Month January 2020

           Light is a form of energy that can travel on its own even through a vacuum. Humans can see visible light, from red to violet, but there are also many other forms of light that cannot be seen with the naked eye. Light consists of energy in the form of electric and magnetic fields, and is therefore referred to as electromagnetic radiation. Light travels like a wave, and light waves come in many sizes. The size of a wave is measured by the distance from one peak to the next, which is called the wavelength. Light waves also come in many frequencies — the number of waves that pass a certain point every second. Gamma rays have the highest frequencies and the shortest wave-lengths, and therefore the most energy.

           It is no accident that humans can ‘see’ light. The detection of light is a very powerful tool for probing the universe around us. As light interacts with matter it can be become altered and by studying light that has originated or interacted with matter, many of the properties of that matter can be determined. It is through the study of light that for example we can understand the composition of the stars light years away or watch the processes that occur in the living cell as they happen

           Matter is composed of atoms, ions or molecules and it is light’s interaction with matter which gives rise to the various phenomena which can help us understand the nature of matter. The atoms, ions or molecules have defined energy levels usually associated with energy levels that electrons in the matter can hold. Light can be generated by the matter or a photon of light can interact with the energy levels in a number of ways.

           We can represent the energy levels in a diagram known as a Jablonski diagram. An example of one is shown in the diagram above. An atom or molecule in the lowest energy state possible known as the ground state can absorb a photon which will allow the atom or molecule to be raised to a higher energy level state or become excited. Hence the matter can absorb light of characteristic wavelengths such as the blue light in the example on the right or the violet light in the example on the left. The atom or molecule won’t stay in an excited state so it relaxes back to the ground state by several ways. In the example on the right, the atom or molecule emits two photons both of lower energy than the absorbed photon. The photons emitted will be a characteristic energy appropriate for a particular atom or compound and so by studying the light emission the matter under investigation can be determined. In the example on the left the excited atom or molecule initially loses energy by not emitting a photon and instead relaxes to the lower energy state by internal processes which typically heat up the matter. The intermediate energy level then relaxes to the ground state by the emission of a photon of orange light. You can find out information on how light is measured by visiting the scientific cameras web section.

          The actual brightness of a star compared to the Sun is called its visual luminosity, which ranges from 100,000 times to 1/100,000 of the Sun’s bright-ness. A star’s apparent magnitude is how bright it appears from Earth. Brighter stars have low, or negative, magnitudes.

          The luminosity of an object in space is the amount of energy that it radiates each second in all directions. Luminosity is also referred to as the absolute magnitude or absolute brightness of an object. It is the real brightness of a celestial object.

          The apparent magnitude or apparent brightness of an object is a measure of how bright an object appears to be to an observer. It is the amount of energy from an object in space which reaches a square centimeter of a detector each second. Apparent magnitude is also referred to as flux. It is a measure of how bright a celestial object appears to us. The apparent magnitude of an object depends upon its real brightness and on its distance from us.

          If you look up at the night sky on a clear night, you will notice that the stars appear to have different levels of brightness – some are bright and some are dim. The apparent brightness of an object is measured in magnitudes. This system was developed over 2000 years ago by the Greek astronomer Hipparchus to rank how bright different stars appeared to the eye. In his magnitude system, the brightest stars were called first magnitude stars, and the dimmest were sixth magnitude stars. So, in this system, brighter objects have lower magnitudes than dimmer objects. Much later, when astronomers were better able to measure the brightness of stars and other celestial objects, they kept the traditional magnitude scale of Hipparchus and added magnitudes that go beyond the range of 1 to 6. Objects which appear to be much brighter than the stars, such as the Sun, Moon and Venus are given negative magnitudes or -26.7, -12.6 and -4.4 respectively. With modern telescopes we can measure objects which appear as faint as +28 magnitudes. Each number on the magnitude scale is about 2.5 times apart in brightness from the next number. For example, first magnitude stars (stars with a magnitude of 1) are about 2.5 times brighter than second magnitude stars (stars with a magnitude of 2).

