A cosmic amethyst in a dying star

On Earth, amethysts can form when gas bubbles in lava cool under the right conditions. In space, a dying star with a mass similar to the Sun is able to produce a structure that lives up to the charm of these beautiful gems.

As stars like the Sun go through their fuel, they throw off their outer layers and the star’s core shrinks. Using NASA’s Chandra X-ray Observatory, astronomers found an ultra-hot gas bubble at the center of one of these expiring stars, a planetary nebula in our galaxy called IC 4593. At a distance of approximately 7,800 light-years. from Earth, IC 4593 is the most distant planetary nebula ever detected with Chandra.

This new image of IC 4593 has Chandra X-rays in purple, invoking similarities with amethysts found in geodes around the world. The bubble Chandra detected comes from gas that has been heated to over a million degrees. These high temperatures were likely generated by material that flew off the star’s shrunken core and crashed into gas that had previously been ejected from the star.

This composite image also contains visible light data from the Hubble Space Telescope (pink and green). The pink regions in the Hubble image are the superposition of the emission from the coldest gas composed of a combination of nitrogen, oxygen and hydrogen, while the green emission comes mainly from nitrogen.

IC 4593 is what astronomers call a “planetary nebula,” a misleading sounding name because this class of objects has nothing to do with planets. (The name was given about two centuries ago because they looked like the disk of a planet when viewed through a small telescope.) Indeed, a planetary nebula forms after the interior of a star with about the mass of the Sun contracting and its the outer layers expand and cool. In the case of the Sun, its outer layers could extend to the orbit of Venus during its red giant phase several billion years in the future.

In addition to the hot gas, this study also finds evidence of a point X-ray source at the center of IC 4593. This X-ray emission has higher energies than the hot gas bubble. The point source could come from the star that discarded its outer layers to form the planetary nebula or it could come from a possible companion star in this system.

A paper describing these findings appears in the April 2020 issue of Royal Astronomical Society Monthly Notices and is available online. The authors are Jesús A. Toalá (Instituto de Radioastronomía y Astrofísica (IRyA) in Michoacan, Mexico); MA Guerrero (Instituto de Astrofísica de Andalucía in Granada, Spain); L. Bianchi (The Johns Hopkins University, in Baltimore, Maryland); Y.-H. Chu (Institute of Astronomy and Astrophysics, Academia Sinica (ASIAA) in Taipei, Taiwan, Republic of China); and O. De Marco (Macquarie University, Sydney, Australia).