Sirius just started his life as a star! All stars begin the same way in the nebula. A nebula is a dust full of gas and dust elements needed to make a star, the gravity pulls all these things into one to create what we call stars. Fun fact: Sirius is the brightest star in the night sky!
Red Giant Stage
Now Sirius is in his protostar stage, which is formed from the collapsing material of the nebula. It's core isn't hot enough for nuclear fusion to take place. The luminosity comes from the heating as the star contracts. There's two elements that is created Sirius's energy which is hydrogen and helium.
White Dwarf Stage
So now Sirius's core is hot enough to enter the main sequence stage of his life. Now that he is a main sequence star, his core will be fusing hydrogen and helium for billions of years. This is also because he is a low mass star. Currently Sirius is 1.19 million km in size and the temperature is about 10,000 degrees Celsius!
Black Dwarf Stage (final stage of life)
Sirius is a medium mass star so that means he will start to lose hydrogen. This makes him turn into a red giant. Which also means he'll begin to cool and contract again at the same time. He'll then become what they call a planetary nebula with a white dwarf in the middle.
When the red giant stage ends Sirius gets spread out so far out it slowly dissipates. That's when he enters the white dwarf stage-the dying remnant of the star. It will continue to emit heat for the next billions of years.
Sirius has now went into the black dwarf stage because the white dwarf stage no longer was emitting heat. So basically the black dwarf is when the star doesn't emit heat or light anymore. This sadly marks the end of his life.