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Exploring The Universe Student Guided Notes Ch. 20 Sec.1 STARS • Objectives: – Classify stars according to chemical and physical properties – Interpret an H-R diagram. What are STARS? • Star – a large celestial body that is composed of gas and that emits light; the sun is a typical star • Light-year – the distance that light travels in one year; about 9.5 trillion kilometers • Stars are huge spheres of hot gas. • The nearest star to the Earth is the sun. • We use the unit light-year to describe a star’s distance from Earth. How do we classify STARS? • Scientists classify stars according to their: – Color – Size – Brightness How do we classify STARS? • Scientists classify stars according to their: – Color – Size – Brightness Copy this table down!!! Above 30,000o Celsius 5000o-6000o Celsius 3000o C BLUE Yellow RED How do we classify STARS? • Scientists classify stars according to their: – Color – Size – Brightness • Apparent Brightness: – Decreases as its distance from you increases • The brightness of a star as it appear from Earth Apparent Brightness: Absolute Brightness: • How bright a star REALLY is regardless of its distance from Earth. • Absolute brightness is a characteristic of a star Apparent Brightness Absolute Brightness How do we classify STARS? • Scientists classify stars according to their: – Color – Size – Brightness • Temperature & Absolute Brightness help us figure out a star’s VOLUME • Gravitational attraction of stars that appear in pairs helps us figure out its MASS • There is a relationship between mass & absolute brightness How do we know what elements are in STARS? • Spectrograph: – An instrument that spreads light from a hot glowing object into a band of colors called a spectrum. • Spectral lines reveal the composition of stars. • The spectra of most stars have dark lines caused by gases in the outer layers that absorb light at that wavelength. • Each element produces a unique pattern of spectral lines. How do we know what elements are in a STAR? • Astronomers can match the dark lines in starlight to the known lines of elements found on Earth. Absorption Lines H-R Diagrams Early 1900’s: • Ejnar Hertzprung & Henry Norris Russell (working independently) Noticed: • Stars can be classified by graphing them according to their surface temperature (color) and absolute brightness. H-R Diagram H-R Diagram • Two factors determine a star’s absolute brightness: – Size – Surface temperature • The H-R diagram shows temperature & absolute brightness – We can use it to infer a star’s size. Types of Stars: • Main Sequence Stars • Supergiants • Giants • White Dwarfs Main Sequence Stars • Range from bright & hot to dim & cool • Over 90% of stars are classified this way • Over 90% of a star’s lifetime is spent in this stage. Diagonal band in the middle of the H-R Diagram •The Sun lies near the middle of this band Super Giants • Very Bright • Much Brighter than main sequence stars of the same temp. • Very Large Red Giants • Large & Bright • Fainter than Supergiants. White Dwarfs • Small, dense remains of low to medium mass stars • Hot, but dimmer than main sequence stars of the same temp. Exploring the Universe Part II Life Cycle of Stars Life Cycles of Stars • Objectives: – Describe how stars form. – Estimate how long a star remains in the main sequence stage. – Predict what happens to a star when it runs out of fuel. Birth of a Star 1. A star begins as a cloud of dust & hydrogen gas. • Cloud of Hydrogen gas NEBULA 2. As gravity pulls the clouds together, a dense, hot core is formed 3. Temps increase in core. Hydrogen atoms smash into each other Cloud pulls Together & Forms a Dense, hot core Birth of a Star 4. When temp is hot enough, hydrogen begin FUSING to form helium. 5. Nuclear Fusion generates huge amounts of energy. 6. Leftover dust & gas dissipates, leaving the star behind. Nuclear…. Fusion begins & the core… Becomes a Star….. Cloud….. Dissipates, Leaving star Behind... Life Cycle of a Star • Lifetime depends on size of star. – Large, heavy stars burn up their hydrogen (fuel) faster than smaller stars do. – Therefore, large (massive) stars burn out faster than small stars. Life Cycle of an Average Star • Death – When Hydrogen is used up, the core collapses on itself causing temp to rise. – Heat from core makes outer layers expand – Any remaining hydrogen is burned up. Life Cycle of an Average Star • Collapse – When all Hydrogen is used up, the star collapses on itself. – Resulting in a WHITE DWARF. Life Cycle of a Massive Star • Death – When Hydrogen is used up, the core collapses on itself causing temp to rise. – Heat from core makes outer layers expand – Any remaining hydrogen is burned up. Life Cycle of a Massive Star • Supernova – When the supergiant starts to collapse, there is a huge & sudden explosion. – Everything except the core is blown outward into space. • This exploded material is used to start new stars & solar systems. Life Cycle of a Massive Star • Collapse – Remaining star’s core becomes a dense, invisible pulsar sending out pulsing radio waves. – When pulsar stops pulsing, it becomes a neutron star. – Neutron star eventually collapses further, becoming a black hole. Exploring the Universe Part III Galaxies The Milky Way and Other Galaxies Galaxies • Galaxies contain millions or billions of stars. • Galaxy a collection of stars, dust, and gas bound together by gravity • Because stars age at