Category Archives: Universe

Sagittarius A* – Our backyard’s black hole

There’s nothing in the whole universe as mysterious and fascinating as black holes. First theorised by John Michell in 1783, and also predicted by Einstein’s Theory of General Relativity in 1916, these dense, dark and elusive objects have been researched and studied for decades, capturing the interest of some of the world’s greatest physicists.

Among these, are three laureates who shared a Nobel Prize in physics in 2020 for their studies on black holes. Roger Penrose, one of the most brilliant minds of our times, demonstrated mathematically that black holes are indeed a direct consequence of the Theory of General Relativity, whereas Reinhard Genzel and Andrea Ghez predicted the existence of the supermassive black hole at the centre of the Milky Way and calculated its mass with fair precision: about 4.3 million solar masses!

Now, the idea of having such a supermassive black hole right at the core of our galaxy could be for many a cause of nightmares but in reality, there is nothing to fear. In fact, this is very common in the universe: it is estimated that there is a supermassive black hole in the centre of virtually every large galaxy.

Do you want to know more about black holes? Read my article: Black Holes: The dark side of gravity.

The supermassive black hole at the core of the Milky Way is called Sagittarius A* (abbreviated as Sgr A* and pronounced “Sagittarius A star”) and is located near the border of the constellations Sagittarius and Scorpius; it has a diameter of around 23.5 million km (14.6 million miles), and it is mostly dormant as it only occasionally feeds on nearby objects.

In May 2022, Sgr A* has made the headlines after its first-ever picture was produced and released to the public by the Event Horizon Telescope (“EHT”), a combination of eight large, ground-based telescopes that had already produced the first image ever of black hole in 2019 (in that case it was the black hole at the core of the Galaxy M87).

First ever image of Sagittarius A*

But how did they manage to achieve this? Obviously, imaging a black hole is a very complex business: while these objects have huge densities, in fact, they are very tiny regions of space. Besides, to observe Sgr A* one must look into the centre of the Galaxy through thick lines of dust, so the best options for observing the Galactic centre are in radio waves and in infrared light.

That is why the EHT used a technique known as Very Long Baseline Interferometry, which allowed astronomers to combine data from eight radio telescopes across the world as though they were one large telescope. Owing to this configuration, the EHT’s maximum baseline — the equivalent to a telescope’s aperture — for observing Sagittarius A* was 10,700 km (6,650 miles) across, effectively making the EHT the largest telescope on Earth!

The Event Horizon Telescope

Since the observation of Sgr A* and M87 were made, three more telescopes have been added to the network, but is it enough to get sharper images or even capture farther away black holes? After all, Sgr A* and M87 were the EHT’s top two targets especially because of their relatively large angular size in the sky whereas supermassive black holes in other galaxies appear far smaller from our viewpoint.

For higher resolution and more imaging power the EHT would need to widen its aperture even further. But how to achieve that given Earth’s limited size? Well, one solution would be to have a space-based addition to the EHT, which would significantly increase the overall angular resolution. How cool would that be?

Black holes: the dark side of gravity

When Galileo Galilei first pointed his homemade telescope toward the sky in 1610, he couldn’t have possibly imagined that he was about to change astronomy forever. His discovery of Jupiter’s moons in particular had two major effects: it provided compelling evidence for the Copernican theory, and it recognised the use of the telescope as a key element in unravelling the mysteries of the universe.

In the centuries following Galileo’s discovery we have built larger and ever-more powerful telescopes, some of which are conveniently placed in space, and have come to map the cosmos with a fair degree of precision. And although there are plenty of things that we still don’t know about the universe, one thing we can be absolutely certain of is that it is an extremely intriguing place.

A place crammed with beautiful galaxies, colourful nebulas and hot stars that are often orbited by planets, which in turn are revolved by rings and moons. A home to fast-spinning pulsars, ultra-compact neutron stars, wandering comets, icy asteroids, and countless other treasures.

But hidden among these jewels is a swarm of elusive, almost invisible objects: black holes, the most mysterious and fascinating objects that we know of, certainly the most feared. After all science fiction has frequently depicted them as evil monsters wandering in the universe vacuuming everything they encounter. But is that really what black holes are?

