First Light--Switching on Stars at the Dawn of Time

Author: Emma Chapman
Publisher: Bloomsbury 2021

This book adopts the convention “all elements other than hydrogen or helium are metals”.  So a “metal-free star” is a star consisting mainly of hydrogen and helium.  In astronomy, "a population I star" is a star that has lots of metals inside it. Population I stars reside in the spiral arms of a galaxy.   Our sun is an example of a population 1 star.  A "population II star" is an older star with fewer metals.  Population II stars reside in the center of a galaxy or in the outer halo of a galaxy.  The first stars that produced all metals that we see today are called "population III stars".  Population III stars are “metal-free”.  

The first stars of the universe or population III stars were beasts with several hundred times the mass of our sun; they lived fast; and died in a few million years.  (The lifespan of a star like our sun is about 10 billion years.)  Even though we know a lot about the history of the universe since the Big Bang, the era of the first stars is a mystery (hardly any clues or data) or a blank space in the timeline of about 1 billion years.  (The universe is about 15 billion years old.)

This book summarizes the challenges in finding population III stars, what little we know about population III stars so far, what we know about early black holes and the formation of the first galaxies.  It also covers the current research on finding population III stars either by doing stelar archaeology or by looking for dying signals from long ago.  These dying signals should be in the radio wave portion of the spectrum due to the expansion of the universe.  

Recently launched James Webb infrared space telescope (JWST) which orbits the L-2 (second Lagrange point) point of Earth at about 1.5 million kilometers from Earth will be looking for population III stars among other things.  The first calibration photo taken by the JWST was released last month (March 2022).  In this photo you can clearly see the otherwise very faint star HD844 which lies around 191 light years from earth.

The electron in the hydrogen atom emits a 21 cm photon when its spin state flips from parallel to ant-parallel.  The first stars emitted high energy UV photons exciting electrons above the energy level indicated by the spin-flip transition.  Some electrons in some hydrogen atoms emit 21cm photons and fall back to the ground state.  Global 21 cm experiments aim to measure this 21 cm radiation at different times in history.  The "Epoch of Reionization"(EoR) is the period when the first stars heated and ionized the surrounding gas.  The Experiment to Detect the Global Epoch of Reionization signature (EDGES) reported the first detection of the 21 radiation from the first stars that ended the "dark ages" in 2018.  

EDGES


It is thought that the population III stars only dominated in galaxies for around 20-200 million years before population II became the dominant population.  First stars lived in tiny halos, before coming together to form dwarf galaxies.  Roughly 5-15 percent of the first tiny galaxies in the galactic neighborhood of our galaxy are thought to be those fossil dwarf galaxies.  Segue 1, a satellite galaxy of the Milky Way is one of the best candidates for a fossil galaxy.  Extremely metal poor stars were found in Segue 1.  Segue 1 was discovered by the Sloan Digital Sky Survey in 2006.  

Segue 1