Stellar

1. Stellar Evolution

2. The stellar evolution involves two opposite forces: on one side, the star’s mass produces the force of gravity, which leads to a contraction, on the other side the nuclear forces in the core produce expansion.

3. BIRTH OF A STAR Stars were born from enourmous cloud of gas and dust. The explosion of a near supernova or a collision between two nebulas marks the beginning of the force of gravity: a protostar is forming.

4. Theseobjectsaren’t realstars, becausetheirinner temperature isn’t enoughto prime nuclearfusion. After a long processofcondensation, if the mass of the protostarislessthan 1/10 ofsolar mass, the protostarisn’t abletoreach the temperature whichpermits the nuclearfusion and itstartstocool, veryslowly. Jupiter, in particolar, but the Earthtoo, are examplesofprotostarswhich continue tobe…protostars, or, aswesay, planets. When the mass of a protostaris more than 1/10 ofsolar mass, temperature in the corereach 10 millions kelvin: nuclearfusionstarts.Thanstars live a periodofinstability: contractions and expansionstrytosorpasseachother, tillthey come to a balance. Hereis the star.

5. Astronomers use to classify stars on the basis of temperature and magnitude, in what they call the H-R Diagram (h-R from hertzsprung-Russel)

6. Herewesee the H-R diagram. In abscissa the surface temperature of the star, in kelvin, and the corrispondentspectralclass are riported. As you can see, from low to high temperature, they are O, B, A, F, G ,K, M. Hereis a tricktorememberthem: they are the initialsofthissentence: Oh Be A Fine Girl, Kiss Me.In the y-axis, the absolutemagnitudeof the stars (nottobeconfusedwith the apparentmagnitude) isreported, with, on the other side of the diagram, the luminositycomparedto the Sun.So, the hottest, brighteststars are at the top leftwhile the coolest, fainteststars are at the bottom right. The diagonal band ofstarsrunningfrom the upper lefttolower right isknownas the MainSequence and includesthosestarswhich are convertinghydrongenintohelium in theircores under stableconditions (90% ofallstarsknown). Red Giants or redSupergiantsrepresent the secondstepof the life of a star, aswe’llsee. Then, on mostoccasions, whitedwarf are the lastestperiodof a common star’s life.

7. Maturity Stars live the 90% oftheir life in the mainsequence, placedaccordingtotheir mass. At a certainpoint in their life the hydrogen in the corefinishes. As a conseguence, the nuclearfusionfinishestoo, and so the star startstocontract under the pressureofitsown mass. Then, the destinyof the star depends on its mass:

8. Starswith mass lessthan 0.5 solarmasses: in this case the core’s temperature doesn’t reachsufficientvaluesto prime the nuclearfusionofhelium. This star isgoingtodie in a whitedwarf.These are littlestars, very hot initially, whichcoolslowlytilltheyswich off completely, in blackdwarf.If a whitedwarfis part of a bynar system, forexamplewith a redgiant, the first one can steal some of the redgiant’s mass and prime the fusionofhydrogen in the externallayers. This cause a a big explosionwhich can beseenfrom the Earth. Thesestars are calledNovae.

9. Stars with mass more than 0.5 solar masses: in this case, the contraction provokes the core’s temperature of 100 million kelvin, which is enough to prime the helium fusion. In the region around the core, instead, temperature is more than 10 million kelvin, and so here the hydrogen fusion starts, causing the expansion of external layers: the Red Giant. The red giant’s life is short, because the helium is less than hydrogen and because the energy produced by helium is less than that produced by hydrogen.

10. The subsequent phases of the life of a star depends, once again, on its mass: • Starswith mass lessthan 1,44 solarmasses:this star can’t have the nuclearfusionofcarbon, and lives a periodofinstability, in whichitexpells the externallayers, madeofcarbon and oxygen, and itbecomes a whitedwarf. Whatweseeis a Planetary Nebula.

11. Starswith mass more than 1,44 solarmasses: the star starts the processofnuclearfusionwhichleadsto the formationofheavier and heavierelements. The star resultsas a compositionoflayersofdifferent density, the lighterones on the top and the heavierones in the core.When the reactionsgettoiron, thingschange: the fusionofirondoesn’t trasform mass in Energy, but Energy in mass. Becauseofthat, the star collapsesintoitself, and esplodesviolently: it’s a Supernova, that can bevisiblefrom the Earthduring the daylight.

12. Heaviest elements If the heaviest elements producted by stars is iron, how about all the others?In this phase, part of the espelled matter creates a bow wave that produce condensation among elements which forms new heavier elements. It’s the only way to produce the heavy elements we can find in nature. That’s why we are called sons of stars.

13. Then the future of the core of the star depends again to its mass. • Core with mass less than 3-4 solar masses: it become a neutron star. It’s a little star in which all the protons and electrons have lost their individuality, and have fused into neutrons. The neutron star has a strong magnetic field and quick rotation. For that reason this type of objects are also known as Pulsar.

14. Core with mass more than 4 solar masses: the contraction goes on till unimaginable density and, at the end, the star becomes a Black Hole, one of the most mysteriousobjects in the universe.