Well, not exactly. Technically, a black hole is a region in space where a massive amount of matter is compressed into a very tiny area, and its gravitational pull is so strong that nothing, not even light, can escape! Black holes, however, do not roam through the universe, randomly swallowing things. Just like any other celestial body, they obey the laws of gravity: they follow precise orbits and are orbited by other bodies.

It is true though that anything swallowed by a black hole is gone for good. This is because at the “event horizon,” the so called “point of no return” or the boundary of the black hole, gravity is so strong that in order to escape, matter would have to travel faster than the speed of light. As this is physically impossible, nothing can escape!

The idea of black holes (initially called “frozen stars”) has been around for centuries although the term “black hole” was first used only in 1967 by the American astronomer John Wheeler. It was in these years that scientific attention shifted back to black holes, with some of the greatest minds such as Wheeler, Oppenheimer, Penrose, Hawking and many others making significant contributions.

Today we are fairly confident that most black holes form from the remnant of large stars. The idea is that when a massive star consumes all its fuel and dies, all it leaves behind is a dense core. If this core has a certain mass (over three times the mass of the Sun), it collapses on itself under the effect of gravity to become a black hole.

But while the theory is elegant and widely accepted, the direct observation of a black holes remained fairly complicated, considering that not even light can escape to reveal them. Astronomers, however, are very clever people and eventually figured out a way: to spot a black hole all they needed to do was observing its effects on nearby matter.

Imagine a black hole passing through a region of interstellar dust or crossing paths with a close star: the extreme gravitational pull would draw matter toward the event horizon and in doing so the matter would accelerate and heat up, emitting fancy x-rays that can be easily observed by astronomers.

That is precisely what astronomers did in 2019 when the Event Horizon Telescope (EHT), a combination of eight ground-based radio telescopes, captured the image of the supermassive black hole at the centre of the galaxy M87 interacting with nearby matter. The first image ever recorded of a black hole was served.

The first image ever of a Black Hole – Credit NASA ®

Black holes are quite common in the universe and, according to recent studies, there may be billions of them in our own galaxy alone. In fact, a supermassive black hole called Sagittarius A* sits at the core of the Milky Way, and astronomers estimate that there is one at the heart of virtually every large galaxy.

So far, scientists and astronomers have identified three types of black holes whereas a fourth type, primordial black holes, is still only theoretical although scientific evidence of their existence is mounting. These are the main classes of black holes:

  • Primordial – While these black holes are still only theoretical, scientists believe they formed in the initial stages of the universe, a fraction of second after the Big Bang. At this stage, the universe was not homogeneous and areas that were denser and hotter than others may have quickly collapsed into black holes.
    According to some theories, primordial black holes may even explain one of the biggest mysteries of the universe: dark matter. Such theories suggest that primordial black holes may in fact be a type of dark matter called MACHOs (MAssive Compact Halo Objects,), the glue that keep galaxies together.
  • Stellar mass (5 to 100 solar masses) – These are the “ordinary” black holes, which form when a star more than thirty times the mass of the Sun explodes into a supernova at the end of its cycle, leaving behind a dense core. This new core than collapses on itself under the effect of gravity and a stellar mass black hole is formed. They are relatively small but incredibly dense, with an impressive gravitational force.
  • Intermediate mass (100 to 10,000 solar masses) – As the name suggest, these black holes represent a midway between stellar mass and supermassive black holes. Scientists ignored their existence for decades until in March 2020 Hubble spotted what it is believed to be a 50,000 solar mass black hole. Intermediate black holes could form when stars in a cluster collide in a chain reaction or from the merger of two or more stellar mass black holes.
  • Supermassive (greater than 1 million solar masses) – These black holes are millions or even billions of times as massive as the Sun and are found at the core of major galaxies: they are the seeds around which the galaxies are constructed. For instance, Sagittarius A*, the supermassive black hole at the core of our galaxy, is roughly four million times the mass of the Sun whereas the black hole photographed in 2019 weighs a billion solar masses. Supermassive black holes are thought to be formed from the merger of hundreds or thousands of tiny black holes.

