Pulsars are the lighthouses of the universe. These rotating dead stars shoot twin jets of radiation from their poles, usually with a predictable rhythm. But sometimes pulsars behave strangely, and one pulsar in particular has had astronomers scratching their heads for years. It’s called PSR J1023+0038, and a decade ago, it shut off its jets and began oscillating between two brightness levels in an unpredictable pattern. Now, scientists think they understand why: it is busy eating a neighboring star.
Artist view of an active supermassive black hole. Credit: ESO/L. Calçada
Black holes swallow everything—including light—which explains why we can’t see them. But we can observe their immediate surroundings and learn about them. And when they’re on a feeding binge, their surroundings become even more luminous and observable.
This increased luminosity allowed astronomers to find a black hole that was feasting on material only 800 million years after the Universe began.
The cross-hairs mark the location of the newly discovered monster black hole. Credit: Sloan Digital Sky Survey/S. Chakrabart et al.
Black holes are among the most awesome and mysterious objects in the known Universe. These gravitational behemoths form when massive stars undergo gravitational collapse at the end of their lifespans and shed their outer layers in a massive explosion (a supernova). Meanwhile, the stellar remnant becomes so dense that the curvature of spacetime becomes infinite in its vicinity and its gravity so intense that nothing (not even light) can escape its surface. This makes them impossible to observe using conventional optical telescopes that study objects in visible light.
As a result, astronomers typically search for black holes in non-visible wavelengths or by observing their effect on objects in their vicinity. After consulting the Gaia Data Release 3 (DR3), a team of astronomers led by the University of Alabama Huntsville (UAH) recently observed a black hole in our cosmic backyard. As they describe in their study, this monster black hole is roughly twelve times the mass of our Sun and located about 1,550 light-years from Earth. Because of its mass and relative proximity, this black hole presents opportunities for astrophysicists.
Protostellar disk around a star at the galactic center. Image credit: Lu et al.
Astronomers using the ALMA Observatory have discovered an unusual, massive star near the center of our galaxy, a star that has two spiral arms. The arms are part of an accretion disk, a broad disk of dust and gas surrounding the protostar. While this is not the first star to be seen with such rare arm-like features, researchers say they believe they can track the formation of the spiral arms to a close encounter the star had with another object.
Fifty years ago, English mathematical physicist and Nobel-prize winner Roger Penrose proposed that energy could be extracted from the space around a rotating black hole. Known as the ergosphere, this region lies just outside an event horizon, the boundary within which nothing can escape a black hole’s gravitational pull (even light). It is also here where infalling matter is accelerated to incredible speeds and emits all kinds of energy.
This became known as the Penrose Process, which many theorists have since expanded on. The latest comes from a study conducted by researchers from Columbia University and the Universidad Adolfo Ibáñez in Chile. With support from organizations like NASA, they demonstrated how a better understanding of the physics at work around spinning black holes could allow us to harness their energy someday.
If we could see the blazar 3C 354.3 up close it would look something like this. A bright accretion disk surrounds a black hole. Twin jets of radiation beam from the center. Credit: Cosmovision
In March 2018 astronomers watched a massive black hole surge in brightness. Then over the following year, its ring of light dimmed to near-invisibility before regaining its former strength. The potential culprit? The black hole swallowing an entire star.
MAXI J1820+070 is a binary pair that has one black hole and one star. The black hole is emitting relativistic jets, and Chandra made a movie of it. Image Credit: Chandra X-Ray Observatory
The Chandra X-Ray Observatory has spotted a distant black hole shooting out jets of material, at close to the speed of light. No worries, this beast is about 10,000 light years away from us. It’s more of a spectacle than a danger.
But it’s a spectacle laden with scientific insights.
Artist's impression of a Star feeding a black hole. Credit: ESO/L. Calçada
Black holes are famous for being inescapable. Within the event horizon of these celestial objects, matter and even light enter and then disappear forever. However, beyond the event horizon, black holes are known to form accretion disks from which light can escape. In fact, this is how astronomers are able to confirm the presence of black holes and determine their properties (i.e. mass, spin rate, etc.)
However, according to a recent NASA-funded study led by researchers from the California Institute of Technology (Caltech), there is evidence that not all light emanating from a black hole’s disk simply escapes. According to their observations, some of the light escaping from the disk is pulled back in by the black hole’s gravity and reflected off the disk again. These observations confirm something astronomers have theorized for about forty years.
