Stellar Evolution PDF

REACH FOR THE STARS Stellar Evolution Stellar Evolution for stars with different masses. Formation Star formation starts in a dense interstellar cloud - a dark dust cloud or a molecular cloud. After some instability or disturbance, the cloud collapses under its own gravity, and form clumps of matter...

REACH FOR THE STARS Stellar Evolution Stellar Evolution for stars with different masses. Formation Star formation starts in a dense interstellar cloud - a dark dust cloud or a molecular cloud. After some instability or disturbance, the cloud collapses under its own gravity, and form clumps of matter. As the clump contracts further, its density grows, its temperature rises, and the clump becomes a protostar. The protostar contracts further and evolves onto the main sequence following the Hayashi track. It enters main sequence when nuclear fusion begins. Main Sequence Stars spend the majority of their lives on the main sequence. Heavier stars spend significantly less time on the main sequence because their rate of fusion is much higher. The radiation pressure from fusion balances with gravity, and the star is in hydrostatic equilibrium. Lower-mass stars fuse Hydrogen into Helium mostly using the proton-proton (p-p) chain. This process starts at around Kelvins. Higher-mass stars fuse Hydrogen into Helium mostly using the CNO Cycle. The CNO cycle uses Carbon, Nitrogen and Oxygen as catalysts, and is dominant in temperatures higher than Kelvins. Post-Main Sequence, Low Mass When the core of a low-mass star is depleted of Hydrogen, nuclear fusion subsides because Helium fusion occurs at much higher temperature. The core contracts, and the heat generated by the contraction heats up the outer layers, creating a Hydrogen-burning shell. The star also expands and becomes a red giant. The core then reaches high enough temperature for helium fusion. For a moment, helium is fused rapidly in a runaway condition known as the helium flash, but then subsides as the star enters the horizontal branch. When a low-mass star runs out of helium, it does not heat up high enough to fuse Carbon into heavier elements, and fusion stops. The core compresses further into a white dwarf, when electron degeneracy pressure prevents it from compressing further, and the outer shell expands into a planetary nebula, heated up by the white dwarf core. The white dwarf slowly cools and becomes a black dwarf. Post-Main Sequence, High Mass When the core of a high-mass star is depleted of Hydrogen, the star expands into a red supergiant. Its luminosity stays roughly the same because the temperature of the outer shell decreases. Helium starts fusing into carbon without a runaway process, so there is no helium flash. A high-mass star is able to fuse elements up to iron, at which point further fusion consumes energy instead. The iron core contracts further. The star explodes in a bright core-collapse supernova. Then, either electrons combine with protons and the core is made of neutron degenerate matter - a neutron star - or the core contracts further into a black hole. Neutron stars rotate very rapidly and have very strong magnetic fields. Some neutron stars are called pulsars and magnetars for their additional features. Spectral Classification Harvard Spectral Classification There are 7 spectral Classes (O,B,A,F,G,K,M). This order is based on decreasing surface temperature. A Class stars have the strongest Hydrogen lines, while M-Class stars have the weakest hydrogen lines. Each class is then subdivided into 10 subdivisions (0-9). The following is a table with properties of each of the spectral classes. Yerkes Spectral Classification The Yerkes Spectral Classification is based on luminosity and temperature. It is also known as luminosity classes. There are seven main luminosity classes: Type Ia: Bright Supergiants Type Ib: Normal Supergiants Type II: Bright Giant Type III: Normal Giant Type IV: Sub-Giants Type V: Main Sequence Type VI: Sub-Dwarf Type VII: White Dwarf Radiation Laws NOTE: This and the following section contain some algebra. If you are not yet comfortable with algebra, you can still read these sections for the theoretical concepts. The radiation laws show relationships between stellar temperature, radius, and luminosity. All three laws are regarding black bodies, ideal objects that absorbs all incoming radiation. Stars, with little incoming radiation, are often approximated as black bodies to simplify calculations. Both Wien's Law and Stefan's Law are proportionality statements, that a change in one quantity is always accompanied by change in other. These can be turned into equations by