Astronomical Distances Blendon Middle School April 13, 2010
Astronomical Distances Blendon Middle School April 13, 2010 Dr. Uwe Trittmann Otterbein College Astronomical Distances Locations in the sky are easy to measure: 2 angles Distances from observer are hard (one
length) Together they give the location of an object in three-dimensional space The Trouble with Angles Angular size of an object cannot tell us its actual size depends on how far away it is Sun and Moon have very nearly the same angular size (30' = ) when viewed from Earth Angles and
Size Without Distances We do not know the size of an object This makes it hard to figure out the inner workings of an object We cant picture the structure of the solar system, galaxy, cosmos The Universe is structured on different length scales
Stars nebulae molecular clouds star clusters THE UNIVERSE clusters and superclusters galaxies
like the Milky Way quasars Solar System black holes pulsars Sun
voids Big ----------------------------- small Powers of Ten From Man to Universe - 100 meters =1 meter The Human Scale
Street Size 103 meters =1000 m = 1 km Harbour City Size 104 meters
= 10,000 m =10 km Chicago Planet Size: thousands of km 1000 km = 1,000,000 m = 1 million meters Star Size: 1,000,000,000 m =1 billion meters
The Sun (a typical star): diameter 1.4 million km. Solar System Scale Venus, Earth, Mars Orbits 1011 meters =100,000,000,000m =100,000,000 km = about 1 A.U. (Astronomical Unit)
Farther out: Nebulae Where stars are born... and die ! How big ARE these? They APPEAR tiny! Black Holes Dead Stars How big is a black hole?
Galaxies How big is a galaxy? Are all galaxies the same size? Clusters of Galaxies What is the distance between galaxies? The Universe
How big is the Universe? Does this question make sense? If yes, can we answer it while living IN the universe? Different lengths scales Different length units Human scale: meters (yards) Human height: 1.8 m Geographical scale: kilometers (miles)
Distance to Cincinnati: 100 mi Solar system scale: Astronomical Unit Distance Earth-Sun: 1 A.U. Intragalactic scale: lightyears (parsecs) Next star: 4 lightyears Intergalactic scale: millions of lightyears (Megaparsecs) Andromeda galaxy: 2.2 million lightyears = 0.67 Mpc
Cosmological Scale: billions of ly (Gigaparsecs) Edge of observable universe: about 15 billion ly Different lengths scales Different length measurements Human scale: yardstick
Geographical scale: triangulation Solar system scale: Radar ranging Intragalactic scale: Close stars: stellar parallax Far: spectroscopic parallax Intergalactic scale: Close: Variable stars Far: Tully-Fisher relation Cosmological Scale: Hubbles Law
Astronomical Distance Measurements Fundamental technique uses triangulation: Objects appear to move with respect to background if looked at from different vantage points Try looking at you thumb with only your left, then right eye
The more the thumb jumps, the closer it is! Measure jump, get distance See: Link, Link 2 Distances to the Stars Measurements year apart! Parallax can be used out to about 100
light years The bigger the parallactic angle, the closer the star! A star with a measured parallax of 1 is 1 parsec away 1 pc is about 3.3 light years
The nearest star (Proxima Centauri) is about 1.3 pc or 4.3 lyr away Solar system is less than 1/1000 lyr Insight Some stars are close to us (4 ly), other are
far away (1000 ly) This means that some stars appear dim but are actually very bright That means that stars have different sizes, temperatures, life expectancy Our Stellar Neighborhood Scale Model If the Sun = a golf ball, then
Earth = a grain of sand The Earth orbits the Sun at a distance of one meter Proxima Centauri lies 270 kilometers (170 miles) away Barnards Star lies 370 kilometers (230 miles) away Less than 100 stars lie within 1000 kilometers (600 miles)
The Universe is almost empty! Hipparcos satellite measured distances to nearly 1 million stars in the range of 330 ly almost all of the stars in our Galaxy are more distant Luminosity and Brightness Luminosity L is the total power (energy per unit time) radiated by the star, actual brightness of star, cf. 100 W lightbulb Apparent brightness B is how
bright it appears from Earth Determined by the amount of light per unit area reaching Earth B L / d2 Just by looking, we cannot tell if a star is close and dim or far away and bright Brightness: simplified 100 W light bulb will look
9 times dimmer from 3m away than from 1m away. A 25W light bulb will look four times dimmer than a 100W light bulb if at the same distance! If they appear equally bright, we can conclude that the 100W lightbulb is twice as far away!
