Suppose that I got a white dwarf and a neutron star and after some trillions of years their temperature are down to just a few °K so cold that they don't emit any appreciable black body radiation.
Now, I send to each one, a robot/spaceship with a huge spotlight and shine a white light into them, possibly even making those old balls as illuminated as the Earth is, even if just for some seconds or minutes.
Also, the robots/spaceships takes photos to be presented as human eyes would see on a distance enough to see them roughly the same angular size that a basketball has in my hand when my arm is extended.
So, let's go to the actual questions.
What would be the color of the stars?
- Would they act like perfect spherical mirrors reflecting not just the spotlight, but everything else nearby?
- Or would they appear white by reflecting all that light diffusely?
- Or maybe pitch black because they would absorve all the light and reflect nothing?
- Perhaps gray, by partially reflecting and partially absorving the light diffusely equally in all wavelengths?
- Or silvery?
- Or perhaps, due to Doppler gravitational reddening, golden?
- Or, maybe I am missing something funny and they are pink, blue or green?
Could I distinguish them only by looking at the photos without measuring density, gravity, size or radiation, considering that the photos show them with the same angular diameter?