Topic: Early September Astronomy Bulletin

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INCOMING OBJECT IS OLD NASA SPACECRAFT
University of Hawaii

With all sorts of rocks flying around willy-nilly in the space around Earth, telescopes around the world are keeping a careful eye on the sky to make sure we're not in any danger. So when the University of Arizona's Catalina Sky Survey and the University of Hawaii's Asteroid Terrestrial-impact Last Alert System spotted what seemed to be a small, incoming object on an impact trajectory with Earth on August 25, scientists sat up and paid attention. However, closer inspection revealed it wasn't an asteroid after all. Almost 56 years after it was launched into space, NASA's Orbiting Geophysics Observatory 1 (OGO-1) was finally falling from the sky. On Saturday at 20:44 UTC, it did so, NASA has confirmed, harmlessly burning up on atmospheric entry in a shower of flaming debris. OGO-1 started its career in early September 1964, when it arrived in equatorial Earth orbit. It, and the other five satellites in the OGO series, were designed to study our planet, its magnetosphere, its atmosphere, the space between Earth and the Moon, and the effect of the Sun on near-Earth space. The satellite spent five years collecting data for its mission before it was no longer operating efficiently enough to justify continuing. It was placed in standby mode in November of 1969, and officially decommissioned in November of 1971.

Since then, the 487-kilogramme satellite has been a lifeless chunk of machinery slowly but surely descending towards Earth. That's because its eccentric orbit brought it close enough to Earth that even minute amounts of atmospheric drag hundreds of kilometres above the planet incrementally brought it lower and lower. This is normal procedure for decommissioned satellites, to remove them from the
crowded space around our planet, thus reducing the risk of collisions in space, which can generate smaller, more dangerous space debris. However, although OGO-1 was the first launched in its series, it was the last OGO in the sky - OGO-2 through OGO-6 safely deorbited starting in 1972, with OGO-5 (launched in 1968) coming down in 2011. All burned up on re-entry, their debris falling into the sea. This is what happened with OGO-1, too. Although its entry was 25 minutes earlier and a little farther east than predicted, it burned up over the South Pacific ocean, around 160 kilometres southeast of Tahiti, in French Polynesia, NASA said.


DOES OUR SUN HAVE A LONG-LOST TWIN?
Center for Astrophysics at Harvard & Smithsonian

The strange configuration of material in the outer reaches of our solar system has led a team of scientists to speculate that the Sun had a companion during its early days. Intriguingly, this scenario could explain the presence of the hypothesized Planet Nine, should it actually exist. Our Sun’s hypothetical twin is long gone, but traces of it can be seen in the overabundance of material located within the outer Oort Cloud according to new research. The Oort cloud is the most distant region
in the solar system, residing much farther than the outer planets and the Kuiper Belt.  Unlike the Kuiper Belt, which is shaped like a doughnut, the Oort cloud is a massive and thick spherical shell that envelopes the entire solar system. The inner Oort cloud starts at around 1,000 AU from the Sun (in which 1 AU is the average distance from Earth to the Sun), while its outer edge stops at around 100,000 AU. This region of space is filled with billions, possibly trillions, of rocky and icy objects left over from the formation of the solar system. According to the new paper, the overabundance of material presumed to exist in the outer Oort cloud is the result of our Sun’s early stint as a binary system. To date, computers trying to simulate the formation of the solar system have failed to reproduce the proportion of objects seen in the outer realms of the Oort cloud and the scattered disc—a specific population of trans-Neptunian objects outside of the Kuiper Belt. As a result, the origin of the outer Oort cloud is “an unsolved mystery. The new paper presents an elegant solution to the overpopulation problem: a second sun.

