Artist's impression of the interior of Mars. Credit: NASA/JPL
For thousands of years, human beings have stared up at the sky and wondered about the Red Planet. Easily seen from Earth with the naked eye, ancient astronomers have charted its course across the heavens with regularity. By the 19th century, with the development of powerful enough telescopes, scientists began to observe the planet’s surface and speculate about the possibility of life existing there.
However, it was not until the Space Age that research began to truly shine light on the planet’s deeper mysteries. Thanks to numerous space probes, orbiters and robot rovers, scientists have learned much about the planet’s surface, its history, and the many similarities it has to Earth. Nowhere is this more apparent than in the composition of the planet itself.
Structure and Composition:
Like Earth, the interior of Mars has undergone a process known as differentiation. This is where a planet, due to its physical or chemical compositions, forms into layers, with denser materials concentrated at the center and less dense materials closer to the surface. In Mars’ case, this translates to a core that is between 1700 and 1850 km (1050 – 1150 mi) in radius and composed primarily of iron, nickel and sulfur.
This core is surrounded by a silicate mantle that clearly experienced tectonic and volcanic activity in the past, but which now appears to be dormant. Besides silicon and oxygen, the most abundant elements in the Martian crust are iron, magnesium, aluminum, calcium, and potassium. Oxidation of the iron dust is what gives the surface its reddish hue.
Composite image showing the size difference between Earth and Mars. Credit: NASA/Mars Exploration
Magnetism and Geological Activity:
Beyond this, the similarities between Earth and Mars’ internal composition ends. Here on Earth, the core is entirely fluid, made up of molten metal and is in constant motion. The rotation of Earth’s inner core spins in a direction different from the outer core and the interaction of the two is what gives Earth it’s magnetic field. This in turn protects the surface of our planet from harmful solar radiation.
The Martian core, by contrast, is largely solid and does not move. As a result, the planet lacks a magnetic field and is constantly bombarded by radiation. It is speculated that this is one of the reasons why the surface has become lifeless in recent eons, despite the evidence of liquid, flowing water at one time.
Despite there being no magnetic field at present, there is evidence that Mars had a magnetic field at one time. According to data obtained by the Mars Global Surveyor, parts of the planet’s crust have been magnetized in the past. It also found evidence that would suggest that this magnetic field underwent polar reversals.
This observed paleomagnetism of minerals found on the Martian surface has properties that are similar to magnetic fields detected on some of Earth’s ocean floors. These findings led to a re-examination of a theory that was originally proposed in 1999 which postulated that Mars experienced plate tectonic activity four billion years ago. This activity has since ceased to function, causing the planet’s magnetic field to fade away.
Map from the Mars Global Surveyor of the current magnetic fields on Mars. Credit: NASA/JPL
Much like the core, the mantle is also dormant, with no tectonic plate action to reshape the surface or assist in removing carbon from the atmosphere. The average thickness of the planet’s crust is about 50 km (31 mi), with a maximum thickness of 125 km (78 mi). By contrast, Earth’s crust averages 40 km (25 mi) and is only one third as thick as Mars’s, relative to the sizes of the two planets.
The crust is mainly basalt from the volcanic activity that occurred billions of years ago. Given the lightness of the dust and the high speed of the Martian winds, features on the surface can be obliterated in a relatively short time frame.
Formation and Evolution:
Much of Mars’ composition is attributed to its position relative to the Sun. Elements with comparatively low boiling points, such as chlorine, phosphorus, and sulphur, are much more common on Mars than Earth. Scientists believe that these elements were probably removed from areas closer to the Sun by the young star’s energetic solar wind.
After its formation, Mars, like all the planets in the Solar System, was subjected to the so-called “Late Heavy Bombardment.” About 60% of the surface of Mars shows a record of impacts from that era, whereas much of the remaining surface is probably underlain by immense impact basins caused by those events.
The North Polar Basin is the large blue low-lying area at the northern end of this topographical map of Mars. Credit: NASA/JPL/USGS
These craters are so well preserved because of the slow rate of erosion that happens on Mars. Hellas Planitia, also called the Hellas impact basin, is the largest crater on Mars. Its circumference is approximately 2,300 kilometers, and it is nine kilometers deep.
The largest impact event on Mars is believed to have occurred in the northern hemisphere. This area, known as the North Polar Basin, measures some 10,600 km by 8,500 km, or roughly four times larger than the Moon’s South Pole – Aitken basin, the largest impact crater yet discovered.
Though not yet confirmed to be an impact event, the current theory is that this basin was created when a Pluto-sized body collided with Mars about four billion years ago. This is thought to have been responsible for the Martian hemispheric dichotomy and created the smooth Borealis basin that now covers 40% of the planet.
Scientists are currently unclear on whether or not a huge impact may be responsible for the core and tectonic activity having become dormant. The InSight Lander, which is planned for 2018, is expected to shed some light on this and other mysteries – using a seismometer to better constrain the models of the interior.
Hellas Planitia extends across about 50° in longitude and more than 20° in latitude. From data from the Mars Orbiter LaserAltimeter (MOLA). Credit: NASA
Other theories claim that Mars lower mass and chemical composition caused it to cool more rapidly than Earth. This cooling process is therefore believed to be what arrested convection within the planet’s outer core, thus causing its magnetic field to disappear.
Mars also has discernible gullies and channels on its surface, and many scientists believe that liquid water used to flow through them. By comparing them to similar features on Earth, it is believed these were were at least partially formed by water erosion. Some of these channels are quite large, reaching 2,000 kilometers in length and 100 kilometers in width.
Yes, Mars is much like Earth in many respects. It’s a rocky planet, has a crust, mantle, and core, and is composed of roughly the same elements. As our exploration of the Red Planet continues, we are learning more and more about its history and evolution. Someday, we may find ourselves settling on that rock, and relying on its similarities to create a “backup location” for humanity.
A collage of Saturn (bottom left) and some of its moons: Titan, Enceladus, Dione, Rhea and Helene. Credit: NASA/JPL/Space Science Institute
Saturn is well known for being a gas giant, and for its impressive ring system. But would it surprise you to know that this planet also has the second-most moons in the Solar System, second only to Jupiter? Yes, Saturn has at least 150 moons and moonlets in total, though only 62 have confirmed orbits and only 53 have been given official names.
Most of these moons are small, icy bodies that are little more than parts of its impressive ring system. In fact, 34 of the moons that have been named are less than 10 km in diameter while another 14 are 10 to 50 km in diameter. However, some of its inner and outer moons are among the largest and most dramatic in the Solar System, measuring between 250 and 5000 km in diameter and housing some of the greatest mysteries in the Solar System.
