Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

The formation of the first low-mass stars from gas with low carbon and oxygen abundances


The first stars in the Universe are predicted to have been much more massive than the Sun1,2,3. Gravitational condensation, accompanied by cooling of the primordial gas via molecular hydrogen, yields a minimum fragmentation scale of a few hundred solar masses. Numerical simulations indicate that once a gas clump acquires this mass it undergoes a slow, quasi-hydrostatic contraction without further fragmentation1,2; lower-mass stars cannot form. Here we show that as soon as the primordial gas—left over from the Big Bang—is enriched by elements ejected from supernovae to a carbon or oxygen abundance as small as 0.01–0.1 per cent of that found in the Sun, cooling by singly ionized carbon or neutral oxygen can lead to the formation of low-mass stars by allowing cloud fragmentation to smaller clumps. This mechanism naturally accommodates the recent discovery4 of solar-mass stars with unusually low iron abundances (10-5.3 solar) but with relatively high (10-1.3 solar) carbon abundance. The critical abundances that we derive can be used to identify those metal-poor stars in our Galaxy with elemental patterns imprinted by the first supernovae. We also find that the minimum stellar mass at early epochs is partially regulated by the temperature of the cosmic microwave background.

Your institute does not have access to this article

Relevant articles

Open Access articles citing this article.

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Required carbon and oxygen abundances (relative to solar values) for cooling a clump of metal-poor gas faster than its free-fall (compressional heating) time.
Figure 2: Observed abundances in low-metallicity Galactic halo stars.


  1. Abel, T., Bryan, G. & Norman, M. The formation of the first star in the universe. Science 295, 93–98 (2002)

    ADS  CAS  Article  Google Scholar 

  2. Bromm, V., Coppi, P. S. & Larson, R. B. The formation of the first stars. I. The primordial star-forming cloud. Astrophys. J. 564, 23–51 (2002)

    ADS  CAS  Article  Google Scholar 

  3. Nakamura, F. & Umemura, M. The stellar initial mass function in primordial galaxies. Astrophys. J. 569, 549–557 (2002)

    ADS  CAS  Article  Google Scholar 

  4. Christlieb, N. et al. A stellar relic from the early Milky Way. Nature 419, 904–906 (2002)

    ADS  CAS  Article  Google Scholar 

  5. McKee, C. F. & Tan, J. C. Massive star formation in 100,000 years from turbulent and pressurized molecular clouds. Nature 416, 59–61 (2002)

    ADS  CAS  Article  Google Scholar 

  6. Clarke, C. J. & Bromm, V. The characteristic stellar mass as a function of redshift. Mon. Not. R. Astron. Soc. 343, 1224–1230 (2003)

    ADS  CAS  Article  Google Scholar 

  7. Omukai, K. Protostellar collapse with various metallicities. Astrophys. J. 534, 809–824 (2000)

    ADS  CAS  Article  Google Scholar 

  8. Bromm, V., Ferrara, A., Coppi, P. S. & Larson, R. B. The fragmentation of pre-enriched primordial objects. Mon. Not. R. Astron Soc. 328, 969–976 (2001)

    ADS  Article  Google Scholar 

  9. Schneider, R., Ferrara, A., Natarajan, P. & Omukai, K. First stars, very massive black holes, and metals. Astrophys. J. 571, 30–39 (2002)

    ADS  CAS  Article  Google Scholar 

  10. Mackey, J., Bromm, V. & Hernquist, L. Three epochs of star formation in the high-redshift universe. Astrophys. J. 586, 1–11 (2003)

    ADS  CAS  Article  Google Scholar 

  11. Schneider, R., Ferrara, A., Salvaterra, R., Omukai, K. & Bromm, V. Low-mass relics of early star formation. Nature 422, 869–871 (2003)

    ADS  CAS  Article  Google Scholar 

  12. Heger, A. & Woosley, S. E. The nucleosynthetic signature of population III. Astrophys. J. 567, 532–543 (2002)

    ADS  CAS  Article  Google Scholar 

  13. Qian, Y.-Z., Sargent, W. L. W. & Wasserburg, G. J. The prompt inventory from very massive stars and elemental abundances in Lyα systems. Astrophys. J. 569, L61–L64 (2002)