different rates, a galaxy may contain many types of stars. The Milky Way and Other Galaxies Galaxies, continued • Gravity holds galaxies together in clusters. • Galaxies are not spread evenly throughout space. The Milky Way and Other Galaxies Galaxies, continued • Cluster a group of stars or galaxies bound by gravity • The Milky Way galaxy and the Andromeda galaxy are two of the largest members of the Local Group, a cluster of more than 30 galaxies. • Clusters of galaxies can form even larger groups, called superclusters. The Milky Way and Other Galaxies Types of Galaxies • We live in the Milky Way galaxy. • Edwin Hubble divided all galaxies into three major types: spiral, elliptical, and irregular. • Most of the objects visible in the night sky are part of the Milky Way galaxy. • Scientists use astronomical data to piece together a picture of the Milky Way galaxy. The Milky Way and Other Galaxies Galaxies, • Types The MilkyofWay is a spiral continued galaxy. • Our galaxy is a huge spiraling disk of stars, gas, and dust. • Our solar system is located within a spiral arm. • The nucleus of the galaxy is dense and has many old stars, while the arms have younger stars. • Interstellar matter the gas and dust located between the stars in a galaxy. Spiral Galaxies The Milky Way and Other Galaxies Types of Galaxies, continued • Eliptical galaxies have no spiral arms. • Elliptical galaxies are spherical or egg shaped. • They contain mostly older stars and have little interstellar matter. • Because older stars are red, elliptical galaxies often have a reddish color. Elliptical Galaxy The Milky Way and Other Galaxies Types of Galaxies, continued • All other galaxies are irregular galaxies. • Irregular galaxies lack regular shapes and do not have a well-defined structure. • Some irregular galaxies may be oddly shaped because the gravitational pull of nearby galaxies distorts their spiral arms. • Contain mostly YOUNG STARS Irregular Galaxy How do Galaxies FORM? • Quasars may be infant galaxies. • In 1960, a faint object was matched with a strong radio signal. This object was called a quasar. • quasar quasi-stellar radio sources; very luminous objects that produce energy at a high rate and that are thought to be the most distant objects in the universe • Each quasar has a huge central black hole and a large disk of gas and dust around it. How do Galaxies FORM? • Galaxies change over time. • Galaxies change as they use up their stores of gas and dust • Galaxies also change as a result of collisions. • As galaxies approach each other, mutual gravitational attraction changes their shape. • Collisions of gas and dust may cause new stars to begin forming. Exploring the Universe Part IV Origins of the Universe Origins of the Universe • Universe the sum of all space, matter, and energy that exist, that have existed in the past, and that will exist in the future. • You are part of the universe, as is Earth and everything on it. Origins of the Universe • We see the universe now as it was in the past. • It takes time for light to travel in space. • The farther away an object is, the older the light that we receive from that object. • Most of the universe is empty space Origins of the Universe • The universe is expanding. • Observations of spectral lines from other galaxies indicated that they were moving away from us Origins of the Universe • Red shift a shift toward the red end of the light spectrum caused when objects move away from the observer • Blue shift a shift toward the blue end of the light spectrum caused when objects move toward the observer Birth of the Universe • Expansion implies that the universe was once smaller. • All of the matter in the universe appears to expand rapidly outward, like a gigantic explosion • Scientists call this hypothetical explosion the big bang. Birth of the Universe • Cosmic background radiation supports the big bang theory. • Cosmic background radiation is a signal from all over the sky in the form of microwave radiation. • Many scientists believe that the microwaves are dim remains of the radiation produced during the big bang. Birth of the Universe • Processes in stars lead to bigger atoms. • Once hydrogen atoms formed, stars and galaxies began to form, too. • All elements other than hydrogen and helium form in stars. The FUTURE of the Universe • The future of the universe is uncertain. • The universe is expanding, • but the gravity of all the mass in the universe is also pulling the universe inward. • So what’s going to happen? The FUTURE of the Universe • The competition between these forces leaves three possibilities: • The universe will keep expanding forever • The expansion of the universe will gradually slow down, and the universe will stay the same size. • The universe will stop expanding and start to fall back in on itself The FUTURE of the Universe • The fate of the universe depends on mass. • If there is not enough mass, the gravitational pull will be too small to stop the expansion. • The right amount of mass, the expansion will continually slow down, but will never stop completely. • Too much mass, gravity will eventually overcome expansion and the universe will contract. The FUTURE of the Universe • There is a debate about dark matter. • There is more matter in the universe than what is visible. • Scientists call this dark matter. • Dark matter may be: • Planets • black holes • brown dwarfs (starlike objects that lack enough mass to begin fusion.)