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Starts With A Bang

Astronomers’ Hopes For Pristine Stars Dashed: They’re Polluted After All

In 2015, the ultra-distant galaxy CR7 was measured to have hydrogen and helium, but no carbon or oxygen. With new ALMA observations, there’s carbon after all.

As brilliant as our Universe’s stars are today, they weren’t the first to arise in time and space.

The visible light spectrum of the Sun, which helps us understand not only its temperature and ionization, but the abundances of the elements present. The long, thick lines are hydrogen and helium, but every other line is from a heavy element that must have been created in a previous-generation star. (Nigel Sharp, NOAO / National Solar Observatory at Kitt Peak / AURA / NSF)

A variety of heavy elements are found inside every star, star cluster, or galaxy ever observed.

Supernova remnants (L) and planetary nebulae (R) are both ways for stars to recycle their burned, heavy elements back into the interstellar medium and the next generation of stars and planets. The truly first, pristine stars need to have been created before supernovae, planetary nebulae, or neutron star mergers polluted the interstellar medium with heavy elements. (ESO / Very Large Telescope / FORS instrument & team (L); NASA, ESA, C.R. O’Dell (Vanderbilt), and D. Thompson (Large Binocular Telescope) (R))

The only way to create elements heavier than helium is through nuclear fusion, requiring the existence of previous generations of stars.

The predicted abundances of helium-4, deuterium, helium-3 and lithium-7 as predicted by Big Bang Nucleosynthesis, with observations shown in the red circles. The first stars in the Universe should have exactly these abundances of the light elements, with nothing heavier. (NASA / WMAP Science Team)

Yet according to the predictions of the Big Bang, the first stars should have been made of pristine material.

A rich nebula of gas, pushed out into the interstellar medium by the hot, new stars formed in the central region. When clouds of gas collapse, they form new stars, but we have yet to find such stars that are made of hydrogen and helium without also being made of carbon and oxygen. (Gemini Observatory / AURA)

Over time, gravitation should pull this gas — made of hydrogen-and-helium only — together, forming unpolluted populations of stars.

The absorption spectra of different populations of gas (L) allow us to derive the relative abundances of elements and isotopes (center). In 2011, two distant gas clouds containing no heavy elements and a pristine deuterium-to-hydrogen ratio (R) were discovered for the first time.(Michele Fumagalli, John M. O’Meara, and J. Xavier Prochaska, via

In 2011, we found the first evidence for unpolluted, pristine gas, but it hadn’t yet collapsed to form stars.

A true image of the galaxy COSMOS Redshift 7, as taken in rest-frame ultraviolet light by the Hubble Space Telescope.(D. Sobral et al. (2015), via

But even bigger news came in 2015, when the galaxy COSMOS Redshift 7 (CR7) was discovered.

When the light from CR7 was broken up into its spectral components, lines corresponding to helium (L) were found, but no evidence of carbon (R) was there, nor (not shown) were the expected nitrogen or oxygen lines. If there were any heavy elements, they were either far fewer in abundance or far lower in ionization than expected.(D. Sobral et al. (2015), via

From 13 billion years ago, helium lines were observed, without any carbon or oxygen lines.

An illustration of the galaxy CR7, which may house multiple populations of stars of various ages (as illustrated), but the hope was that the brightest component was pristine, and had no heavy elements. (M. Kornmesser / ESO)

The hope was that CR7 contained stars made of hydrogen and helium alone.

The Atacama Large Millimeter submillimeter Array (ALMA) are some of the most powerful radio telescopes on Earth. These telescopes can measure long-wavelength signatures of atoms, molecules, and ions that are inaccessible to shorter-wavelength telescopes like Hubble. (ESO/C. Malin)

But recent observations from ALMA have crushed those hopes.

The contours around CR7, which itself has at least 3 components (a major galaxy and two smaller, satellite galaxies that are merging), show that a less-ionized form of carbon than was previously probed exists, and exists in great abundance, in the gas and dust surrounding and penetrating this young, growing galaxy. (J. Matthee et al 2017 ApJ 851 145)

By looking at the dust around the stars, they’ve found carbon after all, and it’s everywhere.

Schematic diagram of the Universe’s history, highlighting reionization. Before stars or galaxies formed, the Universe was full of light-blocking, pristine, neutral atoms. While most of the Universe doesn’t become reionized until 550 million years afterwards, a few fortunate regions are mostly reionized at much earlier times, and some regions may remain pristine for even longer. (S. G. Djorgovski et al., Caltech Digital Media Center)

It’s an accreting, growing galaxy, but it’s not pristine.

We must look elsewhere to find the Universe’s first stars.

Mostly Mute Monday tells the scientific story of a picture, object, or phenomenon in the Universe in images, visuals, and no more than 200 words. Talk less, smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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