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Star formation history of our universe

If we attempt to quantify star-formation history of our Universe then a fascinating figure crops up. 2.21 sextillion stars have formed in the course of the history of our Universe, at least, within the presently observable part of the universe. And of course, that figure is for today: 13.8 billion years post the Big Bang. But point to be noted here is that those stars didn’t form at uniform rate throughout cosmic time. Result of the varying rate of star formation was that only 10% of the current number of stars had formed when our universe was 2.2 billion years old but when universe was 3.3 billion years old 25% of stars had formed. Details of quantified star-formation history of our universe is following:    

  • 98% of the stars in our universe had formed by the time our universe was 12.9 billion years old,
  • 75% by the time it was 7.3 billion years old,
  • 50% by the time it was 4.9 billion years old,
  • 25% by the time it was 3.3 billion years old,
  • 10% by the time it was 2.2 billion years old,
  • 5% by the time it was 1.7 billion years old,
  • 1% by the time it was 1.0 billion years old,
  • 1% by the time it was 500 million years old,
  • and only 0.01% at about ~200 million years post the big bang.

Today, the star-formation rate is a pale shadow of what it once was. According to the most comprehensive studies ever carried out, the star-formation rate has dropped by a whopping 97% since it reached its epitome, 10-to-11 billion years ago. What’s intriguing about our star-formation history is that the greatest uncertainties about it are observed at the initial times: within the first 1.0 billion years. Only around ~1% of all stars formed within that first 1.0 billion year epoch of cosmic past of our universe. The maximum numbers of stars formed when the Universe was about 1.5-to-8 billion years old, and although the star-formation rate has been declining for more than 10 billion years, over the most recent ~5 billion years the decline has accelerated very severely. It’s very much possible that over 95% of the total stars that will ever form have already been created.

Till the gas in the Universe lasts and gravitation is still a thing, there will be prospects of new stars formation. Although the star-formation rate has fallen since the Universe’s early days, it’s not going to drop off to zero until the Universe is many thousands of times its present age. New stars will keep forming for trillions upon trillions of years. But definitely, new stars are much more of a rarity now than they have been any time in our past.


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