Saturn’s moons have such a variety of environments between them that you’d be forgiven for wanting to spend an entire mission just looking at its satellites. From the orange and hazy Titan to the icy plumes emanating from Enceladus, studying Saturn’s system gives us plenty of things to think about. Not only that, the moon discoveries keep on coming. As of April 2014, there are 62 known satellites of Saturn (excluding its spectacular rings, of course). Fifty-three of those worlds are named.
The Cassini spacecraft observes three of Saturn’s moons set against the darkened night side of the planet. Credit: NASA/JPL/Space Science Institute
Discovery and Naming
Prior to the invention of telescopic photography, eight of Saturn’s moons were observed using simple telescopes. The first to be discovered was Titan, Saturn’s largest moon, which was observed by Christiaan Huygens in 1655 using a telescope of his own design. Between 1671 and 1684, Giovanni Domenico Cassini discovered the moons of Tethys, Dione, Rhea, and Iapetus – which he collectively named the “Sider Lodoicea” (Latin for “Louisian Stars”, after King Louis XIV of France).
n 1789, William Herschel discovered Mimas and Enceladus, while father-and-son astronomers W.C Bond and G.P. Bond discovered Hyperion in 1848 – which was independently discovered by William Lassell that same year. By the end of the 19th century, the invention of long-exposure photographic plates allowed for the discovery of more moons – the first of which Phoebe, observed in 1899 by W.H. Pickering.
In 1966, the tenth satellite of Saturn was discovered by French astronomer Audouin Dollfus, which was later named Janus. A few years later, it was realized that his observations could only be explained if another satellite had been present with an orbit similar to that of Janus. This eleventh moon was later named Epimetheus, which shares the same orbit as Janus and is the only known co-orbital in the Solar System.
Collage of Saturn and its largest moons. Credit: NASA/JPL/SSI
By 1980, three additional moons were discovered and later confirmed by the Voyager probes. They were the trojan moons (see below) of Helene (which orbits Dione) as well as Telesto and Calypso (which orbit Tethys).
The study of the outer planets has since been revolutionized by the use of unmanned space probes. This began with the arrival of the Voyager spacecraft to the Cronian system in 1980-81, which resulted in the discovery of three additional moons – Atlas, Prometheus, and Pandora – bringing the total to 17. By 1990, archived images also revealed the existence of Pan.
This was followed by the Cassini-Huygens mission, which arrived at Saturn in the summer of 2004. Initially, Cassini discovered three small inner moons, including Methone and Pallene between Mimas and Enceladus, as well as the second Lagrangian moon of Dione – Polydeuces. In November of 2004,Cassini scientists announced that several more moons must be orbiting within Saturn’s rings. From this data, multiple moonlets and the moons of Daphnis and Anthe have been confirmed.
The study of Saturn’s moons has also been aided by the introduction of digital charge-coupled devices, which replaced photographic plates by the end of the 20th century. Because of this, ground-based telescopes have begun to discover several new irregular moons around Saturn. In 2000, three medium-sized telescopes found thirteen new moons with eccentric orbits that were of considerable distance from the planet.
The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan in the background; Iapetus (top) and Hyperion (bottom). Credit: NASA/JPL/Space Science Institute
In 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons. In 2006, astronomers using Japan’s Subaru Telescope at Mauna Kea reported the discovery of nine more irregular moons. In April of 2007, Tarqeq (S/2007 S 1) was announced, and in May of that same year, S/2007 S 2 and S/2007 S 3 were reported.
The modern names of Saturn’s moons were suggested by John Herschel (William Herschel’s son) in 1847. In keeping with the nomenclature of the other planets, he proposed they be named after mythological figures associated with the Roman god of agriculture and harvest – Saturn, the equivalent of the Greek Cronus. In particular, the seven known satellites were named after Titans, Titanesses and Giants – the brothers and sisters of Cronus.
In 1848, Lassell proposed that the eighth satellite of Saturn be named Hyperion after another Titan. When in the 20th century, the names of Titans were exhausted, the moons were named after different characters of the Greco-Roman mythology, or giants from other mythologies. All the irregular moons (except Phoebe) are named after Inuit and Gallic gods and Norse ice giants.
Saturn’s Inner Large Moons
Saturn’s moons are grouped based on their size, orbits, and proximity to Saturn. The innermost moons and regular moons all have small orbital inclinations and eccentricities and prograde orbits. Meanwhile, the irregular moons in the outermost regions have orbital radii of millions of kilometers, orbital periods lasting several years, and move in retrograde orbits.
Saturn’s moon of Enceladus. Credit: NASA/JPL/Space Science Institute
Saturn’s Inner Large Moons, which orbit within the E Ring (see below), include the larger satellites Mimas, Enceladus, Tethys, and Dione. These moons are all composed primarily of water ice and are believed to be differentiated into a rocky core and an icy mantle and crust. With a diameter of 396 km and a mass of 0.4×1020 kg, Mimas is the smallest and least massive of these moons. It is ovoid in shape and orbits Saturn at a distance of 185,539 km with an orbital period of 0.9 days.
Some people jokingly call Mimas the “Death Star” moon because of the crater on its surface that resembles the machine from the Star Wars universe. The 140 km (88 mi) Herschel Crater is about a third the diameter of the moon itself and could have created fractures (chasmata) on the moon’s opposing side. There are in fact craters throughout the moon’s small surface, making it among the most pockmarked in the Solar System.
Enceladus, meanwhile, has a diameter of 504 km, a mass of 1.1×1020 kg, and is spherical in shape. It orbits Saturn at a distance of 237,948 km and takes 1.4 days to complete a single orbit. Though it is one of the smaller spherical moons, it is the only Cronian moon that is endogenously active – and one of the smallest known bodies in the Solar System that is geologically active. This results in features like the famous “tiger stripes” – a series of continuous, ridged, slightly curved, and roughly parallel faults within the moon’s southern polar latitudes.
Large geysers have also been observed in the southern polar region that periodically releases plumes of water ice, gas, and dust which replenish Saturn’s E ring. These jets are one of several indications that Enceladus has liquid water beneath its icy crust, where geothermal processes release enough heat to maintain a warm water ocean closer to its core.
Dione’s trailing hemisphere, showing the patches of “whispy terrain”. Credit: NASA/JPL
The moon has at least five different kinds of terrain, a “young” geological surface of less than 100 million years. With a geometrical albedo of more than 140%, which is due to it being composed largely of water ice, Enceladus is one of the brightest known objects in the Solar System.
At 1066 km in diameter, Tethys is the second-largest of Saturn’s inner moons and the 16th-largest moon in the Solar System. The majority of its surface is made up of heavily cratered and hilly terrain and a smaller and smoother plains region. Its most prominent features are the large impact crater of Odysseus, which measures 400 km in diameter, and a vast canyon system named Ithaca Chasma – which is concentric with Odysseus and measures 100 km wide, 3 to 5 km deep, and 2,000 km long.
