Have Astronomers Finally Found Population III Stars?

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For about 70 years, astronomers have divided stars into two populations, Population I and Population II. The two stellar populations differ in several ways, but the most important difference is composition. Stars primarily consist of hydrogen and helium—the two lightest elements—with just a small amount of the heavier elements. For convenience, astronomers term the heavier elements metals, though admittedly these metals include some non-metals, such as carbon and oxygen. Population II stars contain far fewer metals than Population I stars. While the metal composition of Population I stars can be few percent by mass, Population II metal composition can be thousands of times less. Astronomers generally explain the composition differences between the two stellar populations as a difference in age. In a big bang origin of the universe, the universe began with only hydrogen and helium and a trifling amount of lithium. Where did metals come from? Stars derive their energy from nuclear fusion, the synthesis of heavier elements from lighter elements. The first and most important nuclear reaction is the fusion of hydrogen into helium, but later reactions can fuse helium into progressively heavier elements. Stars can release the products of this nucleosynthesis through winds or eruptions of supernovae. This processed material then supposedly mixes with gas already in space that in turn forms into new stars, thus repeating the process. According to this theory, stars that form later accumulate ever-increasing amounts of metals. Therefore, Population II (metal poor) stars are older, and Population I (metal rich) stars are younger. However, there is a problem with this understanding. Extreme Population II stars—those stars deemed to be the oldest—contain some metals. Because the big bang model does not produce metals, the very first generation of stars ought not to have contained any metals. Therefore, astronomers hypothesize that the very first generation of stars had no metals. They call these hypothetical stars Population III. There have been extensive searches for Population III stars over the past 40 years, but astronomers have not yet found any of them. There are several theories to explain the absence of Population III stars. One suggestion is that all of the original Population III stars were very massive. Very massive stars have short lifetimes, so if the first generation of stars were very massive, then none of them would be left today. Another suggestion is the Population III stars have been contaminated by processed material ejected from other stars. A recent report suggests that astronomers may have found Population III stars in a very distant galaxy. The galaxy CR71 appears to be at a distance of 12.9 billion light years. If the universe is nearly 13.8 billion years old, as is currently thought within the big bang paradigm, then the light that we are now receiving from this galaxy originated when the universe was not much more than 800 million years old. According to evolutionary cosmic theories, this is near the epoch when the first stars were forming, so one might expect that many of the stars in Galaxy CR7 might be true Population III stars that have not been contaminated yet.

However, confirmation of the status of stars in Galaxy CR7 is fraught with problems. The only direct confirmation that Population III stars are present would be to show that the stars in question have no metals. This requires spectroscopy, the breaking up of light according to wavelength (or color). Different elements emit or absorb light at specific wavelengths, so spectroscopy can unambiguously reveal whether an element is present. The true absence of metals can be established only with a very bright spectrum. CR7 is an incredibly bright galaxy, but it is so far away that its light that we receive from it is feeble, too feeble for the spectroscopy that is necessary for direct confirmation of population III stars. The team of researchers took an indirect route of considering at least three other interpretations for their data and then offering reasons why none of those three interpretations were likely. The authors of the paper are convinced of what they have found, although they used cautionary terms. Given the tentative nature of this study, it is not at all clear that astronomers have at long last found evidence that Population III stars once existed.

Whether or not astronomers amass clear evidence for Population III stars matters little to the creation model. However, the evolutionary model requires that Population III stars exist or once existed in abundance. This is why this paper is so noteworthy, though it is also very tentative. Even if confirmed, discovering Population III stars in Galaxy CR7 does not solve all of the problems. There are many other galaxies and quasars at a similar distance (and hence viewed at a similar very early epoch in a big bang universe) as Galaxy CR7 that show abundance of metals. For instance, many of them show clear evidence of dust, and dust necessarily is made of metals. Dust requires at least two generations of stars: at least one generation to produce the heavier elements and at least one generation to manufacture the dust particles from the metals. Where did the metals in these many other distant objects come from?


  1. The name CR7 is short for Cosmos Redshift 7, but there is a bit of word play here in that discoverers of the galaxy are fans of the Portuguese soccer player, Cristiano Ronaldo, who is popularly known as CR7.

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