A stellar companion to the Sun would increase the chance of trapping objects from the birth cluster of the Sun. The Sun and its companion act as a fishing net that traps objects gravitationally as they pass near one of the two stars and lose energy by kicking it slightly. By birth cluster, astronomers refer to a cluster of stars that arose together in the same molecular cloud, also known as a stellar nursery. Star clusters eventually scatter, either because of strong stellar winds or tidal gravitational forces exerted by the Milky Way galaxy itself. The Sun’s hypothesized twin would have been pulled far, far away. The popular theory associates the origin of the Oort cloud with debris left over from the formation of the solar system. Objects were scattered by the planets to great distances. But this model has difficulty reproducing the observed ratio between the scattered disk population of objects and the more spherical Oort cloud. Our model can account for that ratio. The hypothesized second sun, in order to trap this excess material, would require a mass comparable to our own Sun. So, basically a twin. The two stars would have been roughly 1,000 AU apart, according to the new model. Copious amounts of celestial material would have been lost when the two stars separated, but the team contend that enough material remained to explain the Oort cloud. Passing stars in the birth cluster were likely responsible for separating the Sun from its presumed companion, but not before our solar system captured its outer population of objects, namely the Oort cloud and—quite possibly—the elusive Planet Nine. This gigantic hypothetical planet is thought to exist in the outer solar system owing to the peculiar clustering of certain Kuiper Belt objects. A going origin story for Planet Nine is that it formed as a gas giant in the inner solar system, but it got shoved into the outer solar system after straying too close to Jupiter. The new paper offers an alternative scenario: Planet Nine was captured by our solar system.


COSMIC RAYS TO GET WORSE
Spaceweather.com

Cosmic rays are bad–and they’re going to get worse. That’s the conclusion of a new study entitled “Galactic Cosmic Radiation in Interplanetary Space Through a Modern Secular Minimum” just published in the journal Space Weather. During the next solar cycle, we could see cosmic ray dose rates increase by as much as 75%, which will limit the amount of time astronauts can work safely in interplanetary space.  Cosmic rays are the bane of astronauts. They come from deep space, energetic
particles hurled in all directions by supernova explosions and other violent events.  No amount of spacecraft shielding can stop the most energetic cosmic rays, leaving astronauts exposed whenever they leave the Earth-Moon system. Back in the 1990s, astronauts could travel through space for as much as 1000 days before they hit NASA safety limits on radiation exposure. Not anymore. According to the new research, cosmic rays could limit trips to as little as 290 days for 45-year old male astronauts, and 204 days for females. (Men and women have different limits because of unequal dangers to reproductive organs.) Why are cosmic rays growing stronger? Blame the Sun. The Sun’s magnetic field wraps the entire solar system in a protective bubble, normally shielding us from cosmic rays. In recent decades, however, that shield has been growing weaker–a result of the sputtering solar cycle.  In the 1950s to 1990s, the Sun routinely produced intense Solar Maxima with lots of sunspots and strong solar magnetic fields. Since the heyday of the late 20th century, the 11-year solar cycle has weakened, and the Sun’s magnetic field has weakened with it. Scientists believe we could be entering a Grand Minimum–a long, slow dampening of the 11-year solar cycle, which can suppress sunspot counts for decades. The most famous example of a Grand Minimum is the Maunder Minimum of the 17th century when sunspots practically vanished for 70 years. We are not in a Maunder Minimum, the current situation more closely resembles the Dalton minimum of 1790-1830 or the Gleissberg minimum of 1890-1920.” During those lesser Grand Minima, the solar cycle became weak, but didn’t completely go away.

For years, researchers have been monitoring cosmic rays using CRaTER, a sensor orbiting the Moon on board NASA’s Lunar Reconnaissance Orbiter (LRO). Recent data show that cosmic rays are at very high levels–the highest since LRO was launched in 2009. Researchers took the latest readings from CRaTER and extrapolated them forward into Solar Cycle 25 (the next solar cycle) and found that radiation doses will probably exceed already-high values by 34% for a Gleissberg-like minimum to 75% for a Dalton-like minimum. For astronauts, it begs the question — How much can you get done in 200 days? Barring improvements in shielding technology, future space missions may be limited to only 6 or 7 months, probably too short for a Mars voyage. Lunar exploration could be safer because the body of the Moon itself blocks radiation. But in interplanetary space, the researchers caution, “the next decade or two may be more dangerous than previously realized.


