Bohr’s model is a pillar of the "old quantum theory" – the transitional body of work straddling classical physics and modern quantum mechanics. These early results, which explicitly called for new physics, set physicists on the road to our present understanding of Nature at the atomic and sub-atomic level in terms of quantum mechanics.

Bohr's model was purely phenomenological. It was a hodgepodge of early quantum ideas that fit the data without offering any deeper explanation.

It also preceded de Broglie's hypothesis, which would have allowed Bohr to identify these orbits as integer multiples of an electron's wavelength.

But it seemed to resolve –– or at least relate –– two outstanding puzzles that had baffled scientists. So he was clearly onto something.

Bohr addressed both issues at once by hypothesizing the existence of "stationary orbits" around the nucleus.

Electrons could exist in these stationary orbits without (as expected from classical electrodynamics) radiating energy.

Transitions between these orbits would emit energy in discrete amounts (related to Planck's quanta) dictated by the initial and final level. This reproduced the empirical formulas obtained by Balmer and Rydberg for the wavelengths of the spectral lines.

Classical physics was already having problems with atomic phenomena.

Atoms were known to emit and absorb light preferentially at certain wavelengths. The "spectral lines" fit simple patterns but had no known explanation.

Image: Emission spectrum of Hydrogen

In 1911, scattering experiments by Ernest Rutherford and collaborators had disproven the “plum pudding” model of the atom.

Rather than a blob-like structure with + and - charge spread throughout, experiment indicated atoms have a tiny positive nucleus.

Negative electrons must “orbit” this nucleus. But classically such a configuration is not stable! The accelerating electrons would radiate energy and spiral into the nucleus.

Rutherford’s atom seemed incompatible with known physics!

Danish physicist Niels Bohr was born #OTD in 1885.

Bohr applied the nascent quantum theory to the structure of the Hydrogen atom, offering an explanation (though not a true mechanism) for an outstanding problem that classical physics could not account for.

Image: Atomic Heritage Foundation

About ten years ago, the Hubble Space Telescope was pointed at M31 to record an image of the star. Here's an Earth-based photo of the galaxy with insets showing the region imaged by HST, and the region covered by the Var! plate.

The copy of Hubble's “Var!” plate in this image has been enhanced to make the star much more visible.

Image: R. Gendler, NASA, ESA, Hubble Heritage Team (STScI/AURA)

(Hopefully this gif is displaying correctly for you. It alternates frames at 1s intervals in the preview, but in the actual post my client is showing a much different delay.)

@zleap Yes, that's exactly what these were.

Here is a blink comparator gif of the two plates, switching back and forth with a 1s interval and cropped to show the region with the Cepheid variable.

Do you think you would have spotted it?

The images I have access to have been compressed, and the quality isn't too high, so it becomes almost impossible to see it on the screen. It would have been more apparent (though still very faint) on the original plate.

@HoustonDog There was debate. But definitive proof was collected 100 years ago today. He would announce it a bit later.

You can see three marks "N" on the plate where Hubble identified novae.

Comparing with a plate taken the night before, he realized that the brightness of one was changing. Recognizing a Cepheid variable star, he crossed out the "N" and wrote ‘VAR!’

Here are the plates side-by-side. H331H was taken the previous night, a 40-minute exposure with poor seeing conditions. Plate H335H, which revealed the Cepheid, was a slightly longer exposure under better conditions.

Image: Carnegie Observatories

VAR!

In the early morning hours #OTD in 1923, Edwin Hubble took a photo plate of M31 showing a Cepheid variable star.

Using Henrietta Swan Leavitt’s distance-luminosity relationship, Hubble concluded that M31 is located outside the Milky Way.

This observation established that there are other galaxies besides our Milky Way, that our little island is not the whole Universe.

Image: Carnegie Observatories

Friends, my pal Andrew Gill has put together a great podcast about the world of Christian rock, folks who grew up in it, and their stories. If that’s up your alley, I hope you’ll check it out. If you do, please leave a review!

https://podcasts.apple.com/us/podcast/rock-that-doesnt-roll-the-story-of-christian-music/id1703257857

And if you've never seen "Wanderers,” that's something you should correct right away!

https://vimeo.com/108650530

If you like outer space, you've probably seen ErikWernquist's wonderful short film "Wanderers."

A few days ago he released a new short: "One Revolution Per Minute." It's set aboard a planetary orbiter that uses rotation to simulate gravity.

https://vimeo.com/869858712

The mood in the Pentagon was grim. A young Jeff Goldblum raced down the hallways to let his bosses know the latest intel on the Soviet satellite.

https://youtu.be/kRblCP6r_8I?si=IrQ_aKPwtzo38Doq

But in my opinion this is the best one: NASA's recording of the Sputnik “beep.”

https://soundcloud.com/nasa/sputnik-beep?si=2c7b72d82a224767a7e017cf248b549b

NASA has a terrific collection of resources related to Sputnik 1.

Here's the first US image of the satellite, taken by the Smithsonian Astrophysical Observatory's IGY satellite tracking camera on October 17, 1957.

https://history.nasa.gov/sputnik.html

Image: NASA / SAO

Eleven days later, on the 15th, scientists at the Naval Research Laboratory produced this plot showing where Sputnik 1 would pass over the course of its next few orbits.

This was the first example of scientists tracking and then forecasting the position of a human-made object in space.

Image: Naval Research Laboratory