Same with stars Sirius (white) will look 9 times dimmer from 3 lightyears away than from 1 lightyear away. Vega (also white) is as bright as Sirius, but appears to be 9 times dimmer. Vega must be three times farther away (Sirius 9 ly, Vega 27 ly)
Distance Determination Method Understand how bright an object is (L) Observe how bright an object appears (B) Calculate how far the object is away: B L / d2 So L/B d2 or d L/B
Understand Star Brightness: Classify Stars by their Temperature (Color) Class O B A F G K
M Temperature 30,000 K 20,000 K 10,000 K 8,000 K 6,000 K 4,000 K 3,000 K
Color blue bluish white white yellow orange red Examples Rigel
Vega, Sirius Canopus Sun, Centauri Arcturus Betelgeuse The hotter the bluer! ColorLuminosity Correlation Hertzprung-Russell
Diagram is a plot of absolute brightness (vertical scale) against spectral type or temperature (horizontal scale) Most stars (90%) lie in a band known as the Main Sequence
Spectroscopic Parallax From the color of a main sequence star we can determine its absolute brightness Then, from the apparent brightness compared to absolute luminosity, we can determine the distance d L/B
Insight We now know how far away stars are, so we know how big they are, and we can understand how they work. We understand how big our galaxy is (100,000 ly) and that some nebulae are galaxies like our own Sizes of Stars Dwarfs
Comparable in size, or smaller than, the Sun Giants Up to 100 times the size of the Sun Supergiants Up to 1000 times the size of the Sun
Note: Temperature (Color) changes! Galaxies are close together compared to their size The Local Group The Virgo Cluster Aside: What are stars made out of ? 90% of the universe is Hydrogen
The rest is mostly Helium How do we know? By identifying the fingerprints of the elements, aka the light they send out! Spectral Lines Fingerprints of the Elements Can use this to identify elements on distant objects!
Different elements yield different emission spectra Origin of Spectral Lines: Emission Heated Gas emits light at specific frequencies the positive fingerprints of the elements Origin of Spectral Lines: Absorption
Cool gas absorbs light at specific frequencies the negative fingerprints of the elements Use Spectra to measure the Size of the Universe Measure spectrum of galaxies and compare to laboratory measurement lines are shifted towards red This is the Doppler effect: Red-shifted objects are
moving away from us Using Redshift: Hubbles Law The final rung on the cosmic distance ladder Hubbles observations (1920s): Light from distant galaxies is redshifted The more distant the galaxy, the greater the red-shift Interpretation:
Galaxies are moving away from us More distant galaxies are moving faster The universe is expanding, carrying the galaxies with it! Hubbles Law Velocity = H0 Distance Distance = Velocity /H0 H0 = (65 15) km/sec/Mpc is Hubbles constant
Compare to distance = velocity time Appears the universe exploded from a single point in the past the Big Bang Age of the universe is 1/H0 or about 14 billion years The Latest Surprise Type Ia Supernovae are standard candles Can calculate distance from brightness Can measure redshift
General relativity gives us distance as a function of redshift for a given universe Supernovae are further away than expected for any decelerating (standard) universe Supernova Data magnitude
Solid line is best fit to data redshift Expansion of the Universe Old lore:
Either it grows forever Or it comes to a standstill Or it falls back and collapses (Big crunch) In any case: Expansion slows down! Surprise of the year 1998 (Birthday of Dark Energy): All wrong! It accelerates!
