From Asimov’s ‘Nightfall’

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Gamma-ray Bursts: Black Hole Birth Announcements

Gamma-ray bursts are the brightest, most violent explosions in the universe, but they can be surprisingly tricky to detect. Our eyes can’t see them because they are tuned to just a limited portion of the types of light that exist, but thanks to technology, we can even see the highest-energy form of light in the cosmos — gamma rays.

So how did we discover gamma-ray bursts? 

Accidentally!

We didn’t actually develop gamma-ray detectors to peer at the universe — we were keeping an eye on our neighbors! During the Cold War, the United States and the former Soviet Union both signed the Nuclear Test Ban Treaty of 1963 that stated neither nation would test nuclear weapons in space. Just one week later, the US launched the first Vela satellite to ensure the treaty wasn’t being violated. What they saw instead were gamma-ray events happening out in the cosmos!

Things Going Bump in the Cosmos

Each of these gamma-ray events, dubbed “gamma-ray bursts” or GRBs, lasted such a short time that information was very difficult to gather. For decades their origins, locations and causes remained a cosmic mystery, but in recent years we’ve been able to figure out a lot about GRBs. They come in two flavors: short-duration (less than two seconds) and long-duration (two seconds or more). Short and long bursts seem to be caused by different cosmic events, but the end result is thought to be the birth of a black hole.

Short GRBs are created by binary neutron star mergers. Neutron stars are the superdense leftover cores of really massive stars that have gone supernova. When two of them crash together (long after they’ve gone supernova) the collision releases a spectacular amount of energy before producing a black hole. Astronomers suspect something similar may occur in a merger between a neutron star and an already-existing black hole.

Long GRBs account for most of the bursts we see and can be created when an extremely massive star goes supernova and launches jets of material at nearly the speed of light (though not every supernova will produce a GRB). They can last just a few seconds or several minutes, though some extremely long GRBs have been known to last for hours!

A Gamma-Ray Burst a Day Sends Waves of Light Our Way!

Our Fermi Gamma-ray Space Telescope detects a GRB nearly every day, but there are actually many more happening — we just can’t see them! In a GRB, the gamma rays are shot out in a narrow beam. We have to be lined up just right in order to detect them, because not all bursts are beamed toward us — when we see one it’s because we’re looking right down the barrel of the gamma-ray gun. Scientists estimate that there are at least 50 times more GRBs happening each day than we detect!

So what’s left after a GRB — just a solitary black hole? Since GRBs usually last only a matter of seconds, it’s very difficult to study them in-depth. Fortunately, each one leaves an afterglow that can last for hours or even years in extreme cases. Afterglows are created when the GRB jets run into material surrounding the star. Because that material slows the jets down, we see lower-energy light, like X-rays and radio waves, that can take a while to fade. Afterglows are so important in helping us understand more about GRBs that our Neil Gehrels Swift Observatory was specifically designed to study them!

Last fall, we had the opportunity to learn even more from a gamma-ray burst than usual! From 130 million light-years away, Fermi witnessed a pair of neutron stars collide, creating a spectacular short GRB. What made this burst extra special was the fact that ground-based gravitational wave detectors LIGO and Virgo caught the same event, linking light and gravitational waves to the same source for the first time ever!

For over 10 years now, Fermi has been exploring the gamma-ray universe. Thanks to Fermi, scientists are learning more about the fundamental physics of the cosmos, from dark matter to the nature of space-time and beyond. Discover more about how we’ll be celebrating Fermi’s achievements all year!

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Megalith Cist Burials Found in Southern India

Archaeologists have uncovered a total of 250 cairn circles in southern India’s trade and industrial center of Kodumanal, which was inhabited from the 400s through first century B.C.E.

The cairn circles were made of giant rocks, or megaliths. Most of the cairn circles were around rectangular chambers built of megaliths, which in turn contained burial cists and three or four bowls or pots. The pottery was likely for offerings placed outside the burial cists, showing a belief system that included something after death.

An impressive ten pots and bowls were recently unearthed in a larger circle made of boulders and rectangular-shaped cists made of stone slabs, surrounding a three-chambered burial. This larger, more complex burial might have been intended for someone important in the community.

Sh2-101,  Cygnus 

Hubble’s Guide to Viewing Deep Fields

They say a picture is worth a thousand words, but no images have left a greater impact on our understanding of the universe quite like the Hubble Space Telescope’s deep fields. Like time machines, these iconic images transport humanity billions of light-years back in time, offering a glimpse into the early universe and insight into galaxy evolution!

