Beyond Our Solar System Chapter 24

Transcription

1 Beyond Our Solar System Chapter 24

2 PROPERTIES OF STARS Distance Measuring a star's distance can be very difficult Stellar parallax Used for measuring distance to a star Apparent shift in a star's position due to the orbital motion of Earth Measured as an angle Near stars have the largest parallax Largest parallax is less than one second of arc

3 STELLAR PARALLAX d = 1/p Parallax angle =.77 arc sec d = 1/.77 = 1.3 pc Parsec (pc) = 3.26 ly

4 PROPERTIES OF STARS 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 km (5.87 trillion miles)

5 PROPERTIES OF STARS Stellar brightness Controlled by three factors Size Temperature Distance

6 PROPERTIES OF STARS Magnitude Measure of a star s brightness 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 (the more negative the number the brighter the object is)

7 APPARENT MAGNITUDE Sun Bright - 11 Full Moon Venus (at its brightest) Sirius (brightest star) + 6 Limit of naked eye + 10 Binocular limit + 15 Pluto Dim + 21 Limit of large telescope with naked eye + 30 Limit of large telescope using photography

8 PROPERTIES OF STARS Absolute magnitude "True" or intrinsic brightness of a star Brightness at a standard distance of 32.6 light-years (10 parsecs) Most stars' absolute magnitudes are between -5 and +15 Parsec = 3.26 ly

9 PROPERTIES OF STARS Color and temperature Hot star Temperature above 30,000 K (53,540 F) Emits short-wavelength light Appears blue Cool star Temperature less than 3000 K (4,940.3 F) Emits longer-wavelength light Appears red

10 PROPERTIES OF STARS Between 5000 and 6000 K (8,540.3 F 10,340 F) Stars appear yellow but it s really white, our atm. causes this e.g., Sun Binary stars and stellar mass Binary stars Two stars orbiting one another Stars are held together by mutual gravitation Both orbit around a common center of mass

11 PROPERTIES OF STARS Binary stars and stellar mass Binary stars Two stars orbiting one another Stars are held together by mutual gravitation Both orbit around a common center of mass Visual binaries are resolved telescopically More than 50% of the stars in the universe are binary stars Used to determine stellar mass This means that the Sun is not like most stars we don t have a partner

12 BINARY STARS ORBIT EACH OTHER AROUND THEIR COMMON CENTER OF MASS Figure 24.4

13 PROPERTIES OF STARS Stellar mass Determined using binary stars the center of mass is closest to the most massive star Mass of most stars is between one-tenth and fifty times the mass of the Sun The Sun is just average when it comes to mass

14 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 Temperature

15 HERTZSPRUNG-RUSSELL DIAGRAM Parts of an H-R diagram Main-sequence stars 90% 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 Very large giants are called supergiants Only a few percent of all stars

16 HERTZSPRUNG-RUSSELL DIAGRAM 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% of all stars

17 IDEALIZED HERTZSPRUNG- RUSSELL DIAGRAM Figure 24.5

18 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

19 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 is close to a very hot star Two types of bright nebulae Emission nebula Reflection nebula

20 THE ORION NEBULA IS A WELL- KNOWN EMISSION NEBULA Figure 24.8

21 A FAINT BLUE REFLECTION NEBULA IN THE PLEIADES STAR CLUSTER Figure 24.9

22 INTERSTELLAR MATTER Dark nebula Not close to any bright star Appear dark Contains the material that forms stars and planets

23 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 the cloud Temperature rises Radiates long-wavelength (red) light Becomes a protostar

24 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

25 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% of a star's life is in the main-sequence

26 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

27 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

28 EVOLUTIONARY STAGES OF LOW MASS STARS Figure A

29 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

30 EVOLUTIONARY STAGES OF MEDIUM MASS STARS Figure B

31 H-R DIAGRAM SHOWING STELLAR EVOLUTION Figure 24.11

32 STELLAR EVOLUTION Stages Burnout and death Final stage depends on mass Possibilities Massive star Over 3 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

33 EVOLUTIONARY STAGES OF MASSIVE STARS Figure C

34 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

35 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

36 STELLAR REMNANTS Neutron star Pea size sample 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 supernova)

37 CRAB NEBULA IN THE CONSTELLATION TAURUS Figure 24.14

38 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

39 GALAXIES Milky Way galaxy Structure Determined by using radio telescopes Large spiral galaxy About 100,000 light-years wide Thickness at the galactic nucleus is about 10,000 light-years Three spiral arms of stars Sun is 30,000 light-years from the center

40 FACE-ON VIEW OF THE MILK WAY GALAXY Figure A

41 EDGE-ON VIEW OF THE MILK WAY GALAXY Figure B

42 GALAXIES Milky Way galaxy Rotation Around the galactic nucleus Outermost stars move the slowest Sun rotates around the galactic nucleus once about every 200 million years Halo surrounds the galactic disk Spherical Very tenuous gas Numerous globular clusters

43 GALAXIES Other galaxies Existence was first proposed in mid-1700s by Immanuel Kant Four basic types of galaxies Spiral galaxy Arms extending from nucleus About 30% of all galaxies Large diameter of 20,000 to 125,000 light years Contains both young and old stars e.g., Milky Way

44 GREAT GALAXY, A SPIRAL GALAXY, IN THE CONSTELLATION ANDROMEDA Figure 24.20

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

46 A BARRED SPIRAL GALAXY Figure 24.22

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

48 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

49 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

50 RED SHIFTS Doppler effect 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

51 RAISIN BREAD ANALOGY OF AN EXPANDING UNIVERSE Figure 24.24

52 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

53 BIG BANG THEORY Accounts for galaxies moving away from us Universe was once confined to a "ball" that was Supermassive Dense Hot

54 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 sill stop and gravitational; contraction will follow

55 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

56 END OF CHAPTER 24