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A star is a ball of gas that produces energy in its core by means of nuclear reactions. The nuclear reactions occurring in stars processes billions of kilograms of mass every second, which produces enormous amounts of energy in the form of heat and light. The Sun is a typical star, but appears much brighter because it is so close to us. The stars are located so far away from us that distances from the Earth can be very difficult to grasp. However, if we imagine the Earth as a grain of sand with the Sun situated a metre away, the nearest star would be 270 kilometres away. The furthest stars visible with the naked eye would be located almost half a million kilometres away further than the Moon! And there are millions of other stars even further. It takes light from the Sun about eight minutes to reach the Earth; light from the nearest star takes about 4.3 years. The stars are extremely bright and massive objects, but they are so incredibly distant they appear as mere points of light in our sky.
While the thousands of stars in the night sky appear to be very similar, they are more distinct from one another than their appearance from the Earth would suggest. Stars have various sizes, masses, temperatures, colours, luminosities (power), compositions and lifetimes. The largest stars are 300 times the diameter of the Sun and would engulf the orbit of the Earth, while the smallest stars are smaller than the Earth itself. The most noticeable distinction between stars, however, is the difference in their brightness. The apparent brightness of an object in the sky is denoted by its magnitude, a numeric scale established by Hipparchus around the year 160 BC. Hipparchus stated that the brightest stars in the sky were of first magnitude and the dimmest were of sixth magnitude, making smaller numbers correspond to brighter objects. The scale has been expanded and can now be applied to any object in the sky. The full moon has a magnitude of -12.7, Venus at its brightest is -4.1, the brightest star is -1.46 and the dimmest objects detectable through the largest telescopes are about +29.
Most of the stars in the sky are double stars but usually appear as a single point of light because they are so close together we cannot see them as individual stars. A double star is a pair of stars located in nearly the same position in the sky. The two stars that make up a double star may not actually be close to each other in space, but lie in the same line of sight from the Earth. A system of double stars that are gravitationally bound and are in orbit around each other are called binary stars. Binary stars are often so close together they are only perceivable as two stars by analyzing their combined light. Binary stars are very common and the Sun is actually rare in that it is not part of a double system. There are also a few individual stars that vary in their apparent brightness as seen from Earth, and are called variable stars. The time period of the change in intensity of variable stars can be erratic or can be very regular, ranging from days to years.
Stars are classified by their temperature, which will affect their colour. The stars range in colour from red through yellow and white to blue. The Suns yellow surface is about 5,800 K, while some red stars are 3,000 K and blue stars have surface temperatures of about 30,000 K. When we look up at the stars, they all appear to dazzle bright white, but many stars actually do have colour imperceptible to our eyes. The human eye is not sensitive to colours at low light intensities like the stars. By using a telescope to look at the stars, they appear brighter and the colours become noticeable. Spectroscopy allows for a more detailed classification system using the chemical composition of stars. The light emitted by stars can be broken down into the component colours at various wavelengths. The spectrum will depend primarily on the stars temperature, but it will also contain absorption lines which characterize the elements present in the star. Dark bands appear along the spectrum, and the presence of specific elements will affect their location in the spectrum, and their abundance will affect the width of the band. A spectrum is like a fingerprint and will reveal the chemical abundance within the star.
It was previously thought that stars were stagnant and never changed or evolved. We now know that stars evolve through a life cycle they are created, live long lives and then expire. It is impossible to witness the entire life cycle of an individual star because it is an extremely lengthy process by human standards. But by studying different stars in various stages of development using the best telescopes in the world, astronomers can now establish their life cycle.
Stars form in cold, dark clouds of gas and dust. The cloud must be cold for stars to form because the particles must be moving slowly enough to allow gravity to overcome internal pressure and form clumps of matter. The interstellar cloud must be truly immense, covering billions of kilometres, and must be relatively dense with hydrogen and helium atoms for a star to form. It is thought that a shockwave from a nearby star will trigger a collapse, and the atoms slowly draw together due to the gravitational attraction between them. As the cloud shrinks, it breaks up into smaller fragments known as protostars. An initial interstellar cloud can produce hundreds of protostars. A protostar is a star in its embryonic stage, and although it glows due to the release of gravitational energy, it is not yet hot enough to produce nuclear reactions within its centre. As the protostar continues to collapse due to gravity, it will attract more atoms and continually increase in mass and density. The increased density and gravity will cause the core temperature to rise to about ten million Kelvin, hot enough to convert hydrogen into helium (nuclear fusion). Millions of years after the interstellar cloud first began to collapse, a star is created.
