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Exam #2 Review Looks “quiet in the visible” • The Sun is a huge ball of gas at the center of the solar system The Sun – 100 times Earth diameter, 300,000 earth mass – 1 million Earths would fit inside it! – Releases the equivalent of 100 billion atomic bombs every second! – 1366 watts/square meter at Earth – 15 Tera watts in 62 sq mi • Exists thanks to a delicate balance of gravity and pressure 1 Knowledge of interior based on models which fit observables: •Mass •Radius •Luminosity •Surface Temperature •Image details: granules, spicules, corona, chromosphere The Photosphere • The photosphere is the visible “surface” of our star – Not really a surface, as the Sun is gaseous throughout – Photosphere is only 500 km thick – Average temperature is 5780 K 2 Energy Transport in the Sun • Just below the photosphere is the convection zone. – Energy is transported from deeper in the Sun by convection, in patterns similar to those found in a pot of boiling water (hot gas rises, dumps its energy into the photosphere, and then sinks) • Energy in the convection zone comes from the radiative zone. – Energy from the core is transported outward by radiation – Takes more than 100,000 years for a single photon to escape the Sun! The Solar Atmosphere • Regions of the Sun above the photosphere are called the Sun’s atmosphere • Just above the photosphere lies the chromosphere – Usually invisible, but can be seen during eclipses • Above the chromosphere is the corona – Extremely high temperatures (more than 1 million K!) – Rapidly expanding gas forms the solar wind. 3 The Ideal Gas Law Pressure = Constant × Temperature × Density The Sun’s Energy • The Sun’s energy comes from fusion – the merging of hydrogen nuclei into helium • The reaction releases only a little bit of energy, but it happens a lot! • A hydrogen nucleus has less mass than the four protons (hydrogen nuclei) that fuse • This difference in mass is converted into energy: E = m×c2 The Proton-Proton Chain 4 Sunspots • Sunspots are highly localized cool regions in the photosphere of the Sun – Discovered by Galileo – Can be many times larger than the Earth! – They contain intense magnetic fields. Solar Flares Coronal Mass Ejection 5 • When CME material reaches the Earth, it gets funneled by Earth’s magnetic field and collides with ionospheric particles, close to the poles • The collision excites ionospheric oxygen, nitrogenand which causes it to emit a photon • We see these emitted photons as the aurora, or Northern Lights The Aurora 6 Measuring the Distance to Astronomical Objects using parallax Just a little Trigonometry… 1 parsec = 206265 AU 7 Moving Stars • The positions of stars are not fixed relative to Earth – They move around the center of the galaxy, just as Earth does. – This motion of stars through the sky (independent of the Earth’s rotation or orbit) is called proper motion – Over time, the constellations will change shape! • The speed of a star’s motion toward or away from the Sun is called its radial velocity The Inverse-Square Law • • A star emits light in all directions, like a light bulb. We see the photons that are heading in our direction As you move away from the star, fewer and fewer photons are heading directly for us, so the star seems to dim – its brightness decreases. • The brightness decreases with the square of the distance from the star – If you move twice as far from the star, the brightness goes down by a factor of 22, or 4! • Luminosity stays the same – the total number of photons leaving a sphere surrounding the star is constant. 8 You see this every day! • • • More distant streetlights appear dimmer than ones closer to us. It works the same with stars! If we know the total energy output of a star (luminosity), and we can count the number of photons we receive from that star (brightness), we can calculate its distance L 4 ! B • Some types of stars have a known d= luminosity, and we can use this standard candle to calculate the distance to the neighborhoods these stars live in. Measuring Temperature using Wein’s Law 2.9 #106 K " nm T= ! 9 The Stefan-Boltzmann Law flux = "T 4 Flux is energy / unit area Where, σ= 5.67×10− 8 W·m-2·K-4 L = flux • Area = "T 4 • 4 # r 2 • ! The Stefan-Boltzmann Law links a star’s temperature to the amount of light the star emits ! – Hotter stars emit more! – Larger stars emit more! • • A star’s luminosity is then related to both a star’s size and a star’s temperature We need an organizational tool to keep all of this straight… Photons in Stellar Atmospheres • • • Photons have a difficult time moving through a star’s atmosphere If the photon has the right energy, it will be absorbed by an atom and raise an electron to a higher energy level Creates absorption spectra, a unique “fingerprint” for the star’s composition. The strength of this spectra is determined by the star’s temperature. 