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Sunspot problems in older stars

Sunspots caused by magnetic fields have plagued our Sun for ages. Their frequency changes approximately every eleven years, but even in the worst case they never cover more than 0.4 percent of the Sun’s surface. However, the Sun is pretty big, which you can appreciate from the fact that a sunspot can be about as large as a whole cross section of the Earth.

On a cosmic scale, however, our Sun is only a small light and, just like there are people with more or less freckles, there are also stars that have large numbers of spots. Under certain circumstances, our Sun might even become such a spot-studded star in a good five billion years; on average, that happens to about one out of every ten stars with the Sun’s approximate mass.

What happens then? In its inevitable stage as a red giant, in which the Sun will also swallow up the Earth, its core will be made of helium. It will be too cold in there to turn the helium into carbon. Only the hydrogen shell will still generate energy. That’s why a red giant is relatively cool. But at some point, enough helium will collect in the center so that the pressure and temperature will increase enough to also generate carbon. There will be a sudden helium flash, and, voila, fresh energy! The old star blooms again, hotter than before, even if a bit smaller. The star has landed on the so-called horizontal branch, which refers to the Hertzsprung-Russell diagram. The horizontal branch lies approximately in the middle and runs, as you might’ve guessed, horizontally.

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Where are the very first stars hiding?

Stars like the Sun are made largely from waste – leftover matter ejected billions of years ago during the death of previous star generations. We know this is the case from their content of heavy elements, their metallicity. When the universe was still young, there was only hydrogen, helium, and a little lithium, nothing else. These elements formed the very first stars a long, long time ago. The first stars are called “Population III” stars, while the current generation, which also includes the Sun, is called Population I. Population I was born from the ashes of Population II, just like Population II was created from the remnants of Population III.

However, Population III stars are difficult to find. At first, this seems logical: these stars were huge and had only a short life. Luckily, thanks to the limited speed of light, astronomers can look far into the past with the help of telescopes like Hubble. Objects that are more than ten billion light-years away we can watch as they progress through their youth, even though they have long ago ceased to exist. The gravitational lensing effect is particularly helpful for seeing into the past. Galaxies in the foreground act like lenses and bend the light from objects behind.

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Giant stars prevent the formation of planets

The chance for a young star to raise some planetary offspring apparently depends a great deal on the neighborhood it lives in. This, at least, is one finding that astronomers have discovered with the help of the Hubble Space Telescope. For three years, they observed the open star cluster Westerlund 2 which contains about 5,000 stars, including some real giants, within a relatively small space.

Westerlund 2 is only one to two million years old. That makes it an ideal candidate to test theories on planet formation, because all the stars it contains have just started or are just about to start forming planets. Thus, astronomers are basically able to watch these stars, and some of these are the most massive and hottest young stars in the Milky Way, going through all the gymnastics of pregnancy. Using Hubble’s Wide Field Camera 3 they were also able to find that 1,500 of the total 5,000 stars with 0.1 to 5 solar masses fluctuate in luminosity, which indicates the presence of a protoplanetary disk.

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What a rare ring galaxy reveals about cosmic history

Ring galaxies like the well-known Cartwheel Galaxy can form for two reasons:

  • First – a spiral galaxy is involved in a collision with another galaxy, which punches through the spiral galaxy, thereby clearing away its center.
  • Second – the bar of a barred spiral galaxy becomes unstable because its rotational velocity becomes too high.

Events like these are rare, so ring galaxies themselves are also rare.

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2019 LD2: the unruly comet

Asteroids and comets are generally thought to be different classes of celestial objects. But is the strict distinction really justified? The interstellar visitor ʻOumuamua, for example, was initially thought to be a comet, but didn’t develop either a coma or a tail and was then classified as an asteroid. In the meantime, its trajectory has been calculated so precisely that it must have lost mass – which means it is definitely a comet.

The object 2019 LD2 discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) of the University of Hawaii also appears to be some sort of hybrid. Discovered in June 2019, it was thought at first to be an asteroid, but quickly there were indications that it had comet-like properties. In July 2019, it developed a small tail, which it has kept to today. Therefore, 2019 LD2 must be an active object – a comet.

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How a planet grows up

AB Aurigae, 520 light-years from Earth in the constellation, Auriga (the charioteer), is far from grown up: the star is a so-called Herbig Ae/Be star, which has not yet started to fuse hydrogen in its core. Despite its youthful age of only a few million years, however, it already appears to be concerned with trying to produce offspring. And so, as humans are wont to do, they don’t look away considerately, but instead direct their eyes (and their telescopes) right at the action, full of curiosity.

In doing so, the Very Large Telescope of the European Southern Observatory (VLT) has found clear evidence for the development of a planetary system. In the dense dust and gas disk circling AB Aurigae, the astronomers discovered a distinctive spiral structure with a twist. This appears to mark the spot where a planet is likely forming. This observed feature could thus be the first direct evidence for the formation of a baby planet.

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How common is life in the universe?

The question is basically unanswerable. The well-known Drake equation feigns a certain degree of precision but suffers from the fact that it is nearly impossible to reach agreement on values for any of its seven factors. Right now, we have only one example for intelligent life, and for us to draw conclusions for the entire universe from just our own existence would, indeed, be very human, but would be scientifically problematic.

There is, however, an alternative. We could ask what the likelihood would be for life to develop on Earth if we turned back the clock and started over from the beginning. We know the conditions on Earth rather well, and astronomers are also fairly certain that there are planets somewhere out there that are similar enough to Earth that similar likelihoods would apply to them too.

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