Heavy Metal Sub-dwarfs and the Tear Shaped Star
I've often discussed the standard model for main sequence stars beautifully depicted on the Hertzsprung Russell Diagram, but stars don't always obey the rules, and as you'd expect from a "Heavy Metal" star .. these sub-dwarfs certainly don't.
TThe main sequence shows the stars fusing hydrogen in their core (A), eventually when they run out they bloat and become Red Giants (B) or in some cases super giants (C). The super giants will eventually go supernova and leave a neutron star or a black hole (not represented on this scale), but the smaller stars end up as white dwarfs (D).
Hot stars are generally class O or B (blue stars), and as you'll see from the diagram, they are also very luminous and high mass stars, but not all of them are.
In a few very special cases, some hot stars are classed as sub-dwarf, similar to our Sun, but O or B type. These stars are thought to be super giants that have by interaction with a partner binary star, have lost much of their atmosphere and possibly survived a type 1a explosion, effectively the core remnants that managed to escape, or was left behind somehow.
Some of these incredibly peculiar stars have very high amounts of metals. In astronomy Hydrogen and Helium are considered non-metallic, while the rest of the elements are, but heavy metals will constitute elements from Lead upwards in the periodic table, and it is theorised they may have entire layers of such metals within the atmosphere of the stars themselves.
These metals can only have come from other supernova, probably related to the particular nebula they were first born from.
Astronomers at Warwick University have spotted one such hot sub-dwarf interacting with what is thought to be a white dwarf partner. This interaction is drawing mass from the sub dwarf onto the heavier white dwarf. Eventually the mass of the white dwarf will be sufficient to begin collapse and creating a type 1a supernova.
The effect is to create a star shaped like a tear, and the team in Warwick were able to monitor it and detect the odd shape, while not being able to directly see the white dwarf which is far dimmer.
The star is HD265435 and is 1,500 light years away, meaning that if it did go supernova, it would be unlikely to cause any problems here on Earth, but would light up the sky quite dramatically for a few months.
Source : https://scitechdaily.com/impending-supernova-doom-astronomers-rare-sighting-of-a-teardrop-shaped-star/
