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TRANSCRIPT
Galaxies, Cosmology and the Accelera6ng Universe
Steve Bryson
Class 2: Distances
h>p://stevepur.com/galaxies/
Ques6ons?
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…
Stars are Big!
• The Sun is about 100 6mes the size of Earth
Some Stars are Bigger, Some Smaller
• The Sun is on the small side • Larger stars are much brighter
The Sun
Clusters of Stars
• All about the same distance – So we can see range of brightnesses
• Actually can’t see the dimmest stars
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
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
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
Powers of Ten
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!
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
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
Stellar Parallax
• Replaces your two eyes with the posi6on of the Earth six months apart
Stellar Parallax
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
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
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
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))
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
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
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
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
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!
Variable Star Light Pa>erns
• We make a graph of the star’s brightness over 6me
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
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
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)
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
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
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
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
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
Type 1a Supernova
A Supernova in 2014!
• In the (rela6vely) nearby galaxy M82
Photos taken from San Rafael
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
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!
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…
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
The Cosmic Distance Ladder
• We’ll discuss the last rung, redshiq, next week
To the iPad
• Using the Exoplanet app
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
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
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
Looking into the Far Past
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