Earth Science, 13e Tarbuck & Lutgens

Beyond Our Solar System Earth Science, 13e Chapter 24 Stanley C. Hatfield Southwestern Illinois College

Properties of stars Distance Distances to the stars are very large Units of measurement Kilometers or astronomical units are too cumbersome to use Light-year is used most often Distance that light travels in 1 year One light-year is 9.5 trillion kilometers (5.8 trillion miles) Other methods for measuring distance are also used

Properties of stars Stellar brightness Controlled by three factors Size Temperature Distance Magnitude Measure of a star’s brightness

Properties of stars Stellar brightness Magnitude Two types of measurement Apparent magnitude Brightness when a star is viewed from Earth Decreases with distance Numbers are used to designate magnitudes – dim stars have large numbers and negative numbers are also used

Properties of stars Stellar brightness Magnitude Two types of measurement Absolute magnitude “True” or intrinsic brightness of a star Brightness at a standard distance of 32.6 light-years Most stars’ absolute magnitudes are between –5 and +15

Interstellar matter Between the stars is “the vacuum of space” Nebula Cloud of dust and gases Two major types of nebulae Bright nebula Glows if it close to a very hot star Two types of bright nebulae Emission nebula Reflection nebula

A faint blue reflection nebula in the Pleiades star cluster

Interstellar matter Nebula Two major types of nebulae Dark nebula Not close to any bright star Appear dark Contains the material that forms stars and planets

Hertzsprung-Russell diagram Shows the relation between stellar Brightness (absolute magnitude) and Temperature Diagram is made by plotting (graphing) each star’s Luminosity (brightness) and

Hertzsprung-Russell diagram Parts of an H-R diagram Main-sequence stars 90 percent of all stars Band through the center of the H-R diagram Sun is in the main-sequence Giants (or red giants) Very luminous Large Upper-right on the H-R diagram

Hertzsprung-Russell diagram Parts of an H-R diagram Giants (or red giants) Very large giants are called supergiants Only a few percent of all stars White dwarfs Fainter than main-sequence stars Small (approximate the size of Earth) Lower-central area on the H-R diagram Not all are white in color Perhaps 10 percent of all stars

Idealized Hertzsprung-Russell diagram

Variable stars Stars that fluctuate in brightness Types of variable stars Pulsating variables Fluctuate regularly in brightness Expand and contract in size Eruptive variables Explosive event Sudden brightening Called a nova

Stellar evolution Stars exist because of gravity Two opposing forces in a star are Gravity – contracts Thermal nuclear energy – expands Stages Birth In dark, cool, interstellar clouds Gravity contracts cloud and temperature rises Radiates long-wavelength (red) light Becomes a protostar

Stellar evolution Stages Protostar Gravitational contraction of gaseous cloud continues Core reaches 10 million K Hydrogen nuclei fuse Become helium nuclei Process is called hydrogen burning Energy is released Outward pressure increases Outward pressure balanced by gravity pulling in Star becomes a stable main-sequence star

Stellar evolution Stages Main-sequence stage Stars age at different rates Massive stars use fuel faster and exist for only a few million years Small stars use fuel slowly and exist for perhaps hundreds of billions of years 90 percent of a star’s life is in the main sequence

Stellar evolution Stages Red giant stage Hydrogen burning migrates outward Star’s outer envelope expands Surface cools Surface becomes red Core is collapsing as helium is converted to carbon Eventually all nuclear fuel is used Gravity squeezes the star

Stellar evolution Stages Burnout and death Final stage depends on mass Possibilities Low-mass star 0.5 solar mass Red giant collapses Becomes a white dwarf

Stellar evolution Stages Burnout and death Final stage depends on mass Possibilities Medium-mass star Between 0.5 and 3 solar masses Red giant collapses Planetary nebula forms Becomes a white dwarf

H-R diagram showing stellar evolution

Stellar evolution Stages Burnout and death Final stage depends on mass Possibilities Massive star Over three solar masses Short life span Terminates in a brilliant explosion called a supernova Interior condenses May produce a hot, dense object that is either a neutron star or a black hole

Stellar remnants White dwarf Small (some no larger than Earth) Dense Can be more massive than the Sun Spoonful weighs several tons Atoms take up less space Electrons displaced inward Called degenerate matter Hot surface Cools to become a black dwarf

Stellar remnants Neutron star Forms from a more massive star Star has more gravity Squeezes itself smaller Remnant of a supernova Gravitational force collapses atoms Electrons combine with protons to produce neutrons Small size

Stellar remnants Neutron star Pea size sample Strong magnetic field Weighs 100 million tons Same density as an atomic nucleus Strong magnetic field First one discovered in early 1970s Pulsar (pulsating radio source) Found in the Crab Nebula (remnant of an A.D. 1054 supernova)

Crab Nebula in the constellation Taurus

Stellar remnants Black hole More dense than a neutron star Intense surface gravity lets no light escape As matter is pulled into it Becomes very hot Emits X-rays Likely candidate is Cygnus X-1, a strong X-ray source

Galaxies Types of galaxies Existence was first proposed in mid-1700s by Immanuel Kant Four basic types of galaxies Spiral galaxy Arms extending from nucleus About 30 percent of all galaxies Large diameter up to 125,000 light years Contains both young and old stars e.g., Milky Way

Spiral Galaxy Messier 83

Galaxies Other galaxies Four basic types of galaxies Barred spiral galaxy Stars arranged in the shape of a bar Generally quite large About 10 percent of all galaxies Elliptical galaxy Ellipsoidal shape About 60 percent of all galaxies Most are smaller than spiral galaxies; however, they are also the largest known galaxies

A barred spiral galaxy

Galaxies Other galaxies Four basic types of galaxies Irregular galaxy Lacks symmetry About 10 percent of all galaxies Contains mostly young stars e.g., Magellanic Clouds

Galaxies Galactic cluster Group of galaxies Some contain thousands of galaxies Local Group Our own group of galaxies Contains at least 28 galaxies Supercluster Huge swarm of galaxies May be the largest entity in the universe

Red shifts Doppler effect Change in the wavelength of light emitted by an object due to its motion Movement away stretches the wavelength Longer wavelength Light appears redder Movement toward “squeezes” the wavelength Shorter wavelength Light shifted toward the blue

Red shifts Doppler effect Expanding universe Amount of the Doppler shift indicates the rate of movement Large Doppler shift indicates a high velocity Small Doppler shift indicates a lower velocity Expanding universe Most galaxies exhibit a red Doppler shift Moving away

Raisin bread analogy of an expanding universe

Red shifts Expanding universe Most galaxies exhibit a red Doppler shift Far galaxies Exhibit the greatest shift Greater velocity Discovered in 1929 by Edwin Hubble Hubble’s Law – the recessional speed of galaxies is proportional to their distance Accounts for red shifts

Big Bang theory Accounts for other galaxies moving away from us Universe was once confined to a “ball” that was Supermassive Dense Hot

Big Bang theory Big Bang marks the inception of the universe Occurred about 15 billion years ago All matter and space was created Matter is moving outward Fate of the universe Two possibilities Universe will last forever Outward expansion will stop and gravitational; contraction will follow

Big Bang theory Fate of the universe Final fate depends on the average density of the universe If the density is more than the critical density, then the universe would contract Current estimates point to less then the critical density and predict an ever-expanding, or open, universe

End of Chapter 24