With a diameter and mass of 1,123 km and 11×1020 kg, Dione is the largest inner moon of Saturn. The majority of Dione’s surface is heavily cratered old terrain, with craters that measure up to 250 km in diameter. However, the moon is also covered with an extensive network of troughs and lineaments which indicate that in the past it had global tectonic activity.
It’s covered in canyons, crackings, craters, and is coated from dust in the E-ring that originally came from Enceladus. The location of this dust has led astronomers to theorize that the moon was spun about 180 degrees from its original disposition in the past, perhaps due to a large impact.
Saturn’s Large Outer Moons:
The Large Outer Moons, which orbit outside of the Saturn’s E Ring, are similar in composition to the Inner Moons – i.e. composed primarily of water ice, and rock. Of these, Rhea is the second-largest – measuring 1,527 km in diameter and 23×1020 kg in mass – and the ninth-largest moon in the Solar System. With an orbital radius of 527,108 km, it is the fifth-most distant of the larger moons and takes 4.5 days to complete an orbit.
Views of Saturn’s moon Rhea. Credit: NASA/JPL/Space Science Institute
Like other Cronian satellites, Rhea has a rather heavily cratered surface and a few large fractures on its trailing hemisphere. Rhea also has two very large impact basins on its anti-Saturnian hemisphere – the Tirawa crater (similar to Odysseus on Tethys) and the Inktomi crater – which measure about 400 and 50 km across, respectively.
Rhea has at least two major sections, the first being bright craters with craters larger than 40 km (25 miles), and a second section with smaller craters. The difference in these features is believed to be evidence of a major resurfacing event at some time in Rhea’s past.
At 5150 km in diameter and 1,350×1020 kg in mass, Titan is Saturn’s largest moon and comprises more than 96% of the mass in orbit around the planet. Titan is also the only large moon to have its own atmosphere, which is cold, dense, and composed primarily of nitrogen with a small fraction of methane. Scientists have also noted the presence of polycyclic aromatic hydrocarbons in the upper atmosphere, as well as methane ice crystals.
The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters, evidence of cryovolcanoes, and longitudinal dune fields that were apparently shaped by tidal winds. Titan is also the only body in the Solar System besides Earth with bodies of liquid on its surface, in the form of methane–ethane lakes in Titan’s north and south polar regions.
Image of Titan’s taken by the Cassini spacecraft, showing light passing through the periphery of the moon’s atmosphere. Credit: NASA/JPL-Caltech/Space Science Institute
Titan is also distinguished for being the only Cronian moon that has ever had a probe land on it. This was the Huygens lander, which was carried to the hazy world by the Cassini spacecraft. Titan’s “Earth-like processes” and thick atmosphere are among the things that make this world stand out to scientists, which include its ethane and methane rains from the atmosphere and flows on the surface.
With an orbital distance of 1,221,870 km, it is the second-farthest large moon from Saturn and completes a single orbit every 16 days. Like Europa and Ganymede, it is believed that Titan has a subsurface ocean made of water mixed with ammonia, which can erupt to the surface of the moon and lead to cryovolcanism.
Hyperion is Titan’s immediate neighbor. At an average diameter of about 270 km, it is smaller and lighter than Mimas. It is also irregularly shaped and quite odd in composition. Essentially, the moon is an ovoid, tan-colored body with an extremely porous surface (which resembles a sponge). The surface of Hyperion is covered with numerous impact craters, most of which are 2 to 10 km in diameter. It also has a highly unpredictable rotation, with no well-defined poles or equator.
At 1,470 km in diameter and 18×1020 kg in mass, Iapetus is the third-largest of Saturn’s large moons. And at a distance of 3,560,820 km from Saturn, it is the most distant of the large moons and takes 79 days to complete a single orbit. Due to its unusual color and composition – its leading hemisphere is dark and black whereas its trailing hemisphere is much brighter – it is often called the “yin and yang” of Saturn’s moons.
The two sides of Iapetus, Saturn’s “yin-yang moon”. Credit: NASA/JPL
Saturn’s Irregular Moons:
Beyond these larger moons are Saturn’s Irregular Moons. These satellites are small, have large radii, are inclined, have mostly retrograde orbits, and are believed to have been acquired by Saturn’s gravity. These moons are made up of three basic groups – the Inuit Group, the Gallic Group, and the Norse Group.
The Inuit Group consists of five irregular moons that are all named from Inuit mythology – Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq. All have prograde orbits that range from 11.1 to 17.9 million km, and from 7 to 40 km in diameter. They are all similar in appearance (reddish in hue) and have orbital inclinations of between 45 and 50°.
The Gallic group consists of four prograde outer moons that are named after characters in Gallic mythology – Albiorix, Bebhionn, Erriapus, and Tarvos. Here too, the moons are similar in appearance and have orbits that range from 16 to 19 million km. Their inclinations are in the 35°-40° range, their eccentricities around 0.53, and they range in size from 6 to 32 km.
Last, there is the Norse group, which consists of 29 retrograde outer moons that take their names from Norse mythology. These satellites range in size from 6 to 18 km, their distances from 12 and 24 million km, their inclinations between 136° and 175°, and their eccentricities between 0.13 and 0.77. This group is also sometimes referred to as the Phoebe group, due to the presence of a single larger moon in the group – which measures 240 km in diameter. The second-largest, Ymir, measures 18 km across.
Saturn’s rings and moons Credit: NASA
Within the Inner and Outer Large Moons, there are also those belonging to the Alkyonide group. These moons – Methone, Anthe, and Pallene – are named after the Alkyonides of Greek mythology, are located between the orbits of Mimas and Enceladus, and are among the smallest moons around Saturn. Some of the larger moons even have moons of their own, which are known as Trojan moons. For instance, Tethys has two trojans – Telesto and Calypso, while Dione has Helene and Polydeuces.
Moon Formation:
It is thought that Saturn’s moon of Titan, its mid-sized moons and rings developed in a way that is closer to the Galilean moons of Jupiter. In short, this would mean that the regular moons formed from a circumplanetary disc, a ring of accreting gas, and solid debris similar to a protoplanetary disc. Meanwhile, the outer, irregular moons are believed to have been objects that were captured by Saturn’s gravity and remained in distant orbits.
However, there are some variations to this theory. In one alternative scenario, two Titan-sized moons were formed from an accretion disc around Saturn; the second one eventually broke up to produce the rings and inner mid-sized moons. In another, two large moons fused together to form Titan, and the collision scattered icy debris that formed to create the mid-sized moons.
However, the mechanics of how the moon formed remains a mystery for the time being. With additional missions mounted to study the atmospheres, compositions, and surfaces of these moons, we may begin to understand where they truly came from.
Much like Jupiter, and all the other gas giants, Saturn’s system of satellites is extensive as it is impressive. In addition to the larger moons that are believed to have formed from a massive debris field that once orbited it, it also has countless smaller satellites that were captured by its gravitational field over the course of billions of years. One can only imagine how many more remain to be found orbiting the ringed giant.
