1

Stars, Galaxies, and the Universe

Chapter 30

Earth and Space Science

2

Analyzing Starlight

• Nuclear fusion is the combination of light atomic nuclei to form heavier atomic nuclei

• Astronomers learn about stars by analyzing the light that the stars emit.

• Starlight passing through a spectrograph produces a display of colors and lines called a spectrum.

3

Analyzing Starlight• All stars have dark-line spectra.

• A star’s dark-line spectrum reveals the star’s composition and temperature.

• Stars are made up of different elements in the form of gases.

• Scientists can determine the elements that make up a star by studying its spectrum.

4

The Compositions of Stars• Scientists have learned that stars are

made up of the same elements that compose Earth.

• The most common element in stars is hydrogen.

• Helium is the second most common element in star.

• Small quantities of carbon, oxygen, and nitrogen are also found in stars.

5

The Temperatures of Stars• The temperature of most stars ranges from

2,800˚C to 24,000˚C.

• Blue stars have average surface temperatures of 35,000˚C.

• Yellow stars, such as the sun, have surface temperatures of between 5,000˚C and 6,000˚C.

• Red stars have average surface temperatures of 3,000˚C.

6

The Sizes and Masses of Stars

• Stars vary in size and mass.

• Stars such as the sun are considered medium-sized stars.

• Most stars visible from Earth are medium-sized stars.

7

Stellar Motion

• Two kinds of motion– Actual Motion – Apparent Motion

8

Apparent Motion of Stars• The apparent motion of stars is the

motion visible to the unaided eye. • Apparent motion is caused by the

movement of Earth. • The rotation of Earth causes the

apparent motion of stars sees as though the stars are moving counter-clockwise around the North Star.

• Earth’s revolution around the sun causes the stars to appear to shift slightly to the west every night.

10

Circumpolar Stars• Some stars are always visible in the

night sky. These stars never pass below the horizon.

• In the Northern Hemisphere, the movement of these stars makes them appear to circle the North Star.

• These circling stars are called circumpolar

11

Circumpolar Stars

• The stars of the little dipper are circumpolar for most observers in the Northern Hemisphere.

• At the pole all visible stars are circumpolar.

• As you move off the pole fewer and fewer circumpolar stars exist.

12

Actual Motion of Stars• Most stars have several types of actual

motion.

• Stars rotate on an axis.

• Some stars may revolve around another star.

• Stars either move away from or toward our solar system.

13

Actual Motion of Stars• The spectrum of a star that is moving

toward or away from Earth appears to shift, due to the Doppler effect.

• 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.

14

Actual Motion of StarsDoppler Effect• The spectrum of a star that is moving toward

or away from Earth appears to shift, as shown in the diagram below.

15

Distances to Stars

• Distances between the stars and Earth are measured in light-years.

• light-year the distance that light travels in one year. – about 9.5 trillion

kilometers (5.8 trillion miles).

16

Distance to Stars How big is the universe?

• Proxima Centauri is about 4.3 light-years from the earth.– The light produced by Proxima Centauri

takes about 4.3 years to reach earth.– Light from the sun reaches the earth in

about 8 minutes.

• This fact suggests that the universe is incomprehensibly large.

17

Measuring Distances to the Stars

• Stellar parallax, the extremely slight back-and-forth shifting in a nearby star's position due to the orbital motion of Earth.

– The farther away a star is, the less its parallax.

– Parallax angles are very small.

18

Stellar parallax

20

21

Stellar Brightness

• Three factors control the brightness of a star as seen from Earth: – size (how big), – temperature (how hot), – distance from Earth (how far

away).

22

Stellar Brightness

• Magnitude is the measure of a star's brightness.

• Apparent magnitude is how bright a star appears when viewed from Earth.

• Absolute magnitude is the "true" brightness if a star were at a standard distance of about 32.6 light-years.

• The difference between the two magnitudes is directly related to a star's distance.

23

Apparent magnitude• The lower the number of the star on the

scale shown on the diagram below, the brighter the star appears to observers.

• The sun has an apparent magnitude of –26.8• All other objects are dimmer.

24

End of Section 1

• Answer Questions 1-6 on page 780.

25

Classifying Stars• One way scientists classify stars is by plotting

the surface temperatures of stars against their luminosity.

• The H-R diagram is the graph that illustrates the resulting pattern.

