wasoxygen sent me a half-post, rambling question and comment. With his permission I am posting, answering and finishing his though for him

I am reading a book, originally published in 1962, about relativity. Chapter 11 was added in a 1976 edition, and describes "a series of incredible astronomical discoveries" that began in 1962: quasars, pulsars and black holes. The discovery of pulsars sounds like a great story, and I was thrilled to learn that there are a few "optical pulsars" that flash in visible light. Can you see the Crab Pulsar? I imagined a flashing star would be a bizarre spectacle, until the author pointed out that 30 flashes per second is too fast for the eye to detect.

The Crab Nebula has been photographed with amateur gear, even web cams modified to fit into the eyepiece, that is not the problem. The pulsar itself does not emit as much visible light as it does radio. This site says that the central pulsar is mag +16 in visible light, which is very much doable in my 14".... BUT... the rest of that area is an optical mess with a lot of stuff going on. I've looked at M1, the Crab Nebula many times and I do not recall seeing a faint central star in the cloud of gas. With a 20" or larger telescope and 300x magnification on a clear steady evening, this would be trivial. At this level of visual challenge, the sky becomes the limiting factor more so than the telescope. My 14" telescope has a theoretical limiting magnitude of +18; I've seen Pluto at Mag +14.5 and I've seen one very faint galaxy listed as Mag +15.5 (discussion on these numbers is below). As the pulses are so rapid, the star itself would not blink at you, but instead be a steady beam of starlight not unlike any other faint star. The SEDS site is your go-to for the Messier Objects including their history and some images of what they look like in visible light.

So let's stop and talk about magnitude. This is a logarithmic scale and each number represents the visual brightness of an object. An object can be screaming bright in X-rays and very, very faint in visual light and an object can be bright in the infrared and not at all visible in normal light. A Mag +1 star is exactly 100x brighter than a Mag +6 star. Venus gets to be about Mag -4.5, Sirius is about -1.1, Vega in the Summer Triangle is defined as Mag 0. Jupiter, the very bright unblinking 'star' to the south is about Mag -2.1. The dimmest thing we can see with normal, unaided human eyes on a perfect clear moonless night is about +6.5. My telescope has a listed limit of Mag +16 and I agree. Bigger telescopes collect more light and can see dimmer and dimmer objects, or increase the magnification on brighter objects. (There is a ton of physics and optics here, but the more magnification at the eyepiece, the dimmer you make the image you see.. so bigger scopes and magnify dimmer things.) A Mag +12 star is 100x dimmer than the stuff we see with our eyes, and so on down the line. The dimmest object that I can name off the top of my head that we as a species have seen is a Mag +27 and the Hubble has a theoretical limit of Mag +32, or roughly ten times dimmer still.

Speaking as an aside about flashing stars, Algol is an eclipsing binary that you can watch dim and brighten again in a few hours if you know when to look and have at least an 80mm telescope. It is there, gets crazy dim very quickly, then sort of 'pops' back into view.

I have only the vaguest understanding of what has been or can be observed in the night sky. Obviously all the visible stars have long since been mapped, and presumably all shining objects down to some level of faintness. But the famous Deep Field image shows how much is yet unseen. Can you help me comprehend what our current view of the sky is like? What fraction of the gathered information in the non-visible spectrum? Is a significant quantity of sky permanently invisible because nearby stars or the galactic center?

Considering the recent detection of Gravity Waves and take into account that normal matter is about 4% of the Universe's energy budget? Hell, who knows, man. We are blind deaf and mute to all but a tiny tiny slice of what makes the Universe work. source: https://en.wikipedia.org/wiki/Matter

Nerds in their back yards looking at the sky are still doing good serious work. There are groups who monitor variable star light curves. Guys with spare gear are looking for comets. Amateurs with 8" scopes and web cams just caught an asteroid impacting Jupiter that all the pros missed. There are groups working to time asteroid-star occultations to get size and orbital information. Spectroscopy gear is coming way down in price and there are groups that are using this to get spectra of every star visible in the sky down to Mag +10, something that the Pros do not have the time or money to do, but 10,000 guys in their spare time sure do. the gear is under 3K, which is about what a mid range good CCD costs.

On a clear summer night, Deneb, one of the bright stars in the summer triangle is visible. Deneb is an insanely huge star and arguably the farthest star you can see with the eye on a typical evening. When the light of Deneb began heading this way, Ur was the pinnacle of Human Civilization, horses had just been domesticated and Egypt was not yet an empire. M13, is visible under good conditions at 22,000 light years away. When the light of M13 was leaving and heading our way, we were domesticating wolves and barely beginning to plant our food and the earth was in a major ice age.

Here's a thought experiment that might be a good teaching tool. Suppose I open one browser window to a version of Google Sky that contains all astronomical data and another to Google Maps. I zoom all the way out so I can see the whole sky or earth. Then I zoom in on both maps to interesting locales.

You need a web tool to play with! http://thinkzone.wlonk.com/SS/SolarSystemModel.php Make the earth 30cm in diameter, or 12" in Freedom Units. Play with the Google earth overlays. Neptune at that scale is about a 100 minute drive away! This puts the Sun at about 110 feet in diameter, or bigger than a typical four storey building.

The closest Quasar is 3.5 Billion light years away. It is visible in the constellation Pegasus but the conditions have to be damn near perfect to see it in my 14". But you can see it. For most amateurs this is as far as you can go. I was lucky to pull Einstein's Cross on a special night with someone else's gear, so I've had 8 billion year old photons hit my eye. The individual pips are Mag +17 and at lower power it looks like a galaxy with a weird center. He had to pump up the magnification to see the five individual bright 'pips' but holy hell, I saw it! I consider that a darn good place to put the limit on visual astronomy. Cameras and long duration photography collect more light so that throws tech at the problem, which is why the Gemini, KECK and Hubble can see stuff so faint; they collect lots of bits of photons over time to build an image.

The long and short of it is this: we just realized in the last 60-70 years that there are questions we need to ask about our place in the Universe. Those questions as they get answered, lead to bigger and more mind boggling questions that we did not know we needed to ask even 10 years ago. Dark matter is still an unknown. The proof that the Universe is not just expanding, but accelerating its expansion did not happen until 1998!!!

I've rambled and link dumped enough for the evening, hope that gets you started, man.