Milky Way's spiral was a late addition

Scientists think they have found evidence that the middle of our galaxy formed separately and at a different time to the spiralling arms in which we reside.

Astronomers using the European Southern Observatory's Very Large Telescope (VLT) noticed that the stars in the galactic bulge, as it is known, have a different chemical composition from stars in the arms of the galaxy.

The galactic bulge is made up of only the very oldest stars in the galaxy, dating back 10bn years. The arms, meanwhile, are populated by stars of all ages.

The chemical makeup of stars gives astronomers clues to their pasts. Stars rich in heavier elements such as oxygen and iron, are probably second or even third generation - that is, they have been stars before.

Massive stars can end their lives in a number of different types of supernova. It takes a type II supernova to produce most oxygen, while while iron is forged in type I-a explosions. Thus, the amounts of each element reveal something about the ancestry of the star.

The astronomers studied fifty giant stars in four regions of the galaxy close to the central bulge. They found the amount of oxygen in disc and bulge stars was significantly different, suggesting that the two portions of the galaxy are "genetically different".

“For the first time, we have clearly established a ‘genetic difference’ between stars in the disc and the bulge of our Galaxy,” said Manuela Zoccali, lead author of the paper published in the journal Astronomy and Astrophysics.

“We infer from this that the bulge must have formed more rapidly than the disc, probably in less than a billion years and when the Universe was still very young.”

The team found that for a given amount of iron, stars in the disc contain less oxygen than their bulgier counterparts. This means that bulge stars formed independently, and did not originate in the disc and then migrate inward to build up the bulge, Zoccali concludes. ®