Category The Universe, Exploring the Universe, Solar System, The Moon, Space, Space Travel

Our solar system lays roughly two thirds of the way from the centre of the galaxy, on the inner edge of a spiral arm called the Orion Arm, or the Local Arm. From Earth, the Milky Way appears as a river of milk stretching across the night sky. This is because we are viewing it from inside. The infrared image above gives a clearer view of the Milky Way as it stretches across space.

The Milky Way is a grand spiral galaxy, which astronomers think has four major spiral arms: Perseus, Cygnus, Scutum-Crux, Sagittarius. Some astronomers think we might actually just have two arms, Perseus and Sagittarius. The Sun is located in the inner rim of the Orion Arm, which is thought to be an offshoot of the Sagittarius Arm. The Sun is located about 26,000 light-years away from the center of the galaxy.

Before telescopes, the Milky Way just looked like a bright area in the sky, but when Galileo first turned his telescope on the region in 1610, he realized that it was actually made up of faint stars. The astronomer Immanuel Kant correctly guessed that this might be a cloud of stars held together by gravity, like the Solar System.

The famous astronomer William Herschel attempted to map out the stars in the Milky Way to get a sense of the galaxy’s size and shape, and determine the Sun’s position in it. From Herschel’s first map, it appeared the Sun was at the center of the Milky Way. It was only later on that astronomers realized that gas and dust was obscuring our view to distant parts of the galaxy, and that we were actually in the outer region of the Milky Way.

The astronomer Harlow Shapley accurately determined where the Sun is in the MIlky Way in the early 20th century by noticing that globular clusters were uniformly located above and below the Milky Way, but they were concentrated in the sky towards the constellation Sagittarius. Shapely realized that many globular clusters must be blocked by the galactic core. He created one of the most accurate maps of the Milky Way.

It wasn’t until the 20th century, with the development of larger and more powerful telescopes that astronomers could see the shape of other spiral galaxies, located millions of light-years away. In 1936, Edwin Hubble used cepheid variables as yardsticks to measure the distances to many galaxies, and prove conclusively that the Universe was filled with galaxies, each with as many stars as our own Milky Way.

The Milky Way is our home in the Universe. It is made up of over 200 billion stars, including the Sun, as well as large amounts of gas and dust. It looks like a giant spiral from above, but if it was viewed from the side it would appear as a flat band of stars. This is because the Milky Way is over 100,000 light years long, but only 2000 light years thick. The centre of the Milky Way is made up of a bright nucleus of old, cool stars. Emerging from the central galactic bulge are several spiral arms made up of gas, dust and young stars.

When you look up at the night sky, assuming conditions are just right, you might just catch a glimpse of a faint, white band reaching across the heavens. This band, upon closer observation, looks speckled and dusty, filled with a million tiny points of light and halos of glowing matter. What you are seeing is the Milky Way, something that astronomers and stargazers alike have been staring up at since the beginning of time.

But just what is the Milky Way? Well, simply put, it is the name of the barred spiral galaxy in which our solar system is located. The Earth orbits the Sun in the Solar System, and the Solar System is embedded within this vast galaxy of stars. It is just one of hundreds of billions of galaxies in the Universe, and ours is called the Milky Way because the disk of the galaxy appears to be spanning the night sky like a hazy band of glowing white light

Our Sun (a star) and all the planets around it are part of a galaxy known as the Milky Way Galaxy. A galaxy is a large group of stars, gas, and dust bound together by gravity. They come in a variety of shapes and sizes. The Milky Way is a large barred spiral galaxy. All the stars we see in the night sky are in our own Milky Way Galaxy. Our galaxy is called the Milky Way because it appears as a milky band of light in the sky when you see it in a really dark area.

In 1755, the philosopher Immanuel Kant claimed that some bright objects in space were giant collections of stars, and he named them island universes. The work of Edwin Hubble in the 1920s proved that these island universes were galaxies that lay beyond our own. A galaxy is an enormous collection of stars, dust and gas, held together by its own gravity. Even the smallest galaxies contain hundreds of thousands of stars, and it takes light many thousands of years to travel from one side to the other.

The visible universe is vast. It is 93 billion light years across, and contains more than 100 billion galaxies. The average galaxy contains about 100 billion stars, and untold numbers of planets. Yet a century ago there was serious doubt among many astronomers that the universe was much more than 100,000 light years across. Arguments about whether the universe was small or large became known as the Great Debate.

It is often known as the Shapley-Curtis debate, so named after Harlow Shapley and Heber Curtis, and a public debate they had in 1920. Shapley, you may remember, used observations of globular clusters to correctly show that the Sun is not the center of our galaxy. Curtis was an astronomer who studied nebulae, as well as solar eclipses.

The debate centered on the distance to certain nebulae. At the time, “nebula” referred to anything (excluding comets) that appeared “fuzzy” rather than distinct like a star or planet. So things like the Orion nebula (a stellar nursery), the Crab nebula (a supernova remnant) were considered nebulae just as they are today, but what we now call galaxies were also known as nebulae. The Andromeda galaxy, for example, was known as the Great Andromeda Nebula.

Curtis argued that Andromeda and other spiral nebulae were in fact “island universes”, similar in size to our own Milky Way “universe”. This would mean that not only were these nebulae 100,000 light years across or more, they must be millions of light years away. He based this argument on the fact that more novae were observed in Andromeda alone than were observed in the entire Milky Way. Why would that be the case if Andromeda were small and close? He also noted that some spiral nebulae had rather large redshifts, meaning that they were moving much faster than other objects in the universe.

Shapley argued that what we now call the Milky Way galaxy was the bulk of the universe. Spiral galaxies such as Andromeda must be relatively close and small. He based this view on several points. In 1917 Shapley and others observed a nova in the Andromeda nebula. For a brief time the nova outshined the central region of Andromeda. If Andromeda were a million light years away, as Curtis contended, then this nova (we now know it was a supernova) would need to be far brighter than any known mechanism could produce. There were also observations of the Pinwheel galaxy, seen above, by Adriaan van Maanen. He claimed that the Pinwheel had visibly rotated over the span of years. If the Pinwheel was rotating as van Maanen observed, then it couldn’t possibly be 100,000 light years across. For an object that large to rotate in a matter of years the stars would need to move faster than light.

After the debate the general opinion was that Shapley had won. His own observations of the shape of the Milky Way and the 1917 supernova, and van Maanen’s observations gave the small universe model solid footing. Besides, the idea that objects could be millions of light years away seemed patently absurd.

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.