Chapter 30

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Star - a large celestial body that is composed of gas & emits light.  Astronomers learn about stars by analyzing the light that the stars emit.

 Made up of different elements in the form of gases.  different elements absorb different wavelengths of light  scientists can determine the elements that make up a star by studying its spectrum  Starlight passing through a spectrograph produces a display of colors and lines called a spectrum.

 All stars have dark- line spectra  bands of color crossed by dark lines  reveals the star’s composition & temperature

 Stars are made up of the same elements that compose Earth.  Most common element in stars is hydrogen.  Helium is the second most common element  carbon, oxygen, & nitrogen are also found in stars

 The surface temperature of a star is indicated by its color.  Blue stars have average temp. = 35,000 ˚C  Yellow stars (the sun) have average temp. = 5,500 ˚C  Red stars have average temp. = 3,000 ˚C

Stars vary in size and mass:  Stars such as the sun are considered medium- sized stars. The sun has a diameter of 1,390,000 km.  Most of the stars you can see in the night sky are medium-sized stars.

 The apparent motion of stars is the motion as it appears from Earth  caused by the movement of Earth  The stars seem as though they are moving counter-clockwise around a central star called Polaris, the North Star.

 Most stars have several types of actual motion. 1. Move across the sky (seen only for close stars) 2. Revolve around another star 3. Move away from or toward our solar system

 Doppler effect - an observed change in the frequency of a wave when the source or observer is moving  Causes the spectrum of a star that is moving toward or away from Earth to shift  Stars moving toward Earth are shifted slightly toward blue, which is called blue shift.  Stars moving away from Earth are shifted slightly toward red, which is called red shift.

Doppler Effect

 Distances between the stars and Earth are measured in light-years. Light-year - the distance that light travels in one year. The speed of light is 300,000 km/s Light travels about 9.46 trillion km in one year

Apparent magnitude - the brightness of a star as seen from the Earth  depends on how much light the star emits & how far away it is Absolute magnitude - the brightness that a star would have at a distance of 32.6 light-years from Earth  The brighter a star is, the lower the number of its absolute magnitude.

The lower the number of the star on the scale shown on the diagram below, the brighter the star appears to observers.

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 Scientists classify stars is by plotting the surface temperatures of stars against their luminosity. Luminosity - total amount of energy they give off each second  The Hertzsprung-Russell diagram ( H-R diagram) is a graph that illustrates the luminosity pattern. Main sequence - location on the H-R diagram where most stars lie.

 A star's life cycle is determined by its mass.  larger its mass = shorter its life cycle  A star’s life begins in a nebula. Nebula - large cloud of gas and dust in interstellar space  compressed, some of the particles move close together, pulled by gravity  It begins to spin

 As the gas spins in the nebula, it heats up & becomes as a flatten disk.  The disk has a central concentration of matter called a protostar.  continues to contract & increase in temperature for several million years.

 Temperature reaches 10,000,000 0 degrees  Nuclear fusion occurs in the cloud's core A STAR IS BORN!!!!!  It is now a main sequence star & will remain in this stage for millions to billions of years. ~Our Sun is in this stage

 Main-sequence stage  Second stage & longest (10 billion years)!  energy continues to be generated as hydrogen fuses into helium  Once 20% of the hydrogen atoms have fused into helium, the star moves to the third stage of life.

 As the star’s shell gets bigger, it cools  They begin to glow with a reddish color.  They are called giants Giant - very large and bright star whose hot core has used most of its hydrogen.

 Main-sequence stars that are more massive than the sun will become larger than giants in their third stage.  These highly luminous stars are called supergiants.

 Fusion in the core will stop after the helium atoms have fused into carbon & oxygen.  Star’s outer gases drift away as they are heated.  The gases appear as a planetary nebula  a cloud of gas that forms around a sun- like star that is dying.

