Which came first: stars or galaxies?

After the Big Bang, did stars form and merge into galaxies, or did the first galaxies develop along with the first stars?

The NASA/ESA Hubble Space Telescope has produced an incredibly detailed image of a pair of overlapping galaxies called NGC 3314. While the two galaxies look as if they are in the midst of a collision, this is in fact a trick of perspective: the two are in chance alignment from our vantage point.
Published: February 3, 2022 at 10:46 am
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Astronomers have managed to piece together pretty confidently the story of what occurred after the Big Bang, the point at which the Universe was 'born'.

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We know about the time when the Universe was transformed from a hot, dense primordial soup of ionised gas to the time when the first electrons and protons combined to form the first neutral atoms.

The Universe cooled, the first stars formed and spread light across the Universe.

We know much about what happened during the first few seconds of the Universe, and how the first stars affected the evolution of the Universe.

A region near the centre of the Milky Way galaxy covering 0.5 by 0.25 degrees, which is about twice as wide as the full Moon in the night sky. This image contains over 180,000 stars and shows a region of our galaxy about 220x100 lightyears across. It was captured by the Dark Energy Camera on the Víctor M. Blanco 4-meter Telescope. Credit: CTIO/NOIRLab/DOE/NSF/AURA/STScI, W. Clarkson (UM-Dearborn), C. Johnson (STScI), and M. Rich (UCLA)
A region near the centre of the Milky Way galaxy covering 0.5 by 0.25 degrees. This image contains over 180,000 stars and shows a region of our galaxy about 220x100 lightyears across. Credit: CTIO/NOIRLab/DOE/NSF/AURA/STScI, W. Clarkson (UM-Dearborn), C. Johnson (STScI), and M. Rich (UCLA)

Today we know that of stars, black holes, cosmic dust - and the mysteries substance known as dark matter - make up massive structures known as galaxies.

Some galaxies are spiral-shaped, some are irregular, some are elliptical.

But which came first? After the Big Bang, did stars form first and then gather into galaxies, or did the first galaxies form with stars in place?

It’s something of a chicken-and-egg question, but actually, neither of these things happened. That is if, by galaxies, you mean galaxies as we know them today.

Ram pressure stripping in barred spiral galaxy NGC 4921 HUBBLE SPACE TELESCOPE/ALMA, 29 July 2021 IMAGE CREDIT: ALMA (ESO/NAOJ/NRAO)/S. Dagnello (NRAO), NASA/ESA/Hubble/K. Cook (LLNL), L. Shatz
Barred spiral galaxy NGC 4921, Hubble Space Telescope/ALMA, 29 July 2021. Credit: ALMA (ESO/NAOJ/NRAO)/S. Dagnello (NRAO), NASA/ESA/Hubble/K. Cook (LLNL), L. Shatz

According to the best current estimates, the first objects began to congeal out of the cooling gas of the Big Bang fireball about 200 million years after the beginning of the Universe.

These objects had to be so massive that their self-gravity was strong enough to crush them against the force of the hot gas pushing outwards.

Shortly after the Big Bang it took the gravity of a relatively large mass to shrink a gas cloud because the gas of the Big Bang was so hot.

The Hubble eXtreme Deep Field (XDF) was created by using 10 years of Hubble Space Telescope images and reveals galaxies spanning 13.2 billion years in time: about 0.6 years after the birth of the Universe. The deep field captures such distant light, most of the galaxies are seen as they existed in their infancy. Credit: NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team
The Hubble eXtreme Deep Field (XDF) reveals galaxies spanning 13.2 billion years in time: about 0.6 years after the birth of the Universe. Credit: NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team

Consequently, the first objects in the Universe – dubbed ‘mini-haloes’ – had the mass of roughly a million suns, only about 0.001% of the mass of a big present-day galaxy.

It was in these objects – galactic fragments, if you like – that the first so-called Population III stars are believed to have formed.

Over time, the mini-haloes merged and merged again, eventually making big galaxies like the Milky Way and Andromeda that populate today’s Universe.

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This article originally appeared in the March 2009 issue of BBC Sky at Night Magazine.

Authors

Marcus Chown is an award-winning writer and broadcaster and a former radio astronomer at the California Institute of Technology in Pasadena. He is the author of Breakthrough: Spectacular stories of scientific discovery from the Higgs particle to black holes (Faber & Faber, 2021).

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