          When we look at an object in space we see its apparent brightness. If we know the object’s distance from us, it is easy to calculate its absolute brightness or luminosity.

Absolute magnitude = apparent magnitude – 5 × (distance in parsecs) + 5.

          From this formula, you can see that if a celestial object is 10 parsecs away from us, then its apparent magnitude is equal to its absolute magnitude. The apparent and absolute brightness of objects in space will be different at different wavelengths, for example the infrared magnitude will not be the same as a visible light magnitude, however, and the above formula still applies. Below is a table showing the visible light absolute and apparent magnitudes of some well-known stars.

          Because stars are so far apart from each other, even light can take billions of years to travel between them. The further away a star is from Earth, the longer it takes its light to reach us. This means that when we look up at the stars at night, we are gazing back in time. Even the Sun’s closest neighbour, Proxima Centauri, is more than four light years away, which means that we are seeing it as it was over four years ago.

          Well, here’s something to think about. When you hear a plane, do you ever have trouble spotting it in the sky? If you look up where the sound seems to be coming from, the plane is no longer there. That is because it takes time for the sound waves to reach you, and by the time they do, the plane has moved on. So, in a sense, you are “hearing back in time,” because the sound you hear is the sound of the plane several seconds ago, not the sound of the plane now. When you do spot the plane, you are actually seeing it as it was a tiny fraction of a second ago because just as sound waves take time to travel, so do light waves. Now, nothing can travel faster than light, but as fast as it is, light still needs time to get anywhere, whether it’s across a room, across the span of atmosphere beneath an airplane, or across a big part of the universe.

          The farther away an object is, the longer its light takes to reach us. When you look across the room, you see something as it was a few billionths of a second ago, and when you look at the moon, you see the light that left it a little more than 1 second ago. If our star, the Sun, were to suddenly burn out, we wouldn’t even know it for more than 8 minutes, because the last bit of light that left it would take that long to travel to Earth! Don’t worry, though. The Sun is going to last for another 5 billion years or so! But what about all the more distant stars? It takes much longer for their light to reach us. When you look at the stars at night, you are seeing light that may have taken 20 or 30 or even a few hundred years to reach your eyes. You are looking back in time. The distances in the universe are so vast that scientists use the term “light year” to measure them. A light year is the distance light travels in one year, or 10 trillion kilometers. That’s 1 followed by 13 zeros!

         The Hubble Space Telescope can see objects even more distant than your eyes can. When it takes a picture of a galaxy 100 million light years away, we are seeing the galaxy as it looked 100 million years ago. At the time that light left that galaxy, dinosaurs still roamed Earth and humans would not appear for many millions of years! Because distant galaxies appear to us as they were millions or even billions of years ago, we can study how they change over time.

          By looking at distant galaxies, we see what they looked like when the universe was much younger, as galaxies were first forming. As we look at closer and closer galaxies we see how they change as they age, just as looking at babies, children, teenagers, and then adults can show how we humans change as we age.

          There are countless billions of stars in the Universe, each at different stages of development. Astronomers use a special chart called the Hertzsprung—Russell (H—R) diagram to help understand the different types of stars better. By plotting stars on the H—R diagram based on their temperature and absolute magnitude, astronomers can sort the stars into groups and learn more about them.

          In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences.

          The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere’s temperature.

          Most stars are currently classified under the Morgan-Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest (O type) to the coolest (M type). Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. A8, A9, F0, and F1 form a sequence from hotter to cooler). The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars.

          In the MK system, a luminosity class is added to the spectral class using Roman numerals. This is based on the width of certain absorption lines in the star’s spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class 0 or Ia+ is used for hypergiants, class I for supergiant’s, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class sd (or VI) for sub-dwarfs, and class D (or VII) for white dwarfs. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a surface temperature around 5,800 K.

