Stellar Anatomy
Large, dark features seen on the Sun's surface caused by increased magnetic activity. They appear darker because typically, their temperature is ≈3800 K, whereas the bright regions that surround them are at about 5800 K. They may consist of two parts: an inner darker core, the umbra, and a surrounding less dark region, the penumbra.
What are Sunspots?
[Source: OpenStax Astronomy 15.2 The Solar Cycle]
(1) Massive reservoirs of interstellar matter. They are cold interiors with temperatures of only 10–20 K. These are birthplaces of most stars in our Galaxy. Their masses from a thousand times the mass of the Sun to about 3 million solar masses.
(2) This is what you call a star during the time a dense core is contracting to become a "true star", but before time that the fusion of protons to produce helium begins.
What is (1) Giant Molecular Cloud and (2) Protostar?
[Source: OpenStax Astronomy 21.1 Star Formation]
(1) Mass of high-mass stars
(2) Mass of low-mass stars
(3) Mass of brown dwarfs
What is
(1) > 8M☉,
(2) < 8M☉,
(3) < 0.08 M☉?
[Source: OpenStax Astronomy 23.2 Evolution of Massive Stars: An Explosive Finish, Stellar Life Cycle Chart by R.N. Bailey on Wikimedia in DP Astronomy B2 3.22 Stellar Evolution Notes]
At the temperatures inside the stars with masses smaller than about 1.2 times the mass of our Sun (a category that includes the Sun itself), most of the energy is produced by the reactions we have just described, and this set of reactions is called ______, protons collide directly with other protons to form helium nuclei.
What is the Proton-Proton Chain?
[Source: OpenStax Astronomy 16.2 Mass Energy and the Theory of Relativity]
Extremely dense, it is the source of all of the Sun's energy where nuclear energy is being released. It makes up ≈20% of the size of the solar interior. It is thought to have a temperature of approximately 15 million K, making it the hottest part of the Sun.
What is Core?
[Source: OpenStax Astronomy 15.1 The Structure and Composition of the Sun]
(1) The star derives its energy almost entirely from the conversion of hydrogen to helium via the process of nuclear fusion in its core. Since hydrogen is the most abundant element in stars, this process can maintain the star’s equilibrium for a long time. Thus, all stars remain in this stage for most of their lives.
(2) Marks the time when a star stops contracting, settles onto the main sequence, and begins to fuse hydrogen in its core. It is a continuous line in the H–R diagram that shows where stars of different masses but similar chemical composition can be found when they begin to fuse hydrogen.
What is (1) Main Sequence and (2) Zero-Age Main Sequence?
[Source: OpenStax Astronomy 22.1 Evolution from the Main Sequence to Red Giants]
Besides plotting stellar luminosity / magnitude against surface temperature / spectral class, what else can H-R Diagrams be used to visualize?
What is Evolutionary Track?
[Source: OpenStax Astronomy 21.2 The H–R Diagram and the Study of Stellar Evolution]
Objects with masses less than about 7.5% of the mass of our Sun (about 0.075 M☉) do not become hot enough for hydrogen fusion to take place. This is essentially a “failed star”. They are very difficult to observe because they are extremely faint and cool, and they put out most of their light in the infrared part of the spectrum.
What is Brown Dwarf?
[Source: OpenStax Astronomy 17.3 The Spectra of Stars and Brown Dwarfs]
This is the hottest part of the solar atmosphere, which has a temperature of a million degrees or more. This outermost part of the Sun’s atmosphere was first observed during total eclipses. It extends millions of kilometers above the photosphere and emits about half as much light as the full moon. The reason we don’t see this light until an eclipse occurs is the overpowering brilliance of the photosphere.
What is Corona?
[Source: OpenStax Astronomy 15.1 The Structure and Composition of the Sun]
(1) Over time, lower-mass stars like the Sun become this. They can become so large that if we were to replace the Sun with one of them, its outer atmosphere would extend to the orbit of Mars or even beyond. Most stars generate more energy each second when they are fusing hydrogen in the shell surrounding the helium core than they did when hydrogen fusion was confined to the central part of the star; thus, they increase in luminosity. With all the new energy pouring outward, the outer layers of the star begin to expand, and the star eventually grows and grows until it reaches enormous proportions. Carbon, oxygen, and some other elements are made inside lower-mass stars during this stage, and the vast majority of stars, including our Sun, follow this stage.
