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Our Sun, Sol, and Stars Like It Seen in UV light, SOHO EIT Solar Facts • The sun is 1.4 million km wide (109 times Earth’s diameter) • The sun’s mass is 330,000 times that of Earth; it accounts for about 99.86% of the total mass of the Solar System. About ¾ of the Sun's mass consists of hydrogen, while the rest is mostly helium. Less than 2% consists of heavier elements, including oxygen, carbon, neon, iron, and others. Solar Facts • The Sun is neither a solid nor a gas but is actually a plasma, a state of matter made up of charged particles (protons and electrons). • The sun’s layers rotate at different speeds, with the photosphere rotating at 251/3 days at the sun’s equator, 34 days at its poles. Inner Layers of Our Sun It takes hundreds of thousands of years for energy from the core to reach the surface. • Core—350,000 km, temperature up to 15.7 million K, pressure of 200 billion atm; nuclear reactor where hydrogen fusion takes place • Radiative zone—650,000 km, temperature decreases from 7 to 2 million K, photons of light are emitted, absorbed, re-emitted at lower temps • Convective zone—400,000 km, temp decreases from 2 million to 6000 K, convection currents move heat to surface Nuclear Fusion • Nuclear fusion is the way that the sun produces energy. This reaction converts four hydrogen nuclei into the nucleus of a helium atom, releasing a tremendous amount of energy and photons. • During nuclear fusion, energy is released because some matter is actually converted to energy. E = mc2 • Our sun is about halfway through its expected lifespan of 10 billion years. Outer Layers of our Sun • Photosphere—opaque ‘surface’ we see, 500 km thick, has granules (size of Texas), the tops of convection cells which give the photosphere its grainy appearance, temperature is 5800K • Chromosphere—10,000 km above photosphere, 2000 km thick, red, transparent, hot incandescent gases with average temperature of 15,000K. • Corona—Rarified hot gas extends millions of km in space, temp to 2 million K, gas accelerated by strong magnetic fields, source of x-rays Sun’s Magnetic Field • Our sun has a very complex and intense magnetic field, not a dipole field like Earth’s • Features that relate to intense magnetic activity are sunspots, solar flares, and solar prominences. Sunspots • Dark spots on the sun’s surface • Up to 80,000 km across (5 Earths could fit across) • Cooler than surrounding areas (4000 °C) so they appear dark • Usually occur in pairs, last for days or weeks • 11 year cycle in sunspot maximum Solar Flare Solar prominence CME • A sudden brightening over the sun’s surface when material is heated to tens of millions of degees. It happens when magnetic energy stored in the corona is released as intense radiation. • Associated with loops of plasma called prominences and coronal mass ejections, huge eruptions of material into space with the release of radiation and charged particles. • Solar storms can cause severe electrical and communications blackouts on Earth, affecting air travel, GPS, etc. The sun’s cycle was high in 2013 when intense storms were predicted. Solar Wind • Our sun constantly gives off energetic charged particles that spew outward into space in all directions • These particles carry the sun’s magnetic field with them • When the solar wind reaches Earth, it interacts with our magnetic field to produce auroras in the middle atmosphere, the ionosphere. • Jupiter and Saturn also have auroras. Auroras happen near both the North and South magnetic poles. Lifecycle of Stars Like Our Sun 1. 2. 3. 4. 5. 6. 7. Protostar Main sequence star Red giant Variable star Planetary nebula White dwarf Black dwarf Protostar • A clump of high density gas and/or dust is held together by gravity • As more and more gas and dust collects, the internal pressure rises, and the protostar begins to heat up and radiate in the infrared. • When the temperature reaches 10 million degrees K, hydrogen ‘ignites’ and starts fusing. Nuclear fusion has begun. A star is born! • After hundreds of thousands of years, energy in the form of light reaches the star’s surface. As the starlight streams out into space, it pushes aside any remaining gas and dust. Main Sequence Stars • Stars like our sun spend most of their