For astronomers and physicists alike, the depths of space are a treasure trove that may provide us with the answers to some of the most profound questions of existence. Where we come from, how we came to be, how it all began, etc. However, observing deep space presents its share of challenges, not the least of which is visual accuracy.
In this case, scientists use what is known as Active Optics in order to compensate for external influences. The technique was first developed during the 1980s and relied on actively shaping a telescope’s mirrors to prevent deformation. This is necessary with telescopes that are in excess of 8 meters in diameter and have segmented mirrors.
Definition:
The name Active Optics refers to a system that keeps a mirror (usually the primary) in its optimal shape against all environmental factors. The technique corrects for distortion factors, such as gravity (at different telescope inclinations), wind, temperature changes, telescope axis deformation, and others.
The twin Keck telescopes shooting their laser guide stars into the heart of the Milky Way on a beautifully clear night on the summit on Mauna Kea. Credit: keckobservatory.org/Ethan
Adaptive Optics actively shapes a telescope’s mirrors to prevent deformation due to external influences (like wind, temperature, and mechanical stress) while keeping the telescope actively still and in its optimal shape. The technique has allowed for the construction of 8-meter telescopes and those with segmented mirrors.
Use in Astronomy:
Historically, a telescope’s mirrors have had to be very thick to hold their shape and to ensure accurate observations as they searched across the sky. However, this soon became unfeasible as the size and weight requirements became impractical. New generations of telescopes built since the 1980s have relied on very thin mirrors instead.
But since these were too thin to keep themselves in the correct shape, two methods were introduced to compensate. One was the use of actuators which would hold the mirrors rigid and in an optimal shape, the other was the use of small, segmented mirrors which would prevent most of the gravitational distortion that occur in large, thick mirrors.
The New Technology Telescope (NTT) pioneered the Active Optics. Credit: ESO/C.Madsen. Bacon
Other Applications:
In addition to astronomy, Active Optics is used for a number of other purposes as well. These include laser set-ups, where lenses and mirrors are used to steer the course of a focused beam. Interferometers, devices which are used to emit interfering electromagnetic waves, also relies on Active Optics.
These interferometers are used for the purposes of astronomy, quantum mechanics, nuclear physics, fiber optics, and other fields of scientific research. Active optics are also being investigated for use in X-ray imaging, where actively deformable grazing incidence mirrors would be employed.
Adaptive Optics:
Active Optics are not to be confused with Adaptive Optics, a technique that operates on a much shorter timescale to compensate for atmospheric effects. The influences that active optics compensate for (temperature, gravity) are intrinsically slower and have a larger amplitude in aberration.
Artist’s impression of the European Extremly Large Telescope deploying lasers for adaptive optics. Credit: ESO/L. Calçada/N. Risinger
On the other hand, Adaptive Optics corrects for atmospheric distortions that affect the image. These corrections need to be much faster, but also have smaller amplitude. Because of this, adaptive optics uses smaller corrective mirrors (often the second, third or fourth mirror in a telescope).
Artist's conception of the event horizon of a black hole. Credit: Victor de Schwanberg/Science Photo Library
Ever since scientists first discovered the existence of black holes in our universe, we have all wondered: what could possibly exist beyond the veil of that terrible void? In addition, ever since the theory of General Relativity was first proposed, scientists have been forced to wonder, what could have existed before the birth of the Universe – i.e. before the Big Bang?
Interestingly enough, these two questions have come to be resolved (after a fashion) with the theoretical existence of something known as a Gravitational Singularity – a point in space-time where the laws of physics as we know them break down. And while there remain challenges and unresolved issues about this theory, many scientists believe that beneath veil of an event horizon, and at the beginning of the Universe, this was what existed.
Definition:
In scientific terms, a gravitational singularity (or space-time singularity) is a location where the quantities that are used to measure the gravitational field become infinite in a way that does not depend on the coordinate system. In other words, it is a point in which all physical laws are indistinguishable from one another, where space and time are no longer interrelated realities, but merge indistinguishably and cease to have any independent meaning.
This artist’s impression depicts a rapidly spinning supermassive black hole surrounded by an accretion disc. Credit: ESA/Hubble, ESO, M. Kornmesse
Origin of Theory:
Singularities were first predicated as a result of Einstein’s Theory of General Relativity, which resulted in the theoretical existence of black holes. In essence, the theory predicted that any star reaching beyond a certain point in its mass (aka. the Schwarzschild Radius) would exert a gravitational force so intense that it would collapse.
At this point, nothing would be capable of escaping its surface, including light. This is due to the fact the gravitational force would exceed the speed of light in vacuum – 299,792,458 meters per second (1,079,252,848.8 km/h; 670,616,629 mph).
This phenomena is known as the Chandrasekhar Limit, named after the Indian astrophysicist Subrahmanyan Chandrasekhar, who proposed it in 1930. At present, the accepted value of this limit is believed to be 1.39 Solar Masses (i.e. 1.39 times the mass of our Sun), which works out to a whopping 2.765 x 1030 kg (or 2,765 trillion trillion metric tons).
Another aspect of modern General Relativity is that at the time of the Big Bang (i.e. the initial state of the Universe) was a singularity. Roger Penrose and Stephen Hawking both developed theories that attempted to answer how gravitation could produce singularities, which eventually merged together to be known as the Penrose–Hawking Singularity Theorems.
The Big Bang Theory: A history of the Universe starting from a singularity and expanding ever since. Credit: grandunificationtheory.com
According to the Penrose Singularity Theorem, which he proposed in 1965, a time-like singularity will occur within a black hole whenever matter reaches certain energy conditions. At this point, the curvature of space-time within the black hole becomes infinite, thus turning it into a trapped surface where time ceases to function.
The Hawking Singularity Theorem added to this by stating that a space-like singularity can occur when matter is forcibly compressed to a point, causing the rules that govern matter to break down. Hawking traced this back in time to the Big Bang, which he claimed was a point of infinite density. However, Hawking later revised this to claim that general relativity breaks down at times prior to the Big Bang, and hence no singularity could be predicted by it.
Some more recent proposals also suggest that the Universe did not begin as a singularity. These includes theories like Loop Quantum Gravity, which attempts to unify the laws of quantum physics with gravity. This theory states that, due to quantum gravity effects, there is a minimum distance beyond which gravity no longer continues to increase, or that interpenetrating particle waves mask gravitational effects that would be felt at a distance.
Types of Singularities:
The two most important types of space-time singularities are known as Curvature Singularities and Conical Singularities. Singularities can also be divided according to whether they are covered by an event horizon or not. In the case of the former, you have the Curvature and Conical; whereas in the latter, you have what are known as Naked Singularities.