The young star will continue to gradually collapse until the internal pressure pushing out (caused by heat) equals the inward pull of gravity. This occurs when the central core temperature has increased to about 15 million Kelvin. The star is now in equilibrium, and will continue to process hydrogen for most of its life. This stable period of the stars life will not end until the core becomes depleted of hydrogen, which can vary between millions and billions of years. The Sun is currently converting about 460 billion kilograms of hydrogen into helium every second, and will not exhaust its supply of hydrogen for about another four billion years. The characteristics of a star are determined by its mass, which will depend on the size of the initial fragment of the interstellar cloud. While in its stable core hydrogen-burning phase, higher-mass stars process their fuel of hydrogen (and produce more energy) at a much faster rate than low mass stars. As a result, massive stars burn hotter and brighter, have shorter lifetimes, and will typically have a larger radius. Once the core of a star begins to exhaust its reserve of hydrogen, the star quickly becomes unstable and will evolve from its state of equilibrium. The core is now packed with helium, and a thin spherical shell surrounding the core will begin to process hydrogen. This causes the core to become increasingly dense while the outer layers of the star will expand and cool. The gases will glow red and the star becomes known as a red giant. The hydrogen-burning shell will move outward from the core as it converts the hydrogen into helium, and the core will become progressively compact with helium. This increased pressure will raise the temperature of the core, and will eventually become hot enough to ignite nuclear reactions involving helium. The star now enters another period of equilibrium, and will spend another several million years converting helium into carbon.
The most apparent difference between high mass (10 to 30 solar masses) and low mass (0.5 to 10 solar masses) stars will be the events leading up to the eventual death of the star. Low mass stars do not have the mass required to increase the core temperature enough to allow the carbon to fuse into heavier elements. Once the helium is consumed, the star will die quietly by ejecting their outer layers, creating a planetary nebula. The central carbon core of the star is left behind and continues to shine by stored heat. This remnant is called a white dwarf star, and is about the size of the Earth, but is much more massive. It will gradually cool and dim with time as its stored heat is used up, and the star will become a cold and dark black dwarf, ending the stars evolution. When a white dwarf is part of a binary system with a red giant, its gravity will suck surface material off the red giant. Large amounts of matter falling into the white dwarf will cause instabilities, and explosions will occur in order to release the accumulated material. These explosions are called novae, and the white dwarf star will brighten significantly as seen from the Earth. A nova will last for about one week and then slowly die off and return to its previous brightness.
High-mass stars die much more dramatically in a violent explosion. In contrast to a low-mass star, a high-mass star has enough mass to continually increase the pressure and temperature of its core, which causes a chain of nuclear reactions involving heavier and heavier elements. The nuclear reactions eventually produce iron, but iron nuclei are so compact that they do not release energy in nuclear reactions and produce no heat. With the end of energy production in the core, it no longer produces enough heat to generate adequate inner pressure to match the enormous gravitational pull. At this stage the core is so incredibly dense that it cannot collapse any further and the state of equilibrium comes to an end. The inner core sucks in the surrounding layers and the star will implode and collapse in on itself in a matter of seconds. The material of the collapsing star rebounds off the solid core, producing a shockwave of material that explodes into space. This explosion is called a supernova, and will increase the luminosity of the star by a factor of millions. A supernova is much more powerful than a nova and will be extremely bright for a few weeks or months, until it gradually subsides and dims. What is left after a supernova explosion is called a supernova remnant, an extended nebula of gas. Depending on the initial conditions of the star, it will become either a neutron star, a black hole, or it could simply blow itself completely apart, leaving only the remnant. The time frame for the death of a high mass star is extremely short. While the hydrogen-burning phase lasts for millions of years, the final stages of a stars life leading up to a supernova last for progressively shorter periods, culminating in the core collapse and explosion which last mere seconds. Because the death of a star occurs so rapidly, we can directly witness the process, and supernovae reveal valuable information about stars and our universe. The death of stars is an important part of the stellar life cycle because it promotes star formation. This is because the explosions produce shockwaves that can trigger the collapse of an interstellar cloud into a protostar. Stars also eject their outer layers into space, which produces an interstellar cloud rich in hydrogen and helium atoms. Supernovae release huge amounts of heavy elements into the interstellar medium, which produces perfect conditions for the birth of a star with rocky planets like our solar system. Without the death of stars, it would be very difficult for new stars to form.
In composition and size, the Sun is an average star and is in the middle of its hydrogen-burning stage. It will continue to process hydrogen for another few billion years before swelling into a red giant. Although the Sun is an average-sized star, it is still huge by Earthly standards, having a diameter more than 100 times that of the Earth. The Sun is a ball of gas, and as such does not have a surface like the Earth. The surface of the Sun is called the photosphere, and is where energy from the core is emitted into space. The photosphere has a definite sharp edge as seen from the Earth, and its granular appearance is constantly bubbling as heat from the interior escapes into space. Beyond the photosphere is the lower atmosphere of the Sun, called the chromosphere, and beyond that is the transition zone, where the temperature of the atmosphere rises dramatically. The atmosphere of the Sun is extremely hot and is home to violent events erupting from the photosphere. A prominence is an ejected pillar of glowing gas extending thousands of kilometres from the solar surface. A prominence is visible in photographs and lasts for days or weeks. A solar flare is a more violent eruption from the photosphere that releases an enormous amount of energy. While a prominence will tend to follow the magnetic field lines and loop back down to the photosphere, a flare will shoot off into space. Extending far into space is the corona, a hot and sparse upper atmosphere. The corona is very irregular in appearance, and it is believed that its shape is distorted by the eruption of prominences and flares. As the corona extends further from the Sun, it becomes the solar wind, a very thin gas of charged particles that travels through the solar system. The various levels of the Suns atmosphere cannot be seen unless a special filter is used. The corona is also visible when the body of the Sun is blocked by the Moon during an eclipse (this is explained in greater detail in module 3). The interior of the Sun is composed of a gas made up of about three quarters hydrogen and one quarter helium. The density and temperature of the gas will increase with depth beneath the surface. The regions just below the photosphere are known as the convection zone and the radiation zone, which allow heat and energy to travel up from the core to the surface. The core of the Sun is where the nuclear fusion takes place, and is the energy source of the star.