Luckily for us, the nearest black holes are thousands of light-years away from Earth. At such distance there is no danger for us or our planet; as we saw, black holes obey the law of gravity like any other celestial body. If we were to replace the Sun with an equal-mass black hole, our orbit and that of all planets in the Solar System would remain unchanged.

On the other hand, if someone, say a curious astronaut, was to wander too close to a black hole, this would have catastrophic consequences. As the astronaut approached the black hole, he would be stretched head-to-toe into a long string of particles before even reaching the event horizon, in a process known as “spaghettification.”

So, the bottom line is: unless you want to become spaghetti, better stay away from back holes!

Spaghettification – Source: Wikipedia ®

The James Webb Space Telescope

Since the human race emerged on Earth several hundreds of thousands of years ago, men have been looking up at the sky in wonder, asking some very big questions. Why are we here? What are stars? What is the Universe made of? Are we alone out there?

In the past centuries we made sensational discoveries, pushing the boundaries of our knowledge to unimaginable levels and thanks to some brilliant, visionary minds today we’ve got a fundamental understanding of the Universe. For instance, we know what stars are, how they form, evolve and die; we built tools that helped us mapping the Universe, with its billions of galaxies, quasars, black holes and so on. We sent probes and humans into space…even a dog once, poor little thing!

So do we have all the answers now? No, we most certainly do not but we are still driven by our curious nature and thanks to a joint effort among NASA, the European Space Agency (ESA) and the Canadian Space Agency (CSA), we are now ready to send into space the most advanced telescope ever built, which will help unravel some of the unresolved mysteries of the cosmos.

This is the James Webb Space Telescope (JWST), a 6.5-tonne telescope with a cost tag of nearly 10 billion dollars (8.9 billion euro), which is set to be launched aboard the European Ariane 5 rocket on December 25, 2021 and begin its science observations 6 months after.

Having a telescope in space helps with the two main problems that ground-based astronomers have to face: the weather (clouds are astronomers’ worse nightmare) and the Earth’s atmosphere, which acts like a protective blanket letting only some light through while blocking or distorting some wavelengths. Besides, a space telescope doesn’t have to wait for the Earth to spin to be able to point toward the desired target.

Currently there are several telescopes in space, the most famous being Hubble, which has been orbiting Earth since more than 30 years, delivering iconic images and contributing to more than 15 000 scientific papers. As the JWST is widely considered the successor of Hubble, regardless of some major differences, it is understandable why the excitement around the new observatory grows by the hour within the scientific community.

Credit: NASA ®

The JWST is a powerful observatory, brimming with innovation and complexity. It boasts an impressive primary mirror with a diameter of 6.5 meters, composed of 18 adjustable, gold-plated beryllium segments, which has six times the light-collecting capability of Hubble, and a roughly 15 times wider field of view. It will be equipped with four main scientific instruments, collectively known as the Integrated Science Instrument Module (ISIM):

  • A near-infrared spectrograph (NIRSpec) that can observe 100 objects simultaneously
  • A near-infrared camera (NIRCam)
  • A combined mid-infrared camera and spectrograph (MIRI) with a cryocooler to keep its temperature at –266˚C
  • A fine guidance system and wide-field imager (FGS/NIRISS) that includes a mode for exoplanet spectroscopy

The JWST’s main goal is to look back to 100–250 million years after the Big Bang, when the first stars and galaxies formed, to shed some light on the so-called “end of the dark ages”. According to the Big Bang theory, this is when the Universe began to fill with “first light” from newly ignited stars. By comparison, Hubble was “only” capable of looking back 500 million years after the Big Bang.

Moreover, the mid- and near-infrared cameras will allow the new observatory to peer into the birth of new stars and solar systems and will be sensitive enough to detect oxygen and other so-called biomarkers in the atmosphere of exoplanets (planets orbiting stars outside the Solar System). The JWST will also study the effects of black holes and the origins of life; it will provide a better calculation of the Universe’s size and geometry, getting some insights on dark matter and dark energy, and helping us understand the ultimate fate of the cosmos.

The capabilities of the James Webb Space Telescope are huge but so are its risks. The JWST, in fact, will be folded down to fit inside the Ariane 5 rocket and, once deployed, it will unfold itself, cool down and calibrate in a process that will likely make all those involved in the project lose their sleep.

