The team at Liverpool John Moores University in the UK used the European Southern Observatory's Very Large Telescope (ESO's VLT), situated in the Atacama Desert of northern Chile, to spot the black hole.
The newly found black hole was spotted lurking in NGC 1850, a cluster of thousands of stars roughly 160,000 light years away in the Large Magellanic Cloud -- a neighbour galaxy of the Milky Way. The detection in NGC 1850 marks the first time a black hole has been found in a young cluster of stars (the cluster is only around 100 million years old, a blink of an eye on astronomical scales).
According to Sara Saracino from the Liverpool John Moores University's Astrophysics Research Institute, the black hole is roughly 11 times as massive as our Sun. Astronomers started on the trail of this black hole due to its gravitational influence on the five-solar-mass star orbiting it.
Previously such small, "stellar-mass" black holes have been spotted in other galaxies by picking up the X-ray glow emitted as they swallow matter, or from the gravitational waves generated as black holes collide with one another or with neutron stars.
However, most stellar-mass black holes don't give away their presence through X-rays or gravitational waves.
This is the first time this detection method has been used to reveal the presence of a black hole outside of our galaxy. The method could be key to unveiling hidden black holes in the Milky Way and nearby galaxies, and to help shed light on how these mysterious objects form and evolve, the team said.
"Every single detection we make will be important for our future understanding of stellar clusters and the black holes in them," said co-author Mark Gieles from the University of Barcelona, Spain.
Called Palomar 5, the globular cluster was discovered in 1950. It is in the Serpens constellation at a distance of about 80,000 light years, and it is one of the roughly 150 globular clusters that orbit around the Milky Way.
It is older than 10 billion years, like most other globular clusters, meaning that it formed in the earliest phases of galaxy formation. It is about 10 times less massive and five times more extended than a typical globular cluster and in the final stages of dissolution.
"The number of black holes is roughly three times larger than expected from the number of stars in the cluster, and it means that more than 20 per cent of the total cluster mass is made up of black holes. They each have a mass of about 20 times the mass of the Sun, and they formed in supernova explosions at the end of the lives of massive stars, when the cluster was still very young," said lead author Mark Gieles, from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB).
In the study, published in the journal Nature Astronomy, the team found that Palomar 5 formed with a lower black hole fraction, but stars escaped more efficiently than black holes, such that the black hole fraction gradually increased.
The black holes dynamically puffed up the cluster in gravitational slingshot interactions with stars, which led to even more escaping stars. Just before it completely dissolves -- roughly a billion years from now, the cluster will consist entirely of black holes.
"This work has helped us understand that even though the fluffy Palomar 5 cluster has the brightest and longest tails of any cluster in the Milky Way, it is not unique. Instead, we believe that many similarly puffed up, black hole-dominated clusters have already disintegrated in the Milky Way tides to form the recently discovered thin stellar streams," said Dr Denis Erkal at the University of Surrey.
Tidal streams are streams of stars that were ejected from disrupting star clusters or dwarf galaxies. In the last few years, nearly thirty thin streams have been discovered in the Milky Way halo.
The neutron stars merged in a galaxy called NGC 4993, located about 130 million light years from Earth in the constellation Hydra.
Seven new papers describe the first-ever detection of light from a gravitational wave source.
The event, caused by two neutron stars colliding and merging together, was dubbed "GW170817" because it sent ripples through space-time that reached Earth on August 17.
Around the world, hundreds of excited astronomers mobilised quickly and were able to observe the event using numerous telescopes, providing a wealth of new data.
Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier in October.
However, black hole mergers are not expected to produce any electromagnetic radiation (light), meaning they cannot be detected by conventional telescopes.
In contrast, binary neutron star (NS-NS) mergers have long been expected to produce an energetic explosion and a plume of radioactive material, generating light, but have never previously been detected.
These mergers provide important clues as to how matter behaves under these extreme conditions.
The papers published online in Science describe how light from the NS-NS merger was precisely located, subsequent observations at X-ray, ultraviolet (UV), optical, infrared (IR) and radio wavelengths, and theoretical analysis of the event.
"This is a milestone in the growing effort by scientists worldwide to unlock the mysteries of the universe and of earth," said Professor Ehud Nakar of Tel Aviv University's Raymond and Beverly Sackler School of Physics and Astronomy.
