Galaxies

Chapter 11Page 351

Milky Way

Milky Way

• Our galaxy- figure 11-1 as imagined• “gala”- milky• Enormous assemblage of hundreds

of billions of stars• Gas, dust and other matter• Mutual gravitational attraction

Galaxy = Universe ?

• Prior to 20th century• Astronomers believed Milky Way contained

all the stars in the cosmos (universe)• Galaxy = universe• 1755 Immanuel Kant (philosopher not astronomer)

• Suggested vast collections of stars beyond the Milky Way

Data

• Herschels- William, sister Caroline, son John• Nebulae (nebula, singular)• fuzzy looking objects• William Parsons – Earl of Rosse 1845• 1.8 meter (6ft) mirror telescope• Studied nebulae• Distinct spiral structure• No photographs- just drawings

Whirlpool Galaxy M51 or NGC 5194

One or Only one of many

• Shapley-Curtis debate• 1920• National Academy of Science, Washington• Nothing was decided• Because no measurement of distance to

the spiral nebulae

More Data• 1923 Edwin Hubble--Photographs• Cepheid variables (Chapter 9)• Henrietta Leavitt (1912)• Period of variation determines the absolute

brightness• Apparent brightness determine distance• M31- 2.2 million light years beyond MW• Andromeda Galaxy – most distant object

visible with the naked eye.

Andromeda Galaxy M31

2.5 million light years away and 260,000 light years in diameter

Structure of Milky Way

• Milky Way is actually hard to study since we are in the galaxy.

• William Herschel 1780’s• Map the Milky Way• Counted stars and determined star density the

same in all directions• ERGO we are at the center of the galaxy

Herschel’s Error• 1930’s R.J.Trumpler discovered interstellar gas

and dust which affected Herschel’s count of stars and his estimate of their distance

• Gas and dust obscured stars and made them appear dimmer than actually were.

• Dust is concentrated in the plane of the galaxy• Herschel’s count was low and his estimate of

distance was high• Density of stars greatest toward center• Earth is not the center of the galaxy

Radio Telescopes• Radio, infrared, X-rays and gamma rays

are scattered less by the interstellar gas and dust than are visible wavelengths.

• Studied the galaxy using a combination of wavelengths

Milky Way Structure

• Indicate 200 billion stars• Located in 2 spiral arms and several short

arms• Bar of stars and gas crosses the center• Disk – 100,000 light-years in diameter• 2000 light years thick

Milky WayGrand Design Spiral

Central Bulge

• Flattened sphere of stars• 20,000 ly in diameter• 26,000 ly from us

• Halo of stars extending far beyond the disk

Neighborhood

• A dozen small galaxies orbit our galaxy

• Two smaller galaxies are losing stars to the Milky Way –

• Galactic cannibalism

Motion

• Galaxy is rotating at 828,000 km/hr about the center.

• Galaxy does not rotate like a rigid body• Differential Rotation-stars at different

distances from the center orbit at different rates.

Motion Mysteries• Based on the observed mass of the

galaxy and Kepler’s law stars far from the center of the galaxy should orbit much slower than observed.

• Leads to the conclusion that there must be more mass around the galaxy than accounted for.

• Since we do not “see” this matter at any wavelength it is classed as Dark Matter

10 Things to Remember in 50 Years• 1. Reason for the seasons (not just Christmas)• 2. Solar system – names and order of planets,

classification of planets• 3. Star process – nuclear fusion• 4. Locate and describe a constellation• 5. Difference between astrology and astronomy• 6. Gravity- Newton’s contribution, Einstein’s contribution• 7. Light, electromagnetic spectrum, Doppler effect• 8. Phases of moon – diagram• 9. Difference between comets and meteors• 10. Light year- how to calculate the distance in meters (or

miles)

Types of Galaxies (P365)• Scan pictures and diagrams on pages 365-373)• 2 Comments about Science• Science begins with collecting (observing) and

classifying.• From the classification structure we then hope

to learn the how and why.• Scientist speak very authoritatively about

things which are really very speculative.• “The older I get the fewer things I am sure

about.” Rip Kersey

Types of Galaxies (Page 373)• Spirals (normal)

– Flocculent – Figure 11-23a on page 366– Grand-design- Figure 11-23b, Figure 11-27 page 370

• Barred spirals- Figure 11-28 page 371• Ellipticals Figure 11-29 and 11-31 (P371-372)• Irregulars- Figure 11-32 page 373• Lenticulars – intermediate type

– Look elliptical but have disk and central bulge– No spiral arms

Edwin Hubble- Tuning Fork Classification

Elliptical Galaxies

• Distinctive shape• No spiral arms• Wide range of sizes• Giant ellipticals contain 10 trillion stars• Dwarf ellipticals contain so few stars they are

nearly transparent

Spiral Galaxies (without bars)

• Central bulge• arched lanes of stars and glowing interstellar

clouds - spiral arms• 100 billion stars• 100,000 light years in diameter• Flocculent (fluffy) – fuzzy, poorly defined• Grand-design- well-defined spiral arms

Andromeda with satellites

Sombrero Galaxy

Barred Spiral• Bar of stars crossing through the central bulge• Composed of stars and gas that flow first one

way down the bar and then turn around and flow the other way

• Total mass of normal spiral galaxies prevents bars from forming

• Two main arms in barred spirals typically extend from the ends of the bar rather than from the central bulge.