    ADS  CAS  Article  Google Scholar 

  14. Qian, Y.-Z. & Wasserburg, G. J. Determination of nucleosynthetic yields of supernovae and very massive stars from abundances in metal-poor stars. Astrophys. J. 567, 515–531 (2002)

    ADS  CAS  Article  Google Scholar 

  15. Umeda, H. & Nomoto, K. Nucleosynthesis of zinc and iron peak elements in population III type II supernovae. Astrophys. J. 565, 385–404 (2002)

    ADS  CAS  Article  Google Scholar 

  16. Sneden, C. & Cowan, J. J. Genesis of the heaviest elements in the Milky Way Galaxy. Science 299, 70–75 (2003)

    ADS  CAS  Article  Google Scholar 

  17. Umeda, H. & Nomoto, K. First-generation black-hole-forming supernovae and the metal abundance pattern of a very iron-poor star. Nature 422, 871–873 (2003)

    ADS  CAS  Article  Google Scholar 

  18. Hollenbach, D. & McKee, C. F. Molecule formation and infrared emission in fast interstellar shocks. Astrophys. J. 342, 306–336 (1989)

    ADS  CAS  Article  Google Scholar 

  19. Tumlinson, J. & Shull, J. M. Zero-metallicity stars and the effects of the first stars on reionization. Astrophys. J. 528, L65–L68 (2000)

    ADS  CAS  Article  Google Scholar 

  20. Bromm, V., Kudritzki, R. P. & Loeb, A. Generic spectrum and ionization efficiency of a heavy initial mass function for the first stars. Astrophys. J. 552, 464–472 (2001)

    ADS  CAS  Article  Google Scholar 

  21. Cen, R. The implications of Wilkinson Microwave Anisotropy Probe observations for population III star formation processes. Astrophys. J. 591, L5–L8 (2003)

    ADS  Article  Google Scholar 

  22. Sokasian, A., Yoshida, N., Abel, T., Hernquist, L. & Springel, V. Cosmic reionisation by stellar sources: Population III stars. Mon. Not. R. Astron. Soc. (submitted); preprint at 〈〉 (2003)

  23. Wyithe, J. S. B. & Loeb, A. Was the universe reionized by massive metal-free stars? Astrophys. J. 588, L69–L72 (2003)

    ADS  Article  Google Scholar 

  24. Kogut, A. et al. Wilkinson Microwave Anisotropy Probe WMAP first year observations: TE polarization. Astrophys. J. Suppl. 148, 161–173 (2003)

    ADS  Article  Google Scholar 

  25. Bromm, V. & Loeb, A. Formation of the first supermassive black holes. Astrophys. J. 596, 34–46 (2003)

    ADS  Article  Google Scholar 

  26. Larson, R. B. Early star formation and the evolution of the stellar initial mass function in galaxies. Mon. Not. R. Astron. Soc. 301, 569–581 (1998)

    ADS  Article  Google Scholar 

  27. Rees, M. J. Opacity-limited hierarchical fragmentation and the masses of protostars. Mon. Not. R. Astron. Soc. 176, 483–486 (1976)

    ADS  Article  Google Scholar 

  28. Bromm, V., Yoshida, N. & Hernquist, L. The first supernova explosions in the universe. Astrophys. J. (in the press); preprint at 〈〉 (2003)

  29. Akerman, C. J., Carigi, L., Nissen, P. E., Pettini, M. & Asplund, M. The evolution of the C/O ratio in metal-poor halo stars. Astron. Astrophys. (submitted)

  30. Cayrel, R. et al. Abundance patterns and supernova yields in the early Galaxy from C to Zn. Astron. Astrophys. (submitted)

Download references


We thank R. Barkana and particularly T. Beers for discussions, and are grateful to T. Beers and M. Pettini for making their data available prior to publication. This work was supported in part by NSF, NASA and the Guggenheim foundation (for A.L.).

Author information

Authors and Affiliations


Corresponding authors

Correspondence to Volker Bromm or Abraham Loeb.

Ethics declarations

Competing interests

The authors declare that they have no competing financial interests.

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Bromm, V., Loeb, A. The formation of the first low-mass stars from gas with low carbon and oxygen abundances. Nature 425, 812–814 (2003).

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI:

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing