The yellow ribbon denotes the socalled main sequence stars. These are the stars in which hydrogen is fused into helium. Stars will spend most of their time in fusing hydrogen. That is why most stars can be found on that ribbon in the diagram.

There is a lower mass limit of stars. Below that limit of about 0.07 solar masses there is not enough mass to exert sufficient gravitational pressure in the core to start a fusion process.

The upper mass limit is determined by the amount of energy produced by the star. A star produces radiation pressure which exerts outward pressure to its outer layers. This pressure is counteracted by gravity. For medium size stars the gravity and radiation pressure are in equilibrium. But the heavier the star the more energy and therefore radiation pressure is produced. This equilibrium between gravity and readiation pressure can be maintained up to about 300 solar masses. Stars which are heavier will will simply blow away their outer layers for which gravity is not strong enough until they have reached their upper mass limit.

Spectral classes
The classification of the spectral classes (marked in yellow tan) is based on the Morgan-Keenan classification system.
The absence of an upper temperature limit for spectral class O is one of its main features.

Wolf-Rayet stars

White Dwarfs
The high luminosity of Z Andromedae B is due to the fact that it is capturing hydrogen from its red giant companion, which results in hydrogen fusion on the surface of this white dwarf.

Brown Dwarfs

Luminosity
The Luminosity of a star is proportional to its surface area (4πR²_star) and its surface temperature (T) to the power of four (Stefan-Boltzmann law).

L = σ·4πR²·T⁴ (Watt (Joule/sec))

σ is the Stefan-Boltzmann constant: 5.670 x 10^-8 Wm^-2K^-4

Wien's displacement law
The higher the temperature of a star the more bluish its appearance (the shorter the wavelenght).

b = 2.898 x 10^-3 m K

Magnitude of a star
The magnitude of a star is a measure of its brightness.
This brightness (b) is the amount of energy received per unit area and is inversely proportional to the square of the star distance (d). b Is the measured brightness and is therefore also called apparent brightness
The luminosity (L) of a star is actually an intrinsic property. It is the total amount of energy emitted at its surface

The magnitude is a logarithmic value to accomodate the huge dynamic range of the brightness or luminosity.

The absolute magnitude of a star is by definition related to its apparent brightness as measured from a distance of 10 parsec, 32.6 light-years.
The magnitude scale has been chosen in such a way that a luminosity ratio of 100 results in a magnitude difference of 5.

The luminosity relates to the total emitted energy over the whole electromagnetic spectrum.

Visual and bolometric magnitude
With regard to the absolute magnitude, a distinction is made between the visual magnitude M_v and the bolometric magnitude M_bol. M_v expresses how bright a star is to our eyes, it relates to only that part of the energy which is emitted in the visual part of the electromagnetic spectrum . M_bol expresses how bright a star is to a measuring device integrated over the whole electromagnetic spectrum.

The difference between M_bol and M_v is called the bolometric correction BC and depends on the surface temperature of the star.

Color index
The scale of the color index is based on observational data of stars with surface temperatures between 2 860 and 42 000 Kelvin.
A trend analysis has been used to extrapolate these data to cover a temperature range between 10² and 10⁶ Kelvin.
More information about the analysis can be found on this page.

Life cycle phases of super giant stars
All giant stars will eventually go supernova: a gigantic explosion during which so much energy is released that its luminosity is comparable with that of an entire galaxy containing several hundred billions of sunlike stars. During that explosion much gas is released which still contain an abundant amount of hydrogen to give birth to a next generation of stars.

Neutron star or black hole
Also a very compact object is formed at the very heart of the explosion. It is a neutron star when its mass is less than 2 solar mass. A neutron star is the product of the compressive force as a result of the massive explosion acting on the core of the giant star. The mass density will become so high that normal hydrostatic equilibrium in which the gas pressure is in balance with the gravity cannot be achieved. The core will collapse until its mass density is comparable to that of an atomic nucleus in which nuclear forces are counteracting gravity to reach a new equilibrium. Electron and protons have merged into neutrons.

If the core is havier than 2 solar mass then even nuclear froces are not enough to counteract gravity. It will go through a gravitational collapse to become a black hole.

Two cooling phases of a neutron star
A newly formed neutron star is believed to have a surface temperature of several hundred million Kelvin. In that hot phase it will rapidly loose its thermal energy by radiating high energetic neutrinos. In about 10 000 years the surface temperature will drop to about one million Kelvin. From that moment on the cooling process will increasingly be dominated by radiating photons.