00:00 Stars in the observable universe differ in terms of age, size, color, and composition, and while humans initially had no clue what they were, the idea that stars were like our sun eventually took hold.

02:11 Hermann von Helmholtz developed theories about the sun's energy source, including the idea that it was shrinking, but none of them fully explained the sun's energy output.

03:57 The sun's energy source was determined to be nuclear fusion, where hydrogen atoms fuse together to create helium, and this process is also how stars are made and what happens to them.

05:50 The mass of a star determines its lifespan, with more massive stars having shorter lives, and fusion occurs when there is enough mass, releasing a tremendous amount of energy.

07:47 The classification system for stars is based on their color and temperature, with O stars being the hottest and blue, and G2 stars like our sun being able to burn for billions of years before eventually running out of hydrogen and fusing helium into carbon, turning into a red giant.

09:47 Once a star is left with iron, fusion is no longer possible and the star will collapse inward, with the fate of the star depending on its mass, either becoming a white dwarf if it has 1.44 solar masses or less, or a neutron star if it has a greater mass.

11:46 If a star's mass is more than about 2.1 solar masses, it becomes a black hole, and the process of collapsing into a white dwarf, neutron star, or black hole can result in a supernova, with the remaining mass explosively expelled outward, and there is a theoretical stage called a black dwarf for white dwarf stars where all residual heat dissipates over a very long time, but this stage would take longer than the current age of the universe to reach and would be impossible to see.