GALACTIC BAR PARADOX RESOLVED
RAS

New light has been shed on a mysterious and long-standing conundrum at the very heart of our galaxy. The new work offers a potential solution to the so-called ‘Galactic bar paradox’, whereby different observations produce contradictory estimates of the motion of the central regions of the Milky Way. The majority of spiral galaxies, like our home the Milky Way, host a large bar-like structure of stars in their centre. Knowledge of the true bar size and rotational speed is crucial for understanding how galaxies form and evolve, as well as how they form similar bars throughout the Universe. However our galaxy’s bar size and rotational speed have been strongly contested in the last 5 years; while studies of the motions of stars near the Sun find a bar that is both fast and small, direct observations of the Galactic central region agree on one that is significantly slower and larger. The new study suggests an insightful solution to this discrepancy. Analysing state-of-the-art galaxy formation simulations of the Milky Way, they show that both the bar’s size and its rotational speed fluctuate rapidly in time, causing the bar to appear up to twice as long and rotate 20 percent faster at certain times. The bar pulsations result from its regular encounters with the Galactic spiral arms, in what can be described as a “cosmic dance”. As the bar and spiral arm approach each other, their mutual attraction due to gravity makes the bar slow down and the spiral speed up. Once connected, the two structures move as one and the bar appears much longer and slower than it actually is. As the dancers split apart, the bar speeds up while the spiral slows back down.

The controversy about the Galactic bar can then be simply resolved if we happen to be living at a time when the bar and spiral are connected, giving the illusion of a large and slow bar. However the motion of the stars near the Sun remains governed by the bar’s true, much smaller nature, and so those observations appear contradictory.  Recent observations have confirmed that the inner Milky Way spiral arm is currently connected to the bar, which happens about once every 80 million years according to the simulations. Data from the forthcoming 3rd data release of the Gaia mission will be able to test this model further, and future missions will discover if the dance goes on in other galaxies across the Universe.



RARE ENCOUNTERS BETWEEN COSMIC HEAVYWEIGHTS
W. M. Keck Observatory

A cosmic dance between two merging galaxies, each one containing a supermassive black hole that's rapidly feeding on so much material it creates a phenomenon known as a quasar, is a rare find. Astronomers have discovered several pairs of such merging galaxies, or luminous "dual" quasars, using three Maunakea Observatories in Hawaii -- Subaru Telescope, W. M. Keck Observatory, and Gemini Observatory. These dual quasars are so rare, a research team led by the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo estimates only 0.3% of all known quasars have two supermassive black holes that are on a collision course with each other. In spite of their rarity, they represent an important stage in the evolution of galaxies, where the central giant is awakened, gaining mass, and potentially impacting the growth of its host galaxy. Quasars are one of the most luminous, energetic objects known in the universe, powered by supermassive black holes that are millions to billions times more massive than our Sun. As material swirls around a black hole at the centre of a galaxy, it is heated to high temperatures, releasing so much light that the quasar can outshine its host galaxy. This makes a merging pair of galaxies with quasar activity hard to detect; it is difficult to separate the light from the two quasars because they are in such close proximity to each other. Also, observing a wide enough area of the sky to catch these rare events in sufficient numbers is a challenge.

To overcome these obstacles, the team took advantage of a sensitive wide survey of the sky using the Hyper Suprime-Cam (HSC) camera on the Subaru Telescope.  To make the job easier, the team started by looking at the 34,476 known quasars from the Sloan Digital Sky Survey with HSC imaging to identify those having two (or more) distinct centres. The team identified 421 promising cases. However, there
was still the chance many of these were not bona-fide dual quasars but rather chance projections such as starlight from our own galaxy. Confirmation required detailed analysis of the light from the candidates to search for definitive signs of two distinct quasars. Using Keck Observatory's Low Resolution Imaging Spectrometer (LRIS) and Gemini Observatory's Near-Infrared Integral Field Spectrometer, astronomers identified three dual quasars, two of which were previously unknown. Each object in the pair showed the signature of gas moving at thousands of kilometres per second under the influence of a supermassive black hole. The newly-discovered dual quasars demonstrate the promise of wide-area imaging combined with high-resolution spectroscopic observations to reveal these elusive objects, which are key to better understanding the growth of galaxies and their supermassive black holes.


