The Early Merger that Made the Galaxy's Stellar Halo

Understand

The last two years have seen widespread acceptance of the idea that the Milky Way halo was largely created in an early (8-10 Gyr ago) and massive ($> 10^{10} M_\odot$) merger.

• The roots of this idea pre-date the Gaia mission, but the exquisite proper motions available from Gaia have made the hypothesis irresistible.
• We trace the history of this idea, reviewing the series of papers that led to our current understanding.