Picture Credit : Google

          Light is the fastest thing in the Universe. It travels almost 300, 000km (186,000 miles) in one second. In a single year, light travels 9,500,000,000,000km (5,900,000,000,000 miles) — or 9.5 trillion km (5.9 trillion miles). This distance is called a light year. It is used by astronomers to describe the enormous distances between stars and galaxies in space.

          Early scientists, unable to perceive light’s motion, thought it must travel instantaneously. Over time, however, measurements of the motion of these wave-like particles became more and more precise. Thanks to the work of Albert Einstein and others, we now understand light speed to be a theoretical limit: light speed — a constant called “c” — is thought to be not achievable by anything with mass, for reasons explained below. That doesn’t stop sci-fi writers, and even some very serious scientists, from imagining alternative theories that would allow for some awfully fast trips around the universe.

          The first known discourse on the speed of light comes from the ancient Greek philosopher Aristotle, who penned his disagreement with another Greek scientist, Empedocles. Empedocles argued that because light moved, it must take time to travel. Aristotle, believing light to travel instantaneously, disagreed.

          In 1667, the Italian astronomer Galileo Galilei stood two people on hills less than a mile apart, each holding a shielded lantern. One uncovered his lantern; when the second saw the flash, he uncovered his, as well. By observing how long it took for the light to be seen by the first lantern-holder (and factoring out reaction times), he thought he could calculate the speed of light. Unfortunately, Galileo’s experimental distance of less than a mile was too small to see a difference, so he could only determine that light traveled at least 10 times faster than sound.

          In the 1670s, Danish astronomer Ole Roomer used eclipses of Jupiter’s moon, Io, as a chronometer for the speed of light when he made the first real measurement of the velocity. Over the course of several months, as Io passed behind the giant gas planet, Roomer found that the eclipses came later than calculations anticipated, although over the course of several months, they drew closer to the predictions. He determined that light took time to travel from Io to Earth. The eclipses lagged the most when Jupiter and Earth were farthest apart, and were on schedule as they were closer. According to NASA, “that gave Roomer convincing evidence that light spread in space with a certain velocity.”

          He concluded that light took 10 to 11 minutes to travel from the sun to Earth, an overestimate since it in fact takes eight minutes and 19 seconds. But at last scientists had a number to work with — his calculation presented a speed of 125,000 miles per second (200,000 km/s).

          In 1728, English physicist James Bradley based his calculations on the change in the apparent position of the stars due Earth’s travels around the sun. He put the speed of light at 185,000 miles per second (301,000 km/s), accurate to within about 1 percent.

Picture Credit : Google

          Black holes emit no light. However, scientists can find them if they are located close to another star. The enormous gravitational pull of the black hole will tear gas from the star, pulling in streams of material. This gas will circle the black hole with such force that its temperature can exceed 100 million °C. This is so hot that x-rays will be released. Satellites such as the RXTE are used by astronomers to detect these x-rays.

          Einstein’s General Theory of Relativity predicted that gravity could bend space. This was later confirmed during a solar eclipse when a star’s position was measured before, during and after the eclipse. The star’s position shifted because the light from the star was bent by the sun’s gravity. Therefore, an object with immense gravity (like a galaxy or black hole) between the Earth and a distant object could bend the light from the distant object into a focus, much like a lens can.

           A gravitational lens passed between it and the Earth. When the Hubble Space Telescope looked at the object, it saw two images of the object close together, which indicated a gravitational lens effect. The intervening object was unseen. Therefore, it was concluded that a black hole had passed between Earth and the object.

          When material falls into a black hole from a companion star, it gets heated to millions of degrees Kelvin and accelerated. The superheated materials emit X-rays, which can be detected by X-ray telescopes such as the orbiting Chandra X-ray Observatory.

          The star Cygnus X-1 is a strong X-ray source and is considered to be a good candidate for a black hole. Stellar winds from the companion star, blow material onto the accretion disk surrounding the black hole. As this material falls into the black hole, In addition to X-rays, black holes can also eject materials at high speeds to form jets. Many galaxies have been observed with such jets. Currently, it is thought that these galaxies have supermassive black holes (billions of solar masses) at their centers that produce the jets as well as strong radio emissions.

