Galaxies,)Cosmology)and) the)Accelerang)Universe)stevepur.com/galaxies/GalaxiesCosmology_2.pdf ·...
Transcript of Galaxies,)Cosmology)and) the)Accelerang)Universe)stevepur.com/galaxies/GalaxiesCosmology_2.pdf ·...
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Galaxies, Cosmology and the Accelera6ng Universe
Steve Bryson
Class 2: Distances
h>p://stevepur.com/galaxies/
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Ques6ons?
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Look Up on a Clear Night
• Small points of light – How big are they? – How far away are they?
• Look down – A planet! – Looks bigger…
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Stars are Big!
• The Sun is about 100 6mes the size of Earth
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Some Stars are Bigger, Some Smaller
• The Sun is on the small side • Larger stars are much brighter
The Sun
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Clusters of Stars
• All about the same distance – So we can see range of brightnesses
• Actually can’t see the dimmest stars
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Stars are Really Far Away
• If the Earth were the size of a basketball… – The Moon would be 27 feet away
– The Sun would be about 85 feet across – The Sun would be 1.7 miles away – The nearest star would be 417,000 miles away • 10,000 6mes the distance to the moon
– A typical star in the sky would be a billion miles away • About as far as the real orbit of Uranus
• All this within a small region of the Galaxy
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Galaxies are Big and Far
• If the Earth were the size of a basketball – The center of our Galaxy would be 2.86 billion miles away • About the real orbit of the planet Uranus
– The Andromeda galaxy (a neighbor) would be 220 billion miles away • About 1% of the real distance to the nearest star
• These numbers are ge`ng too big – we need new distance units
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Astronomer’s Big Distance Units
• Astronomical Unit (AU): the average distance from the Earth to the Sun – 93 million miles
• Light Year (ly): the distance light travels in 1 year – 5.878 trillion miles
• Parsec (pc): 3.262 light years – We’ll define parsec in a few minutes
• The closest star to the Sun is 4.3 light years = 1.3 parsecs
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Powers of Ten
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How Do We Measure Such Large Distances?
• We can’t run a measuring tape from here to there
• All we have is starlight • Starlight shows – Posi6on and brightness (today) – Chemistry and mo6on (next week)
• We’ll use posi6on and brightness to measure very large distances, to the furthest galaxies!
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The Cosmological Distance Ladder
• 1st rung: the distance from the Earth to the Sun
• Moving from one rung to the next – Use the rung we’re on to determine distance to the special type of star on the next rung
– Observe something about how the brightness of the special star type depends on something we can observe
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2nd Rung: Parallax
• Parallax measures the distance to a star in exactly the same way that you measure the distance to something in arm’s reach
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Stellar Parallax
• Replaces your two eyes with the posi6on of the Earth six months apart
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Stellar Parallax
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Defini6on of a Parsec
• A Parsec is the distance of a star whose parallax is one second of arc = 3.262 light years – One second of arc = 1/3600 degrees
• A really small angle: the moon is 1800 seconds of arc across in the sky
• Because the angle is small, the distance in parsecs = 1/parallax in arcseconds
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Modern Parallax Measurements
• Hipparcos satellite (1989 – 1993) – Observed 100,000 stars – Capable of measuring parallax as small as 0.001 arcseconds, for distances up to 1000 parsecs
• Launched Jan 8 2014: the GAIA satellite – Observing about a billion stars (!) – Parallaxes as small as 24 millionths of an arcsec (!), for distances up to 40,000 parsecs
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Standard Candles
• A “Standard Candle” is a star type where I know the intrinsic brightness of the star – Usually the brightness is determined by something I can directly observe
• Once I know a star’s intrinsic brightness, I can compare that with its apparent brightness to determine distance – Simple formula, but to complex to show here
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Standard Candles
• A “Standard Candle” is a star type where I know the intrinsic brightness of the star – Usually the brightness is determined by something I can directly observe
• Once I know a star’s intrinsic brightness, I can compare that with its apparent brightness to determine distance – Simple formula, but to complex to show here
• (oops! D = 10(1-‐(M-‐m)/5))
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3rd Rung: Normal Star Brightness
• Heavier stars are brighter (during most of their life6mes) – The brightness at a given mass has a very narrow range
• If I know the mass of the star I can predict its intrinsic brightness
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Measuring the Mass of a Star
• How do we measure the mass of a star? – By watching it orbit other stars! – Half the stars in the night sky are actually two or more stars orbi6ng each other
– Observing the period and size of the orbit tells us the masses of the two stars
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Masses from Binary Star Orbits
• We can directly observe the orbital mo6on over 6me
• The period and size of the orbit is determined by the masses of the stars – Kepler’s and Newton’s laws
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Intrinsic Brightness from Star Mass
• For most of a star’s life, the mass determines the intrinsic brightness – “Mass-‐Luminosity Rela6on” – Not a simple linear rela6on
• A star the mass of the sun will have the same brightness as the sun
• A star twice as massive as the sun will be 16 6mes as bright
• So we can measure the distance to many binary stars – Works out to a few thousand light years, beyond which we can’t observe the individual stars in the binary system
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4th Rung: Variable Stars
• All stars vary in brightness a li>le bit • A “Variable Star” varies a lot in a regularly repea6ng pa>ern
• Different types of variable stars have different pa>erns
• It turns out that for two variable types, that pa>ern tells us the star’s intrinsic brightness!
