Category Science

Much of the space between the stars may be black, but it certainly isn’t empty. Tiny amounts of dust and gas, called interstellar medium, occupy the space between stars. Interstellar medium has an average density of less than one atom per cubic centimetre, but in some places it is concentrated into vast clouds called nebulae. Nebulae come in many different shapes, sizes and colours. Emission nebulae (left) are the most beautiful. Their striking colours come from the presence of hydrogen atoms that release red light. Reflection nebulae (centre) are illuminated by light reflected from nearby stars. They appear blue because the light is scattered by dust grains. Absorption nebulae (right) are dark because there are no nearby stars to light them. They can be spotted because they block out the light from more distant stars.

The interstellar medium is the stuff between the stars. Made up mostly of hydrogen and helium gas, it contains all the material needed to make stars and planets. It is shaped by stellar winds, dying stars, galactic magnetic fields, and supernova explosions. Sure, it’s much emptier than anything here on Earth. But nearly one-sixth of our galaxy’s mass lives here.

The interstellar medium, or ISM, contains the ingredients for making planets, asteroids, and stars. Though tenuous – there is only about one atom in every cubic centimeter – there is enough material here to build entire galaxies.

The ISM is 99% gas. About three-quarters of that gas is hydrogen, the fuel that powers stars for most of their lives. One quarter is helium. Almost all that hydrogen and helium was formed in the first three minutes after the Big Bang. Only a couple percent of the gas is every other element on the periodic table. Carbon, oxygen, magnesium, iron, uranium – all of it formed in the cores of long-dead stars.

The other 1% of the ISM is “interstellar dust”. The dust consists of ices, carbon compounds, and silicate grains formed around red giant stars. Like polluting factories, these stars blow “atomic soot” – carbon, oxygen, silicon – into space, carried aloft by strong stellar winds. Escaping the warm environments of these stars, the soot collects into clouds. There, shielded from the ionizing radiation that bathes the galaxy, the atoms can collect and build complex chains. In these clouds, astronomers have found amino acids – the building blocks of proteins. The stuff of life is everywhere!

The cycle starts in cold, dark clouds. Tens of light-years across, these clouds house enormous quantities of molecular hydrogen. All it takes is a nudge from outside – a passing star cluster, a nearby supernova, the sweep of a galactic spiral arm – and the cloud becomes unstable. Pockets of ever-increasing density flourish, driven by the cloud’s own gravity. From these dark cocoons, stars are born. Upon ignition, they blow away the remaining material and light up the cloud. The Orion Nebula, the Lagoon Nebula, and the Witch Head Nebula are all clouds of gas and dust lit up by nearby young stars.

At the other extreme is the ionized gas. Shocks from powerful supernova explosions heat some of the gas to millions of degrees.  There is enough energy to rip electrons from their atoms.  The gas responds by glowing with x-ray radiation. Some of this gas is even blown free of the galaxy, into intergalactic space. Between supernova shocks, young stellar winds, galactic magnetic fields, and turbulent motion, the ISM has a rich and complex structure. Filaments of gas, dense pockets of hydrogen, and expanding voids connect the network of material threading the galaxy.

Most of this web is invisible.  To map the ISM, astronomers must turn to other parts of the electromagnetic spectrum.  The cold, dark gas emits radio waves.  Warm dust shows up in infrared telescopes. The superheated plasmas glow with x-rays. By putting together observations at all these wavelengths, we can draw a picture of what the interstellar medium around the sun looks like.

Each star produces its own individual light. By splitting the light into a spectrum, astronomers can discover the chemical elements that make up the star. This is because different elements in the star’s atmosphere absorb light of different wavelengths. Sodium atoms, for example, only absorb light from the yellow part of the spectrum. A dark line across this part of the spectrum, called an absorption line, tells scientists that there is sodium in the star. By studying the various lines made on the spectrum, scientists can determine what the star is made up of.

The most common method astronomers use to determine the composition of stars, planets, and other objects is spectroscopy. Today, this process uses instruments with a grating that spreads out the light from an object by wavelength. This spread-out light is called a spectrum. Every element — and combination of elements — has a unique fingerprint that astronomers can look for in the spectrum of a given object. Identifying those fingerprints allows researchers to determine what it is made of.

That fingerprint often appears as the absorption of light. Every atom has electrons, and these electrons like to stay in their lowest-energy configuration. But when photons carrying energy hit an electron, they can boost it to higher energy levels. This is absorption, and each element’s electrons absorb light at specific wavelengths (i.e., energies) related to the difference between energy levels in that atom. But the electrons want to return to their original levels, so they don’t hold onto the energy for long. When they emit the energy, they release photons with exactly the same wavelengths of light that were absorbed in the first place. An electron can release this light in any direction, so most of the light is emitted in directions away from our line of sight. Therefore, a dark line appears in the spectrum at that particular wavelength. 