You’ve probably seen these images before, but what exactly do we see within them? Deep field images are basically core samples of our universe. By peering into a small portion of the night sky, we embark on a journey through space and time as thousands of galaxies appear before our very eyes.

So, how can a telescope the size of a school bus orbiting 340 miles above Earth uncover these mind-boggling galactic masterpieces? We’re here to break it down. Here’s Hubble’s step-by-step guide to viewing deep fields:

Step 1: Aim at the darkness

Believe it or not, capturing the light of a thousand galaxies actually begins in the dark. To observe extremely faint galaxies in the farthest corners of the cosmos, we need minimal light interference from nearby stars and other celestial objects. The key is to point Hubble’s camera at a dark patch of sky, away from the outer-edge glow of our own galaxy and removed from the path of our planet, the Sun, or the Moon. This “empty” black canvas of space will eventually transform into a stunning cosmic mosaic of galaxies.

The first deep field image was captured in 1995. In order to see far beyond nearby galaxies, Hubble’s camera focused on a relatively empty patch of sky within the constellation Ursa Major. The results were this step-shaped image, an extraordinary display of nearly 3,000 galaxies spread across billions of light-years, featuring some of the earliest galaxies to emerge shortly after the big bang.

Step 2: Take it all in

The universe is vast, and peering back billions of years takes time. Compared to Hubble’s typical exposure time of a few hours, deep fields can require hundreds of hours of exposure over several days. Patience is key. Capturing and combining several separate exposures allows astronomers to assemble a comprehensive core slice of our universe, providing key information about galaxy formation and evolution. Plus, by combining exposures from different wavelengths of light, astronomers are able to better understand galaxy distances, ages, and compositions.

The Hubble Ultra Deep Field is the deepest visible-light portrait of our universe. This astonishing display of nearly 10,000 galaxies was imaged over the course of 400 Hubble orbits around Earth, with a total of 800 exposures captured over 11.3 days.

Step 3: Go beyond what’s visible

The ability to see across billions of light-years and observe the farthest known galaxies in our universe requires access to wavelengths beyond those visible to the human eye. The universe is expanding and light from distant galaxies is stretched far across space, taking a long time to reach us here on Earth. This  phenomenon, known as “redshift,” causes longer wavelengths of light to appear redder the farther they have to travel through space. Far enough away, and the wavelengths will be stretched into infrared light. This is where Hubble’s infrared vision comes in handy. Infrared light allows us to observe light from some of the earliest galaxies in our universe and better understand the history of galaxy formation over time.

In 2009, Hubble observed the Ultra Deep Field in the infrared. Using the Near Infrared Camera and Multi-Object Spectrometer, astronomers gathered one of the deepest core samples of our universe and captured some of the most distant galaxies ever observed.

Step 4: Use your time machine

Apart from their remarkable beauty and impressive imagery, deep field images are packed with information, offering astronomers a cosmic history lesson billions of years back in time within a single portrait. Since light from distant galaxies takes time to reach us, these images allow astronomers to travel through time and observe these galaxies as they appear at various stages in their development. By observing Hubble’s deep field images, we can begin to discover the questions we’ve yet to ask about our universe.

Credit: NASA, ESA, R. Bouwens and G. Illingworth (University of California, Santa Cruz)

Hubble’s deep field images observe galaxies that emerged as far back as the big bang. This image of the Hubble Ultra Deep Field showcases 28 of over 500 early galaxies from when the universe was less than one billion years old. The light from these galaxies represent different stages in their evolution as their light travels through space to reach us.

Step 5: Expand the cosmic frontier

Hubble’s deep fields have opened a window to a small portion of our vast universe, and future space missions will take this deep field legacy even further. With advancements in technologies and scientific instruments, we will soon have the ability to further uncover the unimaginable.

Slated for launch in late 2021, NASA’s James Webb Space Telescope will offer a new lens to our universe with its impressive infrared capabilities. Relying largely on the telescope’s mid-infrared instrument, Webb will further study portions of the Hubble deep field images in greater detail, pushing the boundaries of the cosmic frontier even further.

And there you have it, Hubble’s guide to unlocking the secrets of the cosmos! To this day, deep field images remain fundamental building blocks for studying galaxy formation and deepening not only our understanding of the universe, but our place within it as well.