A neutron star forms when a star much bigger and brighter than the sun exhausts its thermonuclear fuel supply and explodes into a violent supernova.
The explosion of neutron stars, which are made almost entirely of neutrons, was detected by multiple telescopes across the electromagnetic spectrum, from gamma rays and visible light to radio waves.
The merger is the first cosmological event observed in both gravitational waves--ripples in the fabric of space-time--and the entire spectrum of light, from gamma rays to radio waves.
Gravitational waves from the event arrived first at the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Hanford, Washington, and Livingston, Louisiana.
Because of the orientation of the neutron star pair, the newly operational Virgo detector, located near Pisa, Italy, observed a weaker signal.
Less than two seconds later, the Gamma-ray Burst Monitor on NASA's Fermi Gamma-ray Space Telescope detected a short burst of gamma rays.
A rapid analysis of these signals enabled the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration to locate the signal in a region covering less than 0.1 per cent of the total sky area as viewed from Earth.
Fermi independently identified a larger area consistent with that identified by LIGO and Virgo.
Astronomers around the globe then directed more than 70 space- and ground-based telescopes toward the event for follow-up observations.
"This is the most intensely observed astronomical event in history," said Peter Shawhan, a professor of physics at University of Maryland (UMD) and an LSC principal investigator.
"For the binary black hole mergers LIGO has already observed, the signals were much shorter--just a fraction of a second," said Alessandra Buonanno, a UMD College Park Professor of Physics and LSC principal investigator.
"We were able to track the evolution of GW170817 over time in a way we could not achieve for two black holes. As we continue to examine the signal, this wealth of data will likely allow us to pursue new and more stringent gravitational tests, which could put Einstein's theory of general relativity in question."
Theorists have predicted that when neutron stars collide, they should give off gravitational waves and gamma rays, along with powerful jets that emit light across the electromagnetic spectrum.
The new observations confirm that at least some short gamma-ray bursts are generated by the merging of neutron stars--something that was only theorized before.
But while one mystery appears to be solved, new mysteries have emerged.
The observed short gamma-ray burst was one of the closest to Earth seen so far, yet it was surprisingly weak for its distance.
Scientists are beginning to propose models for why this might be the case.
"While the results from this event are beyond my wildest dreams, the most exciting part is that this is really just the beginning," said Brad Cenko, principal investigator of the Swift Gamma-ray Burst Mission at NASA's Goddard Space Flight Centre.
"We use the Milky Way and its surroundings to study absolutely everything," said Marla Geha of Yale University in the US and lead author of the paper published in the Astrophysical Journal.
"Hundreds of studies come out every year about dark matter, cosmology, star formation, and galaxy formation, using the Milky Way as a guide. But it's possible that the Milky Way is an outlier," Geha said.
The Milky Way is host to several dozen smaller galaxy satellites.
These smaller galaxies orbit around the Milky Way and are useful in understanding the Milky Way itself.
Early results from the Satellites Around Galactic Analogs (SAGA) Survey, supported by the US National Science Foundation indicate that the Milky Way's satellites are much more tranquil than other systems of comparable luminosity and environment.
Many satellites of those "sibling" galaxies are actively pumping out new stars, but the Milky Way's satellites are mostly inert, the researchers found.
This is significant, according to the researchers, because many models for what we know about the universe rely on galaxies behaving in a fashion similar to the Milky Way.
The SAGA Survey began five years ago with a goal of studying the satellite galaxies around 100 Milky Way siblings.
Thus far it has studied eight other Milky Way sibling systems.
Data from the Gaia-ESO project has provided evidence backing theoretically predicted divisions in the chemical composition of the stars that make up the Milky Way's disc - vast collection of giant gas clouds and billions of stars that give our galaxy its 'flying saucer' shape.
The research suggests that stars in the inner regions of the galactic disc were the first to form, supporting ideas that our galaxy grew from the inside-out.
An international team of astronomers took detailed observations of stars with a wide range of ages and locations in the galactic disc to accurately determine their 'metallicity': the amount of chemical elements in a star other than hydrogen and helium, the two elements most stars are made from.
Immediately after the Big Bang, the universe consisted almost entirely of hydrogen and helium, with levels of "contaminant metals" growing over time.
Consequently, older stars have fewer elements in their make-up - so have lower metallicity, researchers said.
