May 2022 has been an extraordinary month in astronomy and cosmology. From getting a glimpse at what the black hole of our parent galaxy looks like to obtaining the most precise rate of the universe’s expansion, here is a list of top five discoveries that made it to headlines in May 2022.
The first image of the Milky Way’s black hole
On May 12, 2022, astronomers unveiled the first-ever image of Sagittarius A*, the supermassive black hole at the center of our Milky Way galaxy. This picture was produced using sub-millimeter radio waves by the Event Horizon Telescope, a planet-wide array of observatories.
The image has a central dark region which shows the shadow cast by the black hole onto the gas, while the surrounding gas is glowing as a bright ring. The bright spots in the ring typically show the areas of hotter gas that the black hole will probably suck up one day. The image came after a collective effort made by more than 200 scientists and engineers working with telescopes worldwide.
The team used radio telescopes spread across the globe and collected data on Sgr A* for five nights in 2017. The data collected over these five nights was so much that it could not be shared through the internet, and only physical hard drives could be used to transfer it to their destination for processing. Supercomputers further produced millions of different images, each being a different but viable version of the black hole based on the laws of physics. They were then blended to create the final image that the world saw.
Sagittarius A* lies at the heart of the Milky Way in the constellation Sagittarius, almost 26,000 light-years away from us. It is 4.1 million times more massive than the Sun and is thus analogous to many of the hundreds of billions of black holes residing at the center of their host galaxies in the universe. Its deep analysis can help us understand gravity’s behavior at the universal level.
The most precise value ever of the Hubble constant
As an outcome of a scientific collaboration called SH0ES (Supernova, H0, for the Equation of State of Dark Energy), researchers claim to have obtained the most precise value of the Hubble constant ever, placed at 73 ± 1 km/s/Mpc. Represented by Ho, the Hubble constant is a critical number in cosmology. It originates in the Hubble-Lemaître law, which states that the farther a galaxy is in deep space, the faster it’s receding away from us. Mathematically, if v is the velocity of recession and d is the distance to the galaxy, then v ∝ d. The proportionality constant that equates these quantities is called the Hubble constant Ho, and the equation v = Ho d is popularly called Hubble’s law.
Over the years, various observatories looking at different areas of the universe have produced varying results for the value of the Hubble constant. There are two ways to calculate the value of this constant: via observations or mathematical calculations. The observational values consider the parameters like a galaxy’s recessional velocity and its distance from us. While the recessional velocity can be obtained from spectroscopic measurements of the cosmological redshift, the latter’s evaluation uses the Cepheid variable or Type 1a supernovae.
On the other hand, the mathematical predictions are based on the study of the cosmic microwave background data and how the universe expanded just after the big bang. However, the results obtained by both these methods have been quite different, with the mathematical predictions converging around 67-68 Km/s/Mpc and observational values lying between 72-74 km/s/Mpc.
But this time, researchers reviewed all the data, taking over 1,000 Hubble orbits into account. The team analyzed 42 supernova milepost markers that are exploding at about one per year, thereby marking a complete analysis of all the supernovae accessible to the Hubble telescope so far. Finally, they converged on a Hubble constant estimate of 73 ± 1 km/s/Mpc. Although this is still higher than the previous predictions of 67.5 plus or minus 0.5 km/s/Mpc made from the temperature fluctuation measurements of the cosmic microwave background, the measurement is about eight times more precise than Hubble’s expected capability. And given the large Hubble sample size this time, there is only a one-in-a-million chance of the new estimate being wrong. The cause of the discrepancy in the observational and the calculated values remains uncertain, but the new results are expected to open the door to discovering new physics.
Pluto’s chaotic yet stable orbit
Pluto follows a highly elliptical orbit, which is further inclined 17° to the Solar System’s ecliptic plane. Due to the eccentric nature of its trajectory, Pluto orbits closer to the Sun than Neptune for almost 20 years during each orbit. In doing so, Pluto moves inside Neptune’s orbit for a few years. Still, it manages to pass by the ice giant without colliding with it safely. Usually, two phenomena are held responsible for Pluto’s chaotic yet stable orbit. The first is azimuthal libration which describes that whenever Pluto crosses Neptune’s orbit, it is always at least 90 degrees away from Neptune. The second is latitude libration, which ensures that when Pluto reaches its closest point to Neptune or any other giant planets, it always stays high above them and the solar system’s plane.
