Astronomers have determined the pre-explosion mass of a white dwarf star that blew up thousands of years ago and the measurement strongly suggests the explosion involved only a single white dwarf, ruling out a well-established alternative scenario involving a pair of merging white dwarfs.
A set of enigmatic quasar ghosts mark the graves of these objects that flickered to life and then faded.

8 unusual looped structures orbit their host galaxies and glow in a bright and eerie goblin-green hue. The ethereal wisps in these images were illuminated, perhaps briefly, by a blast of radiation from a quasar - a very luminous and compact region that surrounds a supermassive black hole at the center of a galaxy - and Hubble was there to catch it. 

The first object of this type was found in 2007 by Dutch schoolteacher Hanny van Arkel participating in the Galaxy Zoo project, which catalogs the Sloan Digital Sky Survey (SDSS). The bizarre feature was dubbed Hanny's Voorwerp (Dutch for Hanny's object).
Cosmologists may have discovered what could be the precursors of the vast clusters of galaxies that we see today. Galaxies like our Milky Way, with its 100 billion stars, are usually not found in isolation. 13.8 billion years after the Big Bang, many are in dense clusters of tens, hundreds or even thousands of galaxies. 
The study of fluids in motion – now known as hydrodynamics – goes back to the Egyptians so it has been involved in a lot of experiments but now it has provided something new; experimental evidence that stars may generate...sound.
Dark matter is an umbrella term for matter that no one has directly detected but must be out there or physics at the very large scale makes even less sense than it makes now. Since it does not reflect, absorb or emit light, it is invisible, so whatever it 'is' is only known to exist via its gravitational effects on matter as we know it.
We may have Jupiter to thank for our unusual solar system. 

Before the inner planets we now call  Mercury, Venus, Earth, and Mars formed, a great inward-and-then-outward journey that Jupiter made early in the solar system's history may have torn apart a number of super-Earths - planets larger than Earth but smaller than Neptune - and caused their giant remnants to fall into the sun billions of years ago.
One of astronomy's big questions is why galaxies forming as recently as 1 billion years after the Big Bang contain so much dust. The leading hypothesis is that supernovae, stars that explode at the end of their lives, contain large amounts of metal-enriched material that, in turn, harbors key ingredients of dust, like silicon, iron and carbon.
In 1670, the greatest astronomers, including Cassini and Hevelius, the father of lunar cartography, documented the appearance of a new star in the skies.

Hevelius described it as nova sub capite Cygni — a new star below the head of the Swan — and now it is officially known it by the name Nova Vulpeculae 1670.  It lies within the boundaries of the modern constellation of Vulpecula (The Fox), just across the border from Cygnus (The Swan) and is also referred to as Nova Vul 1670 and CK Vulpeculae, its designation as a variable star. 
Historical accounts of novae are rare and Nova Vul 1670 is both the oldest recorded nova and the faintest nova when later recovered.
ESA’s Rosetta spacecraft has made the first measurement of molecular nitrogen at a comet,  Comet 67P/Churyumov–Gerasimenko,  providing clues about the temperature environment in which it formed. 
A new map of the Moon's strangest volcano show that its explosive eruption spread debris over an area in the Compton-Belkovich Volcanic Complex much greater than previously thought.

By mapping the radioactive element thorium which spewed out during the eruption, they discovered that, with the help of the Moon's low gravity, debris from the unnamed volcano was able to cover an area the size of Scotland, or around 70,000 km2. The eruption, which happened 3.5 billion years ago, threw rock five times further than the pyroclastic flow of molten rock and hot gases that buried the Roman city of Pompeii, the researchers added.