The Milky Way GalaxyThe Milky Way Galaxy

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IntroductionIntroduction The pale band of light spangled with stars The pale band of light spangled with stars

stretching across the sky is the stretching across the sky is the Milky WayMilky Way, , a swath of light named by the ancient a swath of light named by the ancient GreeksGreeks

In the 17In the 17thth century, Galileo showed that the century, Galileo showed that the Milky Way is millions of stars too dim to see Milky Way is millions of stars too dim to see individuallyindividually

Today we know the Milky Way is a slow Today we know the Milky Way is a slow revolving disk of stars, a galaxyrevolving disk of stars, a galaxy

We also know today that the Milky Way is We also know today that the Milky Way is filled with stars of various sizes, many of filled with stars of various sizes, many of them found in clusters, and clouds of gas them found in clusters, and clouds of gas and dustand dust

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Overview of the Milky WayOverview of the Milky Way Structure and ContentsStructure and Contents

The Milky Way consists of three partsThe Milky Way consists of three partsDiskDisk

Spiral armsSpiral arms distribution of stars, gas, and dust distribution of stars, gas, and dust with a diameter of about 100,000 light-yearswith a diameter of about 100,000 light-years

Rotation with all objects circling in the same Rotation with all objects circling in the same directiondirection

High density of stars near center to low density High density of stars near center to low density farther out farther out

Dust and gas is nearly 15% of the disc’s massDust and gas is nearly 15% of the disc’s mass Most of galaxy is hidden from Earth due to dust, Most of galaxy is hidden from Earth due to dust,

including the central including the central nucleusnucleus with its dense swarm with its dense swarm of stars and gas in which a massive black hole of stars and gas in which a massive black hole may residemay reside

Radio and infrared telescopes can “see” entire Radio and infrared telescopes can “see” entire galaxygalaxy

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Overview of the Milky WayOverview of the Milky WayStructure and Contents Structure and Contents (continued)(continued)

The Milky Way consists of three parts The Milky Way consists of three parts (continued)(continued)

HaloHalo Roughly spherical regionRoughly spherical region Contains mainly old stars, such as globular Contains mainly old stars, such as globular

clustersclusters Large amounts of dark matter may also be presentLarge amounts of dark matter may also be present

BulgeBulge Flattened collection of stars surrounding dense Flattened collection of stars surrounding dense

core of galaxycore of galaxy About 1/3 the diameter of the galaxyAbout 1/3 the diameter of the galaxy

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Overview of the Milky WayOverview of the Milky Way Mass of the Milky Way and the Number of StarsMass of the Milky Way and the Number of Stars

Assuming that the average star has a mass similar Assuming that the average star has a mass similar to that of the Sun, then based on the Milky Way’s to that of the Sun, then based on the Milky Way’s mass, there are roughly 100 billion starsmass, there are roughly 100 billion stars

From large difference between this mass and what is From large difference between this mass and what is observed, astronomers conclude Milky Way is observed, astronomers conclude Milky Way is embedded in vast halo of material that emits no embedded in vast halo of material that emits no radiation (at any wavelength) – radiation (at any wavelength) – dark matterdark matter

Age of the Milky WayAge of the Milky Way More recent model calculations and observations More recent model calculations and observations

suggests the old star ages are about 11-13 billion suggests the old star ages are about 11-13 billion yearsyears

A rough estimate of the Milky Way then is about 15 A rough estimate of the Milky Way then is about 15 billion yearsbillion years

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Stars of the Milky WayStars of the Milky Way Stellar Censuses and the Mass FunctionStellar Censuses and the Mass Function

Counts that list all known stars of a given type in a Counts that list all known stars of a given type in a region of space is called a stellar censusregion of space is called a stellar census

All star types are represented in the Milky WayAll star types are represented in the Milky Way By analyzing the relative numbers of stars of each type, By analyzing the relative numbers of stars of each type,

predicting the galaxy’s history is possiblepredicting the galaxy’s history is possible From a stellar census one can determine the number From a stellar census one can determine the number

of stars and their masses, technically known as the of stars and their masses, technically known as the mass functionmass function

Mass determines the life cycle of a starMass determines the life cycle of a star The evolution of the Milky Way will then depend on:The evolution of the Milky Way will then depend on:

How many stars of each type it contains (A galaxy with only How many stars of each type it contains (A galaxy with only massive stars will evolve quickly)massive stars will evolve quickly)

How fast each type is created (Fast creation will quickly How fast each type is created (Fast creation will quickly deplete gas resources)deplete gas resources)

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Stars of the Milky WayStars of the Milky WayStellar Censuses and the Mass Function Stellar Censuses and the Mass Function

(continued)(continued)

Some results:Some results:Dividing the number of stars in the Milky Way by Dividing the number of stars in the Milky Way by

its age gives a star creation rate of 3-5 stars per its age gives a star creation rate of 3-5 stars per yearyear

Most numerous stars turn out to be dim, cool, red Most numerous stars turn out to be dim, cool, red dwarfs dwarfs

The average mass for Milky Way stars is 1 The average mass for Milky Way stars is 1 MM(solar mass)(solar mass)

Stars more massive than 30 MStars more massive than 30 M are rare are rare

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Gas and Dust in the Milky WayGas and Dust in the Milky Way IntroductionIntroduction

The space between stars is not empty: it contains The space between stars is not empty: it contains interstellar matterinterstellar matter composed of gas and dust composed of gas and dust

Interstellar clouds seen directly or detected by Interstellar clouds seen directly or detected by their effect on light from distant starstheir effect on light from distant stars

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Gas and Dust in the Milky WayGas and Dust in the Milky Way Interstellar Dust Interstellar Dust

The light from a star dims and reddens as The light from a star dims and reddens as it passes through an interstellar cloud with it passes through an interstellar cloud with dustdust

Light is randomly reflected from the dust Light is randomly reflected from the dust surfaces in a process called surfaces in a process called scatteringscatteringThis scattering is most effective when the light’s This scattering is most effective when the light’s

wavelength is smaller than the dust grainwavelength is smaller than the dust grainThe optimum scattering for interstellar dust and The optimum scattering for interstellar dust and

molecules in the Earth’s atmosphere is for blue molecules in the Earth’s atmosphere is for blue and ultraviolet lightand ultraviolet light

Earth’s sky is blue and the setting Sun red Earth’s sky is blue and the setting Sun red because of scatteringbecause of scattering

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The Galactic CenterThe Galactic CenterBecause the galactic center is not Because the galactic center is not

observable in the visible, astronomers observable in the visible, astronomers must rely on radio, infrared, X-ray, and must rely on radio, infrared, X-ray, and gamma-ray observationsgamma-ray observations

Some of the discoveries are best left to Some of the discoveries are best left to pictures:pictures:

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History of the Milky WayHistory of the Milky Way The Future of the Milky WayThe Future of the Milky Way

Eventually all gas finds its way into stars, Eventually all gas finds its way into stars, which in turn will lock up material in stellar which in turn will lock up material in stellar remnantsremnants

Hundreds of billions of years from now the Hundreds of billions of years from now the Milky Way will fade, slowly spinning in Milky Way will fade, slowly spinning in space, a dark disk of stellar cindersspace, a dark disk of stellar cinders