If stars are primarily made of hydrogen, which is then burned to helium, and then on and on and on down the chain until you either reach iron, or in extreme cases much heavier metals, this then implies that as time goes on, more and more heavy elements will disperse into the universe.

Now, the universe is huge, and there's still a lot of hydrogen left to make stars out of. That being said, in denser star-forming regions, much of this gas is available for recycling as the remnants of old stars get turned into new ones.

Doesn't this all this imply that as time goes on, more and more heavy metals will be included within young stars from the start?

It may be a very small percentile, sure, but it I assume it would increase slowly as the generations go by.

Furthermore, I assume many rocks get swallowed up by a star during the formation of the planetary disc, which likely adds to the impurity.

Does any of this have any adverse effect on the lifespan of the sun? Or are we talking quantities so small in objects so huge in time-spans so large, that even a dip of a hundred years is completely meaningless?

  • 6
    $\begingroup$ Metallicity of a star is a real thing. And for protostar nebula too. Chemical composition enters in the dynamic and balance of both. $\endgroup$
    – Alchimista
    Jan 22, 2021 at 11:28

2 Answers 2


The chemical enrichment of the Universe over time is indeed a thing. The plot below (source) shows observational measurements of the cosmic density of ionised Carbon in the Universe against redshift (higher redshift -> further back in time). The abundance of other heavy elements over time shows a similar trend.

CIV cosmic density

Stars are explicitly classified based on their metallicity, and its direct relation to which generation of stars they are part of. This is known as the Baade chemical classification. The first stars to form from primordial gas are confusingly known as Population III. These stars have yet to be observed, but it's hoped that the James Webb Space Telescope may glimpse them. They are theorised to be very massive and short lived, hence the difficulty in detecting them. The next generation, formed from the enriched material from this first generation of stars are known as Population II. And the subsequent generation, of which our own Sun is a member, are Population I stars. Population I stars have the highest metallicity, due to the fact they formed from material that has already participated in Population III and II star formation.

The metallicity of a star is almost entirely determined by the metallicity of its birth cloud. Stars do swallow up small amounts of material over their lifetimes, but this does not significantly affect the overall metallicity of a star. A stars lifetime is mostly set by it's mass, and whilst metallicity does have a small effect, it doesn't change significantly due to outside effects over a stars lifetime.

  • 13
    $\begingroup$ This is again a nice example of where the astrophysical terms are literally backward due to historical reasons. Nicely explained. $\endgroup$ Jan 22, 2021 at 12:28
  • 5
    $\begingroup$ @planetmaker Still doesn't hold a candle to the negative charge of the electron. $\endgroup$
    – DKNguyen
    Jan 23, 2021 at 3:12
  • 4
    $\begingroup$ Are Population 0 stars or lower possible, and have we witnessed any, or do we think we have? Also, how would you name this category, given that there is no 0 in Roman numerals? $\endgroup$
    – trysis
    Jan 23, 2021 at 20:42
  • 1
    $\begingroup$ @trysis Population 0 stars can be observed by orbiting a black hole and watching the universe end. $\endgroup$ Jan 24, 2021 at 1:50

Yes, although astrophysics does not talk about "purity". The specific term you're looking for is metallicity. In astrophysics, "metals" are all elements heavier than hydrogen and helium, and the metallicity of a star does in fact play a major role in its evolution. It is also on the basis of metallicity that stars are divided into three populations, called I, II, and III, which also correspond to their age, although for historical reasons, Population I stars are the youngest and Population III are the oldest.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .