2020
June
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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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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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