          It is important to remember that black holes are not cosmic vacuum cleaners — they will not consume everything. So although we cannot see black holes, there is indirect evidence that they exist. They have been associated with time travel and worm holes and remain fascinating objects in the universe.

Picture Credit : Google

          If a star’s core after a supernova is more than three times the mass of the Sun, it will collapse in on itself even further than a neutron star, shrinking into an unimaginably small space called a singularity. Its gravity becomes immensely strong, creating a gravitational well in space. If space was a stretched out sheet, a black hole would create such a steep well in the sheet that any object passing too close would he sucked inside forever. The force is so strong that nothing can escape, not even light.

          A black hole is a region of space-time exhibiting gravitational acceleration so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform space-time to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, no locally detectable features appear to be observed. In many ways, a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved space-time predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black hole of stellar mass, making it essentially impossible to observe.

          A black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area – think of a star ten times more massive than the Sun squeezed into a sphere approximately the diameter of New York City. The result is a gravitational field so strong that nothing, not even light, can escape. In recent years, NASA instruments have painted a new picture of these strange objects that are, to many, the most fascinating objects in space.

          The idea of an object in space so massive and dense that light could not escape it has been around for centuries. Most famously, black holes were predicted by Einstein’s theory of general relativity, which showed that when a massive star dies, it leaves behind a small, dense remnant core. If the core’s mass is more than about three times the mass of the Sun, the equations showed, the force of gravity overwhelms all other forces and produces a black hole.

          If a black hole passes through a cloud of interstellar matter, for example, it will draw matter inward in a process known as accretion. A similar process can occur if a normal star passes close to a black hole. In this case, the black hole can tear the star apart as it pulls it toward itself. As the attracted matter accelerates and heats up, it emits x-rays that radiate into space. Recent discoveries offer some tantalizing evidence that black holes have a dramatic influence on the neighborhoods around them – emitting powerful gamma ray bursts, devouring nearby stars, and spurring the growth of new stars in some areas while stalling it in others.

Picture Credit : Google

          When a star explodes into a supernova, all that remains is a very small, extremely dense ball. This star is not made of gas, but rather of a liquid centre of subatomic particles called neutrons, surrounded by a solid iron crust. The matter in a neutron star is packed so tightly that the star is often no bigger than a few kilometres in diameter.

          Neutron stars are city-size stellar objects with a mass about 1.4 times that of the sun. Born from the explosive death of another, larger stars, these tiny objects pack quite a punch. Let’s take a look at what they are, how they form, and how they vary.

          When star four to eight times as massive as the sun explode in a violent supernova, their outer layers can blow off in an often-spectacular display, leaving behind a small, dense core that continues to collapse. Gravity presses the material in on itself so tightly that protons and electrons combine to make neutrons, yielding the name “neutron star.” 

          Neutron stars pack their mass inside a 20-kilometer (12.4 miles) diameter. They are so dense that a single teaspoon would weigh a billion tons — assuming you somehow managed to snag a sample without being captured by the body’s strong gravitational pull. On average, gravity on a neutron star is 2 billion times stronger than gravity on Earth. In fact, it’s strong enough to significantly bend radiation from the star in a process known as gravitational lensing, allowing astronomers to see some of the back side of the star.

          The power from the supernova that birthed it gives the star an extremely quick rotation, causing it to spin several times in a second. Neutron stars can spin as fast as 43,000 times per minute, gradually slowing over time.

          If a neutron star is part of a binary system that survived the deadly blast from its supernova (or if it captured a passing companion), things can get even more interesting. If the second star is less massive than the sun, it pulls mass from its companion into a Roche lobe, a balloon-like cloud of material that orbits the neutron star. Companion stars up to 10 times the sun’s mass create similar mass transfers that are more unstable and don’t last as long. Stars more than 10 times as massive as the sun transfer material in the form of stellar wind. The material flows along the magnetic poles of the neutron star, creating X-ray pulsations as it is heated.

