the planets in the Solar System, as well as a large group of stars, gas, and dust that are bound by gravity. The Milky Way is a large spiral galaxy that hosts all the stars that are seen in the night sky from Earth. In the past, ancient Greeks described the band of light in the night sky as a stream of milk (gala). Hence it is named the Milky Way galaxy.
Although it is hard to count all the stars within the galaxy, Astronomers estimate that it could be made up of approximately 100 billion stars that form a large disk.
The history of the Milky Way galaxy is believed to have started nearly 14 billion years ago when enormous clouds of gas and dust come together due to the force of gravity and later on the two structures became a dense, bright disk from a vast spherical halo. Billions of years later, the Solar System was created inside the galaxy.
However, astronomers have been rewriting the history of the Milky Way galaxy in the past two years. They said that at least one major collision may have happened when the Milky Way was still developing, Wired reported.
Stars in Milky Way Galaxy's Youth
Gaia, a European spacecraft, has released a staggering quantity of information about the cosmos on April 25, 2018. It has years-long data about billions of stars in the galaxy where previous surveys have only managed to map thousands. The information from Gaia has started a new revolution, astronomer Federico Sestito said.
To understand the history of the Milky Way galaxy, astronomers looked at the earliest stars of the galaxy which were composed mainly of hydrogen and helium, the rawest materials.
Astronomers were able to catalog 42 ancient stars that are known as ultra metal-poor stars. Based on the classical story of the Milky Way galaxy, these stars should be swarming throughout the halo, and those stars in the disk should be contaminated with carbon and oxygen.
But the data from Gaia tells another story. Sestito said that most of the earliest stars appeared stuck in the disk which is the youngest region of the Milky Way instead of streaming through the halo as predicted. The researchers did follow-up studies and found that a few hundred of the stars are permanent denizens of the disk, 20% of the earliest stars lie flat in circular sunlike orbits, and stars made up of various metals are moving in flat disk orbits.
But this does not mean that theorists were wrong, they simply lack some piece of the picture, said researcher Tobias Buck.
Milky Way Galaxy Had A Hectic Youth
An analysis from the information brought by Gaia tells the astronomers that the Milky Way galaxy once had a hectic youth with evidence of cataclysmic collisions that left debris in the galaxy. Researchers found that half of the stars in the inner 60,000 light-years of the halo came from this one collision that has boosted the mass of the Milky Way by 10%.
The researchers named the incoming galaxy Gaia-Enceladus, which collided with the Milky Way approximately 10 billion years ago. Study author Paola Di Matteo, an astronomer at the Paris Observatory, said that the two disks in the galaxy might have formed two parts, namely the thin and dark disks, because of the collision by the Gaia-Enceladus.
But aside from that, they also discovered hints of other mergers spotted in bundles of stars, called globular clusters.
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