The JWST folded into the Arian 5 rocket – Credit NASA ®

On top of that, the JWST will position itself beyond the reach of manned repair missions as it will be orbiting the Sun at the Lagrange point 2 (L2), approximately 1.5 million km from us where it will have unobstructed views of the Universe.

Keeping the JWST in orbit around L2, however, requires an astonishing amount of energy and the on-board fuel will guarantee a limited lifetime (5 to 10 years), which should still be enough for some sensational discoveries. So brace yourself and be ready for the main scientific event of the 21st century!

The Pleiades

Located some 440 light years from Earth, in the constellation Taurus, the Pleiades is one of the most beautiful sights in the night sky: whether through the naked eye, an eyepiece or a camera sensor, this unmistakable celestial object never ceases to amaze!

The Pleiades is what astronomers call an “open cluster” or a group of stars formed from the same cloud of gas and dust: under the effect of gravity, these particles are pressed together pushing the temperature up to extreme levels. As the new stars take shape and increase in size, their own gravity becomes stronger, mutually bounding them together.

The cluster contains over three thousand stars (mostly hot B-type stars) although only a handful is visible to the naked eye as a consequence of increasing light pollution. Today, in fact, from most cities it is possible to recognise around 6 or 7 stars, thus the cluster is often referred to as “The Seven Sisters”.

The Pleiades has been known for centuries and there is no shortage of legends about it. According to the Greek mythology, the Pleiades were the seven daughters of the Titan god Atlas who rebelled against Zeus and was thus sentenced to hold up the heavens on his shoulders. Moved by compassion, however, Zeus allowed the seven daughters a place in the sky so they could stay close to their father.

In Japan, the cluster is known as Subaru and was chosen as the name of the 8.2-meter (320 inches) Subaru Telescope of the National Astronomical Observatory of Japan. The name Subaru was also chosen as the brand name of Subaru Automobiles, which has 6 stars in its logo.

The Pleiades is arguably the most famous star cluster in the sky, prominent and unmistakable to the naked eye, but who was the first astronomer to resolve the Pleiades through a telescope? Well, Galileo Galilei of course. The Italian astronomer was the first to discovered that the cluster contains way more stars than those visible to the unaided eye. He published his observations and a sketch of the Pleiades in his “Sidereus Nuncius” in 1610.

However, the nebulosity around the Pleaides was only discovered in 1859 by the German astronomer Ernst Tempel who described it as “a faint stain like a breath on a mirror”, when gazing at the Pleiades through his 4-inch (10cm) telescope. The nebulosity is not directly connected to the star cluster, but it rather seems that the latter is passing through a particularly dusty region, which reflects the light from the brighter stars.

Depending on the observer’s latitude, the Pleiades cluster is visible from October to April and it is best observed in November, when is up the whole night. As mentioned before, the Pleiades is clearly visible to the naked eye but have a look at it through a pair of binoculars or a telescope and I assure you your jaws will drop!

Triangulum Galaxy

Sitting in the constellation Triangulum, the third largest galaxy of the Local Group is a stunning face-on spiral galaxy about half the size of the Milky Way. Home to 40 billion stars, the Triangulum Galaxy may play a role in the collision between Andromeda and our Galaxy!

The Triangulum Galaxy was first discovered by the Italian astronomer Giovanni Battista Hodierna, who described it in his work in 1654 as a “cloud-like nebulosity”. It was only a century later, in 1764, that Charles Messier included it in his catalogue as object 33 and to this day, the Triangulum Galaxy is referred to as M33.

Positioned slightly less than 3 million light-years away from Earth, M33 is the second-closer large galaxy to our own after the Andromeda Galaxy (M31); at half the size of the Milky Way, it is the third-largest galaxy in the Locap Group, and home to around 40 billion stars according to the latest estimations.

While this number is considerably lower than that of our own Galaxy (400 billion stars) and the Andromeda Galaxy (1 trillion), the Triangulum Galaxy has a star-formation rate 10 times that of Andromeda. Several regions of star formation were in fact detected in M33, with the largest of all (NGC 604) being one of the biggest in the Local Group.

