December 1, 2004: Scientists using NASA's Hubble Space Telescope have measured the age of what may be the youngest galaxy ever seen in the universe. By cosmological standards it is a mere toddler seemingly out of place among the grown-up galaxies around it. Called I Zwicky 18, it may be as young as 500 million years old (so recent an epoch that complex life had already begun to appear on Earth). Our Milky Way galaxy by contrast is over 20 times older, or about 12 billion years old, the typical age of galaxies across the universe. This "late-life" galaxy offers a rare glimpse into what the first diminutive galaxies in the early universe look like.See the rest:
The universe is about 14 billion years old. So, when you compare the galaxy with the age of the universe, it is a relative youngster. In fact, if you could compress the evolution of the universe into a 24-hour day, most of the galaxies started forming at 1 a.m. and were fully grown before daybreak. The galaxy I Zwicky 18 did not start forming until 11 p.m. in the evening!
Astronomers know the galaxy is a youngster because they don't see any nomal stars older than 500 million years. Even more conservative estimates nudge the oldest stars to 1 billion years. The galaxy has gone though several sudden bursts of star formation, the latest only 4 million years ago. Also, measuremants show that the galaxy has a paucity of heaver elements beyond helium. This means there have been no successive generations of stars to manufacture the heavier elements through nuclear fusion.
This is quite a mystery with no obvious answer. The galaxy is sort of a Rip Van Winkle that has remained quiescent as the rest of the universe evolved around it. A chance close encounter with another dwarf galaxy may have at last jump-started it into making stars. Gravitational perturbations between the galaxies may have caused hydrogen clouds to collapse and precipitate stars.