StarsStarsChapter 30Chapter 30

Properties of the SunProperties of the Sun The Sun is the largest object in the The Sun is the largest object in the

solar system, in both size and mass. solar system, in both size and mass. – The Sun contains more than 99 percent The Sun contains more than 99 percent

of all the mass in the solar system, of all the mass in the solar system, which allows it to control the motions of which allows it to control the motions of the planets and other objects.the planets and other objects.

Properties of the SunProperties of the Sun The solar interior is gaseous throughout because The solar interior is gaseous throughout because

of its high temperature—about 1 of its high temperature—about 1 ×× 10 1077 K in K in the center. the center.

Many of the gases are in a plasma Many of the gases are in a plasma statestate

The outer layers of the Sun are not The outer layers of the Sun are not quite hot enough to be plasma.quite hot enough to be plasma.

The Sun’s AtmosphereThe Sun’s Atmosphere The The photospherephotosphere, approximately , approximately

400 km in thickness, is the lowest 400 km in thickness, is the lowest layer of the Sun’s atmosphere. layer of the Sun’s atmosphere. – This is the visible This is the visible

surface of the Sun surface of the Sun because most of the because most of the light emitted by the light emitted by the Sun comes from this Sun comes from this layer. layer.

The Sun’s AtmosphereThe Sun’s Atmosphere The The chromospherechromosphere is above the is above the

photosphere.photosphere.

The The coronacorona, which is the top layer , which is the top layer of the Sun’s atmosphere, extends of the Sun’s atmosphere, extends several million kilometers outward several million kilometers outward from the top of the chromosphere.from the top of the chromosphere.

The Sun’s AtmosphereThe Sun’s AtmosphereSolar WindSolar Wind

– Gas flows outward from the corona at Gas flows outward from the corona at high speeds and forms the solar wind high speeds and forms the solar wind which consists of charged particles, or which consists of charged particles, or ions, that flow outward through the ions, that flow outward through the entire solar system. entire solar system.

– The charged particles are trapped in two The charged particles are trapped in two huge rings in Earth’s magnetic field, huge rings in Earth’s magnetic field, called the Van Allen belts, where they called the Van Allen belts, where they collide with gases in Earth’s atmosphere, collide with gases in Earth’s atmosphere, causing an aurora.causing an aurora.

Solar ActivitySolar Activity The Sun’s magnetic field disturbs the solar The Sun’s magnetic field disturbs the solar

atmosphere periodically and causes new features atmosphere periodically and causes new features to appear in a process called solar activity. to appear in a process called solar activity.

SunspotsSunspots are cooler are cooler areas that form on areas that form on the surface of the the surface of the photosphere due to photosphere due to magnetic magnetic disturbances, which disturbances, which appear as dark spots. appear as dark spots.

Solar ActivitySolar ActivitySolar Activity CycleSolar Activity Cycle

– The number of sunspots changes The number of sunspots changes regularly, and on average reaches a regularly, and on average reaches a maximum number every 11.2 years. maximum number every 11.2 years.

– The length of the solar activity cycle is The length of the solar activity cycle is 22.4 years.22.4 years.

– There were severe weather changes on There were severe weather changes on Earth during the latter half of the 1600s Earth during the latter half of the 1600s when the solar activity cycle stopped and when the solar activity cycle stopped and there were no sunspots for nearly 60 there were no sunspots for nearly 60 years.years.

Solar ActivitySolar ActivityOther Solar FeaturesOther Solar Features

– Solar flaresSolar flares are violent are violent eruptions of particles and eruptions of particles and radiation from the surface radiation from the surface of the Sun that are of the Sun that are associated with sunspots.associated with sunspots.– prominenceprominence, , sometimes associated sometimes associated with flares, is with flares, is an arc of gas that is an arc of gas that is ejected from the ejected from the chromosphere, or gas chromosphere, or gas that condenses in the that condenses in the inner corona and rains inner corona and rains back to the surface. back to the surface.

