An H-R diagram plots stars according to their surface temperatures (or spectral types) and luminosities. A stars position along the main sequence depends on its mass: High-mass stars are at the upper left and masses become progressively smaller as we move toward the lower right.

Stars on the main sequence have hydrogen-burning cores. A star’s position on the H-R diagram tells you a great deal of information about it. When stars have exhausted all their hydrogen fuel, they evolve to red giants. Their outer layers of gas expand and cool; therefore, the stars move to the right on the H-R diagram.

The position of each dot on the diagram tells us two things about each star: its luminosity (or absolute magnitude) and its temperature. The vertical axis represents the star’s luminosity or absolute magnitude.

Stars which lie along the top right of the diagram are luminous and cool and are known as supergiants. Knowing a star’s location on the HR diagram allows us to find its mass, its evolutionary state (and age), and its physical size.

The Supergiants are cool stars, which are very large and very bright. They are located towards the top right of the graph. The Giants are cool stars, which are a little smaller and dimmer than the Supergiants. The White Dwarfs are very hot stars, which are small in size and relatively dim.

Red stars

Stars have different colors, which are indicators of temperature. The hottest stars tend to appear blue or blue-white, whereas the coolest stars are red.

Stars in the stable phase of hydrogen burning lie along the Main Sequence according to their mass. After a star uses up all the hydrogen in its core, it leaves the main sequence and moves towards the red giant branch. The most massive stars may also become red supergiants, in the upper right corner of the diagram.

Two of the brightest stars in the evening sky lie at opposite corners of the rectangle: bright orange-red Betelgeuse at the northeastern corner (upper left in the photo) and even brighter Rigel at the southwest (lower right in the photo).

In an H-R diagram the luminosity or energy output of a star is plotted on the vertical axis. This can be expressed as a ratio of the star’s luminosity to that of the Sun; L*/Lsun. Astronomers also use the historical concept of magnitude as a measure of a star’s luminosity.

With an estimated age of seven to nine million years, Rigel has exhausted its core hydrogen fuel, expanded, and cooled to become a supergiant. It is expected to end its life as a type II supernova, leaving a neutron star or a black hole as a final remnant, depending on the initial mass of the star.

Antares

The largest known star in the universe is UY Scuti, a hypergiant with a radius around 1,700 times larger than the sun. And it’s not alone in dwarfing Earth’s dominant star.

Generally, the more massive the star, the faster it burns up its fuel supply, and the shorter its life. The most massive stars can burn out and explode in a supernova after only a few million years of fusion. A star with a mass like the Sun, on the other hand, can continue fusing hydrogen for about 10 billion years.

That’s even hotter. But the hottest known stars in the Universe are the blue hypergiant stars. These are stars with more than 100 times the mass of the Sun. One of the best known examples is Eta Carinae, located about 7,500 light-years from the Sun.

Sirius