How Stars Are Born: Summary. How Stars as Big as the Sun and Bigger Evolve, Characteristics of Stellar Spectra.
How Stars Are Born
Stars form in nebulae which are very rarefied masses of gas (mostly hydrogen) and dust. There are 2 types of nebulae: bright ones that emit starlight ; dark ones that do not transmit light. For a star to be born , matter must become dense , but in nebulae it is not dense because they are very rarefied; matter can densify through the shock wave of the explosion of a nearby star that compresses the gas until a protostar is formed. When the protostar reaches a temperature of 15 million K, hydrogen fusion and various nuclear reactions begin.
In the early stages, the size of the star is unstable because 2 forces act on it: the gravitational force (which tends to compress and therefore shrink the star) and the energy released by nuclear fusion (which tends to expand the volume of the star). The star becomes stable when these 2 forces reach equilibrium. The temperature increases with the constriction of the star and also changes its color:
- the least hot (3,000 K) are red;
- at 6,000 K they are yellow (sun);
- the warmest ones (from 10,000K) are blue-white.
How Stars as Big as the Sun Grow
The Sun is about 4 and a half billion years old and is in a phase of stability that will last for about the same amount of time. Currently, hydrogen is being converted into helium, but at a certain point the hydrogen contained in the nucleus will run out. When this happens, there will be no more fusion reactions and only the force of gravity will act on the star, causing it to compress and increase its temperature . When it reaches 100 million K, the fusion of helium will begin, causing the star to expand, making it become a red giant. In this phase, the Sun will disperse a quantity of matter into space in the form of a planetary nebula and the progressive contraction of the nucleus will cause it to reach the stage of a white dwarf (with a strong increase in surface temperature). In the white dwarf there is not enough matter to fuse carbon, so over time it will cool, becoming a black dwarf, in which the star will shut down.
How Stars Bigger Than the Sun Evolve
In stars larger than the sun (at least 8 times) the initial phases are the same. Later, that is when the helium in the nucleus also runs out, the temperature reaches such values that the fusion of carbon begins. Consequently, this cycle continues until the fusion of iron occurs. Unlike other elements, the fusion of iron does not produce energy but rather consumes it, so the star implodes, temperatures reach very high values until the star explodes completely generating, depending on the amount of matter, a nova or a supernova. In this state, elements heavier than iron are formed. The supernova, however, may not be the last stage of the star, everything depends on how much matter remains after the explosion.
At certain values, a neutron star is formed (since the temperatures in it are high enough to fuse protons and neutrons). Neutron stars are probably objects called pulsars, pulsating stars that rotate rapidly on themselves and emit radiation in only one direction. In cases where the amount of matter that remains is even greater, a black hole is formed. Black holes are bodies in which the density takes on enormous values and generates an enormous gravitational field; even light remains trapped in it, which is why they are not visible. Furthermore, the laws of ordinary physics do not apply to black holes and time and space are different. Since they are not visible, black holes are observed thanks to the effects they produce, so much so that it seems that there is a black hole at the center of our galaxy because its speed is greater.
Stellar Spectra
All the information about stars comes from absorption spectra . From the spectra we can deduce the chemical composition of the star but we can also know the surface temperature. Depending on the surface temperature, stars are classified into 8 classes (from the highest temperature): O; B: A; F; G; K; M; N; each class is divided into 10 subclasses indicated by the numbers from 0 to 9. From the spectra we can also understand if the stars are approaching or moving away through the Doppler effect : if a celestial object is approaching, the wavelength decreases, the radiation in the blue prevails and there is a ” blue shift “; if the star is moving away the wavelength increases, the radiation in the red prevails and there is a ” red shift “. Also thanks to the spectra it was discovered that galaxies rotate and that a galactic recession is taking place, that is, the universe is expanding.
