A death in style

A death in style

Two deaths, actually…

Although it looks like the wings of a butterfly just out of its cocoon opening to life, the image above taken by the NASA/ESA Hubble  Space Telescope, is in fact the death notice of two stars.

The cosmic butterfly  in the image is named “Twin-Jet Nebula”, or more formally, as PN M2-9.  PN  stands for “planetary nebula”. Stars with masses similar to that of the Sun or less, punctuate their lives as planetary nebulae, short -lived but magnificent  displays of light and color.

While the stars with at least four solar masses end their lives as violent supernovae and turn either into a neutron “star” with a diameter of 10-20 km, or a point-like black hole, solar-like stars pass away in more peaceful manner.

 After converting the hydrogen fuel  first to helium and then to carbon and oxygen through fusion reactions at their hot and dense cores, solar-mass stars slowly eject their outer layers of hydrogen and helium to space.  The core, containing about 60 percent of the star’s original mass but  now contracted and heated, emerges into open as a “white dwarf” and ionize the ejected gas of the envelope with intense ultraviolet radiation, causing the expanding ejecta  to glow for 20-30,000 years. And the white dwarf core cools slowly, to eventually turn into a “black dwarf” and become invisible. But since this cooling process lasts longer than the 13.8 billion year age of the universe, no black dwarf is believed to have formed yet.

While the planetary nebulae can have spherical forms, they may take a variety of shapes as they eject their envelopes  in consecutive puffs. They owe their generic name to the discovery of  first examples which appeared as a planet-forming disk of gas – the disk shape being due to the transparency of the hot bubble in Earth’s line of sight –around a central star.

Planetary nebulae with two  lobes are called bipolar nebulae. These usually form when two Sun-like stars in a binary system end their lives in relatively quick succession. That’s what’s happening at the Twin-Jet Nebula. The smaller star of 0.6-1 solar mass has already completed its evolution and become a white dwarf.  The larger member of the system  has neared the end of its life and is rapidly shedding its outer layers. The ultraviolet radiation from the white dwarf  is causing the gas to glow. As the two stars orbit each other in 100 years, the ejected gas does not take a spherical form and instead, forms an expanding two-lobbed structure. From the expansion rate of the lobes, astronomers have calculated the age of the nebula to be 1200 years.

Visible in the image  are jets of matter, extending in  oppsite directions, looking like veins within the colorful, translucent wings. These are made of gas being ejected from the binary system at the center with a velocity of over 1 million kilometers an hour. Jets undergo slight changes in orientation due to magnetic fluctuations that occur in the system.

Raşit Gürdilek