Forget Betelgeuse, the Star V Sagittae Should Go Nova Within this Century

The star V Sagittae is the next candidate to explode in stellar pyrotechnics, and a team of astronomers set the year for that cataclysmic explosion at 2083, or thereabouts. V Sagittae is in the constellation Sagitta (latin for arrow,) a dim and barely discernible constellation in the northern sky. V Sagittae is about 1100 light years from Earth.

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This Star Has Reached the End of its Life

About 10,000 light years away, in the constellation Centaurus, is a planetary nebula called NGC 5307. A planetary nebula is the remnant of a star like our Sun, when it has reached what can be described as the end of its life. This Hubble image of NGC 5307 not only makes you wonder about the star’s past, it makes you ponder the future of our very own Sun.

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This Star Has Been Going Nova Every Year, for Millions of Years

A nova star is like a vampire that siphons gas from its binary partner. As it does so, the gas is compressed and heated, and eventually it explodes. The remnant gas shell from that explosion expands outward and is lit up by the stars at the center of it all. Most of these novae explode about once every 10 years.

But now astrophysicists have discovered one remnant so large that the star that created it must have been erupting yearly for millions of years.

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A Guide to Hunting Zombie Stars

R Aquarii is called a symbiotic star system because of their relationship. As the white dwarf draws in material from the Red Giant, it ejects some if it in weird looping patterns, seen in this Hubble image. Image Credit: By Judy Schmidt from USA - Symbiotic System R Aquarii, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=63473035

Apparently not all supernovas work. And when they fail, they leave behind a half-chewed remnant, still burning from leftover heat but otherwise lifeless: a zombie star. Astronomers aren’t sure how many of these should-be-dead creatures lurk in the interstellar depths, but with recent simulations scientists are making a list of their telltale signatures so that future surveys can potentially track them down.

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You’re Looking at an Actual Image of a White Dwarf Feeding on Material from a Larger Red Giant, 650 Light Years from Earth.

This image is from the SPHERE/ZIMPOL observations of R Aquarii, and shows the binary star itself, with the white dwarf feeding on material from the Mira variable, as well as the jets of material spewing from the stellar couple. Image Credit: ESO/Schmid et al.

The SPHERE planet-hunting instrument on the European Southern Observatory’s Very Large Telescope captured this image of a white dwarf feeding on its companion star, a type of Red Giant called a Mira variable. Most stars exist in binary systems, and they spend an eternity serenely orbiting their common center of gravity. But something almost sinister is going on between these two.

Astronomers at the ESO have been observing the pair for years and have uncovered what they call a “peculiar story.” The Red Giant is a Mira variable, meaning it’s near the end of its life, and it’s pulsing up to 1,000 times as bright as our Sun. Each time it pulses, its gaseous envelope expands, and the smaller White Dwarf strips material from the Red Giant.

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348 Years Ago, a French Astronomer Monk Might have Witnessed the Collision Between a White and Brown Dwarf Star

This hourglass-shaped figure is named CK Vulpeculae. It was discovered by French Monk-Astronomer Per Dom Anthelme in 1670. A new study identifies it as the remnant of a collision between a white dwarf and a brown dwarf. Image Credit: ALMA (ESO/NAOJ/NRAO)/S. P. S. Eyres

There’s something poignant and haunting about ancient astronomers documenting things in the sky whose nature they could only guess at. It’s true in the case of Père Dom Anthelme, who in 1670 saw a star suddenly burst into view near the head of the constellation Cygnus, the Swan. The object was visible with the naked eye for two years, as it flared in the sky repeatedly. Then it went dark. We call that object CK Vulpeculae.

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

For the sake of studying the most distant objects in the Universe, astronomers often rely on a technique known as Gravitational Lensing. Based on the principles of Einstein’s Theory of General Relativity, this technique involves relying on a large distribution of matter (such as a galaxy cluster or star) to magnify the light coming from a distant object, thereby making it appear brighter and larger.

However, in recent years, astronomers have found other uses for this technique as well. For instance, a team of scientists from the Harvard-Smithsonian Center for Astrophysics (CfA) recently determined that Gravitational Lensing could also be used to determine the mass of white dwarf stars. This discovery could lead to a new era in astronomy where the mass of fainter objects can be determined.

The study which details their findings, titled “Predicting gravitational lensing by stellar remnants” appeared in the Monthly Noticed of the Royal Astronomical Society. The study was led by Alexander J. Harding of the CfA and included Rosanne Di Stefano, and Claire Baker (also from the CfA), as well as members from the University of Southampton, Georgia State University, the University of Nigeria, and Cornell University.

A Hubble image of the white dwarf star PM I12506+4110E (the bright object, seen in black in this negative print) and its field which includes two distant stars PM12-MLC1&2. Credit: Harding et al./NASA/HST

To put it simply, determining the mass of an astronomical object is one the greatest challenges for astronomers. Until now, the most successful method relied on binary systems because the orbital parameters of these systems depend on the masses of the two objects. Unfortunately, objects that are at the end states of stellar evolution – like black holes, neutron stars or white dwarfs – are often too faint or isolated to be detectable.

This is unfortunate, since these objects are responsible for a lot of dramatic astronomical events. These include the accretion of material, the emission of energetic radiation, gravitational waves, gamma-ray bursts, or supernovae. Many of these events are still a mystery to astronomers or the study of them is still in its infancy – i.e. gravitational waves. As they state in their study:

“Gravitational lensing provides an alternative approach to mass measurement. It has the advantage of only relying on the light from a background source, and can therefore be employed even for dark lenses. In fact, since light from the lens can interfere with the detection of lensing effects, compact objects are ideal lenses.”

As they go on to state, of the 18,000 lensing events that have been detected to date, roughly 10 to 15% are believed to have been caused by compact objects. However, scientists are unable to tell which of the detected events were due to compact lenses. For the sake of their study then, the team sought to circumvent this problem by identifying local compact objects and predicting when they might produce a lensing event so they could be studied.

Animation showing the white dwarf star Stein 2051B as it passes in front of a distant background star. Credit: NASA

“By focusing on pre-selected compact objects in the near vicinity of the Sun, we ensure that the lensing event will be caused by a white dwarf, neutron star, or black hole,” they state. “Furthermore, the distance and proper motion of the lens can be accurately measured prior to the event, or else afterwards. Armed with this information, the lensing light curve allows one to accurately measure the mass of the lens.”

In the end, the team determined that lensing events could be predicted from thousands of local objects. These include 250 neutron stars, 5 black holes, and roughly 35,000 white dwarfs. Neutron stars and black holes present a challenge since the known populations are too small and their proper motions and/or distances are not generally known.

But in the case of white dwarfs, the authors anticipate that they will provide for many lensing opportunities in the future. Based on the general motions of the white dwarfs across the sky, they obtained a statistical estimate that about 30-50 lensing events will take place per decade that could be spotted by the Hubble Space Telescope, the ESA’s Gaia mission, or NASA’s James Webb Space Telescope (JWST). As they state in their conclusions:

“We find that the detection of lensing events due to white dwarfs can certainly be observed during the next decade by both Gaia and HST. Photometric events will occur, but to detect them will require observations of the positions of hundreds to thousands of far-flung white dwarfs. As we learn the positions, distances to, and proper motions of larger numbers of white dwarfs through the completion of surveys such as Gaia and through ongoing and new wide-field surveys, the situation will continue to improve.”

The future of astronomy does indeed seem bright. Between improvements in technology, methodology, and the deployment of next-generation space and ground-based telescopes, there is no shortage of opportunities to see and learn more.

Further Reading: CfA, MNRAS