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Variable Star Light Pa>erns
• We make a graph of the star’s brightness over 6me
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Useful Variable Stars
• For two kinds of variable stars the period of the pa>ern determines the brightness – RR Lyrae
• Less bright, good in the Galaxy – Cepheid
• Very bright, good for nearby galaxies • Polaris is a Cepheid variable
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Period-‐Luminosity Rela6on
• For Cepheids: – Stars of the same period are the same brightness
– Stars of shorter period are dimmer
– Stars of longer period are brighter
– From the period we can predict the brightness
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Cepheid Period-‐Luminosity Rela6on Discovery
• Henrie>a Levi> finds variable stars in the Magellanic Clouds – Stars that vary in brightness with a regular period
• She no6ces that the variable star’s brightness depends on the period (1908)
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Calibra6ng the Cepheid Period-‐Luminosity Rela6on
• Done in various ways over the last century • Surprise! There are two types of Cepheids – Different brightnesses, period-‐luminosity rela6ons
– Slightly different pa>erns of varia6on – We’ll see later that this was the cause of some bad first es6mates of the age of the universe
• Best values from 2007 (Fritz Benedict) using parallaxes measured by Hubble
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Hubble’s Andromeda Variable
• Because Hubble didn’t know about the two types of Cepheids, his distance to the Andromeda galaxy was half the correct value
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5th Rung: Exploding Stars
• An exploding star is called a nova (for “new”) • A really bright exploding stars is called a supernova – Most massive stars: collapse because they run out of fuel at the end of their lives
– Binary stars exchanging material from one star to another • Type 1a supernova
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Type 1a Supernova
• A binary star system made from – A big gas giant – A white dwarf heavier than the sun, size of the Earth
• Very strong gravity on the surface
• The white dwarf pulls ma>er from the giant star
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Type 1a Supernova
• As the material from the big star builds up on the smaller star, the smaller star gets heavier and heavier
• As the small star gets heavier it gets ho>er, crea6ng large amounts of fusion in a very short 6me – Causing an explosion! – 5 billion 6mes brighter than the sun!
– Bright enough to see in very distant galaxies
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Type 1a Supernova
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A Supernova in 2014!
• In the (rela6vely) nearby galaxy M82
Photos taken from San Rafael
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Intrinsic Brightness of Type 1a SN
• We can determine the intrinsic brightness of a type 1a supernova from how much it dims from its maximum brightness in 15 days
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Type 1a SN Measures Distances to Clusters of Galaxies
• Most galaxies do not have type 1a SN – A couple per century per galaxy – Last only a couple months – Not useful to get distance to most galaxies
• But there are lots of galaxies – Supernova surveys have been a major effort
for the last few decades, hundreds found
• Type 1a SN are useful for determining the distance to clusters of galaxies – Lots of galaxies in a cluster, so a be>er
chance of catching a type 1a SN – The Kepler spacecraq even caught one!
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6th Rung: Brightest Galaxies in Clusters
• Based on previous rungs, we find that the brightest galaxies in clusters tend to be about the same size and brightness – Very approximate…
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The Cosmic Distance Ladder
• 1: Earth-‐Sun distance
• 2: Parallax
• 3: Normal stars in binary systems
• 4: Variable stars
• 5: Type 1a Supernova
• 6: Largest galaxies in clusters
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The Cosmic Distance Ladder
• We’ll discuss the last rung, redshiq, next week
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To the iPad
• Using the Exoplanet app
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Light Has a Finite Speed
• Light is very very fast, but only so fast – 186,262 miles per second – 8.6 minutes for light from the Sun to get to the Earth
– 4.1 hours to the planet Neptune • So when we look far away we are looking backwards in 6me
• The distance in light years is the same as the 6me it takes for light to travel that distance
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Looking Into the Past
• The Andromeda galaxy is 2.2 million light years away – So we’re seeing it as it was 2.2 million years ago
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Looking into the Past
23 million light years
11.4 million light years
40 million light years 2.3 billion light years
333 million light years
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Looking into the Far Past
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The Observable Universe
• If the universe has a finite age (currently believed to be 13.7 billion years) then we can only see 13.7 light years away
• This is the limit of the observable universe – And near this limit we are looking at the universe when it was very young • And indeed galaxies look different then!
• The universe may be much much larger