Solar and Lunar Eclipses in 2016

This year there’s a total solar eclipse on March 9th (visible from Indonesia) and an annular on September 1st (central Africa). But the terrific “tetrad” of total lunar eclipses is over — we’ll see just two barely-there penumbral eclipses, on March 23rd and September 16th.
Any list of nature’s grandest spectacles would certainly include eclipses of the Sun and Moon. Up to seven of them can take place in one year, though the last time that happened was 1982. The fewest possible is four, as will be the case in 2016. Neither of the solar eclipses — one total and one annular — is observable from the Americas. And both lunar eclipses will involve extreme grazes of Earth’s shadow that will be challenging to notice at all.

Few events in nature offer the drama and spectacle of a total solar eclipse. This one occurred over China on August 1, 2008.
S&T: Dennis di Cicco

Why Do Eclipses Happen?
A solar eclipse, such as the one pictured at right, occurs only at new Moon, when the lunar disk passes directly between us and the Sun. Conversely, a lunar eclipse takes place during full Moon, when our satellite passes through Earth’s shadow.
These alignments don’t happen at every new and full Moon because the lunar orbit is tipped about 5° to Earth’s orbital plane — only occasionally do the Sun, Earth, and Moon line up exactly enough for an eclipse to occur. (The technical name for that, by the way, is syzygy.)
Three types of lunar eclipse are possible (total, partial, and penumbral) depending on how deeply the full Moon plunges into or near the umbra, our planet’s dark, central shadow.
If it goes all the way in, we see a total lunar eclipse that’s preceded and followed by partial phases. That was the case during the widely viewed event

Old Stars’ Fossil Fields

Astronomers have confirmed that strong magnetic fields are frozen in place deep inside aging stars.
Stars like the Sun puff up and become red giants towards the end of their lives, so they’re much larger even though the masses don’t really change. The red giants (“old” Suns) of the same mass as the Sun do not show strong magnetic fields in their interior, but for stars slightly more massive, up to 60% host strong magnetic fields. University of Sydney
Stars create magnetic fields through convection, the swirling, Ferris-wheel-like motion of hot, ionized gas (or boiling water, for that matter). Where convection happens in a star depends on how massive the star is: low-mass stars, including the Sun, have convective outer envelopes around a non-convective core, but stars a little bulkier — up to a couple Suns’ worth — do have convective cores.
Recently, Jim Fuller (Caltech) and colleagues found that strong core magnetic fields could explain the oddly weak, on-and-off brightening behavior of a sample of red giant stars. These stars are low- to middle-mass and have stopped fusing hydrogen in their centers, so they don’t have convective hearts. They also often have a mismatched, variable glow, with one hemisphere brightening as the other fades. What was strange about the sample the team looked at was that this group didn’t vary as much in brightness as it should have.
Now, Dennis Stello (University of Sydney and Aarhus University, Denmark), Fuller, and their team has expanded this work to 3,600 red giants, observed with the Kepler spacecraft. The astronomers found that here, too, some red giants had “muffled” variations, but just how much they were suppressed depended on how massive the star was. For stars just above the Sun’s  mass and lighter, the stars looked normal. But for the heftiest of the sample — 1.6 to

Setting Limits to Black Hole Gluttony

Black holes may have a limit to how much they can eat in the public eye.
Artist’s rendering of a supermassive black hole. The black hole itself is dark, but these beasts can be seen from across the observable universe by the light emitted from the accretion disks that feed them. NASA / JPL-Caltech
Even the most gluttonous black hole reaches a point when it pushes itself away from the public buffet line, preferring instead to sneak its treats on the sly..
The gluttony limit of a black hole is around 50 billion times the mass of the Sun, according to calculations by Andrew King (University of Leicester, UK, and University of Amsterdam, The Netherlands). By some deceptively simple reasoning published in the December 10, 2015, Monthly Notices of the Royal Astronomical Society, King shows that once a black hole reaches this mass, the disk of gas that acted as the black hole’s dinner buffet begins to crumble apart, collapsing under its own weight into stars.
The gaseous disk that feeds growing black holes is what enables us to see these dark objects, even from a distant universe less than 1 billion years old. Take away the gas and you take away the visible and ultraviolet light that signals a black hole’s gorging.
“If the black hole is very massive, then the gas disc would have to be correspondingly large and massive,” explains Zoltan Haiman (Columbia University). “The main idea in King’s paper is that above a certain mass, the gas in such a disk would be gravitationally unstable — i.e., it would collapse into clumps under its own weight, before the gas can funnel inward into the black hole.”
In other words, even the immense gravitational pull of a 50 billion solar-mass black hole can’t overcome the self-gravity that clumps up the surrounding matter.
“I find

Brightest Supernova Baffles Astronomers

The most luminous supernova ever discovered, ASASSN-15lh, challenges a popular theory for blazingly bright exploding stars.
The left image shows the yellow-orange host galaxy prior to the supernova’s discovery, captured by the Dark Energy Camera. The righthand image, from Las Cumbres Observatory Global Network, shows the supernova, whose blue light outshines its host. The Dark Energy Survey / B. Shappee / The ASAS-SN Team
About six months ago, we alerted readers to the discovery of the most luminous supernova ever. Now the discovery team is releasing additional information, and it strains even the most extreme physical scenarios.
The explosion of light initially appeared in June 2015 as the faintest of dots in automated images taken by the All-Sky Automated Survey for Supernovae (ASAS-SN), which repeatedly observes the same areas of sky to look for ephemeral bursts of light. Sophisticated software spotted the sudden but subtle influx of light, prompting astronomers to go fishing for follow-up observations at several telescopes. They soon found that the light from the source, dubbed ASASSN-15lh, had traveled for 2.8 billion years to reach Earth.
Due to its distance, ASASSN-15lh only reached about 17th magnitude at its brightest, but its luminosity outshone any supernova yet discovered. Even months later, this single object continues to emit more energy per second than all the stars in the Milky Way.
This image shows two of the 14-cm telescopes used in the All-Sky Automated Survey for SuperNovae that discovered ASASSN-15lh. Wayne Rosing
Subo Dong (Peking University, China) and colleagues released an update to the discovery data in January 15th’s Science. Following a spate of follow-up spectra, the team continued to track the supernova’s goings-on using the Las Cumbres Observatory Global Telescope Network (LCOGT; see the October 2012 issue of Sky & Telescope for more on this ambitious project) and the Swift space telescope. The light curves