How do we know all this stuff about Light and Black Holes and HDE 226868?

Question

http://www.astrosociety.org/education/publications/tnl/24/blackhole2.html

After reading about black-holes. I have a few queries

How do we know HDE 226868 is 10 solar masses?

How do we know that HDE 226868 is in a 5.6 day orbit around "Something dark"

I imagine the experts are just looking at photographs of millions of DOTS, so how the heck can be definitive?

THanks gys

Answer 1

HDE 226868 is a star which forms a double with an X-ray source called Cygnus X-1. Because the stars move around each other the visible one shows variations in position (it moves from left to right and back seen from the earth) and in colour of the emitted light. The latter is the Doppler effect for light: objects moving away from us are redder than objects moving toward us. (Compare with the Doppler effect for sound.)

From these measurements we can deduce the period and the speed of the stars around each other. Applying the laws of gravity this permits to deduce the masses of the stars.  

Answer 2

The star you're asking about is another example of how we know black holes exist.  Cygnus X-1 was the first.  

Using spectroscopy, we can tell a lot about a star.  We can get a good estimate of the star's mass.  We see some stars which are part of a close binary system, orbiting a companion in a period which would require that companion to be of stellar mass.  But there's no visible star there.  So what's making the other star orbit it?  Black holes were theorized.  The model works.  If we're right about black holes, the dark companion in a close double like this should emit intense x-rays.  They do.

Finally, the direct observation of emissions from pulsars also gives strength to the theory.  Since pulsars exist, then it is highly likely that we are right about black holes, which are the result of more massive stars.

As far as what you call "light holes," (white holes), there is no reason to believe they exist.  These were cooked up by someone trying to get attention in the field.