Blue giants and red giants

What is a star?

A star is a sun, which shines through nuclear reactions that occur in the center.
With the exception of the Sun, the stars appear to the naked eye as a bright, shimmering due to atmospheric turbulence, without immediate apparent motion relative to other fixed objects in the sky.
All the stars are considerably more distant from Earth than the Sun.
The nearest star, Proxima Centauri, is located about 4 light years of the solar system, nearly 250 000 times farther than the Sun.
The mass of a star is of the order of 10³⁰ kg and its radius of the order of several million kilometers.
The power radiated by a star like the Sun is about 10²⁶ watts. Stars form due to the contraction of a nebula of gas and dust under the influence of gravity.
If the heating of the material is sufficient, it will trigger the cycle of nuclear reactions in the heart of the nebula to form a star. The energy from these reactions is then sufficient to stop its contraction due to the radiation pressure generated.

The number of stars in the Universe is estimated at between 10²² and 10²³. Apart from the Sun, the stars are too faint to be observed in daylight.

Image: Birth of a Star: composite image from data of X-ray telescope Chandra (blue) and data from the Spitzer Infrared Telescope (red and orange).
At about 4 000 light years from Earth lies RCW 108, a region of the Milky Way, where star formation is active where the presence of clusters of young blue stars in the picture. That we see born, in yellow in the center of the image is deeply rooted in a cloud of molecular hydrogen.

Blue giants and red supergiants

The blue giants and red supergiants are hot and bright. These stars are at least ten times larger than the Sun. The blue giants are extremely luminous in absolute magnitude -5, -6 and more.
Very massive, they quickly consume their hydrogen and their lifespan is very short in the range of 10 to 100 million years ago, very rare in the Milky Way.
When hydrogen in his heart was consumed, then merges the blue giant helium.
Its outer layers swell and its surface temperature drops to become a red supergiant.
The star then produces elements heavier iron, nickel, chromium, cobalt, titanium...
At this point, the fusion reactions stop and the star becomes unstable.
It explodes in a supernova and die. The explosion left behind a strange heart of matters that will remain intact. This corpse is, according to its mass, a neutron star or black hole. Betelgeuse (α Orionis) and Antares (α Scorpii / Alpha Scorpii), Double Star, are red supergiants.

Image: At the heart of the Orion nebula, four giant blue, form a trapezoid and illuminate long-range matter dispersed in space. The atoms absorb the starlight and re-emit according to their own colors, those of oxygen that is in the green, hydrogen and nitrogen in the red.
The Orion Nebula at a distance of 1 350 light years from Earth, offers one of the best opportunities to study the birth of stars, not only because it is the largest and the closest region of star formation, but also because the energy star of the nebula have blown the dust and the surrounding gas, which allows us to see.
We enjoy an intimate vision of a broad spectrum of stars being born and evolve. Some of the brightest stars visible in the sky are blue giants as Rigel (Beta Orionis) and Deneb (Alpha Cygni).