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Lecture 12:Unveiling the Milky Way
Astronomy 1143 – Spring 2014
Key IdeasMapping our Galaxy and Andromeda
Luminosity Distances need Standard Candles • Spectroscopic Parallaxes• Period-Luminosity Relations for Cepheids & RR Lyraes
The Milky Way is our Galaxy• Diffuse band of light crossing the sky• Most of the stars in the Galaxy lie in a disk
Position of Sun in the Galaxy – not the center!• Star Counts: Herschels & Kapteyn• Globular Cluster Distribution: Shapley
Key IdeasGas and Dust lie between the stars
• Dust leads to extinction and reddening• Not accounting for dust led to confusion about size
Nature of Nebulae – important scientific question• Objects inside Milky Way or distant galaxies like MW?• Problems: “nebulae” includes several different phenomena,
inaccurate stellar distances, inaccurate measurements of motion
Accurate stellar distances established…• We are in the Milky Way, a spiral galaxy• Milky Way is one of many galaxies in Universe
Luminosity Distances
Indirect distance estimate:• Measure the object’s Apparent Brightness, B• Assume the object’s Luminosity, L• Solve for the object’s distance, d, by applying
the Inverse Square Law of Brightness
LuminosityAssuming a luminosity is a critical step.
We need to find something that we can observe about an object that tells us its luminosity
For example: • We can look at the color of an object• We can look at the spectrum of an object• We can look at the lightcurve of an object
Then we need to calibrate it by knowing the luminosity of an identical object
Standard CandlesObjects whose Luminosity you know ahead of time.
• Calibrate the Luminosities of nearby objects for which you have distances from Trigonometric Parallaxes.
• Identify distant but similar objects, using a distance-independent property that they share.
• Assume that the distant objects have the same Luminosity as the nearby objects.
Examples of Standard Candles
Normal Stars • Spectral type is the same as a star with a
known luminosity – say a star with a known parallax
Pulsating Stars• Evolved Stars can be unstable• Small Changes in Luminosity• Period-(Average) Luminosity Relationship
All Stars are not like the Sun
Stellar Spectra
Spectroscopic Parallax LimitsDistance Limit:
• Practical limit is few 100,000 pc – need to get spectra of individual stars
Problems:• Stars within each class do not have exactly the
same luminosity
• Depends on composition.
• Faint spectra give poor classifications.
Method works best for clusters of stars, rather than for individual stars.
Periodic Variable Stars
Stars whose brightness varies regularly with a characteristic, periodic pattern.
Distance-Independent Property:Period (repetition time) of their cycle of
brightness variations.
Physics:Period-Luminosity Relations exist for certain
classes of periodic variable stars.
Measuring the Period gives the Luminosity.
Period-Luminosity Relationship
1 5 10 50
102
103
104
Period (days)
Lum
inos
ity
(Lsu
n)
CepheiStars
RR Lyraestars
3 100300.5
Cepheid Variables
Rhythmically Pulsating Supergiant stars:• Found in young star clusters
• Luminosities of ~ 1034 Lsun
• Brightness Range: few % to 23 times• Period Range: 1 day to ~50 days.
Period-Luminosity Relation:• Longer Period = Higher Luminosity
• P = 3 days, L ~ 103 Lsun
• P = 30 days, L ~ 104 Lsun
Typical Cepheid Light Curves
LCB 171P ~ 3 days
LCB 272P ~ 2 days
time time
Bri
ghtn
ess
Bri
ghtn
ess
PeriodPeriod
Easier to get a measurement of brightness than a spectrum, especially for a lot of objects at once
Example: Cepheid with a 10-day period
1 5 10 50
102
103
104
Period (days)
Lum
inos
ity
(Lsu
n)
3 100300.5
L=5011 Lsun
P=10d (observable)
CepheidP-L Relation(calibrated)
Example
You measure the period of a Cepheid to be 5.4 days. What is its luminosity?
Cepheid Variable LimitationsFound only in young star clusters.Distance Limit:
• 3040 Megaparsecs (Hubble Space Telescope)• Crucial for measuring distances to galaxies.
