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.
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.
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.
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.
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.
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.
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.
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.
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.”
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:
Searching the Universe for strange new star systems can lead to some pretty interesting finds. And sometimes, it can turn up phenomena that contradict everything we think we know about the formation and evolution of stars. Such finds are not only fascinating and exciting, they allow us the chance to expand and refine our models of how the Universe came to be.
For instance, a recent study conducted by an international team of scientists has shown how the recent discovery of binary system – a millisecond pulsar and a low-mass white dwarf (LMWD) – has defied conventional ideas of stellar evolution. Whereas such systems were believed to have circular orbits in the past, the white dwarf in this particular binary orbits the pulsar with extreme eccentricity!
To break it down, conventional wisdom states that LMWDs are the product of binary evolution. The reason for this is because that under normal circumstances, such a star – with low mass but incredible density – would only form after it has exhausted all its nuclear fuel and lost its outer layers as a planetary nebula. Given the mass of this star, this would take about 100 billion years to happen on its own – i.e. longer than the age of the Universe.
As such, they are generally believed to be the result of pairing with other stars – specifically, millisecond radio pulsars (MSPs). These are a distinct population of neutron stars that have fast spin periods and magnetic fields that are several orders of magnitude weaker than that of “normal” pulsars. These properties are thought to be the result of mass transfer with a companion star.
Basically, MSPs that are orbited by a star will slowly strip them of their mass, sucking off their outer layers and turning them into a white dwarf. The addition of this mass to the pulsar causes it to spin faster and buries its magnetic field, and also strips the companion star down to a white dwarf. In this scenario, the eccentricity of orbit of the LMWD around the pulsar is expected to be negligible.
However, when looking to the binary star system PSR J2234+0511, the international team noticed something entirely different. Here, they found a low-mass white dwarf paired with a millisecond pulsar which the white dwarf orbited with a period of 32 days and an extreme eccentricity (0.13). Since this defies current models of white dwarf stars, the team began looking for explanations.
“Millisecond pulsar-LMWD binaries are very common. According to the established formation scenario, these systems evolve from low-mass X-ray binaries in which a neutron star accretes matter from a giant star. Eventually, this star evolves into a white dwarf and the neutron star becomes a millisecond pulsar. Because of the strong tidal forces during the mass-transfer episode, the orbits of these systems are extremely circular, with eccentricities of ~0.000001 or so.”
In addition, they consulted recent studies that looked at other binary star systems that show this same kind of eccentric relationship. “We now know [of] 5 systems which deviate from this picture in that they have eccentricities of ~0.1 i.e. several orders of magnitude larger that what is expected in the standard scenario,” said Antoniadis. “Interestingly, they all appear to have similar eccentricities and orbital periods.”
From this, they were able to infer the temperature (8600 ± 190 K) and velocity ( km/s) of the white dwarf companion in the binary star system. Combined with constraints placed on the two body’s masses – 0.28 Solar Masses for the white dwarf and 1.4 for the pulsar – as well as their radii and surface gravity, they then tested three possible explanations for how this system came to be.
These included the possibility that neutrons stars (such as the millsecond pulsar being observed here) form through an accretion-induced collapse of a massive white dwarf. Similarly, they considered whether neutron stars undergo a transformation as they accrete material, which results in them becoming quark stars. During this process, the release of gravitational energy would be responsible for inducing the observed eccentricity.
Second, they considered the possibility – consistent with current models of stellar evolution – that LMWDs within a certain mass range have strong stellar winds when they are very young (due to unstable hydrogen fusion). The team therefore looked at whether or not these strong stellar winds could have been what disrupted the orbit of the pulsar earlier in the system’s history.
Last, they considered the possibility that some of the material released from the white dwarf in the past (due to this same stellar wind) could have formed a short-lived circumbinary disk. This disk would then act like a third body, disturbing the system and increasing the eccentricity of the white dwarf’s orbit. In the end, they deemed that the first two scenarios were unlikely, since the mass inferred for the pulsar progenitor was not consistent with either model.
However, the third scenario, in which interaction with a circumbinary disk was responsible for the eccentricity, was consistent with their inferred parameters. What’s more, the third scenario predicts how (within a certain mass range) that there should be no circular binaries with similar orbital periods – which is consistent with all known examples of such systems. As Dr. Antoniadis explained:
“These observations show that the companion star in this system is indeed a low-mass white dwarf. In addition, the mass of the pulsar seems to be too low for #2 and a bit too high for #1. We also study the orbit of the binary in the Milky way, and it looks very similar to what we find for low-mass X-ray binaries. These pieces of evidence together favor the disk hypothesis.”
Of course, Dr. Antoniadis and his colleagues admit that more information is needed before their hypothesis can be deemed correct. However, should their results be borne out by future research, then they anticipate that it will be a valuable tool for future astronomers and astrophysicists looking to study the interaction between binary star systems and circumbinary disks.
In addition, the discovery of this high eccentricity binary system will make it easier to measure the masses of Low-Mass White Dwarfs with extreme precision in the coming years. This in turn should help astronomers to better understand the properties of these stars and what leads to their formation.
As history has taught us, understanding the Universe requires a serious commitment to the process of continuous discovery. And the more we discover, the stranger it seems to become, forcing us to reconsider what we think we know about it.