Here’s the finest timeline yet for the Milky Way’s huge occasions

A brand-new analysis of almost a quarter million stars puts company ages on the most memorable pages from our galaxy’s life story.

Far grander than most of its next-doorneighbors, the Milky Way emerged long back, as lower galaxies smashed together. Its thick disk — a pancake-shaped population of old stars — stemmed incredibly quickly after the Big Bang and well inthepast most of the outstanding halo that covers the galaxy’s disk, astronomers report March 23 in Nature.

“We are now able to offer a extremely clear timeline of what tookplace in the earliest time of our Milky Way,” states astronomer Maosheng Xiang.

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He and Hans-Walter Rix, both at the Max Planck Institute for Astronomy in Heidelberg, Germany, studied nearly 250,000 subgiants — stars that are growing bigger and cooler after utilizing up the hydrogen fuel at their . The temperaturelevels and luminosities of these stars expose their ages, letting the scientists track how various dates in galactic history generated stars with various chemical structures and orbits around the Milky Way’s .

“There’s simply an amazing quantity of info here,” states Rosemary Wyse, an astrophysicist at Johns Hopkins University who was not included with the researchstudy. “We actually desire to comprehend how our galaxy came to be the method it is,” she states. “When were the chemical components of which we are made produced?”

Xiang and Rix found that the Milky Way’s thick disk got its start about 13 billion years earlier. That’s simply 800 million years after the universe’s birth. The thick disk, which procedures 6,000 light-years from top to bottom in the sun’s area, kept forming stars for a long time, till about 8 billion years back.

During this duration, the thick disk’s iron material shot up 30-fold as blowingup stars enriched its star-forming gas, the group discovered. At the dawn of the thick disk age, a newborn star had just a tenth as much iron, relative to hydrogen, as the sun; by the end, 5 billion years lateron, a thick disk star was 3 times richer in iron than the sun.

Xiang and Rix likewise discovered a tight relation inbetween a thick disk star’s age and iron material. This suggests gas was completely blended throughout the thick disk: As time went on, newborn stars acquired progressively greater amounts of iron, no matter whether the stars formed close to or far from the galactic .

But that’s not all that was occurring. As other scientists reported in 2018, another galaxy when hit our own, offering the Milky Way most of the stars in its halo, which swallowsup the disk (SN: 11/1/18). Halo stars have little iron.

The brand-new work modifies the date of this excellent stellar encounter: “We discovered that the merger tookplace 11 billion years ago,” Xiang states, a billion years earlier than idea. As the burglar’s gas crashed into the Milky Way’s gas, it activated the development of so numerous brand-new stars that our galaxy’s star development rate reached a record high 11 billion years earlier.

The merger likewise sprinkled some thick disk stars up into the halo, which Xiang and Rix determined from the stars’ greater iron abundances. These “splash” stars, the scientists discovered, are at least 11 billion years old, verifying the date of the merger.

The thick disk ran out of gas 8 billion years ago and stopped making stars. Fresh gas around the Milky Way then settled into a thinner disk, which hasactually provided birth to stars ever because — consistingof the 4.6-billion-year-old sun and most of its outstanding next-doorneighbors. The thin disk is about 2,000 light-years thick in our part of the galaxy.

“The Milky Way hasactually been rather peaceful for the last 8 billion years,” Xiang states, experiencing no more encounters with huge galaxies. That makes it various from most of its peers.

If the thick disk truly existed 13 billion years ago, Xiang states, then the brand-new James Webb Space Telescope (SN: 1/24/22) might determine comparable disks in galaxies 13 billion light-years from Earth — pictures of the Milky Way as a young galaxy.

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