The Moon is believed to have formed about 4.6 billion years ago, with the rocks from mare regions sampled by the Apollo astronauts indicating a formation age for these features of about 4.1 billion years. The Earth is dated at 4.7 billion years ago by radioactive dating techniques. The Sun's formation probably continued through the era of planetary formation and ended when it became a T-Tauri star. Its accretion from the rotating core of the proto-solar disk probably began just before 4.7 billion years, and ended around 4.2 billion years.