THE MOST MASSIVE GRAVITATIONAL-WAVE SOURCE YET
Massachusetts Institute of Technology

For all its vast emptiness, the Universe is humming with activity in the form of gravitational waves. Produced by extreme astrophysical phenomena, these reverberations ripple forth and shake the fabric of space-time, like the clang of a cosmic bell. Now researchers have detected a signal from what may be the most massive black hole merger yet observed in gravitational waves. The product of the merger is the first clear detection of an "intermediate-mass" black hole, with a mass between 100 and 1,000 times that of the Sun. They detected the signal, which they have labelled GW190521, on May 21, 2019, with the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO), a pair of identical, 4-kilometre-long interferometers in the United States; and Virgo, a 3-kilometre-long detector in Italy. The signal, resembling about four short wiggles, is extremely brief in duration, lasting less than one-tenth of a second. From what the researchers can tell, GW190521 was generated by a source that is roughly 5 gigaparsecs away, when the Universe was about half its age, making it one of the most distant gravitational-wave sources detected so far. As for what produced this signal, based on a powerful suite of state-of-the-art computational and modelling tools, scientists think that GW190521 was most likely generated by a binary black hole merger with unusual properties. Almost every confirmed gravitational-wave signal to date has been from a binary merger, either between two black holes or two neutron stars.  This newest merger appears to be the most massive yet, involving two inspiralling black holes with masses about 85 and 66 times the mass of the Sun.

The LIGO-Virgo team has also measured each black hole's spin and discovered that as the black holes were circling ever closer together, they could have been spinning about their own axes, at angles that were out of alignment with the axis of their orbit. The black holes' misaligned spins likely caused their orbits to wobble, or "precess," as the two Goliaths spiralled toward each other. The new signal likely represents the instant that the two black holes merged. The merger created an even more massive black hole, of about 142 solar masses, and released an enormous amount of energy, equivalent to around 8 solar masses, spread across the Universe in the form of gravitational waves. The uniquely large masses of the two inspiralling black holes, as well as the final black hole, raise a slew of questions regarding their formation. All of the black holes observed to date fit within either of two categories: stellar-mass black holes, which measure from a few solar masses up to tens of solar masses and are thought to form when massive stars die; or supermassive black holes, such as the one at the centre of the Milky Way galaxy, that are from hundreds of thousands, to billions of times that of our Sun. However, the final 142-solar-mass black hole produced by the GW190521 merger lies within an intermediate mass range between stellar-mass and supermassive black holes -- the first of its kind ever detected. The two progenitor black holes that produced the final black hole also seem to be unique in their size. They're so massive that scientists suspect one or both of them may not have formed from a collapsing star, as most stellar-mass black holes do. According to the physics of stellar evolution, outward pressure from the photons and gas in a star's core support it against the force of gravity pushing inward, so that the star is stable, like the Sun. After the core of a massive star fuses nuclei as heavy as iron, it can no longer produce enough pressure to support the outer layers. When this outward pressure is less than gravity, the star collapses under its own weight, in an explosion called a core-collapse supernova, that can leave behind a black hole.

This process can explain how stars as massive as 130 solar masses can produce black holes that are up to 65 solar masses. But for heavier stars, a phenomenon known as "pair instability" is thought to kick in. When the core's photons become extremely energetic, they can morph into an electron and antielectron pair. These pairs generate less pressure than photons, causing the star to become unstable against gravitational collapse, and the resulting explosion is strong enough to leave
nothing behind. Even more massive stars, above 200 solar masses, would eventually collapse directly into a black hole of at least 120 solar masses. A collapsing star, then, should not be able to produce a black hole between approximately 65 and 120 solar masses -- a range that is known as the "pair instability mass gap." But now, the heavier of the two black holes that produced the GW190521 signal, at 85 solar masses, is the first so far detected within the pair instability mass gap. One possibility, which the researchers consider in their second paper, is of a hierarchical merger, in which the two progenitor black holes themselves may have formed from the merging of two smaller black holes, before migrating together and eventually merging.