Landfill for planets

Ken Tapping, May 22, 2013

In the sky this week…

  • Jupiter, Venus and Mercury are close together in the sunset twilight.
  • Such occurrences are quite rare and worth seeing. Saturn rises before sunset and is well up in the east by dark.
  • The Moon will be Full on the 24th.

At the ends of their lives, really big stars collapse and explode, ejecting their outer layers and lots of newly-formed elements out into space. Their cores get compressed, forming black holes. Stars that are a bit smaller explode almost as energetically, but their cores do not vanish from our space altogether; they collapse into neutron stars, with the mass of a star but only a few kilometres across. Both these classes of cosmic explosion sterilize everything in the neighbourhood but on the plus side the shock waves and material they eject destabilize nearby gas and dust clouds, stimulating the birth of new stars and planets. However, what sort of fate awaits smaller, more ordinary stars like the Sun?

In general, a star has to work with the fuel it collects when it forms. It might pick up a little during its life by for example tearing a partner star to pieces, but usually it has to live on its savings. This means that at some point it will run low on fuel. The first symptom is the star expanding into a red giant. For stars like the Sun, this is followed by the outer parts of the star blowing and sneezing off into space. This leaves the star’s naked core, white hot and with most of the mass of the original star, but compressed to about the size of the Earth, so that a teaspoonful of its material would weigh tonnes. There is no fuel left, so this remnant, known as a white dwarf star, will gradually cool. However, that amount of material will take a very long time to cool off, almost the lifetime of the universe.

Thanks to observations made using the Hubble Space Telescope, a Cambridge-led team of astronomers has added some more detail to the picture. They had a close look at two white dwarf stars located in the constellations of Taurus, “The Bull”. These stars are close to what we expect the Sun to become in a few billion years. They found something rather odd. The light from these stars bore the signature of the element silicon, a lot more of it than might be expected. They suggest this comes from rock dust, from pulverized planets.

In a few billion years the Sun will start to run out of fuel and become a red giant, where Mercury, Venus and possibly the Earth will end up inside. Friction with the material in the expanded star will make the now well-and-truly-fried planets spiral inwards. The Sun’s outer layers will then be sneezed off into space, disrupting the orbits of the other planets. Planets venturing too close to the new white dwarf star will be pulled apart, forming a disc of rock dust. Collisions between the dust particles will lead to some it falling onto the star’s surface. White dwarf stars radiate very little energy, so what’s left of the Solar System will be a very cold place.

Dust, planets and other objects in space usually end up being recycled, being used to form new stars and planets. However, collisions between stars are rare, so it is likely that any planet debris deposited on the surface of a white dwarf is probably going to stay there for the rest of the life of the universe. Thus, white dwarfs, neutron stars and black holes must arguably be the ultimate landfill sites. What is buried there stays put.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355

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