• Astronomers use the H-R diagram to describe the life cycles of stars.

• Most stars fall within a band that runs diagonally through the middle of the H-R diagram.

• These stars are main sequence stars.

26

H-R Diagram - History•A useful astronomical tool which plots stellar temperature (color) against luminosity.

•Independently invented by Henry Russell in 1913 & Ejnar Hertzsprung in 1905 through the study of true brightness and temperature of stars.

•Useful for studying properties & life cycles of stars:• Mass, Luminosity, Surface Temperature, Age

27

H-R Diagram

28

Don’t bother copying…

• Stellar temperature/color also gives rise to “Spectral Classes.”– O (> 30,000 K).– B (10,000 – 30,000 K).– A (7,000 – 10,000 K).– F (6,000 – 7,000 K).– G (5,000 – 6,000 K) – the sun!– K (4,000 – 5,000 K).– M (< 4,000 K).

29

The Hertzsprung-Russell diagram

30

H-R Diagram cont.• Stars located in the upper-right position of an H-R

diagram are called giants, luminous stars of large radius.

• Supergiants are very large. • Very small white dwarf stars are located in the

lower-central portion of an H-R diagram. • Ninety percent of all stars, called main-sequence

stars, are in a band that runs from the upper-left corner to the lower-right corner of an H-R diagram.

31

H-R Diagram

32

Points of Note• Stars spend 90% of their lives on Main Sequence

• Main Sequence stars are burning only Hydrogen

• High mass stars live fast, die young:

• 20 Solar Mass Star - 10 Million Years

• Sun - 10 Billion Years

• Red Dwarf - >100 Billion Years

33

Differences Between High Mass and Low Mass Stars• Stars that are more massive than the Sun

have stronger gravitational forces.• These forces need to be balanced by higher

internal pressures.• These higher pressures result in higher

temperatures which drive a higher rate of fusion reactions.

• The Hydrogen within the core of a high mass star therefore gets used up much faster than in the Sun and “ages” faster.

• Low mass stars “age” slower.

34

Star Formation• A star brings in a nebula.

• As gravity pulls particles of the nebula closer together, the gravitational pull of the particles on each other increases.

• As more particles come together, regions of dense matter begin to build up within the cloud.

35

Nebula• New stars are born out of enormous

accumulations of dust and gases, called nebula, that are scattered between existing stars. Nebula comes from the Latin for “cloud”.

The Orion Star Forming Complex

36

Interstellar Matter

37

Dark Nebula

• When a nebula is not close enough to a bright star to be illuminated, it is referred to as a dark nebula.

• Horsehead Nebula is a dark nebula.

38

• A bright nebula glows because the matter is close to a very hot (blue) star.

• Emission nebulae: derive their visible light from the fluorescence of the ultraviolet light from a star in or near the nebula.

Bright Nebula

39

Bright Nebulae

• Reflection nebulae: relatively dense dust clouds in interstellar space that are illuminated by reflecting the light of nearby stars.

40

Stellar Lifecycles• The process by which

stars are formed and use up their fuel.

• What exactly happens to a star as it uses up its fuel is strongly dependent on the star’s mass.

The Orion Nebula - Birthplace of stars

41

Protostars• Gravity within a nebula compacts it

to form a flattened disk.The disk has a central concentration of matter called a protostar.

• The protostar continues to contract and increase in temperature for several million years and becomes plasma.

42

The Birth of a Star• A protostar’s temperature continually

increases until it reaches about 10,000,000°C.

• At this temperature, nuclear fusion begins.

• The process releases enormous amounts of energy.

• The onset of nuclear fusion marks the birth of a star. Once this process begins, it can continue for billions of years.

43

A Delicate Balancing Act

• As gravity increases the pressure on the matter within the star, the rate of fusion increase.

• In turn, the energy radiated from fusion reactions heats the gas inside the star.

• The outward pressures of the radiation and the hot gas resist the inward pull of gravity.

• This equilibrium makes the star stable in size.

44

The Main-Sequence Stage• Energy continues to be generated in the

core of the star as hydrogen fuses into helium.

• A star that has a mass about the same as the sun’s mass stays on the main sequence for about 10 billion years.

• Scientists estimate that over a period of almost 5 billion years, the sun has converted only 5% of its original hydrogen nuclei into helium nuclei.