introducing a constant known as a proportionality constant. The proportionality statement denotes that if changes by a factor (here, "k" is just an arbitrary variable), also changes by. However, it does not mean that the values are equal to each other: a proportionality constant needs to be added. At Division B it is unlikely that one will perform calculations with these laws, but general questions regarding these laws, such as the proportionality, may be asked. For details about calculations with these laws, visit the Astronomy page. Magnitude and Luminosity Scales The luminosity of a celestial object refers to how much radiation (visible light, infrared, x-ray, etc.) it emits per unit time. Luminosity is measured in Joules per second or Watts. The luminosity, of the sun, for example, is watts. Magnitude scales are different methods to express luminosity. Apparent Magnitude The apparent magnitude, denoted by , denotes the brightness of a celestial object as seen by an observer on Earth. The brighter an object appears, the lower its apparent magnitude. It is a logarithmic scale, not a linear scale, which means that a small decrease in magnitude results in a much greater increase in luminosity. For example, an object with apparent magnitude 5 less than that another would seems 100 times more luminous. Logarithms are very advanced for Division B, but at the very least, it is important to know that a small change in magnitude represents a much larger change in luminosity. Absolute Magnitude The absolute magnitude, denoted by , denotes the brightness of a celestial object as seen by an observer 10 parsecs (about 32.6 light years) away from the object. Similarly, an object with absolute magnitude 5 less that of another would be 100 times more luminous. The absolute magnitude is basically another way of expressing the luminosity of the object. Scientists often consider the absolute bolometric magnitude of an object, meaning that its radiation is being measured across all wavelengths. Inverse Square Law The inverse square law says that a certain quantity is inversely proportional to the square of the distance relating to that quantity. In this case, "inversely proportional" means that an increase of one number causes a decrease in the other number. For example, suppose an astronomer measures a star of some intensity ( I1 ) at a certain distance ( D ) from the source. By the inverse square law, we have the following proportion: This law also applies to Newton's Law of Gravitation. The law states that: Galaxies There are three main types of galaxies: Spiral, Elliptical, and Irregular. However, in the 2013 rules, there are no galaxies on the list. Nevertheless, galaxies are an important part of astronomy, so here is a brief background on the types of galaxies. Spiral Galaxies Spiral Galaxies also have a very large rate of star formation in the spiral arms of the galaxy. Also, almost all spiral galaxies have a galactic halo that surrounds the galaxy. These halos contain stray stars and globular clusters. It is also theorized that many spiral galaxies have supermassive black holes at the center of the galaxy. Our own galaxy, The Milky Way, is a spiral galaxy, and is also theorized to have a supermassive black hole at its center, called Sgr A*. There is also a sub-division of spiral galaxies, known as barred-spiral galaxies. Barred-spirals have a central bar, and then have spiral arms shooting off at each end of the bar. Spirals are classified by presence of a central bar and how tightly the rings are wound. Lenticular Galaxy Lenticular Galaxies are intermediate between spiral and elliptical Galaxies, they contain a large scale disk, but do not have spiral arms. Like Elliptical Galaxies, they contain older Stars and have a low rate of star formation. Elliptical Galaxy Elliptical Galaxies appear just like they sound- they are elliptical/ spherical. Elliptical Galaxies contain mostly old Population II stars, and also, they have a very low rate of star formation because there is barely any interstellar matter in elliptical galaxies. There is the least amount of Elliptical Galaxies in the known Universe. Also, they are classified by how spherical they are, with E followed by a number from zero to seven. Zero indicates perfectly spherical; seven indicates the extremely elongated and cigar-shaped. Irregular Galaxies Irregular also appear just how they sound- they are without a definite shape. They are normally formed by Spiral or Elliptical Galaxies that have been deformed by different forces- such as gravity. They contain a lot of interstellar matter. There are distinctions between "normal" irregular