Event 2

13.5 billion years ago

Star formation
Birth of the first stars. When the universe was less than one billion years old, it went through a stage of intensive star formation, converting primordial hydrogen into a profusion of stars. This image is an artist’s recreation of that process.
Credit: Image is used courtesy of NASA/ESA/A. Schaller (forSTScI)

Soon after the Big Bang (in cosmic terms), perhaps 200 million years later, the matter in the universe had expanded and cooled, and began to coalesce by gravity into denser clouds of gas. As these clouds of gas collapsed even further, they reached sizes, densities, and temperatures large enough to begin nuclear reactions and ignite to become true stars.

The earliest stars in the universe were composed of the lightest and simplest elements, atoms of hydrogen (about 90 percent) and helium (about 10 percent). As gravitational forces began heating up, proto-stars formed. Then when temperatures reached 10 million degrees Kelvin or more, nuclear reactions started, converting hydrogen into helium by fusion and creating huge amounts of energy.


Primordial Stars
This is an artist’s impression of the first primordial stars. Hydrogen and helium gasses create a haze as the first starlight illuminates the Universe.
Credit: Image is used courtesy of Harvard-Smithsonian Center for Astrophysics

Stars range in size from about 1/100 the size of our sun (our sun is just an average-sized star) to supergiants 400 times the size of our sun. If the planet Jupiter were about ten times larger, it would have a large enough mass to ignite and become a star. The larger the star, the shorter its life (we will discuss this further in the next event). All stars start by maintaining a balance between gravity (which tries to compress the star into a smaller and smaller size) and the outward forces of nuclear energy (which try to push the star apart).

Towards the end of a typical star’s life, a range of heavier, more complex elements are forged in the star by helium fusion, including carbon, nitrogen, oxygen, silicon, calcium, and iron (this is discussed in Event 3). Such elements would later become some of the major building blocks of the earth and of life.

Today it is believed that there are about 100 billion stars in a typical galaxy, and 100 billion galaxies in the universe. This means that there are an astonishing 10,000,000,000,000,000,000,000 (ten sextillion!) stars in our universe.


A telescope is, in effect, a time machine. The further away a star is from us, the older it is, and the longer it has taken its light to reach us. (Even the light of our own sun takes eight minutes to reach us). With ever more sophisticated telescopes, we can now see light from stars at the edge of the observable universe.  These stars formed about 13 billion years ago, not very long after the Big Bang. By examining the spectrum of light emitted by stars, it is possible to tell what elements are present.


The processes that created stars also ultimately created our own star, the sun. Every atom in our body and on the earth was originally created in the nuclear furnaces of long-extinct stars that sowed the seeds of our solar system. Stars preserve the main evidence for the evolution and expansion of the universe.





Neil deGrasse Tyson, noted astrophysicist and director of the Hayden Planetarium, discusses the galactic reactions that formed the early stars and the resultant matter that were the building blocks for the origins of life on Earth

How the first stars in the universe came into existence

Scientists see light that may be from the first objects in the universe


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