Explanation: How big is our universe? This question, among others, was debated by two leading astronomers in 1920 in what has since become known as astronomy's Great Debate. Many astronomers then believed that our Milky Way Galaxy was the entire universe. Many others, though, believed that our galaxy was just one of many. In the Great Debate, each argument was detailed, but no consensus was reached. The answer came over three years later with the detected variation of single spot in the Andromeda Nebula, as shown on the original glass discovery plate digitally reproduced here. When Edwin Hubble compared images, he noticed that this spot varied, and on October 6, 1923 wrote "VAR!" on the plate. The best explanation, Hubble knew, was that this spot was the image of a variable star that was very far away. So M31 was really the Andromeda Galaxy -- a galaxy possibly similar to our own. Annotated 100 years ago, the featured image may not be pretty, but the variable spot on it opened a window through which humanity gazed knowingly, for the first time, into a surprisingly vast cosmos.
This is indeed the perfect APOD for 6 October, 2023, exactly 100 years after Edwin Hubble "discovered the universe" (and proved that the Milky Way isn't "all there is").
Click to play embedded YouTube video.
Is that all there is?
It was Harlow Shapley that made Americans believe that "the Milky Way is the Universe" after his Great Debate with Heber Curtis. (Well, I'm sure I've read somewhere that smart Europeans had already figured out that those "island universes" are indeed galaxies of their own, but the Europeans let the Americans live on in their cosmic ignorance. Of course, I can't show you where I read it. )
Anyway, Harlow Shapley was a really interesting character, in spite of his galactic misconceptions. He grew up on a farm in Missouri, only went to elementary school there, studied on his own at home and started working as a reporter - after having no more formal schooling than elementary school! - and later completed a six-year high school program in 1.5 years. Thereafter he went to university to study journalism, but
When he learned that the opening of the School of Journalism had been postponed for a year, he decided to study the first subject he came across in the course directory. Rejecting Archaeology, which Shapley later claimed he could not pronounce, he chose the next subject, Astronomy.
Shapley is famous for using the location of globular clusters to prove that the center of the Milky Way is in Sagittarius, and that the Milky Way is much larger than people had thought. (Well, it had to be large, if it was all there is!)
Shapley was able to (more or less) determine the distance to most globulars in the Milky Way thanks to the presence of variable stars, RR Lyrae stars, in the globulars. Henrietta Swan Leavitt had discovered another type of (brighter) variable stars, Cepheids, in the Small Magellanic Cloud, and she had showed that the period of variability of these stars is directly linked to the intrinsic luminosity of them.
On July 4, 1868, American astronomer Henrietta Swan Leavitt was born. She is best known for her discovery of the relation between the luminosity and the period of Cepheid variable stars. Based on her luminosity-period relation for Cepheids, Edwin Hubble was able to determine that the universe is expanding.
Actually Hubble used redshift and not Cepheids to show that the universe is expanding. But indeed, Hubble's first stepping stone came when he found one of "Henrietta's stars", a Cepheid, in the Andromeda Galaxy, and could show that Andromeda is well outside the limits of the Milky Way. (Well... our halos are probably touching, but that's another matter.)
Quite the historical image! To me Hubble's discovery is way more staggering than all previous major revolutions in astronomy - the Earth is not flat but a sphere, it circles the sun and not the other way around - these seem like kids' stuff compared to trying to fathom that what was once thought to be our "universe" was but a mere galaxy... among billions of others! Humanity is still not through wrapping its head around this one!
Here's another image to go with today's APOD, Hubble showing his stuff to Einstein:
Attachments
Last edited by Christian G. on Fri Oct 06, 2023 2:36 pm, edited 1 time in total.
One of the true pioneers in astronomy. Also a patriot, Major in the 86th Division in WWI. Such a tragic loss, his death at 63 - how much more he might have contributed had he lived another 20 years,as is possible today.
This photo needs better explanation IMO. I followed the links and read the other posts here, but must have missed some important details.
