The Milky Way is much more than just a pretty patch of stars across our nighttime sky.  It is our home galaxy, where the name describes its appearance from Earth: ...a hazy, faint band of cloud-like light formed from stars that cannot be individually distinguished by the naked (unaided) eye.  It is estimated the Milky Way galaxy contains between 100 and 150 Billion stars, encompassing most all the stars we see in our nighttime skies.

A frequently asked question is, "Why doesn't the Milky Way galaxy look like a round, spiral disk?"  The answer lies in where we are located inside the galaxy.  The Milky Way actually DOES mimic a round spiral disk.  It's just that because we are located INSIDE one of the spiral 'arms' of the galaxy appx 1/3 of the way outward from the central core nucleus toward the outer rim of the galaxy, then from our point-of-view, we see the galaxy "edge-on".    As an analogy, think of a dinner plate which appears round when held-up in front of our face.  But when turned to present only the edge of the plate to our eyes, it begins to approximate our view of the Milky Way.

We see portions of the Milky Way galaxy at all times during the year... 
 – In the winter months of December thru February we see the apparent northern end when it appears more diffuse and sparse.  This is because the nighttime skies of Earth are 'turned' such that we are looking outward, away from the central core.  (Or said another way, the 'nucleus' of the galaxy is in the direction 'behind' us, ...on the other side of the Earth.)  This is when we see thru only about 1/3 of the galaxy that lies between us and that region of outer space beyond the rimmed-edge of the galaxy.  Outer space as it exists outside the galaxy is known as "the intergalactic medium", where the expanse of space contains no stars, no nebulae, and no gas or dust clouds.
 – In the captured image, the view of the Milky Way encompasses the apparent southern end of the cloud-like band in the summer-time months of June thru August.  This is the time of year when we see the most interesting parts of our home galaxy.  It is when our view is looking inward toward the central core where the galaxy is much more well defined; and the stars, nebulae , and gas and dust clouds, are much more dense and packed closely together. 



THE MILKY WAY'S BLACK HOLE

Scientists have speculated for decades over what kind of gravitational object might exist at the center of our galaxy that causes all the stars, nebulae , and gas and dust clouds to move in a swirl around it.  Ever since the 1930's, Albert Einstein's theories have pointed toward the possible existence of a super-massive object (...referred to today as a "black hole") that would be so massive, nothing could escape its gravity, not even light itself. 

The pace of discovery in astronomical science has accelerated significantly since just before the COVID-19 pandemic in 2020.  It wasn't until 2022 that scientists proved Einstein was correct...   After finding evidence, in the form of radio emissions, of a super massive black hole at the center of the Milky Way, Scientists have figured-out a way to photograph it. 

Here is a chronology of recent discovery and consequential proof that a supermassive object actually does exist in nature:
 – 1974 – A very strong astronomical radio source is discovered by astronomers Bruce Balick of Denmark and Robert L. Brown, of the USA.  It is an object, very compact and very bright in its emission of electromagnetic radiation (...or "EM" radiation) in the frequency spectrum of radio waves; and it is located appx 27,000 light-years away very near the center of the Milky Way galaxy. 
 – 1982 – The asterisk * is assigned by Brown, who understands that the strongest EM radio emission from the center of the galaxy appears to be due to a compact non-thermal radio object.  The name, "Sagittarius A* " (pronounced, 'Sagittarius A-star') distinguishes the compact source from the larger Sagittarius A region in which it is embedded.
 – 1982 to 2020 – Observations of several stars orbiting Sagittarius A*, particularly star S2, are used to determine the mass and upper limits on the size of the object.  Based on its calculated mass and radius limits, astronomers conclude that Sagittarius A* must be a central supermassive black hole at the center of the Milky Way galaxy.  But at the time, there is still no direct scientific link.  The existence of a black hole remains only a hypothesis. 
 – 2020 (the year of the Covid-19 pandemic) – Three (3) Scientists from different regions of the world collaborate together and discover (by way of measured empirical mathematical proof) that Sagittarius A* is, in fact, a supermassive compact object, for which a black hole is the only plausible explanation at the time.
       • Roger Penrose, Prof. of Mathematics at Oxford Univ. in the U.K., together with
       • Reinhard Genzel, Director of the Max Planck Institute for Extraterrestrial Physics in Germany, and
       • Andrea Ghez, Prof. of Physics and Astronomy, Univ. of California, Los Angeles
All 3 share together in the award of the 2020 Nobel Prize in Physics.
 – May 12, 2022 – the first image of Sagittarius A* is released by the Event Horizon Telescope ("EHT") multi-national collaboration team.  The image, which is based on radio interferometry, confirms that the object contains a black hole.  The image of Sagittarius A* took five years of data collection and calculations to process.  The data was collected from eight (8) radio observatories located at six geographical sites across the Earth, collectively and effectively modeling a single telescope with an aperture as large as the Earth itself.  The technique is referred to as aperture synthesis, where the captured data is collected through all-night-long observations of stable radio sources like Sagittarius A*.


THE SCIENCE OF BLACK HOLES

A black hole emits no light.  In addition, this one is completely obscured from our visible, line-of-sight view by heavy concentrations of galactic gas and dust.   Nonetheless, scientists are able to capture EM radio frequency emissions whose lower-than-visible-light frequencies enable those emissions to penetrate the clouds of dust, and to be detected by radio telescopes here on Earth. 

Scientists have demonstrated that because of the extreme distance between us and Sagittarius A*, the apparent size of the Milky Way's 'beating heart' black hole (...that is, its visual size as we might see it in the sky, if we could, from here on Earth) is appx the size of a doughnut sitting on the surface of the Moon.   ...but the actual physical size of the MW black hole, while larger than our Sun, is actually not as large as scientists first theorized.  Its overall angular diameter approximates the size of the orbit of planet Mercury.  We could certainly consider that to be large; but, it is actually considerably smaller than many of the 'super-giant' stars found across the Milky Way.  

...but what makes this so interesting is that the black hole's mass (...a measure of the total amount of matter crammed inside of it) is over 4 million times that of our Sun.  That conclusion makes the density of our Milky Way's black hole 1,000's of times heavier and more dense than the most dense substance known here on Earth - metallic Osmium.   Osmium is chemically classified as an element, occupying position No. 76 in the periodic table of elements.  It is appx 2x heavier - and more dense - than lead.  We have no such classification for the matter that comprises a black hole; because, it does not exist here on Earth.  But we do understand that if we were to encounter a fist-sized "piece" of the MW Black Hole lying on the ground, we wouldn't even be able to pick it up; because, it would weigh 1,000's of pounds.

One last, amazing fact about the discovery of Sagittarius A* is that the EHT's measurements tested Einstein's theory of relativity more rigorously than has ever been done before; ...and the measured results match perfectly the calculated predictions of Einstein's theory.
120720 Milky Way South End starscape

(under construction)