A Curvature Singularity is best exemplified by a black hole. At the center of a black hole, space-time becomes a one-dimensional point which contains a huge mass. As a result, gravity become infinite and space-time curves infinitely, and the laws of physics as we know them cease to function.
Conical singularities occur when there is a point where the limit of every general covariance quantity is finite. In this case, space-time looks like a cone around this point, where the singularity is located at the tip of the cone. An example of such a conical singularity is a cosmic string, a type of hypothetical one-dimensional point that is believed to have formed during the early Universe.
And, as mentioned, there is the Naked Singularity, a type of singularity which is not hidden behind an event horizon. These were first discovered in 1991 by Shapiro and Teukolsky using computer simulations of a rotating plane of dust that indicated that General Relativity might allow for “naked” singularities.
In this case, what actually transpires within a black hole (i.e. its singularity) would be visible. Such a singularity would theoretically be what existed prior to the Big Bang. The key word here is theoretical, as it remains a mystery what these objects would look like.
For the moment, singularities and what actually lies beneath the veil of a black hole remains a mystery. As time goes on, it is hoped that astronomers will be able to study black holes in greater detail. It is also hoped that in the coming decades, scientists will find a way to merge the principles of quantum mechanics with gravity, and that this will shed further light on how this mysterious force operates.
False-color composite image of the Port-au-Prince, Haiti region, taken Jan. 27, 2010 by NASA’s UAVSAR airborne radar. The city is denoted by the yellow arrow; the black arrow points to the fault responsible for the Jan. 12 earthquake. Image credit: NASA
Every so often, in different regions of the world, the Earth feels the need to release energy in the form of seismic waves. These waves cause a great deal of hazards as the energy is transferred through the tectonic plates and into the Earth’s crust. For those living in an area directly above where two tectonic plates meet, the experience can be quite harrowing!
This area is known as a fault, or a fracture or discontinuity in a volume of rock, across which there is significant displacement. Along the line where the Earth and the fault plane meet, is what is known as a fault line. Understanding where they lie is crucial to our understanding of Earth’s geology, not to mention earthquake preparedness programs.
Definition:
In geology, a fault is a fracture or discontinuity in the planet’s surface, along which movement and displacement takes place. On Earth, they are the result of activity with plate tectonics, the largest of which takes place at the plate boundaries. Energy released by the rapid movement on active faults is what causes most earthquakes in the world today.
The Earth’s Tectonic Plates. Credit: msnucleus.org
Since faults do not usually consist of a single, clean fracture, geologists use the term “fault zone” when referring to the area where complex deformation is associated with the fault plane. The two sides of a non-vertical fault are known as the “hanging wall” and “footwall”.
By definition, the hanging wall occurs above the fault and the footwall occurs below the fault. This terminology comes from mining. Basically, when working a tabular ore body, the miner stood with the footwall under his feet and with the hanging wall hanging above him. This terminology has endured for geological engineers and surveyors.
Mechanisms:
The composition of Earth’s tectonic plates means that they cannot glide past each other easily along fault lines, and instead produce incredible amounts of friction. On occasion, the movement stops, causing stress to build up in rocks until it reaches a threshold. At this point, the accumulated stress is released along the fault line in the form of an earthquake.
When it comes to fault lines and the role they have in earthquakes, three important factors come into play. These are known as the “slip”, “heave” and “throw”. Slip refers to the relative movement of geological features present on either side of the fault plane; in other words, the relative motion of the rock on each side of the fault with respect to the other side.
Tectonic Plate Boundaries. Credit:
Heave refers to the measurement of the horizontal/vertical separation, while throw is used to measure the horizontal separation. Slip is the most important characteristic, in that it helps geologists to classify faults.
Types of Faults:
There are three categories or fault types. The first is what is known as a “dip-slip fault”, where the relative movement (or slip) is almost vertical. A perfect example of this is the San Andreas fault, which was responsible for the massive 1906 San Francisco Earthquake.
Second, there are “strike-slip faults”, in which case the slip is approximately horizontal. These are generally found in mid-ocean ridges, such as the Mid-Atlantic Ridge – a 16,000 km long submerged mountain chain occupying the center of the Atlantic Ocean.
Lastly, there are oblique-slip faults which are a combination of the previous two, where both vertical and horizontal slips occur. Nearly all faults will have some component of both dip-slip and strike-slip, so defining a fault as oblique requires both dip and strike components to be measurable and significant.
Map of the Earth showing fault lines (blue) and zones of volcanic activity (red). Credit: zmescience.com
Impacts of Fault Lines:
For people living in active fault zones, earthquakes are a regular hazard and can play havoc with infrastructure, and can lead to injuries and death. As such, structural engineers must ensure that safeguards are taken when building along fault zones, and factor in the level of fault activity in the region.
This is especially true when building crucial infrastructure, such as pipelines, power plants, damns, hospitals and schools. In coastal regions, engineers must also address whether tectonic activity can lead to tsunami hazards.
For example, in California, new construction is prohibited on or near faults that have been active since the Holocene epoch (the last 11,700 years) or even the Pleistocene epoch (in the past 2.6 million years). Similar safeguards play a role in new construction projects in locations along the Pacific Rim of fire, where many urban centers exist (particularly in Japan).
Various techniques are used to gauge when the last time fault activity took place, such as studying soil and mineral samples, organic and radiocarbon dating.
High concentrations of carbon dioxide (in red) tend to congregate in the northern hemisphere during colder months, when plants can't absorb as much from the atmosphere. This picture is based on a NASA Goddard computer model from ground-based observations and depicts concentrations on March 30, 2006. Credit: NASA's Goddard Space Flight Center/B. Putman/YouTube (screenshot)
What if it were possible to just suck all the harmful pollutants out of the air so that they wouldn’t be such a nuisance? What if it were also possible to convert these atmospheric pollutants back into fossil fuels, or possibly ecologically-friendly bio fuels? Why, then we would be able to worry far less about smog, respiratory illnesses, and the effects that high concentrations of these gases have on the planet.
This is the basis of Carbon Capture, a relatively new concept where carbon dioxide is captured at point sources – such as factories, natural-gas plants, fuel plants, major cities, or any other place where large concentrations of CO² are known to be found. This CO² can then be stored for future use, converted into biofuels, or simply put back into the Earth so that it doesn’t enter the atmosphere.
Description:
Like many other recent developments, carbon capture is part of a new set of procedures that are collectively known as geoengineering. The purpose of these procedures are to alter the climate to counteract the effects of global warming, generally by targeting one of the chief greenhouse gases. The technology has existed for some time, but it has only been in recent years that it has been proposed as a means of combating climate change as well.
Schematic showing both terrestrial and geological sequestration of carbon dioxide emissions from a coal-fired plant. Credit: web.ornl.gov
Currently, carbon capture is most often employed in plants that rely on fossil fuel burning to generate electricity. This process is performed in one of three basic ways – post-combustion, pre-combustion and oxy-fuel combustion. Post-combustion involves removing CO2 after the fossil fuel is burned and is converted into a flue gas, which consists of CO2, water vapor, sulfur dioxides and nitrogen oxide.
