Unit 9: Discoveries About Galaxies and the Universe

What are the word origins for “nebula?”

cloud / mist

• Latin

What are the name origins for the “Large / Small Magellanic Clouds?”

Ferdinand Magellan

• Portuguese

reported nebulae when sailing around Earth

What did the telescope reveal about nebulae in the 1750s?

some are elliptical

What did Immanuel Kant propose about elliptical nebulae in the 17th century?

Island Universe theory

What is the Island Universe theory?

elliptical nebulae = billions of stars grouped in flattened circular system

→ if randomly oriented, random angles

What is a tilted disc’s appearance?

What is an edge-on disc’s appearance?

What is a face-on disc’s appearance?

What does Immanuel Kant explain about the star band?

if we live in elliptical star system,

Milky Way = view through length

What was the main theory before the Island Universe Theory?

our Galaxy is the only galaxy

What is the identity of William Herschel and Caroline Herschel’s son?

John Herschel

What was William Herschel, Caroline Herschel, and John Herschel’s biggest project?

sweep entire north sky & catalogue > 5000 nebulae for 20 years

What was William Herschel’s main study?

cosmology

What did William Herschel build in the 1780s?

20 foot telescope with 18 inch mirror

What was the identity of the largest nebulae catalogue author before the Herschel family?

Charles Messier

• 110 Messier objects (M1)

What was the reason for Charles Messier’s nebulae catalogue?

ensured he didn’t confuse with comet

What is the number of known nebulae before William Herschel’s health began to fail?

2500

What did William Herschel & John Herschel attempt to identify about nebulae?

• distant star system

or

• nearby glowing gas cloud

What did John Herschel do in response to William Herschel’s death?

moved 20 foot telescope & family to Cape of Good Hope

What did John Herschel publish in 1864?

General Catalogue of Nebulae & Clusters of Stars

• 1st all-sky nebulae catalogue

What nebulae catalogue do we use today?

New General Catalogue

• expanded Herschels’ work

What is the largest & brightest nebula in the sky?

Orion Nebula

What was the reason that the Herschels could not come to a conclusion about nebulae composition?

gas nebula

• parts look clumpier than others

• furthest regions are distant to resolve into stars

What are some examples of nebulae that gave John Herschel a lot of trouble?

• Lobster Nebula (M17)

• Triangulum Galaxy (M33)

What map did William Herschel produce?

Solar System from outside

What did William Herschel incorrectly assume?

• brightest stars = nearest

• faintest stars = furthest

What did William Herschel correctly conclude about the Solar System?

Sun embedded in flat star disc

What was the identity of the Irish Earl that attempted to prove that all nebulae are star systems by resolving stars?

William Parsons / Lord Rosse

What did Lord Rosse build in 1845?

Leviathan of Parsons Town

What was the Leviathan of Parsons Town?

reflector with 6 foot diameter mirror

• largest reflector in world for 70 years

What did Lord Rosse’s sketches reveal?

many elliptical nebulae = spiral nebulae

What did Lord Rosses’s sketches incorrectly assume?

bright dot = nebula star

• foreground star

What did Lord Rosse conclude about nebulae?

all nebulae are star systems / gas clouds don’t exist

• cloudy nebulae are so densely packed

What was also discovered in John Herschel’s & Lord Rosse’s Whirlpool Galaxy sketches?

NGC 5195

What is happening to NGC 5195?

galactic cannibalism

What is galactically cannibalizing NGC 5195?

Whirlpool Galaxy

What is galactically cannibalizing the Sagittarius Dwarf?

Milky Way

What are the 2 nearest galaxies?

Large & Small Magellanic Clouds

What did Henrietta Swan Leavitt discover?

thousands of variable stars

What is the definition of “variable star?”

star fluctuating in brightness at regular interval

What was one of Henrietta Swan Leavitt’s jobs aside from analyzing spectra?

measure apparent brightness & assign apparent magnitude

What are a Cepheid’s properties?

• dramatically & rapidly pulsating brightness

  • over few nights, change 1/2 magnitudes

  • detectable by eye

• high-mass

  • 10-20x more massive than Sun

• opaque surface

• variable

What are the name origins of “Cepheid?”

1st variable star type found in 1780s

• Delta Cephei

  • Cepheus constellation

What is the reason for a Cepheid’s opaque surface?

outward photons push surface out

→ expands star & reduces opacity

What was the date of the 1st variable star detection?

shortly after telescope invention

• early 1600s

What do Cepheid spectra show?

unusual amount of helium ionized twice in atmosphere

• electrons & nuclei float together

→ get closer together & denser

What struggle do stars undergo?

• inward gravitational pull

• outward radiation pressure

What is a Cepheid’s pulsation process?

1. photons get trapped by opaque atmosphere

2. outward radiation pressure builds up & breaks balance

3. star inflates

4. helium atoms spread out

5. light releases

6. outward radiation pressure returns to normal levels

7. inward gravitational pull returns to normal size

What is the definition of a “period” on a light curve?

time it takes to go through bright & dim cycle

What did Henrietta Swan Leavitt discover about Cepheids?

brighter = longer P

What method did Henrietta Swan Leavitt use to discover the Period-Luminosity relation for Cephoids?

graphed 25 Cephoid sample in Small Magellanic Cloud minimum & maximum brightness

• roughly same distance from Earth

What is the P-L relation’s use?

calculate star distance

1. observe Cepheid until you know pulsation period

2. P-L relation → Cepheid luminosity

3. use apparent & absolute magnitude

What did Harlow Shapley do with the P-L relation?