The variable star I take it is the one between the two lines near the top and first marked "N" (for nova), but then crossed out and marked as "VAR" instead. There's one other "N"ova between two lines below, and another "N"ova with only one line for some reason.
How did Hubble determine that his previously marked "N"ova was really instead a variable star? Was it just because it increased and decreased in brightness several times, which a nova presumably would not do? Or was it something more specific than that, like a tell-tale spectra?
And, why does the determination of this as being in fact a variable star mean that Andromeda is very much farther away than thought? Because such a variable star has a known (standard) maximum absolute brightness, and it's brightness here was so much less, which must imply it must be farther away (obeying the inverse square law)?
Finally, even if the variable star is now known to be 2.5 Mly away, what said it must be IN Andromeda and not behind it?
Last edited by johnnydeep on Fri Oct 06, 2023 7:38 pm, edited 1 time in total.
-- "To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}
johnnydeep wrote: ↑Fri Oct 06, 2023 6:00 pm
This photo needs better explanation IMO. I followed the links and read the other posts here, but must have missed some important details.
The variable star I take it is the one between the two lines near the top and first marked "N" (for nova), but then crossed out and marked as "VAR" instead. There's one other "N"ova between two lines below, and another "N"ova with only one line for some reason.
How did Hubble determine that his previously marked "N"ova was really instead a variable star? Was it just because it increased and decreased in brightness several times, which a nova presumably would not do? Or was it something more specific than that, like a tell-tale spectra?
And, why does the determination of this as being in fact a variable star mean that Andromeda is very much farther away that thought? Because such a variable star has a known (standard) maximum absolute brightness, and it's brightness here was so much less, which must imply it must be farther away (obeying the inverse square law)?
Finally, even if the variable star is now known to be 2.5 Mly away, what said it must be IN Andromeda and not behind it?
The only way that Hubble could have found the Cepheid in Andromeda was by taking many pictures of Andromeda, maybe even one exposure per night (or two exposures?). Then he could compare many pictures of Andromeda, and inspect them star by star by carefully scrutinizing and comparing them, until he found one star that varied in a regular manner.
johnnydeep wrote: ↑Fri Oct 06, 2023 6:00 pm
This photo needs better explanation IMO. I followed the links and read the other posts here, but must have missed some important details.
The variable star I take it is the one between the two lines near the top and first marked "N" (for nova), but then crossed out and marked as "VAR" instead. There's one other "N"ova between two lines below, and another "N"ova with only one line for some reason.
How did Hubble determine that his previously marked "N"ova was really instead a variable star? Was it just because it increased and decreased in brightness several times, which a nova presumably would not do? Or was it something more specific than that, like a tell-tale spectra?
And, why does the determination of this as being in fact a variable star mean that Andromeda is very much farther away that thought? Because such a variable star has a known (standard) maximum absolute brightness, and it's brightness here was so much less, which must imply it must be farther away (obeying the inverse square law)?
Finally, even if the variable star is now known to be 2.5 Mly away, what said it must be IN Andromeda and not behind it?
Hubble had series of plates and was looking for changes (thus the novas). The period-luminosity relationship of Cepheids had been established for 15 years already. So once he saw that the star was variable and not a nova, he could calculate its distance... which was vastly greater than that of all the previous Cepheids, all of which were in the Milky Way. I suppose it would be possible to interpret this as a star that lay far behind a nearby Andromeda nebula, but the physical proximity, common redshift (which was being measured by 1923) and the simple reality of anything being so far away all pretty clearly pointed to a star in Andromeda. (One thing that distinguishes Cepheids from other types of variables is that their spectral type changes over a cycle, because they are swelling and shrinking. That was also known and measurable in 1923.)
