American Physical Society meeting

Highlights from the April physics meeting, Denver, April 13-16

 Classifying the Crab Nebula supernova

The Crab Nebula, shown in a composite X-ray, visible-light and infrared view, may have finally yielded its secrets. An astronomer proposes that it is the product of a rare kind of supernova. X-ray: J. Hester/ASU, CXC; Optical: J. Hester and A. Loll/ASU, NASA, ESA; Infrared: R. Gehrz/U of Minn., JPL/NASA

In 1054, eyes turned to the sky as a giant star 6,500 light-years away exploded as a supernova. Today, what’s left behind is a colorful shell of gas and dust known as the Crab Nebula. Now an astronomer has laid out a blueprint of just how that star exploded.

The Crab supernova confounds astronomers because it packed less energy than a typical explosion of a star of its size, yet a large portion of that energy seemed to show up as visible light — so much so that Chinese astronomers reported seeing the supernova in the daytime sky for 23 days.

In an April 15 session and a paper posted on, astronomer Nathan Smith from the University of Arizona in Tucson argued that the Crab Nebula is the product of a rare type of supernova called a Type IIn-P. He surmises that a large star rich in oxygen, neon and magnesium exploded and sent out a shock wave that heated up interstellar material to temperatures that maximize the emission of visible light. The brightness of Type IIn-P explosions observed in 1994, 2009 and 2011 plateaued for a few months before plummeting, which Smith says is consistent with observations from 1054.

Fossils show bacteria may have eaten supernova’s iron

Ancient bacteria may have chewed up the radioactive remains of a star that exploded more than 2 million years ago, according to research presented April 14.

Only a few giant stars explode as supernovas close enough to Earth to deposit shrapnel on the planet. A rock recovered from the Pacific Ocean nearly a decade ago suggested one such supernova occurred about 2.2 million years ago. Researchers from the Technical University of Munich in Germany made that determination in 2004 by analyzing the rock’s concentration of iron-60, a radioactive isotope spewed by supernovas.

Shawn Bishop, a physicist from Technical University who was not involved in the prior research, wondered whether he could find any radioactive iron from the supernova in the fossil record. He looked to bacteria that live beneath the seafloor and process iron to produce magnetic crystals.

Bishop and his team obtained a sediment core from the Pacific seafloor that dates back to 3.3 million years ago. They extracted iron-60 from samples of the core and counted the atoms with a mass spectrometer. The radioactive isotope was nonexistent in most of the samples, but it showed up in very small amounts in samples from around the time of the supernova. The findings could indicate that bacteria harvested supernova-produced iron-60 atoms.

Black hole fires particle jet toward Earth

A burst of activity from a nearby supermassive black hole has drawn the gaze of a suite of telescopes, including NuSTAR, NASA’s newest space probe. The jet of energetic particles emanating from the black hole was a hot topic at an April 15 session on NuSTAR’s latest results.

The supermassive black hole, located 400 million light-years away in the galaxy Markarian 421, is inhaling enormous amounts of gas and dust. The black hole’s extreme gravity heats up that material, and some of it escapes at nearly the speed of light in a narrow jet, along with intense radiation. In the case of Markarian 421’s black hole, that jet is pointed straight at Earth.

NuSTAR, which is short for Nuclear Spectroscopic Telescope Array, is an X-ray telescope designed to explore the inner workings of black holes. It has been monitoring Markarian 421 since January, and on April 12 the black hole suddenly started beaming out up to100 times as much radiation as usual. NuSTAR astronomers contacted colleagues around the world through an online forum, and almost immediately as many as 20 telescopes began tracking Markarian 421’s emission of gamma rays, radio waves and visible light.

Caltech’s Fiona Harrison, NuSTAR’s principal investigator, says these combined observations will help astronomers understand where and how matter gets accelerated in the vicinity of an active black hole.

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