Scientists using ALMA have for the first time captured an image of the cool gas near the black hole in the Milky Way. Image Credit: NRAO/AUI/NSF; S. Dagnello
There’s a lot going on at the center of our galaxy. A supermassive black hole named Sagittarius A-Star resides there, drawing material in with its inexorable gravitational attraction. In that mind-bending neighbourhood, where the laws of physics are stretched beyond comprehension, astronomers have detected a ring of cool gas.
An illustration of an accretion disk feeding a central young star, or protostar, and the gaseous jet ejected from the protostar. Credit: Yin-Chih Tsai/ASIAA
According to the Nebula Hypothesis, stars and their systems of planets form from giant clouds of dust and gas. After undergoing gravitational collapse at the center (which creates the star), the remaining matter then forms an accretion disk in orbit around it. Over time, this matter is fed to the star – allowing it to become more massive – and also leads to the creation of a system of planets.
And until this week, the Nebula Hypothesis was just that. Given the distance involved, and the fact that the formation of star systems takes billions of years, being able to witness the process at various stages is quite difficult. But thanks to the efforts of team of researchers from the U.S. and Taiwan, astronomers have now captured the first clear image of a young star surrounded by an accretion disk.
This artist’s concept shows a young stellar object and the whirling accretion disk surrounding it. NASA/JPL-Caltech
The protostellar system in question is known as HH 212, a young star system (40,000 years old) located in the Orion constellation, roughly 1300 light-years from Earth. This star system is noted for its powerful bipolar jet – i.e. the continuous flows of ionized gas from its poles – which is believed to cause it to accrete matter more efficiently. Due to its age and its position relative to Earth, this protostar system has been a popular target for astronomers in the past.
Basically, the fact that it is still in an early phase of formation (and the fact that it can be viewed edge-on) make the star system ideal for studying the evolution of low-mass stars. However, previous searches had a maximum resolution of 200 AU, which meant astronomers were only able to get a hint of a small dusty disk. This disk appeared as a flattened envelope, spiraling towards the protostar at the center.
But with ALMA’s resolution (8 AU, or 25 times higher), the research team was not only able to detect the accretion disk, but also able to spatially resolve its dust emissions at submillimeter wavelength. As Chin-Fei Lee – a research fellow at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) in Taiwan and the lead author on the paper – said in an ALMA press release:
“It is so amazing to see such a detailed structure of a very young accretion disk. For many years, astronomers have been searching for accretion disks in the earliest phase of star formation, to determine their structure, how they are formed, and how the accretion process takes place. Now using the ALMA with its full power of resolution, we not only detect an accretion disk but also resolve it, especially its vertical structure, in detail.”
Jet and disk in the HH 212 protostellar system: (a) A composite image of the jet, produced by combining images from different telescopes. (b) Close-up of the center of the dusty disk at 8 AU resolution. (c) An accretion disk model that can reproduce the observed dust emission in the disk. Credit: ALMA (ESO/NAOJ/NRAO)/Lee et al.
What they observed was a disk that has a radius of roughly 60 astronomical units, which is slightly greater than the distance from the Sun and the outer edge of the Kuiper Belt (50 AU). They also noted that the disk was compromised of silicate minerals, iron and other interstellar matter, and consisted of a prominent equatorial dark layer that was sandwiched between two brighter layers.
This contrast between light and dark sections was due to relatively low temperatures and high optical depth near the central plane of the disk. Meanwhile, the layers above and below the central plane showed greater absorption in both the optical and near-infrared light wavelengths. Because of this layered appearance, the research team described it as looking like “a hamburger”.
These observations are exciting news for the astronomical community, and not just because they are a first. In addition, they also represent a new opportunity to study small disks around the youngest protostars. And with the kinds of high-resolution imaging made possibly by ALMA and other next-generation telescopes, astronomers will be able to place new and stronger constraints on theories pertaining to disk formation.
As Zhi-Yun Li from University of Virginia (the co-author on the study) put it:
“In the earliest phase of star formation, there are theoretical difficulties in producing such a disk, because magnetic fields can slow down the rotation of collapsing material, preventing such a disk from forming around a very young protostar. This new finding implies that the retarding effect of magnetic fields in disk formation may not be as efficient as we thought before.”
A chance to watch stars and planetary systems in their earliest phase of formation and a chance to test our theories about how it’s all done? Definitely not something that happens every day!
And be sure to enjoy this video of the observation, courtesy of ALMA and narrated by Dr. Lee:
