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Starts With A Bang

First Stars Formed No Later Than 250 Million Years After The Big Bang, With Direct Proof

In the big image at left, the many galaxies of a massive cluster called MACS J1149+2223 dominate the scene. Gravitational lensing by the giant cluster brightened the light from the newfound galaxy, known as MACS 1149-JD, some 15 times. At upper right, a partial zoom-in shows MACS 1149-JD in more detail, and a deeper zoom appears to the lower right. This is correct and consistent with General Relativity, and independent of how we visualize (or whether we visualize) space. (NASA/ESA/STSCI/JHU)

The Universe is an enormous place, but we can’t see all the way back to the beginning. Here’s the latest record-breaker.


No matter how far back we look in the Universe, we cannot yet observe the first stars or galaxies directly.

The absorption lines at a variety of redshifts show that the fundamental physics and sizes of atoms have not changed throughout the Universe, even as the light has redshifted due to its expansion. Unfortunately, the most light-blocking material exists at the earliest times, making finding the most distant galaxies an incredible challenge. (NASA, ESA, AND AND A. FEILD (STSCI))

The light they produce is too redshifted and blocked by too much intervening gas to be seen even by Hubble.

The most distant galaxy ever discovered in the known Universe, GN-z11, has its light come to us from 13.4 billion years ago: when the Universe was only 3% its current age: 407 million years old. But there are even more distant galaxies out there, and we at last have direct evidence for it. (NASA, ESA, AND G. BACON (STSCI))

The most distant galaxy ever discovered is already late, dating back to 407 million years after the Big Bang.

Only because this distant galaxy, GN-z11, is located in a region where the intergalactic medium is mostly reionized, can Hubble reveal it to us at the present time. To see further, we require a better observatory, optimized for these kinds of detection, than Hubble. (NASA, ESA, AND A. FEILD (STSCI))

But the very first stars should go back hundreds of million years further.

Various long-exposure campaigns, like the Hubble eXtreme Deep Field (XDF) shown here, have revealed thousands of galaxies in a volume of the Universe that represents a fraction of a millionth of the sky. But even with all the power of Hubble, and all the magnification of gravitational lensing, there are still galaxies out there beyond what we are capable of seeing. (NASA, ESA, H. TEPLITZ AND M. RAFELSKI (IPAC/CALTECH), A. KOEKEMOER (STSCI), R. WINDHORST (ARIZONA STATE UNIVERSITY), AND Z. LEVAY (STSCI))

Sometime between the Cosmic Microwave Background, at 380,000 years, and that first galaxy, the first stars must have formed.

Schematic diagram of the Universe’s history, highlighting reionization. Before stars or galaxies formed, the Universe was full of light-blocking, 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. (S. G. DJORGOVSKI ET AL., CALTECH DIGITAL MEDIA CENTER)

Owing to the second-most-distant galaxy ever found, MACS1149-JD1, we can understand when.

The distant galaxy MACS1149-JD1 is gravitationally lensed by a foreground cluster, allowing it to be imaged at high resolution and in multiple instruments, even without next-generation technology.(ALMA (ESO/NAOJ/NRAO), NASA/ESA HUBBLE SPACE TELESCOPE, W. ZHENG (JHU), M. POSTMAN (STSCI), THE CLASH TEAM, HASHIMOTO ET AL.)

We see MACS1149-JD1 as it was 530 million years after the Big Bang, while inside, it has a special signature: oxygen.

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. The detection of oxygen in this ultra-distant galaxy, along with the galaxy’s brightness, tells us it is already approximately 280 million years since the first stars formed within it.(ESO / VERY LARGE TELESCOPE / FORS INSTRUMENT & TEAM (L); NASA, ESA, C.R. O’DELL (VANDERBILT), AND D. THOMPSON (LARGE BINOCULAR TELESCOPE) (R))

Oxygen is only produced by previous generations of stars, indicating that this galaxy is already old.

The first stars and galaxies in the Universe will be surrounded by neutral atoms of (mostly) hydrogen gas, which absorbs the starlight. We cannot yet observe this first starlight directly, but we can observe what happens after a bit of cosmic evolution, allowing us to infer when stars must have formed in great abundance. The first stars are made of hydrogen and helium alone, but produce copious amounts of oxygen, which show up in later generations of stars. (NICOLE RAGER FULLER / NATIONAL SCIENCE FOUNDATION)

MACS1149-JD1 was imaged with microwave (ALMA), infrared (Spitzer), and optical (Hubble) data combined.

The results indicate that stars existed nearly 300 million years before our observations.

Our entire cosmic history is theoretically well-understood, but only qualitatively. It’s by observationally confirming and revealing various stages in our Universe’s past that must have occurred, like when the first stars and galaxies formed, that we can truly come to understand our cosmos. The Big Bang sets a fundamental limit to how far back we can see in any direction. (NICOLE RAGER FULLER / NATIONAL SCIENCE FOUNDATION)

The very first stars must have arisen no later than 250 million years after the Big Bang.

As we’re exploring more and more of the Universe, we’re able to look farther away in space, which equates to farther back in time. The James Webb Space Telescope will take us to depths, directly, that our present-day observing facilities cannot match. (NASA / JWST AND HST TEAMS)

2021’s James Webb Space Telescope will image them firsthand.


Mostly Mute Monday tells the scientific story of an astronomical phenomenon or discovery 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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