galaxies - with no hint of shape - and peculiar galaxies, that have some hint of form - usually, they were bent out of shape by outside forces or became violently active. The Electromagnetic Spectrum The electromagnetic (EM) spectrum is the range of all types of EM radiation. Radiation is energy that travels and spreads out as it goes – the visible light that comes from a lamp in your house and the radio waves that come from a radio station are two types of electromagnetic radiation. The other types of EM radiation that make up the electromagnetic spectrum are microwaves, infrared light, ultraviolet light, X-rays and gamma-rays. You know more about the electromagnetic spectrum than you may think. The image below shows where you might encounter each portion of the EM spectrum in your day-to-day life. Radio: Your radio captures radio waves emitted by radio stations, bringing your favorite tunes. Radio waves are also emitted by stars and gases in space. Microwave: Microwave radiation will cook your popcorn in just a few minutes, but is also used by astronomers to learn about the structure of nearby galaxies. Infrared: Night vision goggles pick up the infrared light emitted by our skin and objects with heat. In space, infrared light helps us map the dust between stars. Visible: Our eyes detect visible light. Fireflies, light bulbs, and stars all emit visible light. Ultraviolet: Ultraviolet radiation is emitted by the Sun and are the reason skin tans and burns. "Hot" objects in space emit UV radiation as well. X-ray: A dentist uses X-rays to image your teeth, and airport security uses them to see through your bag. Hot gases in the Universe also emit X-rays. Gamma ray: Doctors use gamma-ray imaging to see inside your body. The biggest gamma-ray generator of all is the Universe. Type of Radiation gamma-rays x-rays ultraviolet Visible near-infrared infrared microwaves radio waves Frequency Range (Hz) 1020 - 1024 1017 - 1020 1015 - 1017 4 - 7.5*1014 1*1014 - 4*1014 1013 - 1014 3*1011 - 1013 < 3*1011 Wavelength Range < 10-12 m 1 nm - 1 pm 400 nm - 1 nm 750 nm - 400 nm 2.5 μm - 750 nm 25 μm - 2.5 μm 1 mm - 25 μm > 1 mm Is a radio wave the same as a gamma ray? Are radio waves completely different physical objects than gamma-rays? They are produced in different processes and are detected in different ways, but they are not fundamentally different. Radio waves, gamma-rays, visible light, and all the other parts of the electromagnetic spectrum are electromagnetic radiation. Electromagnetic radiation can be described in terms of a stream of mass-less particles, called photons, each traveling in a wave-like pattern at the speed of light. Each photon contains a certain amount of energy. The different types of radiation are defined by the the amount of energy found in the photons. Radio waves have photons with low energies, microwave photons have a little more energy than radio waves, infrared photons have still more, then visible, ultraviolet, X-rays, and, the most energetic of all, gamma-rays. Measuring electromagnetic radiation Electromagnetic radiation can be expressed in terms of energy, wavelength, or frequency. Frequency is measured in cycles per second, or Hertz. Wavelength is measured in meters. Energy is measured in electron volts. Each of these three quantities for describing EM radiation are related to each other in a precise mathematical way. But why have three ways of describing things, each with a different set of physical units? The short answer is that scientists don't like to use numbers any bigger or smaller than they have to. It is much easier to say or write "two kilometers" than "two thousand meters." Generally, scientists use whatever units are easiest for the type of EM radiation they work with. Astronomers who study radio waves tend to use wavelengths or frequencies. Most of the radio part of the EM spectrum falls in the range from about 1 cm to 1 km, which is 30 gigahertz (GHz) to 300 kilohertz (kHz) in frequencies. The radio is a very broad part of the EM spectrum. Infrared and optical astronomers generally use wavelength. Infrared astronomers use microns (millionths of a meter) for wavelengths, so their part of the EM spectrum falls in the range of 1 to 100 microns. Optical astronomers use both angstroms (0.00000001 cm, or 10-8 cm) and nanometers (0.0000001 cm, or 10-7 cm). Using nanometers, violet, blue, green, yellow, orange, and red light have wavelengths between 400 and 700 nanometers. (This range is just a tiny part of the entire EM spectrum, so the light our eyes can see is just a little fraction of all the EM radiation around us.) The wavelengths of