Dr. Thomas Greif

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First stars

The first stars are thought to have formed a few hundred million years after the Big Bang. The primordial hydrogen and helium gas synthesized in the big bang decouples from the expanding background and collapses into dark matter `minihalos', which are gravitationally bound structures that consist of dark matter and have a mass of about one million times the mass of the Sun. At the center of the halo, molecular hydrogen forms, which allows the gas to cool by exciting its internal degrees of freedom. The collapse proceeds over many orders of magnitude in density until a protostar at the center of the cloud is born. Since the accretion rate onto the protostar is so high, the expected outcome is a star with a mass of about one hundred times the mass of the Sun. The image below shows the initial shock-heating of the gas as it collapses into the dark matter halo on a scale of about 200 pc. The temperature is color-coded from black to white.

Radiative feedback

Due to their high masses, the first stars are more compact than normal stars and have higher surface temperatures. This results in an unusually high flux of ionizing and molecule-dissociating radiation, which affects the chemical and thermal evolution of the gas on intergalactic scales. The image below is a 3D-rendering of the radiation from a number of stars: the blue contours show ionized gas and outline the sizes and shapes of the ionization fronts of the individual stars. The green contours show regions of constant molecular hydrogen abundance. Once the stars turn off, the ionized gas cools and recombines, and facilitates the formation of molecular hydrogen.


The high central temperatures of the first stars facilitate rapid nucleur burning. As opposed to the ten-billion year lifetime of the Sun, they only live a few million years before they explode in extremely bright supernova explosions. The afterglows of these supernovae are likely observable by upcoming instruments such as the James Webb Space Telesope (JWST). Their shock waves clear out the parent halo, and heat and ionize the gas in a large volume around the progenitor star. The gas is enriched with the first heavy elements, which paves the way for the formation of normal stellar populations and the first planets. From left to right, the image below shows the density, temperature, and metallicity of the gas that recollapses into a dark matter halo after such a supernova event. The panels are about 10 kpc on a side, and the inlays show the central 1 kpc.

First galaxies

The first galaxies form in dark mattter halos that are about one hundred times as massive as minihalos. They are the second step in the hierarchy of structure formation that eventually leads to the formation of galaxies as massive as our Milky Way. They typically form from about 10 star-forming minihalos, and are therefore significantly influenced by radiation and metal enrichment. In addition, cold gas accretion along filaments drives supersonic turbulence at the center of the halo. The image below shows the Mach number of the accreting gas, color-coded from blue to yellow. The supersonic accretion of gas along filaments on the left- and right-hand sides of the image is evident. The approximate size of the underlying dark matter halo is indicated by the green dashed line.