Stars in the Early Universe

After 380,000 years, protons and neutrons combined with electrons and formed hydrogen and helium atoms. At that time, the universe became transparent, and was the time at which the pattern of the Cosmic Microwave Background Radiation (CMBR) became fixed in space. Hydrogen atoms joined together and formed molecular hydrogen, H2. Molecular hydrogen and helium gas formed giant dark “molecular” clouds in the early universe. The dark matter strings and clumps attracted clouds of hydrogen and helium gas. For the next 200 million years, the universe was dark.

The original variations in energy and matter density left over from inflation were slight, but they were enough to trigger the clumping of dark and ordinary matter in the universe. Over the next few hundred million years, gravity pulled dark matter together into clumps and then strings that extended throughout the universe. Normally, the pressure in gas clouds prevents their collapse and formation of stars, but with the additional gravity of dark matter, the clouds exceeded the Jeans mass and were able to contract and form stars. Scientists had thought that the first stars formed 400 million years after the beginning of the universe (Figure 3‑2), but recent data indicates that the first stars formed only 200 million years after the beginning of the universe. Scientists think that they were giant blue stars, which then exploded as supernovae and became the black holes at the cores of galaxies. One of these collapsed blue stars became the core of the Milky Way Galaxy. Galaxies and quasars eventually reionized the gases in the universe, which means that they knocked the electrons from atoms, and the nuclei were separated from electrons. Thus, the nuclei had a positive charge, which classifies them as ions. The Epoch of Reionization (Figure 3-2) ended one billion years after the beginning of the universe. In the present universe, helium atoms and molecular hydrogen molecules only form within molecular clouds in galaxies, in which dust protects the interior of the cloud from high energy interstellar radiation.

The Hubble Space Telescope peers into small patches of space for hours to see galaxies as they were 700 million years after the beginning of the universe. The galaxies in Figure 3‑4 are so far away and the light is so redshifted (longer wavelengths have lower energy) that the total light from these galaxies is one four billionth as bright as one visible star in the sky. The star near the center of the image is in our own galaxy, the Milky Way. The rest of the bright spots are distant galaxies, over a trillion times more distant than the star. Deep field images reveal a high number of irregular galaxies in the early universe. More info at https://youtu.be/W4GKf623Exk

Galaxies in deep field images have redshifts in the range of 6 to 9. Galaxies with redshift six emitted the light 900 million years after the Big Bang, and redshift 7 and 8 galaxies are as they were 700 million years after the Big Bang. The earliest galaxies (most redshifted) were small blue galaxies with massive hot stars. They were originally blue, but their wavelengths expanded to the microwave range. [1] Many galaxies with redshifts greater than 1.5 have irregular shapes and many are colliding with each other, which would have taken place in the young universe. The merging of these colliding galaxies caused large star-forming regions. The Hubble Deep Field image had 3,000 galaxies, the smallest of which were the most ancient. Because Hubble takes an image that is the diameter of a straw on the sky, this means that there are tens of billions of galaxies in the universe.

Three types of galaxies formed in the universe: elliptical, irregular, and spiral. Irregular galaxies have a wide variety of shapes. Many elliptical galaxies (egg-shaped) are the result of galaxy mergers. Elliptical galaxies formed almost all their stars in one massive initial starburst. They are generally redder than spiral galaxies because the stars are older (smaller) than spiral galaxies. Large stars have already burned out, and no new stars form within them. Spiral galaxies such as the Milky Way continue to form stars, most of which have planets.

The long-delayed James Webb Space Telescope (JWST) will has infrared radio telescopes with three times the resolution of the Hubble Space Telescope; thus, scientists hope to see even farther back in time (smaller objects) than the Hubble Space Telescope, and view the formation of the first stars in the universe (Figure 3‑5). The JWST is an ambitious project. Hopefully it won’t be like the Herschel’s largest telescope, which never worked. Problems have delayed the launch many times. It is currently supposed to launch in October, 2021.

Quasars (Figure 3‑6) form at the center of galaxies when a supermassive black hole at the center of the galaxy is surrounded by an accretion disk. Their luminosity is thousands of times brighter than the entire Milky Way. Quasars were 100 to 1000 times more frequent in young galaxies that existed 10 billion years ago (redshift = 2) than in galaxies in the present universe. The earliest quasar is at redshift = 7.54 (690 million years after beginning of universe). Strangely enough, quasars emit light because of the emission of light by the movement of the gas. We would not be here without quasars in early galaxy development because they shut down the violent period of rapid star formation.