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Discovery of Other Galaxies: Ch 51, Archives Read Chapter 53, Secrets. Prologue: The Universe as we know it was revealed to astronomers on New Year’s Day 1925. The man responsible, Edwin Hubble, was not present. His historic paper was read to the 33rd annual meeting of the AAS.
Prologue: The Universe as we know it was revealed to astronomers on New Year’s Day 1925.
The man responsible, Edwin Hubble, was not present. His historic paper was read to the 33rd annual meeting of the AAS.
The Milky Way was suddenly humbled, becoming just one of a multitude of galaxies residing in the vast gulfs of space.
At the end of the 18th century,
William Hershel had
cataloged 100’s of spiral nebulae.
He, I. Kant, T. Wright thought
them “Island Universes”, distant
If you observed our solar system from afar, what would look different about our solar system than a spiral nebula?
Can see satellites (i.e., planets)
Can’t see spiral arms of light
Can’t see central brightness
What do you see?
Proto- solar system, collision of a star with a gaseous nebula, or a twin to the Milky Way?
To resolve the scale of the Universe and the nature of the Spiral nebulae
April 26, 1920 at the Smithsonian Museum of Natural History,
arguments written in 1921
Shapley: Milky Way was immense (Sun off-center) and contained the spiral
Nebulae, which themselves were close (inside MW) and small.
Curtis: The spiral nebulae were “island Universes” (hypothesis)
same as MW but far away (Sun at center of MW).
If Andromeda like MW, how far?
200’=1.2x104”=0.06 radians =50 kpc/D
so D=800 kpc (and Opik 1922 got this same from brightness=MW)
Some were wrong, some were right and correctly understood,
and some were right but wrongly interpreted due to lack of
imagination. A truly fascinating debate…like Darwin’s Evolution!
Adrian Van Maanen claimed to see
rotation (said he could see 0.1”/yr)
0.1”=1/(2x106) radians/yr so that’s
D/ (2x106) =0.4 pc/yr=1.3 ly/year!!
Faster than speed of light!! Impossible! Surmised Andromeda
must be much closer, smaller, and inside MW.
Is this spiral nebula actually rotating? (This is M101,
the “pinwheel” galaxy”). Does its form draw you to conclude
Shapley wrote to van Maanan: “Congratulations on the nebulous results! Between
us we have put a crimp in the island universes, it seems, you by bringing the
spirals in and I by pushing the Galaxy out. We are indeed clever, we are.”
The pinwheel looked like it was rotating to me
a) yes b) no
A supernova suddenly appeared in Andromeda
in 1885 (~5.6 mag, naked eye!).
Inverse Square Law:
LSN/ Lbarnard= (DSN/Dbarnard)2 bSN/ bbarnard
If Andromeda at 800 kpc then…
DSN/Dbarnard=800 kpc/1.8pc=4x105 , bSN/ bbarnard=40
LSN/ Lbarnard=5x1012 so LSN= 2x109 Lsun !!!
Shapley could not imagine anything could be as bright as
2 billion Suns so DSN must be much smaller, inside MW, right?!
Agnes Clarke, “a scale of magnitude such as the imagination
recoils from contemplating”. Brighter one in 1895 in NGC 5253!
Map of Spiral Nebulae
Shapley thought MW disk inhibited formation of nebulae.
Curtis reasoned it was dust in the plane of the MW blocking
outside light, thus Spiral Nebulae were well outside MW.
Debate widely viewed a draw.
Then along came Edwin Hubble…
Considered most important telescope…
Incredible engineering feat (90 tons)
First light in Nov. 2, 1917, by George HaleEdwin Powell Hubble
1889-1953, Athlete, Rhodes Scholar, born in
Missouri, basketball coach, lawyer, in WWI.
1917 PhD in astronomy, U Chicago,
“Photographic Investigations of Faint Nebulae”
1919--went to Mt. Wilson, best telescopes
“He was part of a select group in California that for several
decades dominated astronomy’s discoveries in the far Universe
because of its employment of the world’s largest and best
Telescope.” In the fall of 1923, Hubble began a study of
Andromeda, spotting two ordinary novae and a variable star…
Giant, pulsating stars
Light Curve of Cepheid Variable
In ~1900 they could see Cepheids at ~ 1 Megaparsec (106 pc)=2x1011 au
That’s about a million times further than parallax and
Almost a trillion times the distance to the Sun or 3.2 million light years!
Edwin Hubble using the 100” on Mt. Wilson Discovered Cepheids
in Andromeda (Var! upper right) starting with this one.
Page from lab notebook. Saw
that the period was 31 days and that its mean brightness was
4000 times fainter than Barnard’s star (18.65 mag). In paper
claimed 130 platesx35 minutes/plate. At 2 good hrs per night,
that’s 38 nights or ~75 nights due to weather on best telescopes!!
Inverse Square Law:
Dceph=Dbarnard* (bbarnard/ bceph)1/2(Lceph/Lbarnard)1/2
Dceph=1.8pc* (4x103)1/2 *(2.5x107)1/2=1.8pc*3x105=600kpc
=1,800,000 ly. Much bigger than even Shapley’s 50 kpc=
160,000 ly diameter Milky Way! Andromeda must be outside MW!
PHAT HST Survey; Riess et al. 2012
Hubble could eventually see the whole period-luminosity relation.
Saw no Cepheids away from Andromeda so knew
that Cepheids were inside Andromeda.
Van Maanen resisted conceding, but Shapley did.
Hubble hesitated leading to his paper being delayed and read
Curtis: “Recent results by Hubble make the theory doubly certain”
Shapley: Said to Payne-Gaposhkin when letter arrived from
Hubble, “Here is the letter that destroyed my Universe.”
“I believed in van Maanen’s results…after all he was my friend.”
Same idea today with Cepheids 4000 times fainter, galaxies 60 times farther!! ~35 Mpc
Anchor (NGC 4258): Galaxy with Cepheids
whose distance we know by geometry
Supernova Host: Galaxy with Cepheids
and an SN Ia, former calibrates latter
Using a “distance ladder” we knit together progressively longer range
distance indicators to ~300 million light years
Increasing ratio of arms to nucleus
Hubble’s Tuning Fork:A Tool to Classify Galaxies