# Stars

## Key Questions

• The process, known as Hydrogen Burning, is due to a fusion reaction: hydrogen nuclei fuse together to make helium nuclei.
In fusion, light atomic nuclei collide with such violence and frequency in the high-temperature, high-density stellar interior that they fuse into heavier nuclei and release tremendous quantities of energy (such as in a hydrogen bomb).
This reaction requires very high temperature and pressure found mainly in the core of a star and cannot be easily duplicated on Earth, unless using a thermonuclear device (H-bomb).
The great abundance of hydrogen makes it the key constituent in stellar nuclear reactions. The next stable nucleus is helium, "^4He, with atomic weight 4. Since the hydrogen nucleus (one proton) only has atomic weight 1, four protons are required to make one helium nucleus. The atomic weights do not exactly match because the more exact atomic weight of a proton is 1.0078, and four of them add to 4.0312, while the weight of "^4He is 4.0026, leaving a mass defect of 0.0286. This mass is converted to an amount of energy given by Einstein’s equation for the equivalence of mass and energy,
$E = m {c}^{2}$
where c is the speed of light. Because a unit atomic weight is $1.66 \times {10}^{- 27}$ kg, the energy released by the conversion of four "^1H nuclei to one "^4He nucleus is:
$E = 0.0286 \left(1.66 \times {10}^{- 27}\right) \left(9 \times {10}^{16}\right) = 4.3 \times {10}^{- 12} J$

In atomic nuclei, the strong nuclear force overcomes the electrostatic repulsion of the positively charged protons and binds from one to 260 nucleons (protons and neutrons) in a region about ${10}^{- 15}$ m in diameter. Two nuclei will fuse to form one larger nucleus if they approach within ${10}^{- 15}$ m of one another, but their mutual electrostatic repulsion — all nuclei have a positive charge — amounts to a $1$ MeV potential barrier. In contrast, at temperature of ${10}^{7}$ K the average thermal energy of a proton is only $1$ KeV. Classically, protons cannot fuse because of the strong coulombic barrier. Fusion does happen, however, because quantum physics allows the protons to tunnel through the barrier rather than go over it. The easiest fusion reaction involves two protons (hydrogen nuclei); such reactions become significant at temperatures around 10 million K.

(Reference for supporting data and figures: Introductory Astronomy and Astrophysics - M. Zeilik, S. A. Gregory, E. v. P. Smith)

• Well, this is a wonderful question and it is one that had always interested me!

Consider the Interstellar Medium as a volume of space (very big!) full of…dust, radiation, particles and gas (mainly hydrogen). Ok the density is quite low but they are there. So focus on one of these Interstellar Clouds of dust and gas and think that it starts to contract (under the action of gravitational force). You can think of a particle…alone…it passes near (probably billions of kilometres and more) to another particle and attracts it…it is not a lot but they are now tangled and together they can “tangle” other more distant particles (that before were outside their reach) because they combined mass is now higher than before and have more gravitational influence around them.

After a long time our little core of few particles slowly rotating has attracted a lot of interstellar medium and the dust, particles and gas are now forming a dense ball that keeps on attracting stuff and getting denser and hotter, a Protostar .
Dust starts to collide in the confined space of the core producing heat (friction) emitted as infrared radiation. The heat is transported through the star medium up to the surface through the process of convection (as in a pan of boiling water where the movement communicates heat to various parts of the mass of water). The luminosity is quite high as the core keeps on accreting material for ${10}^{5}$ years!
But then the outer layer of our contracting system starts to become denser as well and opaque to infrared radiation and, as a blanket, maintains the interior hot, slowing down the collapse (the time scale to reach this stage is of the order of 1 million years!).
The core still evolves accreting more material and heating up and heating (through radiation of heat now) the outer layers of our Pre-Main Sequence (PMS) star that becomes less opaque and start to shine.

Eventually the core heat up to a few million kelvins and thermonuclear reactions start. When the star gets most of its energy from thermonuclear reactions (rather than from gravitational contraction) the star is born! The structure is said to be in (hydrostatic) equilibrium and becomes a Zero-Age Main-Sequence star. The newborn star settles down in its life converting hydrogen into helium in its core and radiating energy. The total process takes approximately 20 million years.

The place that probably play an essential role in this process of star formation is called a Giant Molecular Cloud (GMC) such as the Star-Forming region near Orion:

In these GMC regions the molecular medium (mainly hydrogen) is bound into giant clouds that at the core are slightly denser than the surrounding medium and at temperatures of the order of 10 K. Young stars are always found near these “cloud” complexes suggesting that they work as “nurseries” for the birthing stars as in the case of H-II region of Eagle Nebula (M16-NGC6611) known as The Pillars of Creation:

Described beautifully as: “Eerie, dramatic new pictures from Hubble Space Telescope show newborn stars emerging from 'eggs' but rather dense, compact pockets of interstellar gas called evaporating gaseous globules (EGGs). Hubble found the EGGs, appropriately enough, in the Eagle nebula, a nearby star-forming region 6,500 light-years away in the constellation Serpens.”
[News Release STScI-1995-44]

REFERENCES:
(Pictures and data reference: M. Zeilik, S. A. Gregory, E. v. P. Smith, Introductory Astronomy and Astrophysics, Saunders College Publishing,1992).