          By 2010, approximately 1,800 pulsars had been identified through radio detection, with another 70 found by gamma-rays. Some pulsars even have planets orbiting them — and some may turn into planets.

Picture Credit : Google

          Large stars, with a mass much greater than our Sun’s, die a very violent death. As the hydrogen in a large star is used up, nuclear reactions produce heavier and heavier elements until a large iron core develops. This core eventually collapses under its own immense gravity, and the force of this collapse creates a tremendous explosion called a supernova. Most of the star’s matter is blown into space by this explosion, leaving a tiny, dense remainder — either a neutron star or a black hole.

          The most massive stars quickly exhaust their fuel supply and explode in core-collapse supernovae, some of the most energetic explosions in the universe. A supernova’s radiation can easily (if only briefly) outshine the rest of its host galaxy. The remnant stellar core will form a neutron star or a black hole, depending on how much mass remains. If the core contains between 1.44 and 3 solar masses, that mass will crush into a volume just 10 to 15 miles wide before a quantum mechanical effect known as neutron degeneracy pressure prevents total collapse. The exact upper limit on a neutron star mass isn’t known, but around 3 solar masses, not even neutron degeneracy pressure can combat gravity’s inward crush, and the core collapses to form a black hole.

          Average stars with up to 1.44 solar masses, such as the Sun, face only a slightly less exotic fate. As they run out of hydrogen to fuse in their cores, they swell into red giant stars before shedding their outer layers. The remnant left behind in these planetary nebulae is a white dwarf star. Like neutron stars, white dwarfs no longer fuse hydrogen into helium, instead depending on degeneracy pressure for support — this time, the electrons are degenerate, packed together and forced into higher energy states, rather than the neutrons.

          Left to their own devices, white dwarfs will eventually fade into black dwarfs. No black dwarfs have been observed yet because a white dwarf takes longer than the current age of the universe to fade away. And if the white dwarf is part of a binary system, it may avoid that fate altogether. By accreting matter from its companion star, the white dwarf can explode in a Type a supernova, leaving no remnant behind.

          The smallest stars in the universe have exceedingly long lives — in fact, none have faced their end yet. Red dwarfs, stars with less than 0.4 solar masses, burn so slowly that they might live to 100 billion years old, much longer than the current age of the universe.

Picture Credit : Google

          A star’s life cycle can last millions, if not billions, of years. All stars begin in the same way — from material in a giant cloud of gas and dust called a molecular cloud. Stars remain alive as long as there is enough hydrogen to make helium, so a star’s lifespan depends on its mass. Stars like the Sun will burn steadily for around 10 billion years before running out of hydrogen. Larger stars convert hydrogen much more quickly and therefore have much shorter lives.

          Stars are formed in clouds of gas and dust, known as nebulae. Nuclear reaction at the centre (or core) of stars provides enough energy to make them shine brightly for many years. The exact lifetime of a star depends very much on its size. Very large, massive stars burn their fuel much faster than smaller stars and may only last a few hundred thousand years. Smaller stars, however, will last for several billion years, because they burn their fuel much more slowly.

          Eventually, however, the hydrogen fuel that powers the nuclear reactions within stars will begin to run out, and they will enter the final phases of their lifetime. Over time, they will expand, cool and change colour to become red giants. The path they follow beyond that depends on the mass of the star.

          Small stars, like the Sun, will undergo a relatively peaceful and beautiful death that sees them pass through a planetary nebula phase to become a white dwarf, which eventually cools down over time and stops glowing to become a so-called “black dwarf”. Massive stars, on the other hand, will experience a most energetic and violent end, which will see their remains scattered about the cosmos in a enormous explosion, called a supernova. Once the dust clears, the only thing remaining will be a very dense star known as a neutron star; these can often be rapidly spinning and are known as pulsars. If the star which explodes is especially large, it can even form a black hole.