Problems:• Few Cepheids with good Trigonometric
Parallaxes• P-L relation may depend on Composition• Two types of Cepheids with different
P-L relations ( Cephei and W Virginis).
Cepheids with HST
RR Lyrae Variables
Pulsating old stars:• Luminosity of ~50 Lsun
• Brightness Range: factor of ~ 23• Period Range: Few hours up to ~ 1 day.• Relatives of Cepheid Variables
Period-Luminosity Relation• Less strong than for Cepheids
RR Lyrae Light Curve
RR Lyrae Star Limitations
Found in old clusters, Galactic bulge & halo
Distance Limit:• ~1 Megaparsec (Hubble)• Limited to our Galaxy & Andromeda
Problems:• No RR Lyrae stars with good Trigonometric
Parallaxes• Less bright than Cepheid stars, so useful only
relatively nearby
The Cosmic Distance ScaleNo single method will provide distances on all cosmic scales:
• Calibrate parallaxes using the AU• Calibrate spectroscopic parallaxes using geometric
parallaxes• Calibrate Cepheid and RR Lyrae star distances
using clusters with spectroscopic or geometric parallaxes
Imprecision at each step carries forward, making subsequent steps less precise.
This is the challenge of measuring distances.
Announcements
Don’t forget to sign the attendance sheet
Homework #1 thoughts
The Milky WayDiffuse band of light crossing the night sky
All cultures have named it:• Celestial River • Celestial Road or Path
Our names are derived from Greek and Latin:
• Greek: Galaxias kuklos = “Milky Band”• Latin: Via Lactea = “Road of Milk”
View from center of a sphere
View from edge of sphere
View from center of disk
View from edge of disk
Star counts and star distances in different directions can tell you the shape of the Milky Way
The Herschels’ Star GaugesWilliam & Caroline Herschel (1785):
• Counted stars along 683 lines of sight using their 48-inch telescope.
• Assumed all stars are the same luminosity, so relative brightness gives relative distance.
• Assumed that they could see all the way to the edges of the system.
Model:• Flattened Milky Way (“grindstone”)• Sun is located very near the center
The Herschels’ Milky Way Map (1785)
Phil. Trans. Roy. Soc. v75, 213 (1785)
The Kapteyn UniverseJacobus Kapteyn (1901 thru 1922):
• Used photographic star counts• Estimated distances statistically based on
parallaxes & proper motions of nearby stars.• Neglected interstellar absorption of starlight
(assumes fainter stars are just farther away).
Model:• Flattened disk 15 kpc across & 3 kpc thick• The Sun is located slightly off center
Kapteyn Milky Way Model (1922)
~17 kpc
~3 kpc
1 kpc = 1 kiloparsec = 1000 pc
Harlow Shapley (1915 thru 1921)
Astronomer at Harvard
Noticed two facts about Globular Clusters:1. Uniformly distributed above & below the Milky
Way on the sky
2. Concentrated on the sky toward Sagittarius
Observations:1. Globular Cluster distances from RR Lyrae stars
2. Used these distances to map the globular cluster distribution in space.
Shapley’s Globular Cluster Distribution
3020101020 40
10
20
10
20
kpc
The Greater Milky Way
Shapley’s Results (1921):• Globular clusters form a subsystem centered
on the Milky Way.• The Sun is 16 kpc from the MW center.• MW is a flattened disk ~100 kpc across
Right basic result, it’s but too big:• Shapley ignored interstellar absorption• Caused him to overestimate the distances
Gas and Dust: the stuff between the stars
The space between the stars is not a vacuum.• Air on Earth – 2.5 x 1019 particles/cm3
• Vacuum pump – 1010 particles/cm3
• Interstellar space – 1 particle/cm3
Composition similar to solar atmosphere• By number: 90% H, 10% He, 0.1% heavy• By mass: 72% H, 26% He, 2% heavy
Mostly atomic H and He, heavies form dust
Dust
About 1% of the interstellar medium is in the form of dust
Very small particles: think soot, not dust bunnies
Composed of carbon, silicon, iron and other heavy elements
Dust is very effective at blocking visible light.
