Astronomers Figure Out How to use Gravitational Lensing to Measure the Mass of White Dwarfs

An international team of scientists has found a way to use white dwarf stars as gravitational lenses, which will allow astronomers to measure their mass.

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Bright Binocular Nova Discovered in Lupus

Source: Stellarium

On September 20, a particular spot in the constellation Lupus the Wolf was blank of any stars brighter than 17.5 magnitude. Four nights later, as if by some magic trick, a star bright enough to be seen in binoculars popped into view. While we await official confirmation, the star’s spectrum, its tattle-tale rainbow of light, indicates it’s a nova, a sun in the throes of a thermonuclear explosion.

The nova, dubbed ASASSN-16kt for now, was discovered during the ongoing All Sky Automated Survey for SuperNovae (ASAS-SN or “Assassin”), using data from the quadruple 14-cm “Cassius” telescope in CTIO, Chile. Krzysztof  Stanek and team reported the new star in Astronomical Telegram #9538. By the evening of September 23 local time, the object had risen to magnitude +9.1, and it’s currently +6.8. So let’s see — that’s about an 11-magnitude jump or a 24,000-fold increase in brightness! And it’s still on the rise.

The star is located at R.A. 15h 29?, –44° 49.7? in the southern constellation Lupus the Wolf. Even at this low declination, the star would clear the southern horizon from places like Chicago and further south, but in late September Lupus is low in the southwestern sky. To see the nova you’ll need a clear horizon in that direction and observe from the far southern U.S. and points south. If you’ve planned a trip to the Caribbean or Hawaii in the coming weeks, your timing couldn’t have been better!

I’ve drawn the map for Key West, one of southernmost locations on the U.S. mainland, where the nova stands about 7-8° high in late twilight, but you might also see it from southern Texas and the bottom of Arizona. Other locales include northern Africa, Finding a good horizon is key. Observers across Central and South America, Africa, India, s. Asia and Australia, where the star is higher up in the western sky at nightfall, are favored.

Nova means “new”, but a nova isn’t a brand new star coming to life but rather an explosion that occurs on the surface of an otherwise faint star no one’s taken notice of – until the blast causes it to brighten 50,000 to 100,000 times.

A nova occurs in a close binary star system, where a small but extremely dense and massive (for its size) white dwarf siphons hydrogen gas from its closely-orbiting companion. After whirling around in a flattened accretion disk around the dwarf, the material gets funneled down to the star’s 150,000 F° surface where gravity compacts and heats the gas until it detonates in a titanic thermonuclear explosion. Suddenly, a faint star that wasn’t on anyone’s radar vaults a dozen magnitudes to become a standout “new star”.

Novae are relatively rare and almost always found in the plane of the Milky Way, where the stars are most concentrated. The more stars, the greater the chances of finding one in a nova outburst. Roughly a handful a year are discovered, many of those in Scorpius and Sagittarius, in the direction of the galactic bulge.

We’ll keep tabs on this new object and report back with more information and photos as they become available. You can follow the new celebrity as well as print out finder charts on the American Association of Variable Star Observers (AAVSO) website by typing ASASSN-16kt in the info boxes.

I sure wish I wasn’t stuck in Minnesota right now or I’d be staring down the wolf’s new star!

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An Old Glass Plate Hints at a Potential New Exoplanet Discovery

Polluted white dwarf

What’s the value to exoplanet science of sifting through old astronomical observations? Quite a lot, as a recent discovery out of the Carnegie Institution for Science demonstrates. A glass plate spectrum of a nearby solitary white dwarf known as Van Maanen’s Star shows evidence of rocky debris ringing the system, giving rise to a state only recently recognized as a ‘polluted white dwarf.’

First, let’s set the record straight. This isn’t, as many news outlets have reported, a new exoplanet discovery per se… or even an old pre-discovery of a known world. Astronomers have yet to nab a bona fide exoplanet orbiting Van Maanen’s Star. But obviously, something interesting is going on in the system that merits closer scrutiny.

The discovery: it all started when astronomer Jay Farihi of University College London requested early plate observations of the star from the Carnegie Institute. Dating from 1917, the plate shows the bar code-looking spectrum of the star. Astronomer Walter Adams captured the image from the Mount Wilson observatory, noting on the sleeve that the ‘ordinary’ looking star (Van Maanen’s Star wasn’t identified as a white dwarf until 1923) was perhaps merely a bit hotter than our own Sun.

