Main-sequence stars form a coherent curve on the Hertzsprung-Russell plot due to their masses dictating their luminosity and spectral signatures. White dwarfs have similar spectral signatures but less luminosity. Stars eventually collapse into white dwarfs, neutron stars, or black holes depending on their mass.
Most stars fall into a categorization class called the main sequence, also known as dwarf stars. In a standard plot of star color versus magnitude, known as a Hertzsprung-Russell plot, main-sequence stars form a coherent curve, unlike the other categories: white dwarfs, subgiants, giants, bright giants, and supergiants. Although not usually included in the chart, black holes, which are gravitationally collapsed stars, could be considered points on the chart with zero luminosity and a spectral signature of 0° K.
The reason main-sequence stars fall on a predictable curve is because their luminosity and spectral signatures are dictated solely by their masses, which range from 0.08 to about 158 solar masses. White dwarfs, stars that have used up their nuclear fuel, have similar spectral signatures to main-sequence stars, but much less luminosity. This is because they do not fuse elements or have a continuous source of energy: their brightness and heat are all residuals. Over billions of years, white dwarfs are predicted to cool and become black dwarfs, or lifeless star carcasses. However, no white dwarf has been around long enough for this to happen yet.
Main-sequence stars fall into several categories: brown dwarfs, with only about 0.08 solar masses, are basically oversized Jupiters with weak fusion reactions in their cores; red dwarfs are slightly hotter and more energetic, with greater mass; these are followed by yellow dwarfs, very common stars of which our Sun is an example.
When stars burn up all their nuclear fuel in the form of hydrogen, they start fusing helium. As old stars begin to form a solid core of molten material, the powerful gravitational forces at the core’s perimeter compress the overlying layers of gas, accelerating the merger and increasing a star’s brightness and size. Through this developmental path, dwarf stars become giants. Depending on their mass, they eventually collapse into white dwarfs, neutron stars, or black holes. The most massive stars cause supernovae, which are huge bursts of energy that escape as fusion ceases in the stellar core and the layers of gas rub vigorously against each other during the final collapse.
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