Makes stars appear fainter, which could fool an observer
Dust causes ExtinctionIf there were as much dust in the air of this room as there is in the gas of the Galaxy, it would be difficult to see your notepad in front of you.
Therefore the dust in the Galaxy is very important and must be understood to understand Galactic structure.
We can see star light from farther away if we look in the infrared
The Milky Way• A flattened disk of stars
with a central bulge• Sun is ~8 kpc from the
center in Sagittarius• ~30 kpc in diameter
and ~1 kpc thick• Galactic Center and
much of the disk is obscured by dust in the plane of the Galaxy
30kpc
If we could see the Milky Way galaxy edge-on from outside:
= 8000 parsecs = 26,000 light-years
=1.7 billion AU
Probing the skies
The return of Halley’s Comet in 1758 made comets very, very popular
All astronomers wanted to discover one, so they used their telescopes to sweep the skies looking for faint, fuzzy objects
If it were a comet, it would move from night to night
If it didn’t move, it was disappointing.
Charles Messier cataloged these objects…
Fuzzy Objects in the Sky:
The Nature of the Nebulae
With telescopes, astronomers found fuzzy things in the sky
Called them “nebulae” -- Latin word for cloud
Were they galaxies like the Milky Way?
Were they clouds of gas inside the Milky Way?
Observations with new and better instruments and new techniques gradually revealed several clues to the nature of these objects.
Observations of NebulaeDuring the 19th century, ever larger telescopes
were built.
Some nebulae were seen to have a spiral structure
Spectra of objects – spiral nebulae had spectra similar to stars
Other nebulae, such as planetary nebulae, had very different spectra – different phenomena
Observations of Nebulae
Bright outbursts observed in spiral nebulae (such as S Andromedae in 1885)
Are these similar to the novae (rapid brightening of individual stars) seen in the Milky Way?
The spiral nebulae in general have large velocities heading away from us.
There were also observations of rotation.
Are the spiral nebulae like the Milky Way?
Shapley-Curtis DebateShapley: spiral nebulae are not galaxies like MW•Distances large, but not large enough
•Milky Way is very large; spiral nebulae aren’t far enough away
•Events like S Andromedae would have to be much more luminous than Milky Way novae
•Observed rotation cannot be explained if at large distances
Curtis: spiral nebulae are galaxies outside MW•Milky Way is not so big; spiral nebulae can easily be outside
•Appearance of nova says spiral nebulae made of stars
•Large speeds away from us not seen for stars & objects that we know are in the Milky Way
•Rotation measurements are wrong
Rotation and SpeedsYour calculation of how far (in kilometers) the spot in the spiral nebula moves depends on how far you think the object is.
Angular size + distance = physical size.
Hubble Ends the Debate
Edwin Hubble (1923):• Using the new 100-inch telescope on
Mt. Wilson in California. • Found a Cepheid Variable in Andromeda• Shapley’s P-L relationship gave a distance of
300 kpc
By 1925:• Hubble had measured 10 Cepheid variables• The Distance to Andromeda: ~1000 kpc.• Size of the Milky Way: 30 kpc
Hubble’s Cepheid in Andromeda
100-inch Telescope(Mt. Wilson)
Current Understanding
With modern technology and more decades of investigation, we know:
Spiral “nebulae” clearly resolved into stars
There are extremely luminous stellar explosions in galaxies called supernova.
The rotation measurements incorrect
The fact that galaxies are moving away from the Milky Way in general is extremely interesting.
Andromeda (M31)Nearest bright galaxy to the Milky Way:
• Distance ~700 kpc
Many similarities to the Milky Way
• Both are large spiral galaxies
• Both have similar stellar and gas contents
Andromeda gives us an approximate outside view of our own Galaxy.
Galaxies come in many
shapes
Our Place in the Neighborhood
Obtaining accurate distances for many stars and galaxies led to our understanding of
• The size and shape of the Milky Way and the Sun’s place in it
• The fact that the Milky Way is one of many galaxies in the Universe
• The properties of galaxies outside of our own
• The expansion of the Universe