However, recently researchers claimed to have obtained a better understanding of these phenomena. They used eight different combinations of giant planet perturbations to run the simulations. The results revealed that as Neptune and Pluto exist in a 3:2 orbital resonance (a property according to which for every two orbits of Neptune around the Sun, Pluto completes two orbits), Neptune has a major influence on Pluto’s azimuthal libration.
But as far as Pluto’s latitudinal libration is concerned, Neptune doesn’t contribute much. On the other hand, Uranus’ gravity destabilizes both the azimuthal and latitudinal constraints. If Pluto’s orbit were only governed by Neptune’s and Uranus’ gravity, it would have become unstable after tens or hundreds of millions of years, leading Pluto to either collide with Neptune or fling entirely out of the solar system.
So the big question was how Pluto still has a stable orbit? And as per the new study, the answer to this question lies in Jupiter’s and a little bit of Saturn’s gravitational pull. It was found that despite being farther from Pluto than Neptune and Uranus, their gravity is still strong enough to dominate the system. Even Jupiter’s solo gravitational influence is enough to keep Pluto’s orbit stable for at least 5 billion years. These simulations can help trace evidence for the existence of lost planets that got ejected from the Solar System billions of years ago and explain the origins of Pluto’s orbit and those of other bodies with high orbital inclinations.
A black hole’s magnetic reversal
For the first time, researchers have probably witnessed an event where a black hole showed magnetic reversal, which means that the north pole became the south pole and vice versa. Researchers observed a galaxy known as 1ES 1927+654, which briefly ceased X-ray emissions for a few months, but after that period, the emissions resumed and increased.
Most galaxies have a supermassive black hole at their center, which accretes the surrounding matter. As the matter gets accumulated in the accretion disc surrounding the black hole, it gets heated up, eventually emitting light in visible, ultraviolet, and X-ray wavelengths as the matter is pushed further inwards. In doing so, the incoming matter forms a cloud of extremely hot particles, a region referred to as corona.
Researchers used NASA’s Neil Gehrels Swift Observatory and the European Space Agency’s XMM-Newton satellite, along with others, to track the changes in ultraviolet, X-rays, radio, and optical outputs of the Galaxy 1ES 1927+654. The X-rays streaming from the heart of galaxy temporarily disappeared for four months and re-emerged in October 2018. Further, the galaxy returned to pre-eruption X-ray emissions in summer 2021.
The new study proposes that the fluctuations were possibly due to the changes in the corona of the galaxy’s central black hole. As the corona began to diminish and the accretion disk grew more compact in the center, it made the UV and visible light increase towards the center of the galaxy. Further, as the flip evolved, the field weakened enough for the corona to no longer support the radiations, thereby allowing the X-ray emissions to cease.
A “Gold standard star” in the Milky Way
Astronomers have discovered a rare star that contains 65 chemical elements, the most number found in any object beyond the solar system. Forty-two (around two-thirds) of these are r-process elements lying at the bottom of the periodic table that can only form in high-energy, dynamic environments such as neutron star mergers.
Usually, elements form in the core of the stars as fusion reactions take place, climbing the ladder from hydrogen to heavier elements. But r-process requires the presence of elements such as iron at the beginning itself. Then, neutrons are added to the nuclei of iron-like elements, creating heavier elements such as selenium, silver, tellurium, platinum, gold, thorium, etc.
HD 222925 is an old star located around 1,460 light-years away. It has passed the red giant stage of its lifetime, which means that it has run out of hydrogen and is fusing helium in its core. This makes it a metal-poor star that should be low in heavier elements. So, detecting the abundance of heavier r-process elements suggests that some neutron star collision or a violent supernova probably occurred back in time, and the observed elements got distributed throughout the molecular cloud of hydrogen and helium from which HD 222925 eventually formed, around 8.2 billion years ago.
The 42 observed r-process elements include gallium, selenium, cadmium, tungsten, platinum, gold, lead, uranium, and others. As HD 222925 demonstrates no other strange behavior in its chemical composition, it can be considered a representative of the outcomes of the r-process.