At the end of their lives, stars glow hotter than ever!

By esiegel on October 11, 2012.

"A colour is a physical object as soon as we consider its dependence, for instance, upon its luminous source, upon other colours, upon temperatures, upon spaces, and so forth." -Ernst Mach

Our Sun, like all Sun-like stars, will come to the end of its life someday. All the hydrogen fuel in its core will eventually burn up, and when this happens, the core itself will begin to contract. When temperatures are finally high enough, the end product of hydrogen fusion -- Helium-4 -- will begin to fuse in the contracted core, and the Sun will expand into a Red Giant.

Image credit: Northwestern University's tutorial on Stellar Evolution.

The outer layers of (mostly) hydrogen will become very diffuse, and will eventually be blown off as the Sun goes through the next phases of its life, culminating in a planetary nebula.

Image credit: NASA and The Hubble Heritage Team (STScI/AURA); Acknowledgment: Dr. Raghvendra Sahai (JPL) and Dr. Arsen R. Hajian (USNO).

The core of the (former) star will eventually contract down to a degenerate white dwarf, while the planetary nebula will expand to be multiple light years in size, eventually cooling and dimming away over the timespan of many tens of thousands of years.

This is the fate of all Sun-like stars in the Universe, as far as we know, and the planetary nebulae that result are varied and beautiful, as many of them have been captured in rich, fascinating detail by the Hubble Space Telescope.

Images credit: NASA / STScI.

These images are all taken in different visible light filters, with different colors highlighting different elements in each image. But in addition to the type of light we're used to, theoretical work told us that it might be possible to create photons of much more energetic wavelengths -- X-rays -- than are normally possible in stars, during this phase of a star's life.

In theory, how is this possible?

Electromagnetic Spectrum Data. Image credit: NASA.

The gas surrounding the white dwarf can become so rarefied that the central, contracting star can super-heat the gas, shocking it, and causing the emission of diffuse X-rays near the central star. The surface of the star, as well, can become an X-ray emitting source, meaning that we can look for both a diffuse X-ray signal near the central region and a strong, X-ray point source at the location of the dying star itself.

And that's exactly what the Chandra X-ray Telescope did.

Image credit: Joel Kastner et al., 2012. From Astronomical Journal, 144, 58.

According to this recent paper, many of the nearest planetary nebulae to us were determined to exhibit exactly this. For the first time, point-source X-rays and diffuse X-rays could be separated out from one another.

In fact, the four planetary nebulae images by Hubble, three images up? They were found to all have diffuse X-ray emissions, and three of them were found to have central point sources, too! See for yourself what the X-ray data looks like.

Image credit: NASA/CXC/RIT/J.Kastner et al.

This is incredibly impressive! In fact, data was taken for 34 different planetary nebulae, to see which ones had this diffuse X-ray emission and which ones didn't.

The results are incredibly educational.

Image credit: Joel Kastner et al., 2012. From Astronomical Journal, 144, 58.

While many of these nebulae of all types had point sources (and many didn't), it's the smallest planetary nebulae, and preferentially the lowest temperature ones as well, that have diffuse X-rays! It's thought that the point sources may have something to do with whether these planetary nebulae originated from binary star systems or not, but what about the diffuse X-rays? They may be because they're the youngest planetary nebulae, still expanding and -- in many cases -- becoming less dense and moving farther away from the central, contracting stellar remnant.

But for these dying stars that have had the privilege of being imaged by both Hubble and Chandra, it means we now have the most enticing, educational and spectrally diverse images ever taken of a planetary nebula. And we have it for a whopping four of them!