Starship Asterisk*

Image Credit: ESA/Hubble & NASA, R. Wade et al.

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This atmospheric Picture of the Week, taken with the NASA/ESA Hubble Space Telescope, shows a dark, gloomy scene in the constellation of Gemini (The Twins). The subject of this image confused astronomers when it was first studied — rather than being classified as a single object, it was instead recorded as two objects, owing to its symmetrical lobed structure (known as NGC 2371 and NGC 2372, though sometimes referred to together as NGC 2371/2).

These two lobes are visible to the upper left and lower right of the frame, and together form something known as a planetary nebula. Despite the name, such nebulae have nothing to do with planets; NGC 2371/2 formed when a Sun-like star reached the end of its life and blasted off its outer layers, shedding the constituent material and pushing it out into space to leave just a superheated stellar remnant behind. This remnant is visible as the orange-tinted star at the centre of the frame, sitting neatly between the two lobes.

The structure of this region is complex. It is filled with dense knots of gas, fast-moving jets that appear to be changing direction over time, and expanding clouds of material streaming outwards on diametrically opposite sides of the remnant star. Patches of this scene glow brightly as the remnant star emits energetic radiation that excites the gas within these regions, causing it to light up. This scene will continue to change over the next few thousand years; eventually the knotty lobes will dissipate completely, and the remnant star will cool and dim to form a white dwarf.

Image Credit: ESA/Hubble & NASA, R. Wade et al

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This Picture of the Week from the NASA/ESA Hubble Space Telescope shows NGC 5307, a planetary nebula which lies about 10000 light years from Earth. It can be seen in the constellation Centaurus (The Centaur), which can be seen primarily in the southern hemisphere. A planetary nebula is the final stage of a Sun-like star. As such, planetary nebulae allow us a glimpse into the future of our own Solar System. A star like our Sun will, at the end of its life, transform into a red giant. Stars are sustained by the nuclear fusion that occurs in their core, which creates energy. The nuclear fusion processes constantly try to rip the star apart. Only the gravity of the star prevents this from happening.

At the end of the red giant phase of a star, these forces become unbalanced. Without enough energy created by fusion, the core of the star collapses in on itself, while the surface layers are ejected outward. After that, all that remains of the star is what we see here: glowing outer layers surrounding a white dwarf star, the remnants of the red giant star’s core.

This isn’t the end of this star’s evolution though — those outer layers are still moving and cooling. In just a few thousand years they will have dissipated, and all that will be left to see is the dimly glowing white dwarf.

Susanna Kohler wrote:

Uranian Ring System ~ Credit ALMA, EM Molter, AJ 2019

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This remarkable image of Uranus, taken at 3.1 mm with the Atacama Large Millimeter/submillimeter Array (ALMA), reveals thermal emission from the dust that makes up Uranus’s narrow ring system (Uranus itself is masked because it’s so bright compared to the rings). We know that Uranus’s rings are made up of centimeter- to meter-sized particles, but little is known about the composition, mass, or the detailed size distribution of these particles or the amount dust in between them. In a new study led by Edward Molter (University of California, Berkeley), a team of scientists presents ALMA and Very Large Telescope observations of the rings’ thermal emission for the first time. These observations indicate that Uranus’s epsilon ring — the primary ring — is devoid of dust. This stands in stark contrast to the rings around Saturn or Jupiter, for instance, which contain lots of micrometer-sized dust — raising the question of why Uranus’s epsilon ring is so unusual! For more information, check out the article below.

• Astronomical Journal 158(1):47 (July 2019) DOI: 10.3847/1538-3881/ab258c
• arXiv.org > astro-ph > arXiv:1905.12566 > 29 May 2019