adult lives on the MAIN SEQUENCE of the Hertzsprung-Russell diagram, burning hydrogen and making helium. • The star has achieved hydrostatic equilibrium (a balance between inward gravity and outward gas pressure). • The time a star spends on the main sequence depends on its mass: low mass stars can remain for 100 billion years, a star like our sun 10 billion, and high mass stars only a few million years. So what’s an H-R diagram? Red Giant Phase • There comes a time when a star has used up almost all of the hydrogen in its core. Nuclear fusion ceases there. • With no gas pressure to keep it balanced, the core begins to collapse under gravity’s influence and gravitational potential energy is converted to heat. Some of this heat radiates outward to the outer layers and increases the rate of hydrogen burning, expanding the outer layers. Red Giant Phase (cont.) • As the outer layers of the star grow larger, they cool off and become reddish in color. (A red star is cooler at its surface than a blue-white star for the same reason that an iron bar, heated until it is white-hot, turns red as it cools.) • We expect our sun to become a red giant in about 5 billion years, when it will swell up and become as large as the orbits of Mars or Jupiter, swallowing the inner planets. Meanwhile…back in the star’s core • The star’s burned-out core continues to collapse and heat up. • When the core temperature reaches 100 million K, helium starts fusing and carbon is produced. • Again hydrostatic equilibrium occurs. The star shrinks and moves back onto the Main Sequence. • Depending on the star’s mass, it may repeat the pattern, becoming a red giant two or more times, each time having its core temperature heating until it starts fusing in turn carbon, neon, oxygen, and then silicon. It becomes a VARIABLE STAR, moving back and forth between the red giant area of the H-R diagram and the main sequence. Variable Stars A Cepheid variable is a star whose brightness varies periodically because it expands and contracts; it is a type of pulsating star. A nova is a star that explosively increases in brightness. (Pictures 2 months apart) Stellar Old Age Depends on Mass • Stars with masses near our Sun’s, when they have completed helium fusing and have a carbon core, expand again into a red giant. • They ‘puff’ off their outer layers which spread out at 20 to 30 km/s, leaving the core behind. These layers are illuminated by light from the star’s core. • This is called a planetary nebula (named by William Herschel about 1800; it has nothing to do with planets!) • A planetary nebula is visible for 50,000 to 100,000 years, a mere blink in stellar lives. About 1600 have been observed. White and Black Dwarfs • Left behind after the planetary nebula is ‘puffed’ off is the stellar core. • It continues to collapse into a white dwarf star, which is extremely hot and dense. • Our sun’s core will collapse into a white dwarf about twice the size of Earth with a gravity about a million times that of Earth. • This is expected to happen in about 5 billion years, so don’t panic! Sorry, our sun will NOT blow up. • Eventually, the white dwarf will radiate away all its stored heat and light and become a corpse—a black dwarf. A cold cinder… Hypothetical, as universe isn’t old enough to have black dwarves. Stingray Nebula NGC 2818 NGC 6751 NGC 6543, Cat's Eye Nebula NGC 7009, Saturn Nebula Low-Mass Stars • The lives of low mass stars (0.25 mass of our sun) are relatively unexciting. • Such a star is small and relatively cool and remains on the main sequence for 100 billion years until it consumes all its hydrogen fuel (no outward pressure from burning, remember) and collapses under its own weight. • It becomes a white dwarf. • Eventually, the white dwarf will radiate away all its stored heat and light and become a corpse—a black dwarf. Deaths of More Massive Stars • High-mass stars (4+ our sun) have sufficient pressure and temperature at their cores to continue fusing, in turn, helium, carbon, oxygen, and silicon as the fuel for making energy, opposing gravity, and keeping the star (briefly) stable as a variable star. Outer layers remain of hydrogen, helium, carbon, etc. so the star is like an onion. Deaths of More Massive Stars • Each stage is less efficient, so each new fusion fuel is