The Solar InteriorThe Solar Interior Fusion occurs within the core of the Sun where Fusion occurs within the core of the Sun where

the pressure and temperature are extremely high.the pressure and temperature are extremely high.

– FusionFusion is the combining of lightweight is the combining of lightweight nuclei, such as hydrogen, into heavier nuclei, such as hydrogen, into heavier nuclei. nuclei.

In the core of the Sun, helium is a In the core of the Sun, helium is a product of the process in which product of the process in which hydrogen nuclei fuse. hydrogen nuclei fuse.

At the Sun’s rate of hydrogen fusing, At the Sun’s rate of hydrogen fusing, it is about halfway through its it is about halfway through its lifetime, with about another lifetime, with about another 5 billion years left.5 billion years left.

The SunThe Sun

SpectraSpectra

A continuous spectrum is produced by a hot solid, A continuous spectrum is produced by a hot solid, liquid, or dense gas. When a cloud of gas is in liquid, or dense gas. When a cloud of gas is in front of this hot source, an absorption spectrum front of this hot source, an absorption spectrum is produced. A cloud of gas without a hot source is produced. A cloud of gas without a hot source behind it will produce an emission spectrum.behind it will produce an emission spectrum.

A A spectrumspectrum is visible light is visible light arranged according to wavelengths. arranged according to wavelengths.

Solar CompositionSolar Composition The Sun consists of hydrogen, about 73.4 percent The Sun consists of hydrogen, about 73.4 percent

by mass, and helium, 25 percent, as well as a by mass, and helium, 25 percent, as well as a small amount of other elements.small amount of other elements.

The Sun’s The Sun’s composition composition represents that represents that of the galaxy as of the galaxy as a whole.a whole.

Groups of StarsGroups of Stars ConstellationsConstellations are the 88 groups of are the 88 groups of

stars named after animals, stars named after animals, mythological characters, or everyday mythological characters, or everyday objects. objects. – Circumpolar constellations can be seen Circumpolar constellations can be seen

all year long.all year long.– Summer, fall, winter, and spring Summer, fall, winter, and spring

constellations can be seen only at constellations can be seen only at certain times of the year.certain times of the year.

Aries, Cancer, Canis Major, Draco, Hercules, Hydra, Leo, Libra, Orion, Pegasus, Pices, Taurus Ursa Minor, Virgo

Groups of StarsGroups of StarsStar ClustersStar Clusters

– A group of stars that are gravitationally bound to A group of stars that are gravitationally bound to each other is called a cluster. each other is called a cluster.

In an open cluster, the stars are not In an open cluster, the stars are not densely packed. densely packed.

In a globular cluster, stars are densely In a globular cluster, stars are densely packed into packed into a spherical shape.a spherical shape.

Groups of StarsGroups of StarsBinariesBinaries

– A A binary starbinary star is two stars that are gravitationally bound is two stars that are gravitationally bound together and that orbit a common center of mass. together and that orbit a common center of mass.

– More than half of the stars in the sky are either More than half of the stars in the sky are either binary stars or members of multiple-star systems. binary stars or members of multiple-star systems.

ConstellationsConstellations

Stellar Position and Stellar Position and DistancesDistances

Astronomers use two units of measure for long Astronomers use two units of measure for long distances.distances.

– A light-year (ly) is the distance that light A light-year (ly) is the distance that light travels in travels in one year, equal to 9.461 one year, equal to 9.461 ×× 10 101212 km. km.

– A parsec (pc) is equal to 3.26 ly, or 3.086 A parsec (pc) is equal to 3.26 ly, or 3.086 ×× 10 101313 km. km.

Stellar Position and Stellar Position and DistancesDistances

To estimate the distance of stars from Earth, To estimate the distance of stars from Earth, astronomers make use of the fact that nearby astronomers make use of the fact that nearby stars shift in position as observed from Earth.stars shift in position as observed from Earth.