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
Sorry to be so obtuse... but the location of the star seemingly labeled VAR! shows only a tiny speck between the diagonal lines and is nowhere near the image of the 'nebula'; it's almost out of the frame. So we assume that a) Hubble must have seen it being brighter in earlier images and now sees only the tiny speck, hence it is variable, and that b) he considered the star to be part of the Andromeda 'nebula'
even though it's widely separated?
If the magnification of the telescope had been slightly higher or the image had been centered slightly differently that location would not have been visible at all.
- Was Hubble using a blink comparator and if so why? Had he been searching for moving objects and noticed a variable star instead?
- He must have made numerous follow-up observations to ascertain the variability and thus the intrinsic brightness of the star, giving the distance. Is it possible he returned to this image, crossed out 'N' and wrote in 'VAR!' sometime later? In other words he had the discovery image before he realized it was the discovery image?
So many questions...
WSODONNELL2 wrote: ↑Sat Oct 07, 2023 12:05 pm
Sorry to be so obtuse... but the location of the star seemingly labeled VAR! shows only a tiny speck between the diagonal lines and is nowhere near the image of the 'nebula'; it's almost out of the frame. So we assume that a) Hubble must have seen it being brighter in earlier images and now sees only the tiny speck, hence it is variable, and that b) he considered the star to be part of the Andromeda 'nebula'
even though it's widely separated?
If the magnification of the telescope had been slightly higher or the image had been centered slightly differently that location would not have been visible at all.
- Was Hubble using a blink comparator and if so why? Had he been searching for moving objects and noticed a variable star instead?
- He must have made numerous follow-up observations to ascertain the variability and thus the intrinsic brightness of the star, giving the distance. Is it possible he returned to this image, crossed out 'N' and wrote in 'VAR!' sometime later? In other words he had the discovery image before he realized it was the discovery image?
So many questions...
What you're missing is the plate scale. The galaxy extends far beyond the edges of the plate. The "nebula" here is just the bright central bulge. The variable star is quite deep inside the galaxy, which Hubble certainly knew.
Hubble was studying variable stars, and was certainly using a blink comparator. The plate seen in this APOD is cataloged as H335H. He identified the Cepheid (initially misidentified as a nova) by comparison with an earlier image, H331H.
_
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
Explanation: How big is our universe? This question, among others, was debated by two leading astronomers in 1920 in what has since become known as astronomy's Great Debate. Many astronomers then believed that our Milky Way Galaxy was the entire universe. Many others, though, believed that our galaxy was just one of many. In the Great Debate, each argument was detailed, but no consensus was reached. The answer came over three years later with the detected variation of single spot in the Andromeda Nebula, as shown on the original glass discovery plate digitally reproduced here. When Edwin Hubble compared images, he noticed that this spot varied, and on October 6, 1923 wrote "VAR!" on the plate. The best explanation, Hubble knew, was that this spot was the image of a variable star that was very far away. So M31 was really the Andromeda Galaxy -- a galaxy possibly similar to our own. Annotated 100 years ago, the featured image may not be pretty, but the variable spot on it opened a window through which humanity gazed knowingly, for the first time, into a surprisingly vast cosmos.
Explanation: How big is our universe? This question, among others, was debated by two leading astronomers in 1920 in what has since become known as astronomy's Great Debate. Many astronomers then believed that our Milky Way Galaxy was the entire universe. Many others, though, believed that our galaxy was just one of many. In the Great Debate, each argument was detailed, but no consensus was reached. The answer came over three years later with the detected variation of single spot in the Andromeda Nebula, as shown on the original glass discovery plate digitally reproduced here. When Edwin Hubble compared images, he noticed that this spot varied, and on October 6, 1923 wrote "VAR!" on the plate. The best explanation, Hubble knew, was that this spot was the image of a variable star that was very far away. So M31 was really the Andromeda Galaxy -- a galaxy possibly similar to our own. Annotated 100 years ago, the featured image may not be pretty, but the variable spot on it opened a window through which humanity gazed knowingly, for the first time, into a surprisingly vast cosmos.
Edwin Hubble Discovers the Universe
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