Still curious about Hubble Deep Fields? Explore more and follow along on Twitter, Facebook, and Instagram with #DeepFieldWeek!

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Sunset on Mars by NASA’s Curiosity Rover

TOP 10 PREHISTORIC OCEAN PREDATORS

10. ANOMALOCARIS (~ 525 Ma) This one metre long invertebrate surely deserves to be included on the list, being one of the first complex oceanic predators to ever have existed. Anomalocaris stalked the Cambrian oceans, viewing the world with a deadly new evolutionary innovation - eyes. Complex eyes allowed this creature to storm its way to the top of the food chain, and with powerful appendages covered in spines it had no trouble devouring prey with tough carapaces. Whilst Anomalocaris is dwarfed by the other contenders on this list, it was still over 10 times larger than any other animal of its time.

9. KRONOSAURUS (125-99 Ma) Kronosaurus, a Cretaceous mosasaur, is named after the Greek titan, Cronus. Its name is well deserved as this ancient beast was a remarkably powerful being. Kronosaurus could reach up to 10 metres long and had a mouth full of sharp, conical teeth. Unlike most other mosasaurs its tail was relatively short, however, evidence shows that Kronosaurus has immensely powerful fins and a pectoral girdle making it an impressive swimmer and hunter.

8. HELICOPRION (290-250 Ma) Helicoprion has astounded scientists since its discovery over 100 years ago. It is iconic for its bizarre spiral of teeth, there are still debates on where exactly these teeth where on the shark with proposals stating they were inside the mouth, on the tip of the tail, the dorsal fin or hanging under the jaw. The most commonly accepted location of the teeth is inside the lower jaw enabling Helicoprion to cleanly slice its prey into pieces.

7. XIPHACTINUS (~110-70 Ma) Xiphactinus was an extraordinary fish that lived during the Cretaceous. It was an esteemed predator that could reach an incredible 6 metres in length and specimens are renowned for their stunning preservation. One such example was 4 metres long and found with another exceptionally well preserved fish just short of 2 metres inside it implying that this particular Xiphactinus individual died shortly after its last feast. Xiphactinus had immensely sharp, slim teeth and an unmistakable underbite which was a possible aid when snaring creatures from below.

6. TYLOSAURUS (86-75 Ma) Tylosaurus is considered a mosasaur and was a vivacious predator all be it smaller than its relative Mosasaurus. Tylosaurus could reach up to 15 metres in length and was one of the apex predators of its day. Fossilised stomach contents of Tylosaurus contain fish, sharks, turtles and other marine reptiles. Despite having an impressive set of teeth, the frontal areas of the jaws exhibit a large reduction in tooth size as well as a more heavily reinforced snout in comparison to other mosasaurs suggesting that Tylosaurus may have rammed into victims with immense force damaging prey internally.

5. MOSASAURUS (70-66 Ma) The mosasaurs ruled the Cretaceous oceans and Mosasaurus was no exception. It could reach up to 17 metres long, longer than most other mosasaurs. Mosasaurus had a strong jaw packed with numerous conical teeth, bite marks of which have been found in huge prehistoric turtles and ammonites suggesting that Mosasaurus was a formidable hunter capable of catching large prey. Mosasaurus was a profound swimmer with strong paddle-like limbs and a huge tail capable of rapidly accelerating the animal when required.

4. DUNKLEOSTEUS (382-358 Ma) Dunkleosteus terrorised the oceans around 370 million years ago and was part of a dynasty known as the placoderm fish (meaning armoured). Dunkleosteus could reach a whopping 6-10 metres in length and probably weighed over a ton. The skull was made up of huge, solid bony plates giving unrivalled protection allowing them to dominate the oceans. Placoderm fish were some of the first organisms to have a mobile jaw, as can be seen in Dunkleosteus’ impressive shearing plates which were used to slice cleanly through prey. Despite an revolutionary jaw, Dunkleosteus could not chew and several fossilised regurgitated remains of its meals have been found that the giant fish simply could not stomach.  

3. DAKOSAURUS (157-137 Ma) Dakosaurus was the largest of a group of marine reptiles that were distant relatives of crocodiles. Dakosaurus could reach up to 5 metres long and had a streamlined body with large paddle-like fins and a long muscular tail implying that is was a very efficient swimmer. The diet of Dakosaurus consisted mostly of fish. The teeth of Dakosaurus are lateromedially compressed and serrated which is a similar morphology to modern killer whales indicating that Dakosaurus was an apex predator of the Jurassic oceans. Skull fenestrae provides evidence that Dakosaurus had very large adductor muscles (which are responsible for the jaw closing) and so it was certainly capable of a forceful bite.

