What causes a star to become a nova?
Most, if not all, novae are members of binary systems of binary systems in which one member is a body like a white dwarf on Black Hole whose intense gravitation draws out a stream of matter from the other body. From time to time such matter is transferred from the second star to the first star which undergoes an unpredictable violent explosion called a nova, often periodically every few decades.
Can a nova be visible to the eye?
Yes. For example a nova visible to the naked eye occurred in August 1975 in the constellation Cygnus. Typically, a nova’s brightness increases by a factor of 10,000 to 100,000 in a few days or even in a few days or even in a few hours, and returns to its original brightness, more or less, over or period of many years.
How can Black Holes be detected?
Since nothing can radiate out of a Black Hole, it is invisible and cannot be directly observed. It can only be observed when the influence of its tremendous gravity is detected on some other visible object near it. For example, in a binary system if one star is an invisible Black Hole, it will certainly influence its visible companion star.
Have any Black Holes been detected?
The best example of a likely Black Hole is Cygnus X-1 which is an X-ray source containing an invisible object too massive to be a white dwarf or neutron star.
How is a Black Hole likely to influence its companion in a binary system?
If one star in a binary system is a Black Hole, its visible companion will be seen to be following an orbit around an invisible object. Moreover due to the Black Hole a steady stream of matter from the visible star will be flowing to the Black Hole, and this matter will emit X-rays when falling into the Black Hole. Hence an X-ray source containing a massive invisible object is likely to be the location of a Black Hole.