Because the wavelengths at which absorption lines occur are unique for each element, astronomers can measure the position of the lines to determine which elements are present in a target. The amount of light that is absorbed can also provide information about how much of each element is present.

The more elements an object contains, the more complicated its spectrum can become. Other factors, such as motion, can affect the positions of spectral lines, though not the spacing between the lines from a given element. Fortunately, computer modeling allows researchers to tell many different elements and compounds apart even in a crowded spectrum, and to identify lines that appear shifted due to motion. 

          Stars in the night sky appear to glow in a variety of different colours. This is because they have different temperatures and emit light with different wave-lengths. Hot stars, with temperatures greater than 28,000°C (50,400°F), glow blue. Stars like our Sun, which have a surface temperature of around 5500°C (9900°F), appear yellow, whereas cooler stars glow red. Astronomers divide stars into seven spectral types: 0 (hottest), B, A, F, G, K and M (coolest).

          Throughout history mankind has gazed up at the stars in awe and wonder. To the naked eye, most of the stars appear white. As the light from the stars comes through the earth’s atmosphere, they appear to be twinkling. Until about two hundred years ago, everyone that studied the stars thought that all stars were white. The astounding part is stars come in almost all of the shades of the rainbow.

          When scientists started learning more about light and light waves, they realized that there are various kinds of light and the wavelengths can be wide or tightly packed. As they studied the planets they began to recognize that light can be perceived in different shades of color based on the wavelength, and that wavelength can change based on a star’s temperature.

          A type of science physics called ‘blackbody radiation’ was developed and they continued to examine the various temperatures and colors. It seems that the stars with ‘cooler’ temperatures have energy that is radiated in the red tones of the electromagnetic color spectrum, while those with ‘hotter’ temperatures had energy that is radiated in the blue and white tones of the electromagnetic color spectrum. This makes the cooler stars appear red and the stars with the higher temperatures appear blue or white. From cool to hot, the colors can appear red, orange, yellow, green and blue. If you remember the colors of the rainbow, you will see that these are in the same order.

          There is another important factor that can alter a stars color. If the star has any elements in its atmosphere it can change the light wavelength and that will cause a change in the color that we measure or observe. This may explain why there are so many different colors in the stars that are being studied.

          The coolest stars are the red stars and their temperature is around 3,000 degrees C. Our own sun has a temperature of around 6,000 degrees C and glows orange/yellow. Green stars have a temperature of about 10,000 degrees C and the blue stars, which are the hottest, are about 25,000 degrees C.

          The largest stars in the universe expend all of their energy much more quickly the smaller stars. This means that they have a very short lifespan. Our sun in considered to be a medium-sized star and it is half-way through its lifecycle. But it also has millions and millions of years left to shine brightly for us.

          So, as you can see, the color of a star depends upon the temperature as well as any atmospheric contributions it may have to distort the measurable temperature. Scientists have developed very sensitive equipment that works with the telescopes to observe and note the rainbow colors of stars that we can see. This is the science of spectroanalysis and the scientists can detect not only the star’s color, but what the star is actually made up of. The elements of a star will help as we classify the solar systems and galaxies that we discover.

       Scientists have to know how far away a star is before they can begin to analyze details such as its age, size, temperature and mass. The most effective way of measuring a star’s distance from Earth is called the parallax method. If you are travelling in a car and looking out of the window, nearer objects seem to pass by much more quickly than distant ones. In the same way, as Earth orbits the Sun, nearer stars appear to move more quickly through the sky than those further away. The angle through which a certain star moves over a period of six months is called its parallax. This angle is used by astronomers to work out how far away the star is.

         Parallax is “the best way to get distance in astronomy,” said Mark Reid, an astronomer at the Harvard Smithsonian Center for Astrophysics. He described parallax as the “gold standard” for measuring stellar distances because it does not involve physics; rather, it relies solely on geometry.

          The method is based on measuring two angles and the included side of a triangle formed by the star, Earth on one side of its orbit and Earth six months later on the other side of its orbit, according to Edward L. Wright, a professor at the University of California, Los Angeles.

         It works like this: hold out your hand, close your right eye, and place your extended thumb over a distant object. Now, switch eyes, so that your left is closed and your right is open. Your thumb will appear to shift slightly against the background. By measuring this small change and knowing the distance between your eyes, you can calculate the distance to your thumb.

          To measure the distance of a star, astronomers use a baseline of 1 astronomical unit (AU), which is the average distance between Earth and the sun, about 93 million miles (150 million kilometers). They also measure small angles in arcseconds, which are tiny fractions of a degree on the night sky.