 The planetary nebula disperses (spreads out)  Gravity causes the left over matter to collapse inward  A hot, extremely dense core of matter is left. White dwarf - small, hot, dim star that is the leftover center of an old sun- like star  shine for billions of years before they cool completely into a black dwarf

 If a white dwarf star revolves around a red giant, the white dwarf may capture gases from the red giant.  As these gases accumulate & pressure begins to build up.  may cause large explosions, called a nova. Nova - star that suddenly becomes brighter

 A white dwarf may also become a supernova. Supernova - star that has such a tremendous explosion that it blows itself apart.  thousand times more violent than novas  completely destroys the white dwarf star & much of the red giant

After the supergiant stage, the star collapses  producing high temperatures  nuclear fusion begins again  This time, carbon atoms in the core fuse into heavier elements until the core is almost entirely made of iron.  The star’s core begins to collapse under its own gravity.  The outer layers to explode outward with tremendous force.

 After a star explodes as a supernova, the core may contract into a neutron star. Neutron star - star that has collapsed under gravity to the point that the electrons and protons have smashed together to form neutrons

 Some neutron stars emit a beam of radio waves Pulsar - rapidly spinning neutron star that emits pulses of radio and optical energy.  Some massive stars produce leftovers too massive to become a stable neutron star.  These stars contract, & the force of the contraction leaves a black hole. Black hole - an object so massive and dense that even light cannot escape its gravity

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Constellation - a group of stars organized in a recognizable pattern  In 1930, astronomers around the world agreed upon a standard set of 88 constellations.  You can use a map of the constellations to locate a particular star.

 When two or more stars are close together, they form multiple-star systems.  Binary stars - pairs of stars that revolve around each other & are held together by gravity.

 Sometimes, nebulas collapse to form groups of hundreds or thousands of stars called clusters. 2 types: 1. Globular clusters have a spherical shape and can contain up to one million stars. 2. Open cluster is loosely shaped and rarely contains more than a few hundred stars.

Galaxy - collection of stars, dust, and gas bound together by gravity  the major building blocks of the universe.  May have a diameter of 100,000 light- years  May contain more than 200 billion stars.  Astronomers estimate that the universe contains hundreds of billions of galaxies.

Galaxies are classified by shape: 1. spiral galaxy - has a nucleus of bright stars & flattened arms that spiral around the nucleus. 2. elliptical galaxies - vary in shape, spherical to elongated, extremely bright in the center, & do not have spiral arms. 3. irregular galaxy - no particular shape, and is fairly rich in dust & gas.

Elliptical Spiral Irregular

The Milky Way Galaxy:  spiral galaxy with hundreds of billions of stars (including our sun)  It takes the sun 225 million years to complete one orbit around the galaxy.  The Large Magellanic Cloud & Small Magellanic Cloud, are our closest neighbors.  Within 5 million light-years of the Milky Way are about 30 other galaxies.

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Cosmology - the study of the origin, properties, processes, and evolution of the universe  Cosmologists and astronomers can use the light given off by an entire galaxy to create the spectrum for that galaxy.  Edwin Hubble used galactic spectra to uncover new information about our universe.

 Found that the spectra of galaxies were shifted toward the red end of the spectrum  Determined the speed at which the galaxies were moving away from Earth  Using Hubble’s observations, astronomers have been able to determine that the universe is expanding.

Cosmologists have proposed several different theories to explain the expansion of the universe. Big bang theory - the theory that all matter & energy in the universe was compressed into an extremely small volume that 3 to 15 billion years ago exploded and began expanding in all directions  By the mid-20th century, almost all astronomers and cosmologists accepted the big bang theory.

Cosmic background radiation - radiation uniformly detected from every direction in space; considered a remnant of the big bang  Astronomers believe that cosmic background radiation formed shortly after the big bang.  The background radiation has cooled after the big bang, and is now about 270 °C below zero.

Timeline of the Big Bang

 About 23% of the universe is made up of a type of matter that does not give off light but that has gravity.  This type of matter is called dark matter.  Most of the universe is made up of an unknown material called dark energy.  Scientists think that dark energy acts as a force that opposes gravity & that some form of undetectable dark energy is pushing galaxies apart.