45

Leaving the Main Sequence• When almost all of the hydrogen

atoms within its core have fused into helium atoms the core of the star contracts because of gravity.

• As the temperature rises the last of the hydrogen atoms fuse and send energy into the outer shell.

46

Giant Stars• A star enters its third stage when

almost all of the hydrogen atoms within its core have fused into helium atoms.

• A star’s shell of gases grows cooler as it expands. As the gases in the outer shell become cooler, they begin to glow with a reddish color. These stars are known as giants.

47

Supergiants• 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.

• These stars appear along the top of the H-R diagram.

• Despite the high luminosity these stars are relatively cool.

48

The Final Stages of a Sunlike Star

• When all the helium has been used up, the fusion will stop.

• With no energy available the star will enter its last stages.

49

Planetary Nebulas

• As the star’s outer gases drift away, the remaining core heats these expanding gases.

• The gases appear as a planetary nebula, a cloud of gas that forms around a sunlike star that is dying.

50

The Sun’s Planetary Nebula• When it runs out

of Helium fuel it begins to contract and heat up.

• The Sun increases its luminosity.

• The outer layers of the Sun expand, cool and redden again.

• The outer layers of the Sun start streaming away from the core.

• This material forms a nebula surrounding the Sun.

51

White Dwarfs• As a planetary nebula disperses,

gravity causes the remaining matter in the star to collapse inward.

• A hot, extremely dense core of matter - a white dwarf - is left.

• White dwarfs shine for billions of years before they cool completely.

52

Novas and Super novas

• When a star explosively brightens, it is called a nova (new star). Excessively large explosions are called supernovas.

• During the outburst, the outer layer of the star is ejected at high speed.

• After reaching maximum brightness in a few days, the nova slowly returns in a year or so to its original brightness.

53

Novas and Supernovas

• Some white dwarfs revolve around red giants. When this happened, the gravity of the white dwarf may capture gases from the red giant.

• As these gases accumulate on the surface of the white dwarf, pressure begins to build up.

• This pressure may cause large explosions. These explosions are called novas.

54

Supernova

• Stars more than three times the mass of the Sun terminate in a brilliant explosion called a supernova.

55

The Final Stages of Massive Stars• The result of a star that exploded in 1054 AD.

• This spectacular supernova explosion was recorded by Chinese and (quite probably) Anasazi Indian astronomers.

The Crab Nebula

56

Supernovas in Massive Stars• Massive stars become supernovas as part

of their life cycle.

• After the supergiant stage, the star collapses, producing such high temperatures that nuclear fusion begins again.

• When nuclear fusion stops, the star’s core begins to collapse under its own gravity. This causes the outer layers to explode outward with tremendous force.

57

Neutron Stars• Stars more massive than the sun do not

become white dwarfs.

• After a star explodes as a supernova, the core may contract into a neutron star.

• A star that has collapsed under gravity to the point that the electrons and protons have smashed together to form neutrons

58

Pulsars• Variable stars fluctuate in brightness.

• Some neutron stars emit a beam of radio waves that sweeps across space and are detectable here on Earth.

• These stars are called pulsars. For each pulse detected on Earth, we know that the star has rotated within that period.

59

Black Holes

Supernovae events can produce small, extremely dense (A pea-sized sample of matter would weigh 100 million tons) neutron stars, composed entirely of subatomic particles called neutrons; or even smaller and more dense black holes, objects that have such immense gravity that light cannot escape their surface.

60

Section 3: Star Groups• We can only see some of the trillions of

stars that make up the universe.

• Most of the ones we see are within 100 light-years of Earth.

• In the constellation Andromeda there is a hazy region that is actually a collection of stars that are 2 million light-years from Earth.

61

Dividing Up the Sky

• In 1930, astronomers around the world agreed upon a standard set of 88 constellations which the sky has been divided in order to describe the locations of celestial objects.

• You can use a map of the constellations to locate a particular star.

62

Naming Constellations

• Many of the modern names we use for the constellations come from Latin.

• Some constellations are named for real or imaginary animals, such as Ursa Major (the great bear) or ancient gods or legendary heroes, such as Hercules or Orion.

63

The Constellation Orion

64

Multiple-Star Systems• Over half of all observed stars form

multiple-star systems.

• Binary stars are pairs of stars that revolve around each other and are held together by gravity.