Milky Way- artist conception

Irregulars

• Rich in interstellar gas and dust• Both young and old stars

• Some are highly distorted and completely asymmetrical (perhaps a collision)

• Typically smaller and less massive than spirals

Clusters and Superclusters

• Galaxies are not scattered lonely objects• They occur in groups• Orbit each other • Collide• Surround “voids”• Superclusters contain dozens of individual

clusters

Local Group

Antlia Discovered in 19973 million ly away1,000,000 stars

Clusters

• Poor Cluster < 1000 galaxies• Rich Cluster > 1000 galaxies

• Regular Cluster- distinctly spherical• Irregular cluster – randomly scattered

• Our local group = poor irregular

Coma Cluster

• Rich Regular Cluster• 300.000.000 ly away• 1000’s of galaxies• Mostly elliptical and lenticular• Central region 1.5 million light years across

Hercules Cluster

• Irregular• 700,000,000 light years away• Mostly spiral galaxies• Pairs and groups of galaxies

Super Clusters

• Hugh associations• Dozens of individual clusters• 150 million light year diameter• Not gravitationally bound• Clusters in the supercluster are moving away

from each other• Superclusters are moving away from other

superclusters.

Super Clusters “Near” us

Size of the Universe

• We don’t know how big the universe is.• Infinite? Finite?

• We are limited in how far we “see” • Volume 13.5 Billion Light Years in Radius• Because universe is thought to be 13.5 Billion

years old• Light from farther than 13.5 Billion LY would

not have reached us yet.

Galaxies in Motion

Galaxy Collisions (P378)• Two or more galaxies collide (occasionally)

– Stars rarely collide, great distance between stars.– Clouds of gas and dust do collide.– Heats the gas – hot intergalactic gas– Detect hot gas with x-rays– Hot gas does not create stars

• Other sources of gas (cool gas)– Leak from supernova explosions in dwarf galaxies– Near miss collisions can compress gas in a galaxy– Stimulate galaxy wide star formation- – Starburst galaxies

Galaxy Mergers (P379)

• Collision of galaxies– Fling some stars away- isolated stars– Stars slow down and collect in a merged galaxy– Black holes combine to form massive BH– One galaxy robs stars from the other galaxy– Galactic Cannibalism– Suspect that giant elliptical were formed by

merger– Giant galaxies occupy centers of rich clusters

Formed when one galaxy passed through another causing star formation. Note blue and white stars shown in visible light.

Star Burst GalaxyFormed perhaps by NGC1510 compressing gas in NGC1512

Galaxies connected by streamers of gas caused by collisions

Distortion caused by nearby orbiting galaxy

Spiral galaxies in process of merging

Merger simulation

Super Clusters in Motion

Page 383

Edwin Hubble, Again• Hubble’s Tuning Fork Classifications of

galaxies.• Expansion of the Universe and Hubble’s Law• All clusters and superclusters are moving away

from us.• The farther away a cluster is the faster it is

moving away.• Doesn’t mean we are the center-

Moving Galaxies (P383)

• 1914 Slipher observed redshift of “spiral nebulae”

• 1920’s Edwin Hubble and Milton Humason using 100 inch Mt. Wilson telescope recorded spectra of many galaxies

• Measured distance to these galaxies• Galaxies in distant clusters and superclusters

are moving away from us more rapidly than galaxies in near by clusters and superclusters.

Expansion of the UniverseWhat it is not!

• The earth isn’t expanding.• The solar system isn’t expanding.• The galaxy isn’t expanding.• The local group (cluster) isn’t expanding.

• But clusters are moving away from each other.

• On the large scale the universe is expanding.

Dark Matter, Dark EnergySupposition, not certainty.

Evidence for Dark MatterPage 382

• Galaxies in clusters and superclusters are not drifting away.

• Speed of Rotation of Galaxies• Gravitational Lensing

Mysteries at the Galactic Fringe (P363)

• Kepler’s Law• Objects farther from center orbit more slowly• Orbital speed actually increases at the edge of

the Milky Way

Effect of mass on light

• Microlensing- small scale distortions of objects within the Milky Way

• Gravitational Lensing- distortion of light from distant galaxies due to mass between us and the distant object.

• If we cannot detect a “visible” object astronomers hypothesize dark matter.

Examples of lensing due to visible mass

Dark Matter Lensing

• If we observe lensing effect but don’t see the object causing the gravity that is bending the light then assume dark matter.

Active Galactic Nuclei

Active Galaxies (P387- )• Quasars- quasi-stellar radio sources• Look like very distant stars• Intense energy emitted- more than any

star could emit• Thought to be supermassive blackholes

at the center of galaxies.

Similar Objects

• Hugh red-shifts - so large that at first astronomers did not recognize the spectra.

• Hugh Red-shift indicates very fast movement away from us.

• Fast movement away indicates very distant• Intense radiation (visible or radio) indicates

high energy• Theory (speculative) super massive black hole