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Solid black lines: Well calibrated ladder step. Dashed black lines: Uncertain calibration ladder step. The cosmic distance ladder (also known as the extragalactic distance scale ) is the succession of methods by which astronomers determine the distances to celestial objects.
Nuclear fusion reaction of two helium-4 nuclei produces beryllium-8, which is highly unstable, and decays back into smaller nuclei with a half-life of 8.19 × 10 −17 s, unless within that time a third alpha particle fuses with the beryllium-8 nucleus [3] to produce an excited resonance state of carbon-12, [4] called the Hoyle state, which ...
A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species and also in some species of Ctenophores, which are not related to cnidarians at all. Some groups of Nemerteans also produce larvae that are very similar to the planula, which are called planuliform larva.
Planetary habitability in the Solar System is the study that searches the possible existence of past or present extraterrestrial life in those celestial bodies. As exoplanets are too far away and can only be studied by indirect means, the celestial bodies in the Solar System allow for a much more detailed study: direct telescope observation, space probes, rovers and even human spaceflight.
The first step toward a theory of Solar System formation and evolution was the general acceptance of heliocentrism, which placed the Sun at the centre of the system and the Earth in orbit around it. This concept had been developed for millennia ( Aristarchus of Samos had suggested it as early as 250 BC), but was not widely accepted until the ...
In the CNO cycle, four protons fuse, using carbon, nitrogen, and oxygen isotopes as catalysts, each of which is consumed at one step of the CNO cycle, but re-generated in a later step. The end product is one alpha particle (a stable helium nucleus), two positrons , and two electron neutrinos .
The first step in all the branches is the fusion of two protons into a deuteron. As the protons fuse, one of them undergoes beta plus decay , converting into a neutron by emitting a positron and an electron neutrino [ 7 ] (though a small amount of deuterium nuclei is produced by the "pep" reaction, see below):
Throughout human history, astrometry played a significant role in shaping our understanding of the structure of the visible sky, which accompanies the location of bodies in it, hence making it a fundamental tool to celestial cartography.