• In star systems that have more than two stars, two stars may revolve rapidly, while a third star revolves more slowly at a greater distance from the pair.

65

Spot Question

• What percentage of stars are in multiple-star systems?

• More than 50% of all stars are in multiple-star systems.

66

Star Clusters

• Sometimes, nebulas collapse to form groups of hundreds or thousands of stars called clusters.

• Globular clusters have a spherical shape and can contain up to 100,000 stars.

• An open cluster is loosely shaped and rarely contains more than a few hundred stars.

67

Galaxies• Galaxies are the major building

blocks of the universe. Astronomers estimate that the universe contains hundreds of billions of galaxies.

• A typical galaxy, such as the Milky Way, has a diameter of about 100,000 light-years and may contain more than 200 billion stars.

68

Types of Galaxies• Galaxies are classified by shape into three main

types.

• A spiral galaxy has a nucleus of bright stars and flattened arms that spiral around the nucleus.

• Elliptical galaxies have various shapes and are extremely bright in the center and do not have spiral arms.

• An irregular galaxy has no particular shape, and is fairly rich in dust and gas.

69

Galaxy Types

• Spiral galaxies are typically disk-shaped with a somewhat greater concentration of stars near their centers, often containing arms of stars extending from their central nucleus. (30% of all galaxies)

70

Galaxy Types

• Elliptical galaxies are the most abundant type, 60% of all galaxies, which have an ellipsoidal shape that ranges to nearly spherical, and lack spiral arms.

71

Galaxy Types

• Irregular galaxies, which lack symmetry and account for only 10% of the known galaxies.

72

Our Galaxy

• The Milky Way Galaxy is a large, disk-shaped, spiral galaxy about 100,000 light-years wide and about 10,000 light-years thick at the center.

• There are three distinct spiral arms of stars, with some showing splintering.

• The Sun is positioned in one of these arms about two-thirds of the way from the galactic center, at a distance of about 30,000 light-years.

73

The Milky Way Galaxy

74

Quasars

• Quasars appear as points of light, similar to stars.

• Quasars are located in the centers of galaxies that are distant from Earth.

• Quasars are among the most distant objects that have been observed from Earth.

75

Section 4: The Big Bang Theory

• The study of the origin, structure, and future of the universe is called cosmology.

• There are many scientific theories and un scientific theories to the origin and evolution of the universe.

76

Hubble’s Observations

• 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.

77

The Doppler Effect• By applying the Doppler Effect (the apparent

change in wavelength of radiation caused by the motions of the source and the observer) to the light of galaxies, galactic motion can be determined.

• Large Doppler shift indicates a high velocity• Small Doppler shift indicates a lower velocity• It was soon realized that an expanding

universe can adequately account for the observed red shifts.

78

Doppler cont.

• Most galaxies have Doppler shifts toward the red end of the spectrum, indicating increasing distance.

• The amount of Doppler shift is dependent on the velocity at which the object is moving.

79

Doppler cont.

• Because the most distant galaxies have the greatest red shifts, Edwin Hubble concluded in the early 1900s that they were retreating from us with greater recessional velocities than more nearby galaxies.

80

The Expanding Universe

The “Raisin Bread” Theory of an Expanding Universe

81

The “Big Bang” Theory

• The belief in the expanding universe led to the widely accepted Big Bang Theory of the origin of the universe.

• According to this theory, the entire universe was at one time confined in a dense, hot, super massive concentration.

• About 20 billion years ago, a cataclysmic explosion hurled this material in all directions, creating all matter and space.

• Eventually the ejected masses of gas cooled and condensed, forming the stellar systems we now observe fleeing from their place of origin.

82

Cosmic Background Radiation

• 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.

83

Ripples in Space• Maps of cosmic background

radiation over the whole sky show ripples.

• These ripples are irregularities caused by small fluctuations in the distribution of matter in the early universe, and may indicate the first stages in the formation of the universe’s first galaxies.

84

A Universe of SurprisesDark Matter

• Analysis of the ripples in the cosmic background radiation shows that the matter that humans, the planets, the stars and the matter between the stars makes up only 4% of the universe.

• 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.

85

A Universe of SurprisesDark Energy• 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.

• Many scientists think that some form of undetectable dark energy is pushing galaxies apart.

86

The End