Chapter 2: The Solar System and the Stars Lesson 1 (PDF)

HENSLER – Earth and Space Science Chapter 2 – The Solar System and the Stars Lesson 1 – Formation of the Solar System A Collapsing Interstellar Cloud Stars and planets form from interstellar clouds, which exist in space between the stars...

HENSLER – Earth and Space Science Chapter 2 – The Solar System and the Stars Lesson 1 – Formation of the Solar System A Collapsing Interstellar Cloud Stars and planets form from interstellar clouds, which exist in space between the stars o Clouds consist mostly of hydrogen (H2) and helium (He) gas with small amounts of other elements and dust o Appear as blotches of light and dark depending on how light interacts with dust and clouds Astronomers think that solar system forms from interstellar clouds as gravity slowly pulls matter together until it is concentrated enough to form a star and possibly planets o Study planets around other stars to get clues to the formation of our solar system Collapse Accelerates At first, collapse of interstellar cloud is slow, but it gradually accelerates and cloud becomes much denser at its center o If rotating, cloud spins faster as it contracts ▪ As collapsing cloud spins it becomes flattened turning into a rotating disk with a dense concentration of matter at center Matter Condenses Sun formed when dense concentration of gas and dust as center of rotating disk reached a temperature and pressure high enough to fuse hydrogen into helium o Rotating disk surrounding young Sun became solar system Area closest to dense center was warm while outer edge of disk was cold o Results in different elements and compounds condensing, depending on their distance from Sun o Also affected distribution of elements in forming planets 1  HENSLER – Earth and Space Science Planetesimals Tiny grains of condensed material started to accumulate and merge, forming larger particles o These grew as grains collided and stuck together and as gas particles collected on their surfaces o Eventually formed planetesimals ranging from 1 km to hundreds of kilometers in diameter As planetesimals grew, eventually formed planets Gas Giants Form First large planet to develop was Jupiter o Increased in size through merging of icy planetesimals that contained mostly lighter elements Saturn and other gas giants formed similarly, but could not become as large because Jupiter had collected so much of available material As each gas giant attracted material from surroundings, disk formed around them much like disk of early solar system o Disk matter clumped together to form rings and satellites Terrestrial Planets Form Terrestrial planets formed by merging of planetesimals in inner part of main disk, near young Sun o Composed mainly of elements that don’t vaporize quickly, so inner planets are rocky and dense Scientists think that solar wind swept away much of gas in area of inner planets and prevented them from acquiring much of this material from surroundings Debris Debris is material that remained after formation of planets and satellites Some debris that was not ejected or did not crash into planets became icy objects known as comets Other debris formed rocky bodies known as asteroids o Most are found in area between Jupiter and Mars known as asteroid belt ▪ Jupiter’s gravitational force prevented them from merging to form a planet 2  HENSLER – Earth and Space Science Modeling the Solar System Ancient astronomers initially assumed that Sun, planets, and stars orbited stationary Earth in geocentric, or Earth-centered model o However, this model could not explain some other aspects of planetary motion o For example, planets might appear farther to east one evening, against background of stars, than they had previous night ▪ Sometimes planet seems to reverse direction and move back to west ▪ Apparent backward movement of planet is called retrograde motion Search for simple explanation of retrograde motion motivated astronomers to keep searching for better explanation for design of solar system o Now we know that retrograde motion is due to the fact that planets orbit the sun at different speeds ▪ Example: for every full orbit Mars completes, Earth completes two Heliocentric Model In 1543, Polish scientist Nicolaus Copernicus suggested that Sun was center of solar system o In this Sun-centered, heliocentric model, Earth and all other planets orbit Sun o In this model, increased gravity due to proximity to Sun causes inner planets to move faster in orbit than do outer planets ▪ Explains simple explanation for retrograde motion Kepler’s Laws After the confirmation of heliocentric model, German astronomer Johannes Kepler discovered three laws to explain planetary motion: o 1st