APOD from 2025-06-16

APOD is 30 Years Old Today

Celebrating 30 years, today's APOD forms Van Gogh's "Starry Night" from 1,836 past images arranged into 32,232 tiles. Thanks to all contributors and readers for sharing the cosmic journey.

HD image at https://apod.nasa.gov/apod/ap250616.html#space #astronomy #planet #earth

This is the 100 year anniversary of humans having an idea of what the heck the sun and all the stars actually are. If you had asked a leading astronomer in 1925 what the sun was, they would say that it's basically the same as Earth, but very hot.

In Cecilia Payne's doctoral thesis she was the first to say, from spectral data, that the sun was overwhelmingly made of hydrogen and helium.

It was later described as "the most brilliant PhD thesis ever written in astronomy".

https://en.wikipedia.org/wiki/Cecilia_Payne-Gaposchkin

Physicist Helen Quinn was born #OTD in 1943. She is best known for Peccei-Quinn theory, a proposed explanation of the Strong CP problem of quantum chromodynamics.

Peccei-Quinn theory also implies the existence of a very light particle –the Axion– which is a pretty good dark matter candidate.

Image: H. Quinn/ @QuantaMagazine

Astronomers just found the smallest galaxy ever

With just a mere 16 solar masses worth of stars, galaxy Ursa Major III/UNIONS 1 is by far the smallest galaxy ever discovered.

But is it truly a galaxy, or just a collection of stars?
https://bigthink.com/starts-with-a-bang/astronomers-smallest-galaxy-ever/