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Detection of the First Stars in the Universe

This article appeared in the March 9, 2018 edition of the Davis Enterprise.

Detection of the First Stars in the Universe

The Murchison Radio-astronomy Observatory in Western Australia. Photo by Pete Wheeler

 

By Vinita Domier

Special to the Enterprise

 

On February 28, 2018, scientists from the Experiment to Detect the Global EoR Signature (EDGES) announced that they have detected radio signals at the Murchison Radio-astronomy Observatory in Western Australia from which they can infer the existence of stars in the universe as early as 180 million years after the cataclysmic Big Bang origin of the universe that is theorized to have occurred 13.8 billion years ago.

 

If these observations are confirmed by further investigations, it will conclusively prove that the very first stars formed around 13.6 billion years ago, and this Epoch of Reionization (EoR), when the universe was bathed in high-energy starlight heralding the cosmic dawn, began as early as 180 million years after the Big Bang rather than the previous estimate of 400 million years. These radio signals then would be the indirect observations of the oldest and farthest matter observed in the universe.

 

Hydrogen is the simplest and most abundant element in the universe, created during the Big Bang and amounting to 75% of all normal matter. It can exist in different forms: atomic hydrogen comprising one proton in the nucleus surrounded by one orbiting electron, molecular hydrogen comprising two hydrogen atoms bonded together, and ionized hydrogen comprising one proton in the nucleus and one free electron.

 

Atomic hydrogen is electrically neutral and abundantly present in relatively cold interstellar areas known as HI regions, and is detectable by the presence of emission line at the 21-cm wavelength (1420 MHz frequency) in the radio region of the electromagnetic spectrum. Molecular hydrogen clouds are present in relatively cold interstellar medium, are neutral and stable, and are detected by looking for carbon monoxide spectral lines.

 

Ionized hydrogen is found in extremely hot areas known as HII regions where there is enough energy available for the electrons to overcome the attractive forces of the protons, resulting in plasmas of electrically charged ions and free electrons. Atomic hydrogen becomes and stays ionized in active star-forming and galactic regions which produce high-energy ultraviolet radiations, and this transformation is inferred by the absence of the 21-cm spectral line.

 

All the hydrogen created in the Big Bang and permeating the early universe was ionized due to extreme ambient temperatures. It took the next 380,000 years for the temperatures to cool sufficiently to allow protons and electrons to combine and form atomic hydrogen. After millions of years, matter in localized areas collapsed to form the first massive hot stars that released ultraviolet radiations and reionized the atomic hydrogen enveloping them.

 

The EDGES scientists detected the tell-tale 21-cm spectral line dip in the electromagnetic spectrum centered around 78 MHz, which was redshifted due to the expansion of the universe from the at-rest value of 1420 MHz. They inferred from the dip that atomic hydrogen was being ionized by ultraviolet radiation from nearby stars, and from the redshift they calculated that these stars formed 13.6 billion years ago.

 

Join the Davis Astronomy Club on Saturday, March 10, starting at 7pm at the Explorit Science Center (3141 5th Street, Davis) when we will discuss the genesis of the first stars in the universe and the Big Bang theory. Everyone is invited to the free meeting indoors, followed by a star party outdoors (weather permitting).

 

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