how does astronomers come up with such specific pictures of a galaxy or planet?

Hi Purna!

You touched the answer in your question: predictable physics allows for us to do observe these stellar objects.

While humans are confined to the visible light range, telescopes can take advantage of the full spectrum of light (radio, microwave, infrared, visible, x-ray, gamma) to observe stellar objects. Additionally, astronomers can figure out the chemical composition of a stellar object using spectroscopy (if you pass the light from an object through a prism, you can see its components). This helps 'fingerprint' stellar objects from a distance and without needing a precise visual picture.

In the recent black hole picture, we already knew that the galaxy M87 is an 'active galaxy', meaning it has large black hole at its core. Galaxies also move very slowly as it appears to us, so we could predict the location of this black hole.

Some further reading if you're interested:

• NASA, "How Scientists Captured the First Image of a Black Hole"
• SpaceTelescope.org, "Black Holes, Quasars, and Active Galaxies"
• Wikipedia, "Astronomical spectroscopy" (more dense reading)