When the gases travel through a smokestack or chimney, CO² is captured by a “filter” which actually consists of solvents that are used to absorb CO2 and water vapor. This technique is effective in that such filters can be retrofitted to older plants, avoiding the need for a costly power plant overhaul.
Benefits and Challenges:
The results of these processes have so far been encouraging – which boast the possibility of up to 90 % of CO² being removed from emissions (depending on the type of plant and the method used). However, there are concerns that some of these processes add to the overall cost and energy consumption of power plants.
According to 2005 report by the Intergovernmental Panel on Climate Change (IPCC), the additional costs range from 24 to 40% for coal power plants, 11 to 22% for natural gas plants, and 14 to 25% for coal-based gasification combined cycle systems. The additional power consumption also creates more in the way of emissions.
Vehicle emissions are one of the main sources of carbon dioxide today. Credit: ucsusa.org
In addition, while CC operations are capable of drastically reducing CO², they can add other pollutants to the air. The amounts of kind of pollutants depend on the technology, and range from ammonia and nitrogen oxides (NO and NO²) to sulfur oxides and disulfur oxides (SO, SO², SO³, S²O, S²O³. etc.). However, researchers are developing new techniques which they hope will reduce both costs and consumption and not generate additional pollutants.
Examples:
A good example of the Carbon Capture process is the Petro Nova project, a coal-fired power plant in Texas. This plant began being upgraded by the US Dept. of Energy (DOE) in 2014 to accommodate the largest post-combustion carbon-capture operation in the world.
Consisting of filters that would capture the emissions, and infrastructure that would place it back in the Earth, the DOE estimates that this operation will be capable of capturing 1.4 million tons of CO2 that previously would have been released into the air.
In the case of pre-combustion, CO² is trapped before the fossil fuel is even burned. Here, coal, oil or natural gas is heated in pure oxygen, resulting in a mix of carbon monoxide and hydrogen. This mix is then treated in a catalytic converter with steam, which then produces more hydrogen and carbon dioxide.
When complete, the US Department of Energy’s (DoE) Petro Nova will be the largest post-combustion carbon capture operation in the world. Credit: DOE
These gases are then fed into flasks where they are treated with amine (which binds with the CO² but not hydrogen); the mixture is then heated, causing the CO² to rise where it can be collected. In the final process (oxy-fuel combustion), fossil fuel is burned in oxygen, resulting in a gas mixture of steam and CO². The steam and carbon dioxide are separated by cooling and compressing the gas stream, and once separated, the CO² is removed.
Other efforts at carbon capture include building urban structures with special facilities to extract CO² from the air. Examples of this include the Torre de Especialidades in Mexico City – a hospital that is surrounded by a 2500 m² facade composed of Prosolve370e. Designed by Berlin-based firm Elegant Embellishments, this specially-shaped facade is able to channel air through its lattices and relies on chemical processes to filter out smog.
China’s Phoenix Towers – a planned-project for a series of towers in Wuhan, China (which will also be the world’s tallest) – is also expected to be equipped with a carbon capture operation. As part of the designers vision of creating a building that is both impressively tall and sustainable, these include special coatings on the outside of the structures that will draw CO² out of the local city air.
Then there’s the idea for “artificial trees“, which was put forward by Professor Klaus Lackner of the Department of Earth and Environmental Engineering at Columbia University. Consisting of plastic fronds that are coated with a resin that contains sodium carbonation – which when combined with carbon dioxide creates sodium bicarbonate (aka. baking soda) – these “trees” consume CO² in much the same way real trees do.
A cost-effective version of the same technology used to scrub CO² from air in submarines and space shuttles, the fronds are then cleaned using water which, when combined with the sodium bicarbonate, yields a solution that can easily be converted into biofuel.
In all cases, the process of Carbon Capture comes down to finding ways to remove harmful pollutants from the air to reduce humanity’s footprint. Storage and reuse also enter into the equation in the hopes of giving researchers more time to develop alternative energy sources.
X-47B conducting a midair refueling run in the Atlantic Test Ranges. Credit: US Navy
The field of aviation has produced some interesting designs over the course of its century-long history. In addition to monoplanes, jet-aircraft, rocket-propelled planes, and high-altitude interceptors and spy craft, there is also the variety of airplanes that do away with such things as tails, sections and fuselages. These are what is known as Flying Wings, a type of fixed-wing aircraft that consists of a single wing.
While this concept has been investigated for almost as long as flying machines have existed, it is only within the past few decades that its true potential has been realized. And when it comes to the future of aerospace, it is one concept that is expected to see a great deal more in the way of research and development.
Description:
By definition, a flying wing is an aircraft which has no definite fuselage, with most of the crew, payload and equipment being housed inside the main wing structure. From the top, a flying wing looks like a chevron, with the wings constituting its outer edges and the front middle serving as the cockpit or pilot’s seat. They come in many varieties, ranging from the jet fighter/bomber to hand gliders and sailplanes.
A clean flying wing is theoretically the most aerodynamically efficient (lowest drag) design configuration for a fixed wing aircraft. It also offers high structural efficiency for a given wing depth, leading to light weight and high fuel efficiency.
A Junkers G 38, in service with Lufthansa. Credit: SDASM Archives
History of Development:
Tailless craft have been around since the time of the Wright Brothers. But it was not until after World War I, thanks to extensive wartime developments with monoplanes, that a craft with no true fuselage became feasible. An early enthusiast was Hugo Junkers who patented the idea for a wing-only air transport in 1910.
Unfortunately, restrictions imposed by the Treaty of Versailles on German aviation meant that his vision wasn’t realized until 1931 with the Junker’s G38. This design, though revolutionary, still required a short fuselage and a tail section in order to be aerodynamically possible.
A restored Horten Ho 229 at Steven F. Udvar-Hazy Center. Credits: Cynrik de Decker
Flying wing designs were experimented with extensively in the 30’s and 40’s, especially in the US and Germany. In France, Britain and the US, many designs were produced, though most were gliders. However, there were exceptions, like the Northrop N1M, a prototype all-wing plane and the far more impressive Horten Ho 229, the first jet-powered flying wing that served as a fighter/bomber for the German air force in WWII.
This aircraft was part of a long series of experimental aircraft produced by Nazi Germany, and was also the first craft to incorporate technology that made it harder to detect on radar – aka. Stealth technology. However, whether this was intentional or an unintended consequence of its design remains the subject of speculation.
After WWII, this plane inspired several generations of experimental aircraft. The most notable of these are the YB-49 long-range bomber, the A-12 Avenger II, the B-2 Stealth Bomber (otherwise known as the Spirit), and a host of delta-winged aircraft, such as Canada’s own Avro-105, also known as the Avro Arrow.