The surface of the Sun was originally thought to be perfect and uniform, but we now know the photosphere is marked by numerous irregularly shaped dark patches called sunspots. Sunspots are depressed areas on the Sun which have a lower temperature than the surrounding surface. They are typically about the size of the Earth, and are composed of a dark central region called the umbra surrounded by a lighter coloured ring called the penumbra. They are temporary features and constantly alter the appearance of the photosphere. Sunspots are closely tied to the solar magnetic field and often occur in groups or in pairs of opposite polarity. The rotation period of the Sun would be very difficult to determine without the aid of sunspots. Because the Sun is not solid, it experiences differential rotation. The surface rotates at different speeds depending on the latitude, with the equatorial regions rotating faster than the polar regions. The number of visible sunspots varies year to year, and the frequency follows a regular 11-year cycle between times of maximum and minimum. During times of maximum, hundreds of sunspots are visible, but during a minimum, the photosphere can be absent of any sunspots. Complex sunspot groups cause the eruption of solar flares, which produce a substantial release of solar particles into the solar wind. Because charged particles from the Sun cause the aurora on Earth, these displays are directly affected by the number of sunspots. During a sunspot maximum like in 2001, we will tend to see amazing auroral displays, and during minimums the aurora are essentially non-existent.
Because the Sun is so incredibly bright, we cannot safely look at it without damaging our eyes. The Sun can be viewed with the use of special filters or via projection. Filters can be fitted onto telescopes to block out most of the incoming light, leaving images astronomers can safely view and study. Various filters allow astronomers to observe different areas of the sun, including sunspots and prominences. Image projection is a simple method which involves the projection of the Sun through a small telescope onto a piece of paper. This method does not show any of the solar atmosphere, but sunspots will be visible. We must never look directly at the Sun without safety precautions, but with them in place, our star is a wonderful object to study.
How do we know what we do about the stars? Much of our knowledge of stars is obtained by studying our own star, the Sun. Astronomers have used complex mathematical models to investigate the solar interior, but observing the Sun in different wavelengths and with different filters can give them valuable information as well. One of the most important methods in studying the interior of the Sun is called helioseismology which involves the observation of the bubbles on the photosphere as they rise and fall. Because the Sun is an average star, we can assume that the processes that drive it will also be present in other similar stars.
Billions of stars populate our universe. The nuclear reactions within their core release incredible amounts of energy, and they would appear much brighter if it were not for their considerable distances. While looking up at the night sky, the only perceivable difference between stars is their apparent magnitude, but stars each have their own characteristics. Although difficult to detect, stars shine different colours depending on their temperature. Spectroscopy is an accurate method of determining a star's colour, and more importantly, a star's spectrum will reveal the relative abundance of elements within its atmosphere. Many stars are part of a double star system, and some will vary their brightness. Stars evolve through a life cycle that begins with their creation in an interstellar cloud. The cloud slowly collapses due to gravity, a protostar is formed and soon the internal temperature rises high enough to ignite nuclear fusion. A star processes hydrogen for the majority of its life before dying quietly as a planetary nebula or violently as a supernova. The Sun is in the middle of its hydrogen-burning stage, and will live another few billion years before dying. The solar atmosphere is composed of three main layers: the chromosphere, the transition zone and the corona. Prominences and flares erupt into the atmosphere, releasing energy and particles into the corona and eventually extending into the solar wind. The Suns energy is generated within its core, and the internal regions transport this energy to the surface. The surface of the Sun is called the photosphere and is yellow and granular in appearance. Randomly covering the photosphere are dark patches cooler than the rest of the surface. These sunspots are temporary, and their numbers follow an 11-year cycle between times of maximum and minimum. They often occur in complex groups and are associated with the aurora on the Earth because they are the origin of solar flares. The Sun is an important object to study, but because it is so luminous it is extremely dangerous to look at the Sun without adequate protection. The use of a special filter or the method of projection allows the Sun to be studied safely to better understand the processes within the stars. The Sun and the stars are incredible objects, and without them, life on Earth would not exist.