The Triangulum Galaxy is a spiral galaxy whose structure implies it had few interactions with nearby galaxies and is therefore considered an “introvert” galaxy. However, in recent years scientist found mounting evidence that suggests that the Triangulum Galaxy may actually be orbiting the Andromeda Galaxy and could therefore be involved in the collision between Andromeda and the Milky Way, about 4.5 billion years from now.

Andromeda Galaxy

Lurking in the Andromeda constellation, south-west of Cassiopeia, the Andromeda Galaxy is a barred spiral galaxy, similar to the one we leave in. Formed roughly 10 billion years ago, Andromeda is the largest galaxy of the Local Group, which includes the Triangulum Galaxy and our Milky Way.

At nearly 2.5 million light years from Earth, Andromeda is the closest large galaxy to the Milky Way, and the farthest celestial object visible to the naked eye. First observations, in fact, date back to the year 964, long before the invention of the telescope.

The record belongs to the Persian astronomer Abd al-Rahman al-Sufi who included Andromeda in his book about “fixed stars” although referring to it as a “small cloud”. For centuries, in fact, Andromeda was considered as a nebula or a gaseous region, and it was only in the 20th century that astronomers were able to resolve the Andromeda spiral nebula into individual stars.

This then led to what is known as “the great debate”: for decades scientists argued on whether spiral nebulas where nearby objects within the Milky Way or large, distant galaxies of their own. It was only in the early 1920s that the debate was finally closed, thanks to Edwin Hubble’s calculations.

Credit: NASA ®

Today, after decades of research and studies, we know much more about Andromeda. Scientist believe it formed roughly 10 billion years ago from the collision and merger of several smaller protogalaxies, which formed most of the galaxy’s galactic halo and extended disk. Andromeda today is home to approximately one trillion stars and its diameter is about 200,000 light-years across.

We also know that Andromeda is on a collision course with the Milky Way, with the two galaxies approaching at a speed of around 110km per second. Should this speed remain constant, the merge will happen in about 4.5 billion years and will last around 2 billion years, resulting in a new giant galaxy that astronomers like to call Milkomeda. But don’t worry. Mergers like this are quite common in the universe and besides, humans will be long gone by the time this happens!

Orion Nebula

Shining bright in the constellation of Orion the Hunter, the Orion Nebula, with its gorgeous colours and distinctive features, is arguably one of the most beautiful areas of the winter night sky. A masterpiece painting in the museum called Universe!

Winter in the northern hemisphere is definitely the season of Orion the Hunter, the legendary constellation sparkling bright and proud against the dark sky. The three bright stars that form a short, straight row known as the Orion’s Belt are among the most distinctive features in the winter night sky. One that I’m sure, everyone has glanced at.

Just beneath the Belt lays the Orion Nebula, a diffuse nebula that obviously takes the name from its hosting constellation. This is one of the brightest nebulas we know of; so bright that is visible to the naked eye, even from light polluted areas. It is in fact one of the most observed and photographed targets among the astronomy community.

The discovery of the Orion Nebula is generally credited to Nicolas-Claude Fabri de Peiresc, who first recorder its observation through a refracting telescope in 1610. According to some speculations, however, the nebula was first observed by the Mayans who referred to it as a “cosmic fire of creation”.

In 1880, it became the first ever nebula to be photographed. The American doctor and amateur astronomer Henry Draper, in fact, used the new dry plate photographic process to take a long exposure photograph of the Orion Nebula. For this reason, Draper is considered a pioneer of modern astrophotography.

Later inserted in its catalogue by Charles Messier as object 42, the Orion Nebula is a stellar nursery, a place where new stars are born out of collapsing clouds of interstellar gas and dust. Its central region hosts four massive stars known as The Trapezium: a very young open cluster that illuminates the nebula.

Always escorted by its loyal companion, the Running Man Nebula, the Orion Nebula is the closest star-formation region to our planet, approximately 1,350 light years away from Earth. It extends for roughly 30 light years across and has a mass of around 2,000 that of our Sun. Perhaps for this reason, it is often referred to as the Great Orion Nebula.