Recent Developments:
More recent examples of aircraft that incorporate the flying wing design include the X-47B, a demonstration unmanned combat air vehicle (UCAV) currently in development by Northrop Grumman. Designed for carrier-based operations, the X-47B is a result of collaboration between the Defense Advanced Research Projects Agency (DARPA) and the US Navy’s Unmanned Combat Air System Demonstration (UCAS-D) program.
The X-47B first flew in 2011, and as of 2015, its two active demonstrators successfully performed a series of airstrip and carrier-based landings. Eventually, Northrop Grumman hopes to develop the prototype X-47B into a battlefield-ready aircraft known the Unmanned Carrier-Launched Airborne Surveillance and Strike (UCLASS) system, which is expected to enter service in the 2020s.
Another take on the concept comes in the form of the bidirectional flying wing. This type of design consists of a long-span, low speed wing and a short-span, high speed wing joined in a single airframe in the shape of an uneven cross. The proposed craft would take off and land with the low-speed wing across the airflow, then rotate a quarter-turn so that the high-speed wing faces the airflow for supersonic travel.
The design is claimed to feature low wave drag, high subsonic efficiency and little or no sonic boom. The low-speed wings have likely a thick, rounded airfoil able to contain the payload and a wide span for high efficiency, while the high-speed wing would have a thin, sharp-edged airfoil and a shorter span for low drag at supersonic speed.
In 2012, NASA announced that it was in the process of funding the development of such a concept, known as the Supersonic Bi-Directional Flying Wing (SBiDir-FW). This came in the form of the Office of the Chief Technologist awarding a grant of $100,000 to a research group at the University of Miami (led by Professor Gecheng Zha) who were already working on such a plane.
Since the Wright Brothers first took to the air in a plane made of canvas and wood over a century ago, aeronautical engineers have thought long and hard about how we can improve upon the science of flight. Every once in awhile, there are those who will attempt to “reinvent the wheel”, throwing out the old paradigm and producing something truly revolutionary.
An aurora seen over the South Pole, from the ISS. Credit: Doug Wheelock, NASA.
For many people around the world the ability to see the Aurora Borealis or Aurora Australis is a rare treat. Unless you live north of 60° latitude (or south of -60°), or who have made the trip to tip of Chile or the Arctic Circle at least once in their lives, these fantastic light shows are something you’ve likely only read about or seen a video of.
But on occasion, the “northern” and “southern lights” have reached beyond the Arctic and Antarctic Circles and dazzled people with their stunning luminescence. But what exactly are they? To put it simply, auroras are natural light displays that take place in the night sky, particularly in the Polar Regions, and which are the result of interaction in the ionosphere between the sun’s rays and Earth’s magnetic field.
Description:
Basically, solar wind is periodically launched by the sun which contains clouds of plasma, charged particles that include electrons and positive ions. When they reach the Earth, they interact with the Earth’s magnetic field, which excites oxygen and nitrogen in the Earth’s upper atmosphere. During this process, ionized nitrogen atoms regain an electron, and oxygen and nitrogen atoms return from an excited state to ground state.
High-speed particles from the Sun, mostly electrons, strike oxygen and nitrogen atoms in Earth’s upper atmosphere. Credit: NASA
Excitation energy is lost by the emission of a photon of light, or by collision with another atom or molecule. Different gases produce different colors of light – light emissions coming from oxygen atoms as they interact with solar radiation appear green or brownish-red, while the interaction of nitrogen atoms cause light to be emitted that appears blue or red.
This dancing display of colors is what gives the Aurora its renowned beauty and sense of mystery. In northern latitudes, the effect is known as the Aurora Borealis, named after the Roman Goddess of the dawn (Aurora) and the Greek name for the north wind (Boreas). It was the French scientist Pierre Gassendi who gave them this name after first seeing them in 1621.
In the southern latitudes, it is known as Aurora Australis, Australis being the Latin word for “of the south”. Auroras seen near the magnetic pole may be high overhead, but from farther away, they illuminate the northern horizon as a greenish glow or sometimes a faint red. The auroras are usually best seen in the Arctic and Antarctic because that is the location of the poles of the Earth’s magnetic field.
The South Pole Telescope under the aurora australis (southern lights). Credit: Keith Vanderlinde
Names and Cultural Significance:
The northern lights have had a number of names throughout history and a great deal of significance to a number of cultures. The Cree call this phenomenon the “Dance of the Spirits”, believing that the effect signaled the return of their ancestors.
To the Inuit, it was believed that the spirits were those of animals. Some even believed that as the auroras danced closer to those who were watching them, that they would be enveloped and taken away to the heavens. In Europe, in the Middle Ages, the auroras were commonly believed to be a sign from God.
According to the Norwegian chronicle Konungs Skuggsjá (ca. 1230 CE), the first encounter of the norðrljós (Old Norse for “northern light”) amongst the Norsemen came from Vikings returning from Greenland. The chronicler gives three possible explanations for this phenomena, which included the ocean being surrounded by vast fires, that the sun flares reached around the world to its night side, or that the glaciers could store energy so that they eventually glowed a fluorescent color.
Auroras on Other Planets:
However, Earth is not the only planet in the Solar System that experiences this phenomena. They have been spotted on other Solar planets, and are most visible closer to the poles due to the longer periods of darkness and the magnetic field.
Image of Saturn’s aurora taken by the Huddle Space Telescope and seen in ultraviolet wavelengths. Credit: ESA/NASA/Hubble
For example. the Hubble Space Telescope has observed auroras on both Jupiter and Saturn – both of which have magnetic fields much stronger than Earth’s and extensive radiation belts. Uranus and Neptune have also been observed to have auroras which, same as Earth, appear to be powered by solar wind.
Auroras also have been observed on the surfaces of Io, Europa, and Ganymede using the Hubble Space Telescope, not to mention Venus and Mars. Because Venus has no planetary magnetic field, Venusian auroras appear as bright and diffuse patches of varying shape and intensity, sometimes distributed across the full planetary disc.
An aurora was also detected on Mars on August 14th, 2004, by the SPICAM instrument aboard Mars Express. This aurora was located at Terra Cimmeria, in the region of 177° East, 52° South, and was estimated to be quite sizable – 30 km across and 8 km high (18.5 miles across and 5 miles high).
Mars has magnetized rocks in its crust that create localized, patchy magnetic fields (left). In the illustration at right, we see how those fields extend into space above the rocks. At their tops, auroras can form. Credit: NASA
Though Mars has little magnetosphere to speak of, scientists determined that the region of the emissions corresponded to an area where the strongest magnetic field is localized on the planet. This they concluded by analyzing a map of crustal magnetic anomalies compiled with data from Mars Global Surveyor.
