Time and gravity have intermingled our galaxy’s many stellar generations, scattering its oldest stars across the galactic disc. ” The stars that made up the ancient, proto-galaxy that became the Milky Way were mostly pristine-but during the last 13 billion years, the Milky Way has merged with many of its neighbors, and it has eaten scores of smaller satellite galaxies. The oldest are composed almost entirely of hydrogen.
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Older generations of stars have less metal. With any luck, not all of them will be human. In that time, many astronomers may lay eyes on it. The star may live for another 10 billion years. When astronomers run sunlight through their special prisms, they find all kinds of fine lines. The gas clouds that birthed young stars like our sun were dusty and metal-rich enough to form rocky planets. The elements that make up the universe became ever heavier. The metallic shrapnel from these explosions seeded the hydrogen clouds that spawned succeeding generation of stars, which made still more metals in their cores. In the cores of these stars, atoms fused together, making metals that exploded out into the universe when they died. Let me explain: Shortly after the Big Bang, the universe was a sea of hydrogen gas, cold clouds of which condensed to form the first stars. These fine lines reveal a star’s metallicity-and generally, the more metallic the star, the newer it is. If they spot a suspect-looking star, they can run its light through a special prism, and look for fine lines in the resulting rainbow.