The team have shown that older, 'metal-poor' stars inside the Solar Circle - the orbit of our Sun around the centre of the Milky Way - are far more likely to have high levels of magnesium.
The higher level of the element inside the Solar Circle suggests this area contained more stars that "lived fast and die young" in the past, researchers said.
The stars that lie in the outer regions of the galactic disc are predominantly younger, both 'metal-rich' and 'metal-poor', and have surprisingly low magnesium levels compared to their metallicity.
This discovery signifies important differences in stellar evolution across the Milky Way disc, with very efficient and short star formation timescales occurring inside the Solar Circle; whereas, outside the Sun's orbit, star formation took much longer.
"We have been able to shed new light on the timescale of chemical enrichment across the Milky Way disc, showing that outer regions of the disc take a much longer time to form," said Maria Bergemann from Cambridge University's Institute of Astronomy, who led the study.
The study appears in the journal Astronomy and Astrophysics.
ARIES Director Wahab Uddin described the finding of the new variable stars, whose luminosity or brightness keeps changing, as a "rare achievement".
It is for the first time that variable stars have been identified in Globular cluster NGC 4147 in the constellation of Coma Berenices, said former director of ARIES, Anil Pande, who now works at the institute as a senior scientist.
"The discovery by ARIES scientists could be crucial in advancing knowledge about the composition of globular clusters in general," Pande told PTI.
Besides the detection of new variable stars, the study also provides important insights into the internal structure of NGC 4147 which is located closer to the Earth than previously thought, he said.
"The ARIES team of researchers led by Dr. Sneh Lata and Dr. A K Pandey performed photometric observations of globular cluster NGC 4147 using 3.6 metre Devasthal Optical Telescope (DOT) established in 2016 near Nainital," Pande said.
A variable star is a star whose luminosity or brightness changes.
"This change may be caused due to periodical swelling and shrinking of the star or occasional block of light due to an eclipse by some other solar body," he said.
NGC 4147 was discovered by British astronomer William Herschel in 1784, who described the globular cluster as "very bright, pretty large, gradually brighter in the middle."
This is a relatively small globular cluster, ranking 112th in luminosity among the Milky Way globular cluster population, Pande said.
A globular cluster is a spherical collection of stars that orbit a galactic core, as a satellite.
Globular clusters are very tightly bound by gravity, which gives them their spherical shapes, and relatively high stellar densities towards their centres, he said.
Such clusters are found in the halo of the galaxy and contain considerably more and older stars, according to Pande.
Although it appears that globular clusters contain oldest population of the galaxy, their origins and their role in galactic evolution are still unclear, he said.
The detailed findings of the research will be published in the August issue of the Astronomical Journal.
The star, called AG Carinae, is a few million years old and resides 20,000 light-years away inside our Milky Way galaxy. It is estimated to be up to 70 times more massive than our Sun and shines with the blinding brilliance of one million suns.
The price for its opulence is “living on the edge.” It is waging a tug-of-war between gravity and radiation to avoid self-destruction.
Hubble’s sharp vision revealed the most prominent features of AG Carinae — filamentary structures shaped like tadpoles and lopsided bubbles. These structures are dust clumps illuminated by the star’s reflected light.
The tadpole-shaped features, most prominent at left and bottom, are denser dust clumps that have been sculpted by the stellar wind.
The image was taken in visible and ultraviolet light. Ultraviolet light offers a slightly clearer view of the filamentary dust structures that extend all the way down toward the star. Hubble is ideally suited for ultraviolet-light observations because this wavelength range can only be viewed from space.
The mammoth star was created from one or more giant eruptions about 10,000 years ago. The star’s outer layers were blown into space — like a boiling teapot popping off its lid. The expelled material amounts to roughly 10 times our Sun’s mass.
These outbursts are the typical life of a rare breed of star called a luminous blue variable.
Like many other luminous blue variables, AG Carinae remains unstable. It has experienced lesser outbursts that have not been as powerful as the one that created the present nebula.
Although AG Carinae is quiescent now, as a super-hot star it continues pouring out searing radiation and powerful stellar wind (streams of charged particles). This outflow continues shaping the ancient nebula, sculpting intricate structures as outflowing gas slams into the slower-moving outer nebula.
Massive stars, like AG Carinae, are important to astronomers because of their far-reaching effects on their environment.