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First Stars

The first stars in the Universe (also termed Population III), are thought to have formed only a few hundred million years after the Big Bang. The primordial gas collapses into so-called dark matter minihalos, which are roughly spherical, gravitationally bound structures with a mass of about one million times the mass of the Sun. At the center of the halo, molecular hydrogen (H2) is reformed, which allows the gas to cool as the internal degrees of freedom of H2 are excited. The gas contracts to higher and higher densities, and eventually a protostar forms that accretes from the surrounding gas cloud. Since the accretion rate is so high, at least one star with a mass of about one hundred times the mass of the Sun is expected to form. The image on the right shows the temperature of the gas as it collapses into a dark matter minihalo, color-coded from black (cool) to white (hot).

Stellar Radiation

Due to their high masses, the first stars are more compact than other stars and have higher surface temperatures. This results in a strong flux of photodissociating and ionizing radiation that affects the chemical and thermal evolution of the intergalactic medium on very large scales. The image below is a 3D-rendering of the radiation from a number of Population III stars: the blue isocontours show ionized gas and outline the sizes and shapes of individual HII regions, and the green isocontours show molecular hydrogen. Once the stars turn off, the gas in the relic HII region can cool and recombine, forming an even higher molecule fraction than before.


The high central temperatures of the first stars allows them to burn their nucleur fuel very rapidly. Compared to the ten billion years of the Sun, they only live a few million years before they explode in extremely bright supernova explosions that might be discovered by the latest generation of space telescopes. The shock waves from the energetic supernova remnants clear out the parent halo, and heat and ionize the gas in a large volume around the progenitor. The gas is enriched with the first heavy elements, which paves the way for the formation of normal stellar populations and the first rocky 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.

First Galaxies

The first galaxies are dark mattter halos about one hundred times as massive as minihalos, and are the next step in the hierarchy of structure formation. They typically assemble from about ten Population III star formation sites, and are therefore significantly influenced by radiation and metal enrichment. In addition, the gas accreting onto the halo from the intergalactic medium flows at supersonic speeds, which drives substantial turbulence at the center of the halo. The image on the left shows the Mach number of the accreting gas, which is high at the center and in the flows coming from the left and right side of the image. The approximate size of the underlying dark matter halo is indicated by the green circle.