Respuesta :
Halo stars formed before disk stars and the protogalatic cloud(s) contained essentially no elements besides hydrogen and helium.
Halo stars
The hierarchical growth of huge galaxies like the Milky Way results in the natural by-product of extended star haloes. Our galaxy's star halo should contain traces of past merger events if this process is a significant contributor to the galaxy's expansion. However, it is difficult to reliably identify real halo stars. Aims. We have entered a new phase of galactic astronomy with the launch of the Gaia space telescope. Over two million stars the majority of which are in the solar neighbourhood have their locations, parallaxes, and proper motions listed in the initial Gaia data release. This sample will be expanded to include more than 1.5 billion stars in the second Gaia data release, of which 5 million will be fully phase-space characterized stars.
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