ParallaxParallax is the apparent shift in is the apparent shift in position of an object caused by the position of an object caused by the motion of the observer. motion of the observer. As Earth moves from one side of its As Earth moves from one side of its orbit to the opposite side, a nearby orbit to the opposite side, a nearby star appears to be shifting back and star appears to be shifting back and forth. forth.

Stellar Position and Stellar Position and DistancesDistances

The distance to a star, up to 500 pc using the The distance to a star, up to 500 pc using the latest technology, can be estimated from its latest technology, can be estimated from its parallax shift.parallax shift.

Basic Properties of StarsBasic Properties of Stars

The diameters of stars range from as The diameters of stars range from as little as little as 0.1 times the Sun’s diameter to 0.1 times the Sun’s diameter to hundreds of times larger.hundreds of times larger.

The masses of stars vary from a little The masses of stars vary from a little less than 0.01 to 20 or more times less than 0.01 to 20 or more times the Sun’s mass. the Sun’s mass.

Basic Properties of StarsBasic Properties of StarsApparent MagnitudeApparent Magnitude

– The ancient Greeks established a classification system based on the brightnesses of stars. The ancient Greeks established a classification system based on the brightnesses of stars. – The brightest stars were given a ranking of +1, the next brightest +2, and so on. The brightest stars were given a ranking of +1, the next brightest +2, and so on.

– In this system, a difference of 5 magnitudes corresponds to a factor of 100 in brightness. In this system, a difference of 5 magnitudes corresponds to a factor of 100 in brightness.

– Negative numbers are assigned for objects brighter than magnitude +1. Negative numbers are assigned for objects brighter than magnitude +1.

Basic Properties of StarsBasic Properties of StarsAbsolute MagnitudeAbsolute Magnitude

– Apparent magnitude does not actually indicate how bright a star is, because it does not take distance Apparent magnitude does not actually indicate how bright a star is, because it does not take distance into account. into account.

– Absolute magnitudeAbsolute magnitude is the brightness an object would have if it were placed at a distance of 10 pc. is the brightness an object would have if it were placed at a distance of 10 pc.

Basic Properties of StarsBasic Properties of Stars

SpectaSpecta

– LuminosityLuminosity is the energy output from the surface is the energy output from the surface of a star per second.of a star per second.

Stars also have dark absorption lines in their Stars also have dark absorption lines in their spectra and are classified according to their spectra and are classified according to their patterns of absorption lines.patterns of absorption lines.

LuminosityLuminosity

Spectra of StarsSpectra of StarsClassification by SpectraClassification by Spectra

– All stars, including the Sun, have nearly identical compositions—about 73 percent All stars, including the Sun, have nearly identical compositions—about 73 percent of a star’s mass is hydrogen, about 25 percent is helium, and the remaining 2 of a star’s mass is hydrogen, about 25 percent is helium, and the remaining 2 percent is composed of all the other elements.percent is composed of all the other elements.

B5 star

F5 star

K5 star

M5 star

– The differences in the appearance of their spectra are The differences in the appearance of their spectra are almost entirely a result of temperature effects. almost entirely a result of temperature effects.

Spectra of StarsSpectra of StarsWavelength ShiftWavelength Shift

– Spectral lines are shifted in wavelength by motion between Spectral lines are shifted in wavelength by motion between the source of light and the observer due to the Doppler effect.the source of light and the observer due to the Doppler effect.

If a star is moving toward the observer, If a star is moving toward the observer, the spectral lines are shifted toward the spectral lines are shifted toward shorter wavelengths, or blueshifted. shorter wavelengths, or blueshifted.

If the star is moving away, the If the star is moving away, the wavelengths become longer, or wavelengths become longer, or redshifted.redshifted.

Spectra of StarsSpectra of StarsH-R DiagramsH-R Diagrams

– A A Hertzsprung-Russell diagram,Hertzsprung-Russell diagram, or H-R diagram, or H-R diagram, demonstrates the relationship between mass, luminosity, demonstrates the relationship between mass, luminosity, temperature, temperature, and the diameter of stars.and the diameter of stars.