Additional Material Powers of Ten From Man to Universe - 100 meters =1 meter The Human Scale Powers of Ten
From Man to Universe - 101 meters =10 meters Lawn and Blanket Powers of Ten From Man to Universe - 102 meters
=100 meters Highway and Boats Powers of Ten From Man to Universe - 103 meters =1000 m Harbour
Powers of Ten From Man to Universe - 104 meters =10 km Chicago Lakeshore Powers of Ten From Man to Universe -
105 meters =100 km Chicago & L. Michigan Powers of Ten From Man to Universe - 106 meters =1000 km Lake
Michigan Powers of Ten From Man to Universe - 107 meters =10000 km The Earth Powers of Ten From Man to Universe -
108 meters =100000 km Earth in Space Powers of Ten From Man to Universe - 109 meters =1000000km
Moon Orbit Powers of Ten From Man to Universe 1010 meters Part of Earths Orbit around the Sun Powers of Ten From Man to Universe -
1011 meters = ca. 1 A.U. (Astronomical Unit) Earths Orbit Powers of Ten From Man to Universe - 1012 meters
Inner Planets Orbits Powers of Ten From Man to Universe - 1013 meters Outer Planets Orbits Powers of Ten
From Man to Universe - 1014 meters Solar System in Space Powers of Ten From Man to Universe - 1015 meters The Sun a bright star
Powers of Ten From Man to Universe - 1016 meters = ca. 1 ly (light year) The Sun just another star Powers of Ten
From Man to Universe - 1017 m = ca. 10 ly Distinct Stars Powers of Ten From Man to Universe - 1018 m = 100 ly
Sun in center; Arcturus ( Tauri) Powers of Ten From Man to Universe - 1019 m = 1000 ly A cloud of Stars - making up constellations
Powers of Ten From Man to Universe - 1020 m = ca.10000 ly Clouds - made out of Stars Powers of Ten From Man to Universe 1021 m =100000 ly
The Milky Way Our Galaxy Powers of Ten From Man to Universe - 1022 m =1,000,000 ly The Milky Way in Space
Powers of Ten From Man to Universe - 1023 m = 10 x 106 ly The Local Group of Galaxies Powers of Ten From Man to Universe -
1024 m = 108 ly The Virgo Cluster of Galaxies (incl. the local Group) Powers of Ten From Man to Universe 1025 m = 109 ly The Universe: Many clusters of
galaxies and even more empty space The old Planets Mercury Venus Mars Jupiter
Saturn The new Planets Uranus (1781) Neptune (1846) Pluto (1930) (dwarf planet since 2006)
Keplers Third Law: Relating Orbits The square of a planets orbital period is proportional to the cube of its orbital semi-major axis: 2 3 Jupiter: 53 / 122 = 125/144 ~ 1 P a a
1.001 The Problem with Keplers Third Law The square of a planets orbital period is proportional to the cube of its orbital semi-major axis: P 2 a3 But: everything is expressed in Earth units, i.e. one Earth year, and one Earth-Sun distance. Problem: How big are these units?
Practical Problem: Determine the Suns Diameter? Trickier than you might think We know only how big it appears It appears as big as the Moon Need to measure how far it is away Keplers laws dont help (only relative distances) Without knowing its size, we dont know
how much energy it can produce, so we cant figure out how the Sun works Solution: relate to distances on Earth or fundamental constants Use two observations of Venus transit in front of Sun Captain Cook in the 1700s Hard and not very precise (100,000 km) Modern way: bounce radio signal off of
Venus (measure traveling time of light) In the 1960s, very precise (few centimeters) Insight Sun is 109 times bigger than Earth Up to the 1930s no mechanism was known to produce so much energy Know now that the Sun fuses hydrogen to helium More Insight: Understanding
Variable Stars yields another Method Two useful types: Cepheids RR Lyrae Again, method uses insight to get absolute brightness, then concludes distance from apparent brightness Cepheids
Named after Cephei Period-Luminosity Relations Used as standard candles yard-sticks for distance measurement Cepeids in Andromeda Galaxies established
the extragalacticity of this nebula Cepheids Henrietta Leavitt (1908) discovers the period-luminosity relationship for Cepheid variables Period thus tells us luminosity, which then tells us the distance Since Cepheids are brighter than RR Lyrae, they can be used to
measure out to further distances Properties of Cepheids Period of pulsation: a few days Luminosity: 200-20000 suns Radius: 10-100 solar radii Properties of RR Lyrae Stars Period of pulsation: less than a day Luminosity: 100 suns