law – each planet orbits Sun in a shape called an ellipse, rather than a circle o 2nd law – planets move faster when they are closer to the Sun o 3rd law – explains the mathematical relationship between size of a planet’s ellipse and its orbital period ▪ The orbital period is the length of time it takes for a planet or other body to travel a complete orbit around the Sun ▪ In other words, the farther a planet is from the sun, the longer its orbital period 3  HENSLER – Earth and Space Science Each planet has its own elliptical orbit around the Sun o Earth’s average distance from the Sun is used as a unit to measure distances within solar system, known as astronomical unit (AU), which equals 1.496 x 108 km ▪ Example: Mars is 1.52 AU from Sun Planet in elliptical orbit does not orbit at constant distance from Sun o Shape of elliptical orbit defined by eccentricity The orbits of most planets are not very eccentric, in fact some are almost perfect circles Gravity Isaac Newton first developed an understanding of gravity by observing falling objects o He described falling as downward acceleration produced by gravity, an attractive force between two objects o Determined that both masses and distances between two bodies determined force between them as expressed in law of universal gravitation Newton realized that attractive force could explain why planets move according to Kepler’s laws o Moon’s direction of motion changes because of gravitational attraction of Earth ▪ In a sense, Moon is constantly falling toward Earth Newton also determined that each planet orbits a point between it and Sun called the center of mass o For any planet, the center of mass is just above or within surface of Sun because Sun is much more massive than any planet Present-Day Viewpoints Traditionally, astronomers divided planets into two groups, four smaller, rocky, inner planets and four outer gas planets o Pluto did not fit classification because it was different from gas giants in composition and orbit In early 2000s, astronomers discovered vast number of small, icy bodies inhabiting outer reaches of solar system o Now solar system is defined in three zones: the inner terrestrial planets, the outer gas giant planets, and the dwarf planets and comets 4  HENSLER – Earth and Space Science Chapter 2 – The Solar System and the Stars Lesson 2 – The Sun Properties of the Sun Sun is largest object in solar system, in both diameter and mass o It would take 109 Earths, or almost 10 Jupiters lined up edge to edge, to fit across the sun o The mass is about 330,000 times the mass of Earth and 1048 times the mass of Jupiter o Contains 99% of all the mass in the solar system Like many other stars, Sun’s interior is gaseous throughout because of high temperature o At center of Sun, composed of atomic nuclei and electrons Sun produces equivalent of 4 trillion trillion 100-W lightbulbs of light each second The Sun’s Atmosphere Atmospheric Thickness Temperature Important Information Layer Photosphere 400 km 5,800 K Visible surface of Sun Innermost layer of Sun’s atmosphere Chromosphere 2500km 15,000K Outside of photosphere Visible only during solar eclipse when photosphere is blocked Appears red Radiation emitted in UV wavelengths Corona Several 3-5 million K Outermost layer of Sun’s atmosphere million km It is so dim that can only be seen during an eclipse Temperature so high that radiation emitted is in X rays 5  HENSLER – Earth and Space Science Solar Activity Features on stars change over time in a process called solar activity Sun’s magnetic field disturbs solar atmosphere periodically and causes new features to appear Solar Information Activity Features Sunspots Dark spots on the surface of photosphere Appear darker because they are cooler than surrounding area Located in regions where Sun’s intense magnetic fields penetrate photosphere Occur in pairs with opposite magnetic polarity – with north and south poles similar to magnet Coronal Only detectable in X-ray photography holes Located over sunspot groups Areas of low density in the corona and are main regions from which particles that comprise solar wind escape Solar wind Plasma that flows outward from corona at high speeds Bathes planets in a flood of particles Not uniform – streams between 300 km/s to 800 km/s Causes the aurora lights that we see on Earth in the polar region Solar flares Violent eruptions of particles and radiation from surface of Sun Associated with sunspots Largest recorded solar flare occurred in November 2003 Prominence Sometimes associated with solar flares Arc of gas ejected from the chromosphere, or is gas that condenses in the inner corona and rains back to the surface Can reach temperatures greater than 50,000K and last