But to Farihi’s trained eye, something was up with Van Maanen’s star. Specifically, it was the presence of the third set of absorption lines between the standard pair that showed evidence of calcium, magnesium and iron —materials that should have long since sunk down to the dense core of the degenerate star. Somehow, these heavy — remember, to an astronomer, the periodic table consists of hydrogen, helium and ‘metals’ — were being replenished from above.

“The unexpected realization that this 1917 plate from our archive contains the earliest recorded evidence of a polluted white dwarf system is just incredible,” says Carnegie Observatory director John Mulchaey in a recent press release. “And the fact that it was made by such a prominent astronomer in our history as Walter Adams enhances the excitement.”

The very fact that this crucial bit of evidence was sitting on a plate locked away in a vault for a decade is amazing. We now know that rocky rings of debris around white dwarf stars can give rise to what’s known as polluted white dwarfs. And where there’s debris, there are often planets. As newer exoplanet hunters such as TESS, JWST, WFIRST, LSST and the Gemini Planet Imager begin to scour the skies, we wouldn’t be at all surprised if Van Maanen’s Star turned out to have planets.

The Carnegie Institute maintains a collection of 250,000 glass plates taken from the Las Campanas, Mount Wilson and Palomar observatories dating back over century. These stellar spectra were painstakingly all examined by ‘Mk-1 eyeball,’ and enabled early astronomers such as Annie Jump Cannon and Henrietta Swan Leavitt to categorize stars by color and temperature and identify standard distance candles known as Cepheid variables. Both concepts are still used by astronomers today.

Finding Van Maanen’s Star

Located 14 light years distant, the high proper motion of Van Maanen’s star was first noted by Adriaan Van Maanen in 1917, the same year the plate was made. A high proper motion hints that a star is located near our solar neighborhood. Van Maanen’s Star is the third white dwarf discovered (after Sirius B and 40 Eridani B) and the third closest to our Sun (after Sirius B and Procyon B). Van Maanen’s Star also holds the distinction of being the closest solitary white dwarf to our solar system.

Located in the constellation Pisces, Van Maanen’s Star shines at magnitude +12.4. It also made our handy list of white dwarf stars for backyard telescopes.

Many false alarms of claimed exoplanet discoveries dot the history of 20th century astronomy. One of the most notorious were the claims of a planet orbiting Barnard’s Star, betrayed by supposed wobbles detected in its high proper motion. The first true modern exoplanet was actually a trio discovered orbiting the pulsar PSR B1257+12 in 1994. Ironically, though the exoplanet tally now sits at 2108 and counting, no known worlds have been identified around Barnard’s star.

What other future secrets do those old glass plates hold? “We have a ton of history sitting in our basement,” says Mulchaey in this month’s press release. “Who knows what other finds we might unearth in the future?”

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Weekly Space Hangout – May 15, 2015: Finding, Studying and Visiting Other Worlds!

Host: Fraser Cain (@fcain) Guests: Jolene Creighton (@jolene723 / Brian Koberlein (@briankoberlein / Dave Dickinson (@astroguyz / Morgan Rehnberg ( / @MorganRehnberg ) Alessondra Springmann (@sondy) (…)Read the rest of Weekly Space Hangout – May 15, 2015: Finding, Studying and Visiting Other Worlds! (501 words) © Fraser for Universe Today, 2015. | […]

New Binocular Nova Discovered in Sagittarius

Looks like the Sagittarius Teapot’s got a new whistle. On March 15, John Seach of Chatsworth Island, NSW, Australia discovered a probable nova in the heart of the constellation using a DSLR camera and fast 50mm lens. Checks revealed no bright asteroid or variable star at the location. At the time, the new object glowed at the […]

Two Stars On A Death Spiral Set To Detonate As A Supernova

Two white dwarfs circle around one other, locked in a fatal tango. With an intimate orbit and a hefty combined mass, the pair is ultimately destined to collide, merge, and erupt in a titanic explosion: a Type Ia supernova. Or so goes the theory behind the infamous “standard candles” of cosmology. Now, in a paper published in today’s issue of […]

End the Year with a Bang! See a Bright Supernova in Virgo

A 14th magnitude supernova discovered in the spiral galaxy NGC 4666 earlier this month has recently brightened to 11th magnitude, making it not only the second brightest supernova of the year, but an easy find in an 8-inch or larger telescope. I made a special trip into the cold this morning for a look and saw […]