Video Night - Dr. Emily Levesque from the University of Colorado on The Weirdest Stars in the Universe

Tuesday 18th September, 2018

The Society's September meeting featured a video about "The Weirdest Stars in the Universe" by Dr Emily Levesque from the University of Washington in Seattle, USA. She studies massive stars using data from some of the largest telescopes on Earth such as the Gemini and Keck observatories on Mauna Kea, Hawaii as well as the Hubble Space Telescope.

The presentation began with a slide showing five pictures of different stars that are associated with either extreme or unexplained behaviour. She promised the audience that by the end of her talk they would be able to identify each one, and on her final slide she added labels to each of the objects. The labels were supernova/neutron stars, red supergiants, gamma ray bursts/black holes, luminous blue variables, Thorne-Zytkow Objects.

Dr Levesque then covered each of these objects in turn beginning with red supergiants, which are huge red stars about 900 times the size of our own Sun. They do not start out this large but expand to these volumes when a massive star about 10 times the size of the Sun uses up all its hydrogen fuel at its centre. The star then switches to processing helium at its core with a surrounding shell of hydrogen. This shell acts as fuel for further nuclear reactions and that in turn heats up the outer layers of the star causing the star to expand.

She then explained that this state of affairs does not continue indefinitely but as the star ages it builds up layers of heavier elements somewhat like a nested set of Russian dolls or layers of an onion. The lightest elements are on the outside of the star and the heaviest elements at its core. This layering comes to a grinding halt when the core tries to react the element of Iron as this process requires energy rather than making energy. At this point the star can collapse and blow itself apart. This event is called a supernova and in 1054 Chinese astronomers recorded such an event that was so bright it was visible during the day. Sometimes, in the short period before the star explodes it can become very unstable. Its light output varies dramatically and it can eject huge amounts of material into space. In this part of its lifecycle astronomers call the star a luminous blue variable.

Depending on the mass of giant stars the core of the star sometimes survives a supernova explosion and the remnant is called a neutron star. However, for larger stars the collapse can continue further to the point where it becomes a strange object called a black hole. These are called "black" as any light that comes too near them is swallowed up and renders them invisible.

Dr Levesque then continued by saying that, just like the Earth, stars are spinning and so as the star is collapsing to form a black hole some of the infalling material is channelled along the star's poles and escapes. In this violent environment the material is heated to extraordinary temperatures and gives off radiation in the form of gamma rays. When these are spotted from Earth they are known as gamma ray bursts.

She finished her talk by saying that one of the strangest stars is a Thorne-Zytkow Object and is the result of a red supergiant star merging with a neutron star. The red supergiant star's core ends up being replaced by a neutron star.