Measuring Distances in the Cosmos

Newton

• 17th century calculated that Sirius (one of the brightest stars was 1 million times further away than the sun (1 million AU)– Compared brightness of Sirius to brightness

of Saturn

• Actual distance to Sirius = 550,000 AU

Measuring with Triangulation and Parallax

TriangulationMethod of measuring

distance indirectly Done by creating an

imaginary triangle between an observer and an object who’s distance needs to be discovered

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Triangulation

A) Create a baseline– In this case line A to C

B) Measure angles from the two points on the baseline– In this case Angle A and Angle C

C) Make a scale drawing of the triangle using a protractor and a ruler

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Greeks

• Used triangulation 2000 years ago to calculate the distance between the Earth and Moon– Baseline was 300 km– Also knew they would have to take into

account the curvature of the Earth.

Parallax

Is the apparent shift in position of a nearby object when it is viewed from two different points.– Eg: Point finger close one

eye then the other eye.

• Using Earths orbit – Sightings are taken 6

months apart– If a star is close enough it

will appear to move relative to more distant stars

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Light Years

Represents the distance light travels in one year (63,240 AU)– Closest star to Earth = Proxima Centauri

(272000 AU)• 4.28 light years away

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Cepheid Variables

Are stars that change size and brightness (they pulsate)– Cepheids pulsate in predictable fashion– Two Cepheids would pulsate (go bright then dim) at

the same time• They would have to reach the same brightness• If they didn’t the reason was due to distance

By classifying Cepheids according to their maximum brightnessAstronomers have been able to use them as a

standard against which to analyze other variable stars• Helped measure accurate distance of milky way galaxy

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The Milky Way

Appears as a white hazy band extending from south horizon across overhead sky– May look like thin haze of clouds– Actually a vast accumulation of 400 billion

stars

GalaxyIs a collection of stars, gas and dust held

together by gravity

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Star Clusters

• Some stars occur in clustersOpen Clusters: are collections of 50 to 1000 stars that

appear dispersed along the main band of the Milky Way

• Eg: Pleiades

Globular Clusters: are collections of 100 000 to 1 million stars arranged in distinct spherical shapes. Eg: Hercules• Appear not along the band of the milky way but in the

southern region of the sky

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Mapping the Size and Shape

• Harlow Shapley (1918)– Mapping galaxy using Cepheid Variables– Resulted in new picture of galaxyMilky Way Galaxy

Disk shapedHalo of Globular Clusters surrounding centerSun was nowhere near the center of the galaxy

– 75,000 Light-Years in diameter– Sun is 25,000 Light-Years from center of galaxy

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Andromeda and Beyond

Most distant object seen by the unaided eye is constellation Andromeda– 1925 Edwin Hubble spotted stars in the

nebulaRealized it was not a cloud of dust and gas

but another Galaxy• Scientists have now found hundereds of galaxies

(“island universes” – as called by Hubble)

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Types of Galaxies• Three major types

EllipticalMost commonShaped like a footballMade of old stars, and little inter-stellar dust and gas

Spiral Flat pinwheels with arms spiralling outward from a central

regionComposed of dust gas and young blue stars (star formation

still occurring)Irregular

No shape, mixture of young and old stars embedded in dust and gas

Smaller and less common than the other two types

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Galaxy Clusters

Galaxies also occur in clusters throughout the universe– Eg: Milky Way and

Andromeda Galaxy are two of 30 galaxies that make up a cluster called the “Local Group”

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The Cosmic Speedometer

Spectroscopes also allow us to tell how fast an object is moving towards or away from us– Eg: You may have noticed

that the siren on an ambulance sounds different as the vehicle approaches, passes then moves away

This is called the Doppler Effect

– Same technology used by Police radar guns

When object is not moving the sounds have uniform pitch in all directions because waves are uniform

When the object is moving the distance between sound waves is changed so the pitch changes Higher as it moves toward something, Lower as it moves away.

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Cosmic Speedometer• Light like sounds moves

in waves so the Doppler effect can be used to measure the speed of light emitting objects– If a star is approaching

the wavelength becomes compressed and the stars spectrum becomes shifted towards the blue end (blue-shift)

– If a star is moving away the wavelengths become red-shifted (more elongated)

Discovery of the Expanding Universe

• 1929 Edwin Hubble estimated the distance to 46 Galaxies. Discovered all galaxies were red-

shiftedThis means they were all moving

away from earthThe furthest away were moving

the fastest…the closest galaxies were moving slower This became known as HUBBLES

LAW

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Determining the Beginning of Time

• Origin of MatterBig Bang theory

15-20 billion years agoUniverse is expanding therefore it must have

started off small and dense

– Echos of the Big Bang• 1960 microwave antenna kept detecting

background noise• Picking up radiation given off by original Big Bang

event

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Quasars1960 astronomers discovered

star-like objects that emit great amounts of radio wavesResult of explosions produced

by colliding galaxies• Collisions of galaxies forced huge

amounts of star material into a central black hole.

• This was converted into light energy

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Gamma Ray BurstsPowerful burst of

energy (more energy in seconds than the sun can emit in it’s entire life)Predict:

Two stars collide or collapse forming a black hole releasing a large amount of energy

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Missing MassGalaxies occur in clusters

Astronomers added up the mass of all the matter in the Galaxies (60-70 percent of estimated mass was missing)

• Suggest there may be burned out stars we can’t see that have mass

• Empty space contains neutrinos – elementary particles that carry no charge

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