Ciro Santilli likes to learn astronomy a bit like he learns geography: go down some lists of "stuff that seems most relevant in some criteria to us!", possibly at different size scales e.g.:
We can't see individual stars outside of the Milky Way: https://earthsky.org/tonight/can-we-see-stars-outside-our-milky-way-galaxy/.
The Large Magellanic Cloud stands out as the brightest thing we can see from outside the Milky Way by far!
Some stars are so close that we can actually see their angles move with time due to the relative motion between them and the Sun, e.g. Proxima Centauri!
A fancy name for astronomy ;-)
Sometimes it feels like this could be how we finally make experiments to see what the theory of everything looks like, a bit like the first high energy experiments were from less exotic cosmic rays.
Surely You're Joking, Mr. Feynman chapter An Offer You Must Refuse (a play on words on The Godfather (1972)) has an interesting historical mention from the early 1950s while at Caltech:
The next day, I had the greatest luck in making a decision. God must have set it up to help me decide. I was walking to my office, and a guy came running up to me and said, "Hey, Feynman! Did you hear what happened? Baade found that there are two different populations of stars! All the measurements we had been making of the distances to the galaxies had been based on Cephid variables of one type, but there's another type, so the universe is twice, or three, or even four times as old as we thought!"I knew the problem. In those days, the earth appeared to be older than the universe. The earth was four and a half billion, and the universe was only a couple, or three billion years old. It was a great puzzle. And this discovery resolved all that: The universe was now demonstrably older than was previously thought. And I got this information right away - the guy came running up to me to tell me all this.
The existence of this is quite mind blowing.
It is basically perfectly black-body radiation, with a very faintly by measurable anisotropy (slightly less or slightly more some regions) due to quantum fluctuations of the early universe.
To make sense of it, you can think of the universe as the expanding raisin bread model, but it expands faster than light (thus the existence of the cosmological event horizon), so we are still receiving light form the middle, not the borders.
The ansiotropies of CMB are likely the best astronomical compass we will ever have, as it is the thing with the least proper motion.
What an awesome list the dude compiled. Contains many of the features we care the most about of the sky, since of course, apparent magnitude is a big determinant of that.
Interesting to note that there are quite a few nearer than Sagittarius A, as of 2022 we know of one at 1500 kly.
Cover up the entire sky in a compatible way with the traditional constellations. They are also very square, the boundaries consisting only of vertical and horizontal lines on the sphere.
Basically a mini-Constellation.
Composed mostly of the Virgo cluster and the Local group.
Some major ones:https://noirlab.edu/public/images/noao-m49/?nocache=true also lists: M58, M59, M60, M61, M84, M85, M86, M87, M88, M89, M90, M91, M98, M99, and M100 so lots of large and easily observable galaxies in the area.
The basically composed of only the Andromeda Galaxy and the Milky Way. Every other galaxy is a satellite of those two.
First proper nearest galaxy to the Milky Way. Everything in the middle in the Local group is either a satellite of the Milky Way or Andromeda.
Many Andromeda satellite galaxies are simply numbered Andromeda II, Andromeda III and so on.
As described on Wikipedia, the observational history of Andromeda is fascinating. Little by little, people noticed that it had a different nature to many other objects observed on the sky, and the hypothesis that there are other galaxies like ours grew in force.
Part of our fascination with Andromeda is due to how similar in size and shape and close it is to the Milky Way.
It is clearly the only thing so large and so close.
Andromeda is, without a doubt, our sister galaxy.
The first proper galaxy near the Milky Way is the Andromeda Galaxy. Everything else in the middle is a satellite of either of of those.
Another important one to know is the
One of the brightest natural objects in the sky, and by far the brightest not in the Milky Way! This is partly because it is relatively close to us.
It is so close that we can notice its proper motion, and its distance to us will vary significantly across a few tens of thousands of years!
This is quite close! But as mentioned at: stars nearest to the Sun, there are several others nearby. Notably Sirius at 9 ly, the brightest star in the sky as of 2020.
Some notable ones:
- Proxima Centauri, the nearest one, at 4 ly. It is part of the Alpha Centauri star system, which contains two other stars at very similar distances as well, and their relative distances to earth will change positions in a few tens of thousands of years.
- Sirius, the brightest star in the sky at 9 ly
As usual, blame the Russians.
archean/proterozoic barrier.
The term "visible life" refers to multicellular from before people knew there was life in the proterozoic.
This period is similar to the Quaternary, but it also includes tool usage by close relatives of humans which were not humans yet.
It ends together with the pleistocene.
Agriculture is not the official definition of the age. But it is good enough. Likely related to the official end of glaciations thing.
No life, earth too hot, until formation of water.
Made up mosty of calcium carbonate.
Not done yet as of 2020! Will be done one day for sure.
The first planet not known since anitquity.
Quite cool how it was discoverd by the perturbation of Uranus' orbit.