– An H-R diagram An H-R diagram plots the absolute plots the absolute magnitude on the magnitude on the vertical axis and vertical axis and temperature or temperature or spectral type on the spectral type on the horizontal axis.horizontal axis.

Spectra of StarsSpectra of StarsH-R DiagramsH-R Diagrams

– The The main sequencemain sequence, which runs diagonally from the , which runs diagonally from the upper-left corner to the upper-left corner to the lower-right corner of an lower-right corner of an H-R diagram, represents about 90 percent of stars.H-R diagram, represents about 90 percent of stars.

– Red giants are large, cool, Red giants are large, cool, luminous stars plotted at the luminous stars plotted at the upper-right corner.upper-right corner.

– White dwarfs are small, dim, hot White dwarfs are small, dim, hot stars plotted in the lower-left stars plotted in the lower-left corner. corner.

Basic Structure of StarsBasic Structure of Stars The mass and the composition of a The mass and the composition of a

star determine nearly all its other star determine nearly all its other properties. properties. – Hydrostatic Hydrostatic equilibrium is the equilibrium is the balance between balance between gravity squeezing gravity squeezing inward and pressure inward and pressure from nuclear from nuclear fusion and radiation fusion and radiation pushing outward.pushing outward.

Basic Structure of StarsBasic Structure of StarsFusionFusion

– Inside a star, the density and temperature increase toward the center, where energy is generated by nuclear fusion. Inside a star, the density and temperature increase toward the center, where energy is generated by nuclear fusion.

– Stars on the main sequence all produce energy by fusing hydrogen into helium, as the Sun does. Stars that are not on Stars on the main sequence all produce energy by fusing hydrogen into helium, as the Sun does. Stars that are not on the main sequence either fuse different elements in their cores or do not undergo fusion at all.the main sequence either fuse different elements in their cores or do not undergo fusion at all.

Stellar Evolution and Life Stellar Evolution and Life CyclesCycles

Star FormationStar Formation– A A nebulanebula (pl. nebulae) is a cloud of interstellar gas (pl. nebulae) is a cloud of interstellar gas

and dust.and dust.– Star formation begins when the nebula collapses on itself as a result of its own gravity. Star formation begins when the nebula collapses on itself as a result of its own gravity.

– A A protostarprotostar is a hot condensed object that forms at the center of the disk that will become a is a hot condensed object that forms at the center of the disk that will become a new star. new star.

Stellar Evolution and Life Stellar Evolution and Life CyclesCycles

Fusion BeginsFusion Begins– Eventually, the temperature inside a protostar becomes hot enough Eventually, the temperature inside a protostar becomes hot enough

for nuclear fusion reactions to begin converting hydrogen to helium.for nuclear fusion reactions to begin converting hydrogen to helium.

– When the hydrogen in its core When the hydrogen in its core

is gone, a star has a helium is gone, a star has a helium center and outer layers made center and outer layers made of hydrogen-dominated gas.of hydrogen-dominated gas.

– Some hydrogen continues to Some hydrogen continues to react in a thin layer at the react in a thin layer at the outer edge of the helium core outer edge of the helium core causing the outer layers to causing the outer layers to expand forming a red giant.expand forming a red giant.

The Sun’s Life CycleThe Sun’s Life Cycle What happens during a star’s life What happens during a star’s life

cycle depends on its mass. cycle depends on its mass.

The Sun’s Life CycleThe Sun’s Life Cycle– While the star is a red giant, it loses gas While the star is a red giant, it loses gas

from its outer layers while its core from its outer layers while its core becomes hot enough for helium to react becomes hot enough for helium to react and form carbon. and form carbon.

– When the helium in the core is all used When the helium in the core is all used up, the star is up, the star is left with a core made of carbon.left with a core made of carbon.

– The outer layers expand once again and are The outer layers expand once again and are driven off entirely by pulsations that develop, driven off entirely by pulsations that develop, becoming a shell of gas called a planetary becoming a shell of gas called a planetary nebula.nebula.