          If we divide the baseline of one AU by the tangent of one arcsecond, it comes out to about 19.2 trillion miles (30.9 trillion kilometers), or about 3.26 light years. This unit of distance is called a parallax second, or parsec (pc). However, even the closest star is more than 1 parsec from our sun. So astronomers have to measure stellar shifts by less than 1 arc second, which was impossible before modern technology, in order to determine the distance to a star.

          We know that earth is bombarded by thousands of meteorites every day, none of which does our planet much damage. Any meteorite up to 10m (33ft) in diameter will normally burn up in the atmosphere before it reaches Earth, separating into tiny fragments. If a meteorite larger than this falls to Earth, it can cause considerable damage — impacting with the energy of five nuclear warheads. Approximately once every 1000 years, a larger meteorite does fall to Earth, and several large craters caused by such impacts can still be seen. One such was the nickel – iron meteorite that created the Barringer Crater in Arizona, USA. The meteorite was an incredible 45m (148ft) wide, creating a crater nearly 1.5km (1 mile) in width. However, it would take an impact by an object roughly 5km (3 miles) wide to cause mass extinctions and threaten life on Earth.

          Most meteorites that are found on the ground weigh less than a pound. While it may seem like these tiny pieces of rock wouldn’t do much damage, a 1-lb. (0.45 kilograms) meteorite traveling upward of 200 mph (322 km/h) can fall through the roof of a house or shatter a car windshield. 

          When the Grimsby meteorite landed in Ontario, Canada in 2009, for example, it broke the windshield of an SUV. In another incident, meteorites crashed into the back end of a Chevy Malibu in Peekskill, New York, in 1992, Cooke and Moorhead said. Thankfully, no one was injured during these events. 

          However, the pieces of rock falling from the sky are not even the greatest concern regarding meteor impacts, Cooke said.

          “What causes the most damage is the shock wave produced by the meteor when it breaks apart in [Earth’s] atmosphere,” Cooke said. “So, you don’t have to watch for the falling rocks — you have to worry about the shockwave.”

          For example, the Chelyabinsk meteor — an asteroid the size of a six-story  building that entered Earth’s atmosphere in February 2013 over Russia — broke apart 15 miles (24 km) above the ground and generated a shock wave equivalent to a 500-kiloton explosion, Cooke said. It injured 1,600 people.

          Another major collision was the Tunguska meteorite, which was larger than Chelyabinsk and 10 times more energetic. The meteorite exploded over the Tunguska River on June 30, 1908, and flattened 5000,000 acres (2,000 square km) of uninhabited forest. Because of its remote location, the event is an example of a meteorite that would have gone undetected had it not been so large, Cooke and Moorhead explained. 

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          Unlike many of the planets, moons and smaller bodies in the Solar System, Earth appears to be covered by very few craters. In the early days of the Solar System, Earth was as much a target for meteorites as any other planet, and suffered intensive cratering in the first one billion years of its existence. However, unlike bodies such as Mercury and the Moon, Earth has many geological processes that “hide” craters. Constant weathering and erosion from winds and water wear away or cover up craters. Some may also be hidden by vegetation or lie under the sea, although in the last hundred years, aerial photography and other forms of imaging have given us a clearer view of many remaining craters.

          Impact craters leave quite an impression on the surface of planets and moons — just think of Earth’s moon, which gets its distinctive appearance from millions of encounters of asteroids over the centuries. But Earth is a different story altogether, with only 128 impact craters recorded in the most recent count. That can’t be right, can it?

          He reports that a new study shows that the low number found by past scientists isn’t “just the result of lazy searching”: it’s the surprising truth about a planet that’s astonishingly crater-free.

          The study looked at the ways Earth erosion affects existing craters and concluded that the current count of 70 craters larger than 6 km (3.7 miles) in diameter should be just about right. That’s a rare instance of a complete geologic record, writes Hand — and one that may discourage people on the hunt for new craters.

          But don’t put away your crater-catching gear just yet. The study’s authors note that just because we’ve already found all of the likely large impact craters on Earth don’t mean there aren’t more to discover. The real opportunity, they write, lies in smaller craters: they estimate that more than 90 craters between .6 miles and 3.7 miles in diameter should still be undiscovered and more than 250 between 0.1 miles and .6 miles.

          NASA notes that Earth is equipped with three processes that eat up craters relatively quickly: erosion, tectonics, and volcanism. These forces leave only the largest scars from meteorites or asteroids — unlike, say, the moon, which can’t gobble up craters. Hand writes that the parameters of the study also play a part in the low number — it looks at just surface craters, not those that lie beneath sediment. And the study also didn’t look at volcanic craters, which formed some of Earth’s most distinctive basins and lakes.

Picture Credit : Google