from a few hours to a few months Number of sunspots changes in a predictable and set pattern called sunspot cycle o Changes from minimum to maximum in 11-year cycles correlated with reversals of sun’s magnetic field ▪ This is half of the solar activity cycle – so an entire cycle is approximately 22 years 6  HENSLER – Earth and Space Science Solar Interior Energy that causes solar activity and light comes from fusion in the core of the Sun o Fusion is the combination of lightweight, atomic nuclei into heavier nuclei, such as hydrogen fusing into helium This is opposite of fission which is splitting of heavier atomic nuclei into lighter nuclei, like uranium into lead In core of the Sun, helium is product of process in which hydrogen nuclei fuse o Mass lost in fusion of hydrogen to helium is converted to energy, which powers Sun At current rate of fusion, Sun is about halfway through lifetime and has about 5 billion years left o Has only used about 3% of its hydrogen Energy Transport Two zones in solar interior that transfer energy produced in core to surface: o Radiation zone is inner portion of Sun extended to 86% of radius ▪ Energy transferred by radiation o Convection zone extends from radiation zone to photosphere ▪ Energy transferred by gaseous convective currents Leaving sun’s outermost layer, energy moves in variety of wavelengths in all directions o Tiny fraction eventually reaches Earth Spectra When light shines through a prism, a rainbow often appears o This rainbow is a spectrum which is visible light arranged according to wavelengths There are three types of spectra: o Continuous spectra has no breaks in it ▪ Produced by glowing solid or liquid or by highly compressed glowing gas o Emission spectra produced by non-compressed gas has bright lines at certain wavelengths ▪ Lines are called emission lines 7  HENSLER – Earth and Space Science ▪ Wavelengths of visible lines depend on element being observed because each element has its own characteristic emission spectrum o Absorption spectra are produced when there is a cooler gas in front of source that emits continuous spectrum ▪ Creates dark spectral lines caused by different chemical elements that absorb light at specific wavelengths Comparing laboratory spectra enables scientists to identify elements that make up Sun’s outer layer Solar Composition Scientists have learned great deal about Sun’s composition from the lines of its absorption spectra Sun consists of 71% hydrogen (H) by mass, 27.1% helium (He) by mass and small amounts of other elements o This is similar to composition of gas giant planets ▪ Suggests that sun and gas giants represent the composition of interstellar cloud that solar system formed from o Heavier elements composition of terrestrial planets probably came from contribution to interstellar cloud from long-extinct stars Sun’s composition represents that of galaxy as a whole o Most stars have proportions similar to that of Sun Chapter 2 – The Solar System and the Stars Lesson 3 – Measuring the Stars Patterns of Stars Ancient civilizations named star groups after animals, mythological characters, or everyday objects o These groups are called constellations Today astronomers recognize 88 constellations Types of constellations: o Circumpolar constellations: visible in all year round in certain hemispheres, such as Ursa Major in the northern hemisphere, which includes the Big Dipper o Seasonal constellations: visible during specific seasons based on Earth’s orbit ▪ For example: Orion is visible in the winter and Hercules is visible in the summer in the northern hemisphere 8  HENSLER – Earth and Space Science o Zodiac constellations: located along ecliptic planes, seen in both hemispheres, and associated with astrology ▪ Ecliptic plane is the apparent path that the planets take around the Sun Star Clusters Although stars in constellations appear close to each other, most of them are not physically related o For example, two stars that seem next to each other may be separated by hundreds of trillions of kilometers Groups of stars that are gravitationally bound to each other are called clusters o These stars formed together from the same molecular cloud and have similar ages and compositions Types of clusters: o Open clusters: loosely bound groups of stars that formed from the same nebula but have dispersed slightly over time ▪ Example: The Pleiades (also called the Seven Sisters) in the constellation Taurus o Globular clusters: densely packed spherical groups of stars that contain hundreds of thousands or even millions of stars ▪ Example: M13 (the Great Hercules Cluster) in the constellation Hercules Binaries Binary stars are two stars that are gravitationally bound to each other and