Radius: 5 solar radii Extends the cosmic distance ladder out as far as we can see Cepheids about 50 million ly In 1920 Hubble used this technique to measure the distance to Andromeda
(about 2 million ly) Works best for periodic variables Distance Measurements with variable stars Cepheids and RR Lyrae: Yard-Sticks Normal stars undergoing a phase of instability Cepheids are more massive
and brighter than RR Lyrae Note: all RR Lyrae have the same luminosity Apparent brightness thus tells us the distance to them! Recall: B L/d2 Insight: How does our Galaxy look like? Other Galaxies
Edwin Hubble identified single stars in the Andromeda nebula (turning it into a galaxy) Measured the distance to Andromeda to be 1 million Ly (modern value: 2.2 mill. Ly) Conclusion: it is 20 times more distant than the milky ways radius Extragalacticity! Old theory (Milky Way is the universe) falsified! The Tully-Fisher Relation
A relation between the rotation speed of a spiral galaxy and its luminosity The more mass a galaxy has the brighter it is the faster it rotates the wider the spectral lines are Measuring rotation speed allows us to estimate luminosity; comparing to observed (apparent) brightness then tells us the distance Electromagnetic Spectrum Energy:
low medium high
Electromagnetic Radiation: Quick Facts There are different types of EM radiation, visible light is just one of them EM waves can travel in vacuum, no medium needed The speed of EM radiation c is the same for all types and very high ( light travels to the moon in 1 sec.) The higher the frequency, the smaller the wavelength ( f = c) The higher the frequency, the higher the energy of
EM radiation (E= h f, where h is a constant) Visible Light Color of light determined by its wavelength White light is a mixture of all colors Can separate individual colors with a prism
Three Things Light Tells Us Temperature from black body spectrum Chemical composition from spectral lines Radial velocity from Doppler shift Temperature Scales
Fahrenheit Centigrade Absolute zero Kelvin 459 F 273 C 0K
32 F 0 C 273 K Human body temperature 98.6 F
37 C 310 K Water boils 212 F 100 C 373 K
Ice melts Black Body Spectrum Objects emit radiation of all frequencies, but with different intensities Ipeak Higher Temp.
Ipeak Ipeak Lower Temp. fpeak
(600K, fpeak in infrared) The Suns surface (6000K, fpeak in visible) Hot stars in Omega Centauri (60,000K, fpeak in ultraviolet) The higher the temperature of an object, the higher its Ipeak and fpeak
Activity: Black Body Radiation Pick up a worksheet Form a group of 3-4 people Work on the questions on the sheet Fill out the sheet and put your name on top
Hold on to the sheet until weve talked about the correct answers Hand them in at the end of the lecture or during the break Ill come around to help out ! Kirchhoffs Laws: Bright lines Heated Gas emits light at specific frequencies the positive fingerprints of the elements
Kirchhoffs Laws: Dark Lines Cool gas absorbs light at specific frequencies the negative fingerprints of the elements Kirchhoffs Laws 1. A luminous solid or liquid (or a sufficiently dense gas) emits light of all wavelengths: the black body spectrum 2. Light of a low density hot gas consists of a series of discrete bright emission lines: the positive
fingerprints of its chemical elements! 3. A cool, thin gas absorbs certain wavelengths from a continuous spectrum dark absorption ( Fraunhofer) lines in continuous spectrum: negative fingerprints of its chemical elements, precisely at the same wavelengths as emission lines. Spectral Lines Origin of discrete spectral
lines: atomic structure of matter Atoms are made up of electrons and nuclei Nuclei themselves are made up of protons and neutrons Electrons orbit the nuclei, as planets orbit the sun Only certain orbits allowed Quantum jumps!
The energy of the electron depends on orbit When an electron jumps from one orbital to another, it emits (emission line) or absorbs (absorption line) a photon of a certain energy The frequency of emitted or absorbed photon is related to its energy E=hf (h is called Plancks constant, f is frequency)
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