A short take on astronomy news finds a cosmic butterfly testing theories of galactic evolution, a new idea for the formation of Mars's moons, and the discovery of a stellar thief that survived a supernova explosion.

Cosmic Butterfly Tests Galaxy Evolution

Two galaxies collided to form the butterfly-shaped remnant NGC 6240. At its heart, two supermassive black holes still orbit each other, 2,200 light-years apart. Millions of years from now, this system will collide too, sending out a burst of gravitational waves. But for now, this galactic merger makes the perfect laboratory for testing a prominent theory of galaxy evolution: that when two galaxies merge, feedback halts star formation.

New Hubble Space Telescope observations of the system show two distinct winds. One of the feeding black holes emits a cone of ionized gas in the northeast direction, while recently formed stars drive a bubble-shaped wind of excited hydrogen gas in the northwest direction. The black hole-driven outflow contains more than twice the mass and up to 15 times the power of the starburst-driven gas.

Nevertheless, Müller-Sánchez (University of Colorado) and colleagues report in the April 19 Nature that both contributions are necessary to halt star formation in the surrounding galaxy. Read more in The University of Colorado’s press release.

NGC 6240: Cosmic Butterfly
NGC 6240 as seen by the Hubble Space Telescope.
NASA / ESA / Hubble Heritage (STScI / AURA)-ESA / Hubble Collaboration / A. Evans (Univ. of Virginia, Charlottesville / NRAO / Stony Brook University)

— Monica Young

A Twist on Mars Moon Formation

Planetary scientists have long debated how Mars acquired its two diminutive moons, Phobos and Deimos. Although they look a lot like asteroids, their nearly circular orbits around Mars’s equator suggest they formed from a disk of debris the Red Planet coughed out when slammed by an interplanetary intruder, similar to Earth’s own natural satellite. But it’s been tough to make the numbers come out right.

Reporting April 18th in Science Advances, Robin Canup and Julier Salmon (Southwest Research Institute, Boulder) think a much smaller impactor might solve the problem. Using new computer simulations, they found that a wide-angle hit from an object with a mass between that of Vesta and two times that of Ceres—about one-tenth as in earlier studies—would throw the right amount of stuff into orbit, creating a disk from which the two moons ultimately formed. The energies involved might match the largest basins on Mars, including Utopia, Hellas, and maybe even the vast northern Borealis.

But confirmation will require knowing what Phobos and Deimos are made of: The simulations predict that most of their material came from Mars, not the impacting asteroid, and scientists don’t know for sure what the moons’ compositions are. The planned Japanese Martian Moons Exploration mission aims to nab a sample from Phobos and bring it back to Earth for study.

This 3-D simulation show that the impact initially produces a disk of orbiting debris primarily derived from Mars:

— Camille M. Carlisle

Stellar Thief Survives Supernova

Astronomers have long seen a particular type of supernova that’s lacking hydrogen — an odd characteristic, considering that stars start out with mostly hydrogen and helium. New Hubble observations confirm an old idea for why these stars would have lost their hydrogen gas before going boom.

As a star runs out of hydrogen to fuse in its core, it starts to burn heavier elements, such as helium, carbon, and oxygen. Meanwhile, around the core are layers of successively lighter elements; the outermost layer is hydrogen. If the dying star has a close companion, the companion can steal the outermost hydrogen layer, leaving only heavier elements for the eventual blast.

A competing idea is that a singleton might lose its outermost hydrogen layer via a fast stellar wind. Such stars probably do exist but can’t account for all hydrogen-poor supernovae.

Observations of a galaxy 40 million light-years away, NGC 7424, showed a star going supernova in 2001 (an event dubbed SN 2001ig). Its spectrum revealed little hydrogen. Twelve years later, long after the supernova had faded away, a Hubble image showed that a fainter star remained, having survived the explosion after gobbling its dying companion’s hydrogen. Stuart Ryder (Australian Astronomical Observatory) and colleagues estimate in the March 20th Astrophysical Journal that companions are responsible for roughly half of all stripped-envelope supernovae, with stellar winds being responsible for the rest. You can find more information in the Hubble press release.

Stellar Thief Survives Supernova Explosion
Seventeen years ago, astronomers witnessed supernova 2001ig go off 40 million light-years away in the galaxy NGC 7424, in the southern constellation Grus, the Crane. As the supernova’s glow faded, scientists focused Hubble on that location in 2016. They pinpointed and photographed a surviving companion, providing the best evidence yet that the companions had survived the blast.
NASA / ESA / S. Ryder (Australian Astronomical Observatory) / O. Fox (STScI)

— Monica Young


Image of misha17


April 30, 2018 at 8:39 pm

I think there a "typo" in this sentence: "Observations of a galaxy 40 million light-years away, NGC 7424, showed a star going supernova in 2001 (an event dubbed SN 2011ig)."

The supernova's designation contain the year it was discovered - it should be SN 2001ig if it was discovered in 2001.

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Image of Monica Young

Monica Young

May 2, 2018 at 11:54 am

Yes, that is a typo, thanks for the catch! I'll change the text.

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