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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Cosmic dawn is the early-universe era when the first stars and galaxies formed, ending the long, dark interval after the universe became transparent. Gravity drew matter into structures, gas collected and formed stars, and the stars’ ultraviolet light began ionizing the surrounding hydrogen. NASA places cosmic dawn approximately 50 million to one billion years after the Big Bang, but astronomers do not know the exact moment the first stars appeared.
What happened during cosmic dawn?
After the Big Bang, the universe cooled enough for electrons and protons to combine into neutral atoms. Light could then travel freely through space. But there were not yet stars to shine, so this stretch is known as the cosmic dark ages. Cosmic dawn began when the first stars and galaxies emerged and filled space with light.
NASA uses an approximate span of 50 million to one billion years after the Big Bang for cosmic dawn. This is a broad period, not a precisely dated event: the first stars’ timing and formation process are still unknown. The oldest known galaxies existed less than 300 million years after the Big Bang, according to NASA’s Early Universe overview.
How did the first galaxies form?
Gravity amplified small concentrations of matter in the early universe. Gas gathered in these structures, cooled, and formed stars; collections of stars and gas became the earliest galaxies. Dark matter is part of this formation framework, helping provide the gravitational structure in which ordinary matter assembled.
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Those first galaxies did not appear fully formed or remain unchanged. Stars produced heavier elements and energetic radiation as they evolved. Gas continued to collect, new stars formed, and galaxies grew and changed. The relative importance of processes such as gas accretion, mergers, and episodes of rapid star formation remains an active research question.
How do astronomers observe galaxies from cosmic dawn?
Light from distant galaxies is stretched to longer wavelengths as the universe expands. Ultraviolet and visible light emitted by early galaxies can therefore reach us as infrared light. NASA’s Early Universe overview explains why the James Webb Space Telescope (JWST), built for infrared observations, can detect this ancient light.
Images help astronomers find possible distant galaxies. They then use spectroscopy to examine a target’s light for spectral features that can confirm its redshift—the amount by which its light has been stretched. A candidate identified from imaging is not equivalent to a spectroscopically confirmed distance.
JADES-GS-z13-1: a confirmed example
Webb imaging identified JADES-GS-z13-1 as a promising very distant galaxy, and follow-up NIRSpec spectroscopy confirmed a redshift of 13.0. The resulting view shows the galaxy as it was about 330 million years after the Big Bang, according to ESA/Webb.
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ESA/Webb also reports unusually strong Lyman-alpha emission from hydrogen in this galaxy. Neutral hydrogen absorbs or scatters this signal, so its presence raises questions about how ionized the galaxy’s surroundings were and how reionization was progressing. The observation is established; its explanation is not settled.
How did the first galaxies change the universe?
Ultraviolet radiation from the first stars and galaxies ionized neutral hydrogen in nearby space. As ionized regions expanded and overlapped, the universe became more transparent to light. This process is called reionization. NASA describes it as extending from the end of the dark ages toward roughly the universe’s first billion years.
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Webb observations indicate that small galaxies helped clear surrounding regions near the end of reionization. NASA reports regions extending to about 2 million light-years in radius in the results described on its Webb Science: Galaxies Through Time page. That figure describes those observed regions; it is not a fixed size for every galaxy’s ionized bubble. The timing, sources, and progression of reionization remain under investigation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What has Webb revealed—and what remains uncertain?
JWST is finding early galaxies that are brighter and more numerous than astronomers anticipated. Its infrared observations also help researchers study dust, star formation, and galaxy growth. NASA notes that these results have raised new questions but have not contradicted current best models; see Webb Science: Galaxies Through Time.
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One example of the distinction between a field result and a universe-wide count comes from NASA’s EIGER team. Combining Webb NIRCam imaging with slitless spectroscopy, the team reported identifying 117 galaxies in its first field—more than it had expected. That is the reported count for that field, not a total count of early galaxies; see NASA’s Webb Proves Galaxies Transformed the Early Universe.
Researchers are still investigating why some early galaxies appear unexpectedly bright; how quickly stars and heavier elements formed; how dust accumulated; which sources supplied enough ionizing ultraviolet radiation; and how early black holes formed and affected their host galaxies. A 2025 review describes how combining JWST and ALMA observations helps examine stars alongside gas, dust, kinematics, morphology, possible active galactic nuclei, and star formation. It also identifies improved angular resolution and sensitivity as important to future progress: The early Universe with JWST and ALMA.
- Candidate versus confirmation: imaging can suggest a high-redshift object; spectroscopy can confirm its redshift.
- Observation versus explanation: a measured spectral feature does not by itself establish the physical cause.
- Infrared versus other views: JWST detects ancient infrared-shifted light, while ALMA provides complementary observations of gas and dust.
As NASA-quoted astronomer Steven Finkelstein of the University of Texas at Austin put it, Webb allows scientists to “accurately measure” distant black holes and galaxies, building on what had largely been theoretical research. The statement appears on NASA’s Webb Science: Galaxies Through Time page.
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