2. LIOPLEURODON (160-155 Ma) Liopleurodon stormed the Jurassic oceans, its huge 7 metre long frame effortlessly cruised through the water. The skull itself could reach a massive 1.5 metres long with a jaw that was packed with teeth up to 10cm long and was capable of an immense bone-crushing force. Liopleurodon was a remarkable hunter with the ability to swim with its nostrils open and so could use its powerful sense of smell to track prey from afar, much like sharks do. Liopleurodon most likely had good camouflage such as a lighter underside and a darker topside so it would blend in with the water to prey above and below.  

1. MEGALODON (~16-2.6 Ma) Megalodon rightfully deserves the top position of the greatest prehistoric ocean predators, ruling the seas for an incredible 14 million years. Megalodon has been estimated to reach up to 18 metres in length and weighing over 40 tonnes. Megalodon is known for its huge 6 inch teeth which were serrated on both sides for an efficient slicing action. Fossils of Megalodon’s prey have also been found, the shark appeared to have adapted its hunting tactics for different sized prey; for smaller prey they would just use their bone crushing bite to pulverise internal organs, but for larger prey they would bite or rip flippers off of creatures to immobilise them and then go in for the kill. The exact bite force of Megalodon has been estimated at around 110,000 N which was more than enough to shatter even the most robust bones. The hunting methods of Megalodon will unfortunately remain a mystery but it was been hypothesised that they swam at great depths and used short bursts of speed to swim up and tear into their preys vulnerable underbelly. Sharks have existed for over 420 million years and still continue to be some of the most successful predators alive, Megalodon is a perfect example of how deadly they can be.

Ten interesting facts about Uranus

Like the classical planets, Uranus is visible to the naked eye, but it was never recognised as a planet by ancient observers because of its dimness and slow orbit. Sir William Herschel announced its discovery on 13 March 1781, expanding the known boundaries of the Solar System for the first time in history and making Uranus the first planet discovered with a telescope.

Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both have different bulk chemical composition from that of the larger gas giants Jupiter and Saturn.

(The five largest moons of Uranus) Like all of the giant planets, Uranus has its share of moons. At present, astronomers have confirmed the existence of 27 natural satellites. But for the most part, these moons are small and irregular. 

Uranus’ moons are named after characters created by William Shakespeare and Alexander Pope. These include Oberon, Titania and Miranda.  All are frozen worlds with dark surfaces. Some are ice and rock mixtures.  The most interesting Uranian moon is Miranda; it has ice canyons, terraces, and other strange-looking surface areas.

Only one spacecraft in the history of spaceflight has ever made a close approach to Uranus. NASA’s Voyager 2 conducted its closest approach to Uranus on January 24th, 1986, passing within 81,000 km of the cloud tops of Uranus. It took thousands of photographs of the gas/ice giant and its moons before speeding off towards its next target: Neptune.

Uranus has rings: All the gas and ice giants have their own ring systems, and Uranus’ is the second most dramatic set of rings in the Solar System.

Uranus makes one trip around the Sun every 84 Earth years. During some parts of its orbit one or the other of its poles point directly at the Sun and get about 42 years of direct sunlight. The rest of the time they are in darkness.

All of the planets in the Solar System rotate on their axis, with a tilt that’s similar to the Sun. In many cases, planet’s have an axial tilt, where one of their poles will be inclined slightly towards the Sun. But the axial tilt of Uranus is a staggering 98 degrees! In other words, the planet is rotating on its side. 

Uranus is approximately 4 times the sizes of Earth and 63 times its volume.

Uranus is blue-green in color, the result of methane in its mostly hydrogen-helium atmosphere. The planet is often dubbed an ice giant, since 80 percent or more of its mass is made up of a fluid mix of water, methane, and ammonia ices.

Uranus hits the coldest temperatures of any planet. With minimum atmospheric temperature of -224°C Uranus is nearly coldest planet in the solar system. While Neptune doesn’t get as cold as Uranus it is on average colder. The upper atmosphere of Uranus is covered by a methane haze which hides the storms that take place in the cloud decks.

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Images credit: NASA/ wikipedia

The orbit of Jupiter protects the Earth from asteroids.

Caldwell 14 by NASA Hubble