The significance is the following:
1) When lots of stars are plotted in the HR diagram, the vast majority star appear along a curve. This curve is known as the main sequence.
2) It turns out that those are basically the stars that burn hydrogen-1, converting it into helium-4.
its located somewhere
On the main sequence.
The temperature and luminosity of stars.
The location on the Hertzsprung-Russell (HR) diagram where most stars lie is known as the main sequence. The HR diagram is a plot of stellar luminosity against surface temperature. The main sequence is a prominent band that extends diagonally across the HR diagram from high temperature and high luminosity to low temperature and low luminosity. The majority of stars, approximately 90% of all stars, are situated along the main sequence on the HR diagram. These stars are often referred to as main-sequence stars. They exhibit a smooth relationship between surface temperature and luminosity, with varying sizes and masses but sharing this common characteristic of lying on the diagonal band from the upper left to the lower right of the HR diagram. Source: Teach Astronomy - The Hertzsprung-Russell Diagram
On the HR diagram the Sun's spectrum is of type G2 which makes it yellow.
The most massive type of main sequence star on the HR diagram would be indicated by the main sequence area at the top left of the HR diagram, which would be blue or violet in color. Zeta Puppis is an example of such a star. It comes in at 40 solar masses. It is classified as a type "O" star.
Of course they are on the HR diagram. They are simply not on the main sequence.
The HR diagram contains only stars - so everywhere.
HR diagram
The red dwarves.
Up at the top right. If you look on the Australian Telescope National facility website, their HR diagram shows this clearly.
The main reason that the HR Diagram is so useful and important to scientists is, you can tell the size of the star by plotting it on the HR Diagram. The different sizes of stars form a pattern on the HR diagram.
its located somewhere
All stars.
Slightly to the right of centre on the main sequence. If you look on the Australian Telescope National facility website, their HR diagram shows the position of the sun.
The colors of the HR Diagram are:BlueBlue-WhiteYellowOrangeRed-OrangeRed
Its called an HR diagram or a Hertzsprung-Russell diagram.