More recently, an aurora was observed on Mars by the MAVEN mission, which captured images of the event on March 17th, 2015, just a day after an aurora was observed here on Earth. Nicknamed Mars’ “Christmas lights”, they were observed across the planet’s mid-northern latitudes and (owing to the lack of oxygen and nitrogen in Mars’ atmosphere) were likely a faint glow compared to Earth’s more vibrant display.
In short, it seems that auroras are destined to happen wherever solar winds and magnetic fields coincide. But somehow, knowing this does not make them any less impressive, or diminish the power they have to inspire wonder and amazement in all those that behold them.
Take a look at Venus in even the most powerful telescope, and all you’ll see is clouds. There are no surface features visible at all. It wasn’t until the last few decades, when radar equipped spacecraft arrived at Venus, that scientists finally had a chance to study the geology of Venus in great detail.
Spacecraft like NASA’s Magellan mission are equipped with radar instruments that let it penetrate down through the clouds on Venus and reveal the surface below. Magellan found that the surface of Venus does have many impact craters and evidence of past volcanism. But the total number of craters showed that the surface of Venus is actually pretty young. It’s likely that some catastrophic event resurfaced Venus about 300-500 million years ago, wiping out old craters and volcanoes.
Unlike Earth, Venus doesn’t have plate tectonics. It’s possible that the planet had them in the ancient past, but rising temperatures shut them down and helped the planet go into a runaway greenhouse cycle. Carbon on Earth is trapped by plants, and is then recycled into the Earth through plate tectonics. But on Venus, the tectonic system shut down, so carbon was able to build up to tremendous levels. This cycle thickened the atmosphere, raised temperatures with its greenhouse effect, releasing more carbon, raising temperatures even higher… etc.
There are volcanoes on Venus; scientists have identified more than 100 isolated shield volcanoes. And there are thousands and maybe even millions of smaller volcanoes less than 20 km across. Many of these have a strange dome-shaped structure, believed to have formed when plumes of magma thrust the crust upward and then collapsed.
Scientists can’t be exactly sure what the internal structure of Venus is like, but based on its density, Venus is probably similar to Earth in composition. It’s believed to have a solid or liquid core of metal 3,000 km across. This is surrounded by a mantle of rock 3,000 km thick, and then a thin crust of solid rock about 50 km thick.
One big difference between Earth and Venus is the lack of a planetary magnetic field at Venus. It’s believed that the Earth’s magnetic field is driven by the convection of liquid metal at the Earth’s core. If true, it means that Venus probably doesn’t have the same kind of temperature differences at its core, and lacks the convection to sustain a planetary magnetic field.
Much like any living being, stars go through a natural cycle. This begins with birth, extends through a lifespan characterized by change and growth, and ends in death. Of course, we’re talking about stars here, and the way they’re born, live and die is completely different from any life form we are familiar with.
For one, the timescales are entirely different, lasting on the order of billions of years. Also, the changes they go through during their lifespan are entirely different too. And when they die, the consequences are, shall we say, much more visible? Let’s take a look at the life cycle of stars.
Molecular Clouds:
Stars start out as vast clouds of cold molecular gas. The gas cloud could be floating in a galaxy for millions of years, but then some event causes it to begin collapsing down under its own gravity. For example when galaxies collide, regions of cold gas are given the kick they need to start collapsing. It can also happen when the shockwave of a nearby supernova passes through a region.
As it collapses, the interstellar cloud breaks up into smaller and smaller pieces, and each one of these collapses inward on itself. Each of these pieces will become a star. As the cloud collapses, the gravitational energy causes it to heat up, and the conservation of momentum from all the individual particles causes it to spin.
Protostar:
As the stellar material pulls tighter and tighter together, it heats up pushing against further gravitational collapse. At this point, the object is known as a protostar. Surrounding the protostar is a circumstellar disk of additional material. Some of this continues to spiral inward, layering additional mass onto the star. The rest will remain in place and eventually form a planetary system.
Depending on the stars mass, the protostar phase of stellar evolution will be short compared to its overall life span. For those that have one Solar Mass (i.e the same mass as our Sun), it lasts about 1000,000 years.
T Tauri Star:
A T Tauri star begins when material stops falling onto the protostar, and it’s releasing a tremendous amount of energy. They are so-named because of the prototype star used to research this phase of solar evolution – T Tauri, a variable star located in the direction of the Hyades cluster, about 600 light years from Earth.
A T Tauri star may be bright, but this all comes its gravitational energy from the collapsing material. The central temperature of a T Tauri star isn’t enough to support fusion at its core. Even so, T Tauri stars can appear as bright as main sequence stars. The T Tauri phase lasts for about 100 million years, after which the star will enter the longest phase of its development – the Main Sequence phase.
Main Sequence:
Eventually, the core temperature of a star will reach the point that fusion its core can begin. This is the process that all stars go through as they convert protons of hydrogen, through several stages, into atoms of helium. This reaction is exothermic; it gives off more heat than it requires, and so the core of a main sequence star releases a tremendous amount of energy.
This energy starts out as gamma rays in the core of the star, but as it takes a long slow journey out of the star, it drops down in wavelength. All of this light pushes outward on the star, and counteracts the gravitational force pulling it inward. A star at this stage of life is held in balance – as long as its supplies of hydrogen fuel lasts.
The life cycle of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant on the right after billions of years. Credit: ESO/M. Kornmesser
And how long does it last? It depends on the mass of the star. The least massive stars, like red dwarfs with half the mass of the Sun, can sip away at their fuel for hundreds of billions and even trillions of years. Larger stars, like our Sun will typically sit in the main sequence phase for 10-15 billion years. The largest stars have the shortest lives, and can last a few billion, and even just a few million years.
Red Giant:
Over the course of its life, a star is converting hydrogen into helium at its core. This helium builds up and the hydrogen fuel runs out. When a star exhausts its fuel of hydrogen at its core, its internal nuclear reactions stop. Without this light pressure, the star begins to contract inward through gravity.
This process heats up a shell of hydrogen around the core which then ignites in fusion and causes the star to brighten up again, by a factor of 1,000-10,000. This causes the outer layers of the star to expand outward, increasing the size of the star many times. Our own Sun is expected to bloat out to a sphere that reaches all the way out to the orbit of the Earth.
The temperature and pressure at the core of the star will eventually reach the point that helium can be fused into carbon. Once a star reaches this point, it contracts down and is no longer a red giant. Stars much more massive than our Sun can continue on in this process, moving up the table of elements creating heavier and heavier atoms.
White Dwarf:
A star with the mass of our Sun doesn’t have the gravitational pressure to fuse carbon, so once it runs out of helium at its core, it’s effectively dead. The star will eject its outer layers into space, and then contract down, eventually becoming a white dwarf. This stellar remnant might start out hot, but it has no fusion reactions taking place inside it any more. It will cool down over hundreds of billions of years, eventually becoming the background temperature of the Universe.