1. used nearby Cepheid sample with known distances

2. found Cepheid stance in most distant star clusters

3. revealed our star system edges

What did Harlow Shapley incorrectly conclude about SMC?

in our Galaxy

What was the reason for Harlow Shapley’s incorrect conclusion about SMC?

measured 200 000 lightyear distance

What did Harlow Shapley incorrectly conclude about nebulae?

all are systems in our Galaxy

What was a globular cluster’s most important feature to Harlow Shapley?

some of most distant things in Galaxy

What did Harlow Shapley measurements reveal about the disc?

length

• 50 kiloparsecs + 30 kiloparsecs = 80 kilo parsecs / 265 000 → 300 000 light years

What was the reason for Harlow Shapley’s incorrect measurements?

didn’t know about dust between Milky Way & globular clusters

• appear fainter = more distant

What did Harlow Shapley correctly conclude?

• Galaxy’s not heliocentric

• Galaxy’s surrounded by sphere halo containing star clusters

• Milky Way centre → Sagittarius direction

What did spiral nebulae maps reveal in the 1860s?

spirals aren’t seen near Milky Way disc

What was Harlow Shapley’s response to spiral nebulae maps’ revelations in the 1860s?

spirals are pushed from disc by radiation pressure from high star density

→ Zone of Avoidance

What did Heber Curtis do with spiral nebulae?

completed high-res survey & revealed observing matter (dust)

What did Heber Curtis correctly conclude about spiral nebulae?

• believed Island Universe theory

  • our Galaxy = spiral galaxy

• spirals are not seen in Zone of Avoidance because they are obscured by dust band

• if spirals move faster than our Galaxy’s star, outside our Galaxy

What was Heber Curtis’ reason for concluding each spiral nebula is its own spiral galaxy?

observing matter resembled Milky Way dark patchiness across sky

What makes up our Galaxy’s disc?

microscopic metallic grains from dying stars

→ absorbing starlight

• carbon & silicon & iron

• micron in size

What is proof that our disc is filled with dust?

when heated, glow red in infrared light

What did Vesto Slipher’s 1st spiral nebula spectrum measure in 1913?

large redshift in calcium absorption line

→ spiral nebulae were made of stars

• absorption line is produced by star

What was Vesto Slipher’s method of measuring radial velocity?

plug wavelength shift distance in Doppler equation

What did Vesto Slipher produce in 1913?

1st spiral nebula spectrum

What is the average spiral velocity relative to stellar velocity?

25:1

What debate was held in 1920?

Shapley-Curtis debate / Great Debate of 1920

What was the Shapley-Curtis debate outcome?

neither was conclusive

What was Heber Curtis’ debate document?

The Spiral Nebulae as Island Universes

What were Heber Curtis’ arguments in The Spiral Nebulae as Island Universes?

• spiral radial velocity > star velocity

• spiral spectra = solar systems

  • absorption lines

• spiral novae appear fainter

• dust explains Zone of Avoidance

What was proven by Edwin Hubble 3 years after the Shapley-Curtis debate?

Island Universe theory

What did Edwin Hubble find in 1923?

Cepheid novae in Andromeda Nebula (M31)

What are the name origins of the Andromeda Nebula?

located in Andromeda constellation

What are the word origins of “Andromeda?”

Greek princess

What did Edwin Hubble find of the Andromeda Nebula with the PL relation?

distance of 1 000 000 lightyears

What was Edwin Hubble’s observation process of the Andromeda Nebula?

1. got light curve

2. measured pulsation P

• 31 days

3. got distance

What was Edwin Hubble’s response to his results in February 1924?

wrote Harlow Shapley a letter

What was launched in honour of Edwin Hubble’s discovery in 1990?

Hubble Space Telescope

What does the James Webb Space Telescope improve from the Hubble Space Telescope?

• bigger primary mirror

  • 6x larger

• spectroscope

  • probe galaxy chemical composition

What is the definition of “S galaxy?”

spiral

• flat disc containing spiral arms & possibly central bulge

What is the definition of “SB galaxy?”

barred spiral

• spiral with central bar

What is the S & SB galaxy subclassification?

arm tightness & bulge brightness

What is the definition of “Hubble Sequence” or “Hubble tuning fork?”

galaxy classification system by appearance

What is the percentage of S galaxies relative to known galaxies?

30%

What is the percentage of SB galaxies relative to S galaxies?

60%

What is the reason that the Milky Way is a SB galaxy?

further away from bulge = less stars

• infrared

What are some possible reasons for SB galaxies?

• temporary & recurring phenomenon

• collision

What is the current reason for S galaxies?

2 non-S galaxies merge

What is the definition of “S//B0 galaxy?”

lenticular

• flat disc & possible central bulge

• no spiral

What is the percentage of S0 galaxies relative to known galaxies?

15%

What are some possible reasons for S//B0 galaxies?

• old spiral whose arms faded away

• new spiral that didn’t develop arms yet

What is the definition of “E galaxy?”

elliptical

• elongated spheroid

What is the percentage of E galaxies relative to known galaxies?

5%

What do E galaxy numbers assign?

higher number = more elongated

What is a possible reason for E galaxies?

• 2 approximately equal mass galaxies merge