– In the center of a planetary nebula, the core of In the center of a planetary nebula, the core of the star remains as a white dwarf made the star remains as a white dwarf made of carbon. of carbon.

The Sun’s Life CycleThe Sun’s Life CycleA Nebula Once AgainA Nebula Once Again

The Sun’s Life CycleThe Sun’s Life Cycle

Pressure in White DwarfsPressure in White Dwarfs– A white dwarf is stable because it is supported by the resistance of electrons being squeezed close together and does A white dwarf is stable because it is supported by the resistance of electrons being squeezed close together and does

not require a source of heat to be maintained. not require a source of heat to be maintained.

– A star that has less mass than that of the Sun has a similar life cycle, except that helium may never form carbon in the A star that has less mass than that of the Sun has a similar life cycle, except that helium may never form carbon in the core, and the star ends as a white dwarf made of helium. core, and the star ends as a white dwarf made of helium.

A massive star A massive star undergoes many undergoes many reaction phases and reaction phases and produces many produces many elements in its elements in its interior. interior.

Life Cycles of Massive Life Cycles of Massive StarsStars

A massive star begins its life high on the A massive star begins its life high on the main sequence with hydrogen being converted main sequence with hydrogen being converted to helium. to helium.

The star becomes a The star becomes a red giant several red giant several times as it expands times as it expands following the end of following the end of each reaction stage. each reaction stage.

A massive star loses A massive star loses much of its mass much of its mass during during its lifetime. its lifetime.

White dwarf White dwarf composition composition is determined by is determined by how many reaction how many reaction phases the star went phases the star went through before through before reactions stopped. reactions stopped.

Life Cycles of Massive Life Cycles of Massive StarsStars

As more shells are formed by the fusion of As more shells are formed by the fusion of different elements, the star expands to a different elements, the star expands to a larger size and becomes a supergiant.larger size and becomes a supergiant.

Life Cycles of Massive Life Cycles of Massive StarsStars

Neutron stars and PulsarsNeutron stars and Pulsars– A star that begins with a mass between about 8 and 20 times the Sun’s mass will end up with a core that is too A star that begins with a mass between about 8 and 20 times the Sun’s mass will end up with a core that is too

massive to be supported by electron pressure. massive to be supported by electron pressure.

– Once no further energy-producing reactions can Once no further energy-producing reactions can occur, the core of the star violently collapses in on occur, the core of the star violently collapses in on itself and protons and electrons in the core merge to form a neutron star. itself and protons and electrons in the core merge to form a neutron star.

Life Cycles of Massive Life Cycles of Massive StarsStars

SupernovaeSupernovae– A neutron star has a mass of 1.5 to 3 times the Sun’s A neutron star has a mass of 1.5 to 3 times the Sun’s

mass but a radius of only about 10 km.mass but a radius of only about 10 km.– Infalling gas rebounds when it strikes the hard surface of the neutron star and Infalling gas rebounds when it strikes the hard surface of the neutron star and

explodes outward.explodes outward.

– A A supernovasupernova is a massive explosion in which the entire outer portion of the is a massive explosion in which the entire outer portion of the star is blown off and elements that are heavier than iron are created.star is blown off and elements that are heavier than iron are created.

Life Cycles of Massive Life Cycles of Massive StarsStars

Black HolesBlack Holes– A star that begins with more than about 20 times the Sun’s mass will not be able to form a neutron star. A star that begins with more than about 20 times the Sun’s mass will not be able to form a neutron star.

– The resistance of neutrons to being squeezed is not great enough to stop the collapse, so the core of the star simply continues to The resistance of neutrons to being squeezed is not great enough to stop the collapse, so the core of the star simply continues to collapse forever, compacting matter into a smaller and smaller volume. collapse forever, compacting matter into a smaller and smaller volume.

– A A black holeblack hole is a small, extremely dense remnant of a star whose gravity is so immense that not even light can escape its gravity field. is a small, extremely dense remnant of a star whose gravity is so immense that not even light can escape its gravity field.