ultraviolet, X-ray, and gamma-ray regions of the EM spectrum are very small. Instead of using wavelengths, astronomers that study these portions of the EM spectrum usually refer to these photons by their energies, measured in electron volts (eV). Ultraviolet radiation falls in the range from a few electron volts to about 100 eV. X-ray photons have energies in the range 100 eV to 100,000 eV (or 100 keV). Gamma-rays then are all the photons with energies greater than 100 keV. Why do we put telescopes in orbit? Illustration showing how far into the atmosphere different parts of the EM spectrum reach The Earth's atmosphere stops most types of electromagnetic radiation from space from reaching Earth's surface. This illustration shows how far into the atmosphere different parts of the EM spectrum can go before being absorbed. Only portions of radio and visible light reach the surface. (Credit: STScI/JHU/NASA) Most electromagnetic radiation from space is unable to reach the surface of the Earth. Radio frequencies, visible light and some ultraviolet light makes it to sea level. Astronomers can observe some infrared wavelengths by putting telescopes on mountain tops. Balloon experiments can reach 35 km above the surface and can operate for months. Rocket flights can take instruments all the way above the Earth's atmosphere, but only for a few minutes before they fall back to Earth. For long-term observations, however, it is best to have your detector on an orbiting satellite and get above it all! What Different Types of Light Tell Us To study the universe, astronomers employ the entire electromagnetic spectrum. Different types of light tell us different things. Radio waves and microwaves, which have the lowest energies, allow scientists to pierce dense, interstellar clouds to see the motion of cold gas. Infrared light is used to see through cold dust; study warm gas and dust, and relatively cool stars; and detect molecules in the atmospheres of planets and stars. Most stars emit the bulk of their electromagnetic energy as visible light, that sliver of the spectrum our eyes can see. Hotter stars emit higher energy light, so the color of the star indicates how hot it is. This means that red stars are cool, while blue stars are hot. Beyond violet lies ultraviolet (UV) light, whose energies are too high for human eyes to see. UV light traces the hot glow of stellar nurseries and is used to identify the hottest, most energetic stars. X-rays come from the hottest gas that contains atoms. They are emitted from superheated material spiraling around a black hole, seething neutron stars, or clouds of gas heated to millions of degrees. Gamma rays have the highest energies and shortest wavelengths on the electromagnetic spectrum. They come from free electrons and stripped atomic nuclei accelerated by powerful magnetic fields in exploding stars, colliding neutron stars, and supermassive black holes. Milky Way Our Sun is one of at least 100 billion stars in the Milky Way, a spiral galaxy about 100,000 light-years across. The stars are arranged in a pinwheel pattern with four major arms, and we live in one of them, about two-thirds of the way outward from the center. Most of the stars in our galaxy are thought to host their own families of planets. Thousands of these exoplanets have been discovered so far, with thousands more candidates detected and awaiting confirmation. Many of these newly discovered planetary systems are quite different from our own. All of the stars in the Milky Way orbit a supermassive black hole at the galaxy's center, which is estimated to be four million times as massive as our Sun. Fortunately, it is a safe distance from Earth, at around 28,000 light-years away. Our galaxy is one of the billions in the universe, each having millions, or more frequently billions, of stars of its own. Our Sun Our Sun is a 4.5 billion-year-old star – a hot glowing ball of hydrogen and helium at the center of our solar system. The Sun is about 93 million miles (150 million kilometers) from Earth, and without its energy, life as we know it could not exist here on our home planet. The Sun is the largest object in our solar system. The Sun’s volume would need 1.3 million Earths to fill it. Its gravity holds the solar system together, keeping everything from the biggest planets to the smallest bits of debris in orbit around it. The hottest part of the Sun is its core, where temperatures top 27 million degrees Fahrenheit (15 million degrees Celsius). The Sun’s activity, from its powerful eruptions to the steady stream of charged particles it sends out, influences the nature of space throughout the solar system.