used up in a shorter time. • For a star that is 25 times the mass of our sun, this would mean: – Hydrogen fusion lasts 7 million years – Helium fusion lasts 500,000 years – Carbon fusion lasts 600 years – Neon fusion lasts 1 year – Oxygen fusion lasts 6 months – Silicon fusion lasts 1 day Deaths of More Massive Stars • Eventually, however, the massive star's core becomes composed of the element iron, which sets off a final collapse and catastrophe. Why? Nuclear reactions involving iron do not release energy. Death Throes • When the core of a massive star collapses, its powerful gravity takes it right through the white dwarf stage to produce one of two extremely bizarre objects—either a neutron star or a black hole. • In most cases, the rest of the star then blows up, in a gargantuan explosion called a supernova. SUPERNOVAE Type Ia Type II Supernovae remnants Neutron Star • The violent end of a massive star of at least 8 solar masses (with a core of between 1.4 and 3 solar masses) produces so much pressure that the atoms in the core experience a remarkable subatomic change: electrons are actually squeezed into the nuclei and "join" with protons to become neutrons, creating a neutron star. • This process also removes all the space within the atom, leading to a fantastic compression of the star's remains to about 20 km across. • One teaspoonful would weigh a billion tons. Pulsars • A pulsar (pulsating star) is a highly magnetized, spinning neutron star with jets of particles moving almost at the speed of light streaming out above its magnetic poles. • These jets produce very powerful beams of light. • The precise periods of pulsars make them useful tools to astronomers. PLM/Pulsars: Jocelyn Bell Black Holes • However, if the star was more than 20 times our sun’s mass, the result of its supernova explosion is a completely collapsed object called a black hole. • In these objects gravity is so strong that nothing, not even light, can escape. Space, time, matter, and energy are all trapped within a tiny region. • The event horizon is the boundary where the inward pull of gravity is so overwhelming that no information about the black hole's interior can escape to the outer universe. Black Holes (cont.) NGC 4261 Put these in order of lifestage C A B D F G E Red giant H K J K H, B, J, K, F, E, C, A, G, D Constellations • There are 88 named constellations in the sky • Constellations are patterns of stars named in honor of mythological creatures or heroes • Stars in a constellation are not associated with each other in any physical way Binary Stars So how do we know how far away stars are? Stellar parallax is the apparent change in a star’s position observed when the star is sighted from opposite sides of Earth’s orbit. Parallax is used to find distance Is parallax the only method? • Astronomers have LOTS of ways to measure distances • These methods overlap so they ‘check’ each other FYI Distances to Some Stars • • • • • • • • • Nearest is Alpha Centauri (binary) @ 4.3 ly Next is Sirius (binary) @ 8.6 ly Procyon (binary) @ 11.4 ly Vega @ 25 ly Aldebaran @ 67 ly Mizar (binary binary) @ 78 ly Antares @ 550 ly Betelguese @ 570 ly Rigel @ 860 ly Apparent and Absolute Magnitude Some stars’ magnitudes Star Spectral Class, Temperature, Mass and Color Spectral class Surface Temperature (K) Solar mass (solar mass) O > 30,000 > 16 B 10,000-30,000 2.1 - 16 Blue-white to blue A 7,500-10,000 1.4 - 2.1 White F 6,000-7,500 1.04 – 1.4 Yellow to white G 5,000-6,000 0.8 – 1.04 Yellow K 3,500-5,000 0.45 – 0.8 Orange M <3,500 < 0.45 Apparent color Blue Red Hertzsprung-Russell Diagrams • A Hertzsprung–Russell diagram shows the relationship between the luminosity (absolute magnitude) and temperature of stars. • H–R diagrams have been helpful in formulating and testing models of stellar evolution. • They are also useful for illustrating the changes that take place in an individual star during its life span. • Most stars spend the majority of their lives on the ‘main sequence’, the diagonal band from upper left to lower right Hertzsprung-Russell Diagram Hyperlink to animated star’s lifecycle. More H-R diagrams • You will do a W/S on H-R diagrams, but before, let’s practice interpreting H-R diagrams. http:/www.pbs.org/seeinginthedark/astronomy-topics/lives-ofstars.html