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