Nebulae- An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star.A planetary nebula, abbreviated as PN or plural PNe, is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated to planets or exoplanets. An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star. The ions move inside the gas and force the atoms in the gas to gain or lose electrons which results in ionization of these gases.A reflection nebula is a cloud of interstellar gas and dust that reflects the light from other stars. This happens in the surroundings of stars that are not hot enough to excite the hydrogen atoms of the cloud (black dwarfs)(as is the case for emission nebulae that are emitting light, not just reflecting the light from stars).A dark nebula or absorption nebula is a type of interstellar cloud that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection nebulae.The nebulae pull each other due to their gravitational forces and form clumps of gases known as protostars. This force(gravitational potential)changes to heat energy. Over millions of years , the gases gain enough heat to start the process of fusion of hydrogen which is distributed throughout the protostar. The layers of gases exert pressure at the centre which is the core and hence the core is hot.Simce the core is pressurized, it contracts and the atoms come closer. These atoms collide with each other and gain energy(collisions of macroscopic bodies, some kinetic energy is turned into vibrational energy of the atoms, causing a heating effect, and the bodies are deformed.) , they become hot. At last , the temperature reaches where hydrogen fuses into helium(Nuclear fusion requires so much heat because it is needed to overcome the electrostatic repulsion between the nuclei in the reaction. A tokamak reactor uses strong magnetic fields to contain the fusion reaction.) The hydrogen is not only at the core but also at the parts forming an envelope of non- burning hydrogen.Small stars with mass of 1 and a half mass of sun fuse hydrogen to helium and this releases energy which is radiation pressure and counteracts the force of gravity and keeps the star from absorbing the points above the point of gravity. Slowly the core becomes hot enough to warm the outer layers and so they expand becoming a red giant / when the core becomes hot enough the outer envelope of hydrogen is fused which is present just outside the core. As gravity crushes the star, I understand that the star heats up as gravity crushes it. As a result, although the stellar core remains “dead” (no fusion occurs), a “shell” of gas around the stellar core becomes hot enough to begin fusing helium. Since the fusion occurs as a “shell” around the stellar core, the outward-push from the fusion is what pushes the star’s outer layers further. The result is that the star grows into a Red Giant. The radiation pressure by the core reduces in energy as it travels till the outer layers. The core becomes a helium inert core but the gas is still hot and at high pressure and hence the core immediately doesn't contract under the effectof its own gravity.The core contracts and atoms are brought near to each other and collide with each other more due to brownian motion , this increases the energy possessed by the atoms and this energy converts to energy and heats the star.The helium requires high amount of heat and this provides ideal conditions for helium to fuse into carbon followed by many elements till they reach iron. Once iron is reached, fusion is halted since iron is so tightly bound that no energy can be extracted by fusion. Iron can fuse, but it absorbs energy in the process and the core temperature drops. The star's gravity prevails and compresses the stars. Now , the fate of the star depends on its mass. A low or medium mass star (with mass less than about 8 times the mass of our Sun) will become a white dwarf. A typical white dwarf is about as massive as the Sun, yet only slightly bigger than the Earth. This makes white dwarfs one of the densest forms of matter, surpassed only by neutron stars and black holes.The white dwarf is supported by the paulie exclusion principle in which - In a degenerate gas, all quantum states are filled up to the Fermi energy. Most stars are supported against their own gravitation by normal thermal gas pressure, while in white dwarf stars the supporting force comes from the degeneracy pressure of the electron gas in their interior. In neutron stars, the degenerate particles are neutrons.A fermion gas in which all quantum states below a given energy level are filled is called a fully degenerate fermion gas. The difference between this energy level and the lowest energy level is known as the Fermi energy. In an ordinary fermion gas in which thermal effects dominate, most of the available electron energy levels are unfilled and the electrons are free to move to these states. As particle density is increased, electrons progressively fill the lower energy states and additional electrons are forced to occupy states of higher energy even at low temperatures. Degenerate gases strongly resist further compression because the electrons cannot move to already filled lower energy levels due to the Pauli exclusion principle. Since electrons cannot give up energy by moving to lower energy states, no thermal energy can be extracted. The momentum of the fermions in the fermion gas nevertheless generates pressure, termed "degeneracy pressure". The electrons occupy the lower energy levels first according to aufbau principle and then fill the higher orbitals , when they are pressurized into smaller states , the gravity cannot further compress the electrons due to same quantum numbers:-
1 Principal quantum number (n)
2 Azimuthal quantum number (ℓ)
3 Magnetic quantum number (mℓ)
4 Spin quantum number (s)
The white dwarf stars are white as they are still hot from their activities, eventually becoming cold by the radiation loss in the form of infrared(the atoms become hot and want to lose energy in the fastest way hence lose energy by infrared which is the feeling of warmth when we come closer to a hot object )
A red giant star with more than 7 times the mass of the Sun is fated for a more spectacular ending.
These high-mass stars go through some of the same steps as the medium-mass stars. First, the outer layers swell out into a giant star, but even bigger, forming a red supergiant. Next, the core starts to shrink, becoming very hot and dense. Then, fusion of helium into carbon begins in the core. When the supply of helium runs out, the core will contract again, but since the core has more mass, it will become hot and dense enough to fuse carbon into neon. In fact, when the supply of carbon is used up, other fusion reactions occur, until the core is filled with iron atoms.
Up to this point, the fusion reactions put out energy, allowing the star to fight gravity. However, fusing iron requires an input of energy, rather than producing excess energy. With a core full of iron, the star will lose the fight against gravity.
The core temperature rises to over 100 billion degrees as the iron atoms are crushed together.The electrons are compressed till the nucleus where they are attracted by the protons and the electrons collide with the protons forming neutrons and releasing a neutrino and the newly created neutrinos go flying outward, expelling the outer layers of the star in a gigantic explosion called a supernova (to be precise, a type II or core collapse supernova).See , the neutrons are in very large numbers as the star is very big. Now , the remnants of the supernova also depends on mass and then they are either neutron stars or black holes. Black holes are very dense and bend spacetime , even light can't escape.
Do you know? - hot stars are blue !
The hotter the star, the shorter the wavelength of light it will emit. Since it is in middle of the radiation chain , it emits Blue light and uv rays. SInce less hot stars donot have much energy to emit blue light hence it emits light of red light.
