Small mass stars evolution From birth to death

The place stars are forming • Stars are forming in great molecular clusters (GMC), like very big clouds from Hydrogen and a little of dust. • When a part of the cloud is getting more dense, the gravitation takes over. The part of the cloud is breaking in pieces, and every piece will form a new star.

Creation- protoplanetary disk • When the piece of the cluster is dense enough, the pressure stops the gravitational collapse. • A protoplanetary disk is forming around the star.

The planets • The planets are forming in the protoplanetary disk. • We can see a protoplanetary disk only in infrared light, because of the dust in it.

Three stages of star forming At stage 1 the star is in equilibrium (pressure against gravity). At stage 2 the star has a planetary nebula and polar jets (T-Tauri star). The star is still accreting mass from the disk. At stage 3 the star is fusing Hydrogen to Helium in its core. The star is now stable, and will be in that condition for almost its hole lifetime.

The main sequence • If we put al the stars on a diagram of luminosity and temperature, we will see that there is a belt where the most stars are located. We call it the main sequence, the state the stars are burning Hydrogen to Helium in their cores. The surface temperature and the luminosity of main sequence stars remain stable (for the 90% of its lifetime).

After the main sequence • When the star has fused all the Hydrogen in its core to Helium, it is starting to fuse Hydrogen to Helium outside the core. So the pressure is increasing and the star is expanding • This is the red giant stage. A star like the Sun will expand 100 times. It becomes colder (3000 K surface temperature).

The giant branch The red giant stage has a duration of hundreds of thousand Years. • After the Hydrogen burning in the shell the star is burning Helium to Carbon in its core. The Helium burning is 10 times less efficient, so this stage’s duration is a relative short time.

The asymptotic branch • The small mass stars can not fuse the carbon to heavier elements. The fusion stops, and the core is collapsing to a white dwarf. • The strong stellar winds have displaced the half of the star mass (outside layers) to a cloud we call planetary nebula.

Πλανητικό νεφέλωμα και λευκός νάνος • The remain of the star (white dwarf) has the size of the Earth, but the half of the star mass. Its surface temperature is 30000 K and it is heating the planetary nebular around it.

Stars recycling • A planetary nebular is very rich in complex molecules and heavy elements. So when it is colliding with a molecular nebular the newborn stars, and also the planets, will be chemical enriched. Στο πλανητικό νεφέλωμα αναπτύσσεται πλούσια χημεία, απλή ανόργανη αλλά και οργανική, με πολύπλοκα μόρια.