2019
May
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The first star explosions were gigantic – and asymmetrical

After a star with significantly more mass than the Sun has consumed all its fuel, it decays into a massive firework display, a supernova. In today’s universe, that is not a very common sight, because the greatest percentage of stars is made up of red dwarfs, which end their lives not nearly so spectacularly. Our Sun is also not destined to turn into a supernova. It will grow into a red giant and then, at the end, only a harmless white dwarf will remain.

In the early universe, however, things were much different. At that time, there were neither red dwarfs nor stars around the size of our Sun. Instead, the much smaller universe at that time was filled with giant stars that today would be classified in so-called Population III. They were made only of what the big bang had supplied for them: hydrogen, helium, and a bit of lithium. But the composition of the cosmos changed as these early stars ended their short, but energetic lives. Their explosions created the first heavy elements that would accumulate to form stars of the younger Populations II and I.

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Two values for one constant – impossible, but true

The universe is expanding. That’s something astronomers have agreed on for a long time. Edwin Hubble, an American astronomer, was the first to discover that light from distant galaxies was shifted toward red frequencies by the time it reached us – which meant that the source of the light was moving away from us. The Hubble constant, which expresses how quickly the universe is expanding, was named in Hubble’s honor. It has a value of approximately (more on this later) 70 kilometers per second per megaparsec. For example, if an object is one million parsecs (3.26 million light-years) farther away from us than a second object, then it is moving 70 kilometers per second faster away from us than the second object.

This constant can be measured in different ways. Space telescopes, such as Hubble or Gaia, for example, measure the brightness of certain variable stars, for which the relationship between change in luminosity and luminosity is known. In this way, the distance to galaxies containing these so-called standard candles can be calculated and the velocity at which these galaxies are moving away from us can be determined.

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