orbit a common center of mass o More than half of the stars visible in the night sky are part of binary or multiple- star systems o Most appear as single point of light to naked eye due to close proximity to each other Methods to detect binary stars: o Orbital movement: if only one star is visible, its position might shift periodically, indicating the presence of an unseen companion o Eclipsing binaries: when the eclipsing plane is aligned with Earth’s line of sight, the two stars block each other, causing fluctuations in brightness 9  HENSLER – Earth and Space Science Doppler Shifts Astronomers use the Doppler effect to study the motion of stars, particularly in binary systems o When a star moves toward an observer, its light waves compress, causing a blueshift (shift toward shorter wavelengths) o When a star moves away, the light waves stretch, causing a redshift (shift to longer wavelengths) The changes in wavelength help an astronomer determine the speed and direction of a star’s motion o The higher the speed, the larger the shift Doppler shift can only be used for a star’s motion that is directed toward or away from Earth – not at right angles Even if two stars are too close to be distinguished by telescopes, the periodic Doppler shifts in their spectral lines reveal the presence of a companion star (in a binary) o Stars identified in this way are called spectroscopic binaries Parallax Astronomers use two primary units to measure distances in space: o Light-year (ly): the distance that light travels in one year, about 9.461 trillion kilometers o Parsec (pc): a larger unit of measurement, equal to 3.26 light-years, or approximately 30.86 trillion kilometers Parallax is the apparent shift in the position of a nearby star against the distant background when viewed from different positions in Earth’s orbit o The closer the star, the larger the shift o This shift can be measured to calculate the distance to stars up to 100 parsecs away 10  HENSLER – Earth and Space Science Basic Properties of Stars Stars are primarily characterized by their: o Mass: determines most other stellar properties, including lifetime and energy output o Diameter: size of the star, which influences its luminosity o Luminosity: the amount of energy a star emits per second o Temperature: estimated by analyzing a star’s spectrum and controls its nuclear reaction rate Magnitude Systems Apparent magnitude is the brightness of a star as seen from Earth, without accounting for distance o Ancient Greek astronomers assigned rankings from +1 (brightest) to +6 (dimmest) o Modern astronomers have refined this system, and now a magnitude difference of 5 corresponds to a 100-fold difference in brightness ▪ Magnitude +1 star is 100 time brighter than a magnitude +6 star Absolute magnitude is the actual brightness of a star if it were placed 10 parsecs from Earth, allowing for direct comparisons o This system corrects for the distortions caused by varying distances between stars and Earth Luminosity of Stars Luminosity is the total energy output of a star per second, measured in watts o The Sun’s luminosity is about 3.85x1026 Watts, enough to power trillions of light bulbs Stars’ luminosities vary greatly, from 0.0001 to over 1 million times the Sun’s luminosity Luminosity depends on both the star’s intrinsic energy output and its distance Classification of Stars Spectral classification: o Stars are classified by their spectral lines, which indicate their temperature and composition o The classification system uses O, B, A, F, G, K, and M, with O stars being the hottest (up to 50,000K) and M stars being the coolest (as low as 2,000K) ▪ Each class subdivided into more specific divisions from 0 to 9 11  HENSLER – Earth and Space Science o The Sun is a G2-type star with a surface temperature of 5,800K Composition: o All stars have a similar composition: ~73% hydrogen, ~25% helium, and ~2% other elements o The differences in spectral lines are largely due to temperature, not composition H-R Diagram and Stellar Evolution The Hertzsprung-Russell diagram is a graphical representation of stars, plotting absolute magnitude (brightness) against surface temperature Most stars fall along the main sequence, where they spend the majority of their lives fusing hydrogen into helium o About 90% of stars, including the sun are main sequence stars o More massive stars burn their hydrogen fuel faster, resulting in shorter lifespans ▪ Exist on the main sequence a shorter amount of time o Stars like the Sun evolve off the main sequence once they run out of hydrogen in their core Red giants and white dwarfs on the HR diagram: o Red giants are large, cool, luminous stars found in the upper-right corner of the H-R diagram ▪ Red giants have expanded as they exhaust their hydrogen fuel, increasing in size and luminosity despite a cooler surface temperature o White dwarfs are small remnants of stars that have exhausted their nuclear fuel ▪ Found in the lower-left corner of the H-R diagram, white dwarfs are hot but very dim due to their small size 12  HENSLER – Earth and Space Science Chapter 2 – The Solar System and the Stars Lesson 4 – Stellar Evolution The Basic Structure of Stars Mass governs star properties: o Mass is the fundamental factor influencing a star’s temperature, luminosity, and diameter o Both mass and composition of a star determine nearly all of its other properties Hydrostatic equilibrium: o The balance between inward gravity and outward pressure (due to nuclear fusion and compression) keeps a star stable o More massive stars have a greater gravitational pull inward so the hotter and the denser the star must be inside to balances its own gravity ▪ Produce more energy, hence higher luminosity 13  HENSLER – Earth and Space Science o Too much gravity and star collapses while too much outward pressure and star expands Nuclear fusion: o Main-sequence stars fuse hydrogen into helium o Stars not on the main sequence either fuse heavier elements or stop fusing altogether Stellar Evolution Change in composition over time: o As stars age, nuclear fusion converts lighter elements into heavier ones, altering their internal composition o This leads to increases in density, temperature, and luminosity o As long as a star is fusing hydrogen into helium, it remains on the main sequence End of hydrogen fusion: o Once hydrogen runs out, the star’s core begins to fuse heavier elements (e.g., helium) or contracts due to gravity o Stars evolve as they burn through different elements or when fusion ceased Star Formation Stars form from clouds and dusts, called nebula, which collapse under gravity As the cloud contracts, it forms a protostar at the center of a rotating disk Friction and gravitational forces heat the protostar until it reaches the temperature needed for nuclear fusion When the star is hot enough for hydrogen fusion, the protostar becomes a main- sequence star with its mass determining its position on the sequence Life Cycle of Sun-like Stars Main sequence lifetime: o Stars like the Sun take about 10 billion years to convert all core hydrogen into helium Red Giant phase: o After hydrogen runs out in the core, fusion continues in a shell around the core ▪ Energy produced in this layer forces the outer layers of the star to expand and cool o The star becomes larger and more luminous, but its surface temperature decreases due to expansion 14  HENSLER – Earth and Space Science o While in this phase, the red giant loses gas from its outer layers due to a low surface gravity o Meanwhile, core becomes hot enough for helium to react and form carbon ▪ Star contracts back to normal size where it becomes stable again for awhile ▪ Helium-reaction phase only lasts about 1/10th as long as hydrogen- burning phase Final stages: o A star with same mass as Sun never becomes hot enough for carbon to fuse, so energy production ends o Outer layers expand again and are expelled from star creating a shell of gas called a planetary nebula o In core of nebula is a small, hot object about size of Earth ▪ Star is a white dwarf made of carbon This can remain stable as long as the remaining core is less than 1.4 times the mass of the Sun o Over time, the white dwarf cools and fades, becoming an undetectable black dwarf Life Cycle of Massive Stars Main-sequence phase: o More massive stars are brighter and have shorter lifespans due to faster consumption of nuclear fuel Supergiant phase: o Massive stars undergo multiple stages of fusion, creating heavier elements and expanding into a supergiant ▪ Example: Betelgeuse Supernova formation: o Once reactions in core of star have created iron, no further energy-producing reactions can occur and core violently collapses leading to supernova explosion ▪ This releases energy and creates elements heavier than iron Neutron stars and pulsars: o Stars with a mass between 8 and 20 times that of the Sun form neutron stars after a supernova ▪ These have a mass of 1.4 to 3 times the Sun’s mass but a diameter of only about 20km o As neutron stars rotate, they emit beams of light that pulse called pulsars Black holes: 15  HENSLER – Earth and Space Science o Stars with a mass of more than 20 times that of the Sun and a core after supernova that exceeds 3 times the mass of the Sun will collapse into a black hole ▪ This is an object with a gravity so strong that not even light can escape ▪ Only thing astronomers can observe is X-ray-emitting gas that spirals into it 16