10 Classify: 1- variation in H line strengths…. “Spectral types” based on H lines strength: A B C D E F… 1901, Annie Jump Cannon > spectral classification 11 Spectral Classification • Spectral classification system – Arranges star classifications by temperature • Hotter stars are O type • Cooler stars are M type • New Types: L and T – Cooler than M • From hottest to coldest, they are O-B-A-F-G-K-M – Mnemonics: “Oh, Be A Fine Girl/Guy, Kiss Me – Or: Only Bad Astronomers Forget Generally Known Mnemonics A convenient tool for organizing stars • • In the previous unit, we saw that stars have different temperatures, and that a star’s luminosity depends on its temperature and diameter The Hertzsprung-Russell diagram lets us look for trends in this relationship. 12 Stars come in all sizes… • • A star’s location on the HR diagram is given by its temperature (x-axis) and luminosity (y-axis) We see that many stars are located on a diagonal line running from cool, dim stars to hot bright stars – • – – • The Main Sequence Other stars are cooler and more luminous than main sequence stars Must have large diameters (Red and Blue) Giant stars Some stars are hotter, yet less luminous than main sequence stars – – Must have small diameters White Dwarf stars • So what’s going on here? The Mass-Luminosity Relation 13 Stars come in all sizes… • L vs T b= L 4 "d 2 b=brightness, d=distance away 2.9 "10 6 K # nm T= $ ! ! Stellar Evolution on the Main Sequence 14 The Main-Sequence of a Star High-mass starsLifetime 10 mpg Low-mass stars 60 mpg • The length of time a star spends fusing hydrogen into helium is called its main sequence lifetime – Stars spend most of their lives on the main sequence – Lifetime depends on the star’s mass and luminosity – More luminous stars burn their energy more rapidly than less luminous stars. – High-mass stars are more luminous than low-mass stars – High mass stars are therefore shorter-lived! • • Cooler, smaller red stars have been around for a long time Hot, blue stars are relatively young. A (temporary) new lease on life • The triple-alpha process provides a new energy source for giant stars • Their temperatures increase temporarily, until the helium runs out • The stars cool, and expand once again • The end is near… 15 Evolution to red giant phase Fuel runs out Core pressure drops Gravity compresses core Core temperature rises Shell burning Pressure puffs outer layers Core heats up more Shell burning grows stronger Atmosphere expands and cools further • The star is expanding and cooling, so its luminosity increases while its temperature decreases • Position on the HR diagram shifts up and to the right… 16 Helium Fusion • Normally, the core of a star is not hot enough to fuse helium – Electrostatic repulsion of the two charged nuclei keeps them apart • The core of a red giant star is very dense, and can get to very high temperatures – If the temperature is high enough, helium fuses into Beryllium, and then fuses with another helium nuclei to form carbon. Main Sequence Turn-off Main sequence What are we looking at? a) Stars of the same mass? b) Stars of the same color? c) Stars of the same magnitude? d) Stars of the same age? 17 Main Sequence Turn-off Main sequence Mass at turn-off What does the mass of the Main Sequence-Turn-off tell us? a) The Mass of the cluster? b) The Age of the cluster? c) The Distance of the cluster? d) The Brightness of the cluster? The Life-path of the Sun 18 Formation of Planetary Nebula • As a red giant expands, it cools – Outer layers cool enough for carbon flakes to form – Flakes are pushed outward by radiation pressure – Flakes drag stellar gas outward with them – This drag creates a high-speed stellar wind! – Flakes and gas form a planetary nebula White Dwarf Stars • At the center of the planetary nebula lies the core of the star, a white dwarf – Degenerate material – Incredibly dense • Initially the surface temperature is around 25,000 K • Cools slowly, until it fades from sight. 19 The Lifespan of a Massive Star Layers of Fusion Reactions • As a massive star burns its hydrogen, helium is left behind, like ashes in a fireplace • Eventually the temperature climbs enough so that the helium begins to burn, fusing into Carbon. Hydrogen continues to burn in a shell around the helium core • Carbon is left behind until it too starts to fuse into heavier elements. • A nested shell-like structure forms. • Once iron forms in the core, the end is near… 20 Stellar Corpses • A type II supernova leaves behind the collapsed core of neutrons that started the explosion, a neutron star. • If the neutron star is massive enough, it can collapse, forming a black hole… Azimuthal Radial velocity velocity •Most luminous EM events in the universe •Short (few sec) gamma ray burst •Longer optical afterglow •2 kinds (short and long) pressure density •(1)Massive star death •(2)Collisions of dead stars 21 Supernova Remnant • The supernova has left behind a rapidly expanding shell of heavy elements that were created in the explosion. • Gold, uranium and other heavies all originated in a supernova explosion! 22