(2) These massive stars evolve in much the same way that the Sun does (but more quickly) up to the formation of a carbon-oxygen core - the difference is that for stars with more than about twice the mass of the Sun, helium begins fusion more gradually, rather than with a sudden flash. When more massive stars become red giants, they become so bright and large that we call them ________. Such stars can expand until their outer regions become as large as the orbit of Jupiter. These stars can start additional kinds of fusion in their centers and in the shells surrounding their central regions. The outer layers of a star with a mass greater than about 8 solar masses have a weight that is enough to compress the carbon-oxygen core until it becomes hot enough to ignite fusion of carbon nuclei. Carbon can fuse into still more oxygen, and at still higher temperatures, oxygen and then neon, magnesium, and finally silicon can build even heavier elements. Iron is the endpoint because iron fusion produces products that are more massive than the nuclei that are being fused and therefore the process requires energy, as opposed to releasing energy, which all fusion reactions up to this point have done. This required energy comes at the expense of the star itself, which is now on the brink of death.
What is (1) Red Giant and (2) Red Supergiant?
[Source: 22.1 Evolution from the Main Sequence to Red Giants, 22.4 Further Evolution of Stars, 22.5 The Evolution of More Massive Stars]
Very common red, cool, low-luminosity stars at the lower end of the main sequence are much smaller and more compact than the Sun. Its diameter is only 1/10 that of the Sun. A star with such a low luminosity also has a low mass (about 1/12 that of the Sun). This combination of mass and diameter means that it is so compressed that the star has an average density about 80 times that of the Sun.
What is Red Dwarf?
[Source: OpenStax Astronomy 18.4 The H-R Diagram]
(1) Chandrasekhar Limit - the maximum mass of a core that can become a white dwarf
(2) Oppenheimer-Volkhoff Limit - the maximum mass of a neutron star
(3)A mass beyond the Oppenheimer-Volkhoff Limit will collapse and form a Black Hole.
What is (1) < 1.4 Mʘ (white dwarf), (2) <3 Mʘ (neutron star), and (3) >3 Mʘ (black hole)?
[Source: DP Astronomy B2 3.22 Stellar Evolution Notes]
(1) Layer where the Sun becomes opaque and marks the boundary past which we cannot see. Energy that emerges from this region was originally generated deep inside the Sun and comes in the form of photons, which make their way slowly toward the solar surface. Astronomers have found that the solar atmosphere changes from almost perfectly transparent to almost completely opaque in a distance of just over 400 kilometers; it is this thin region that we call the ______, a word that comes from the Greek for “light sphere.”
(2) Region of the Sun’s atmosphere that lies immediately above the photosphere. The name is Greek for “colored sphere,” given to its red streak. Observations made during eclipses show that it is about 2000 to 3000 kilometers thick, and its spectrum consists of bright emission lines, indicating that this layer is composed of hot gases emitting light at discrete wavelengths. Its reddish color arises from one of the strongest emission lines in the visible part of its spectrum - the bright red line is caused by hydrogen, the element that dominates the composition of the Sun. Its temperature is 10,000 K.
What is (1) Photosphere and (2) Chromosphere?
[Source: OpenStax Astronomy 15.1 The Structure and Composition of the Sun]
(1) Contain strong sources of stellar winds and ultraviolet radiation, which sweep outward into the shells of material ejected when the star was a red giant. The winds and the ultraviolet radiation heat the shells, ionize them, and set them aglow. Its name is derived from the fact that a few of these have a round shape bearing a superficial resemblance to planets, even though they have nothing to do with planets.
(2) Massive stars finally exhaust their nuclear fuel, and they most often die in this spectacular explosion. They play a crucial role in enriching their galaxy with heavier elements, allowing the chemical elements that make up earthlike planets and the building blocks of life. The elements built up by fusion during the star’s life are now “recycled” into space by the explosion, making them available to enrich the gas and dust that form new stars and planets. They produce a flood of energetic neutrons that barrel through the expanding material, and these neutrons can be absorbed by iron and other nuclei where they can turn into protons. Thus, they can build up elements that are more massive than iron, possibly including such gold, silver and uranium.
(3) The word means “new” in Latin. Before telescopes, when a star too dim to be seen with the unaided eye suddenly flared up in a brilliant explosion, observers concluded it must be a brand-new star. It is a class of exploding stars whose luminosity temporarily increases from several thousand to as much as 100,000 times its normal level. It reaches maximum luminosity within hours after its outburst and may shine intensely for several days or occasionally for a few weeks, after which it slowly returns to its former level of luminosity. It mostly occurs in binary star systems in which members revolve closely around each other. Both members of such a system, commonly called a close binary star, are aged: one is a red giant and the other a white dwarf. Sometimes, the red giant expands into the gravitational domain of its companion - the gravitational field of the white dwarf is so strong that hydrogen-rich matter from the outer atmosphere of the red giant is pulled onto the smaller star. When a sizable quantity of this material accumulates on the surface of the white dwarf, a nuclear explosion occurs there, causing the ejection of hot surface gases.
What is (1) Planetary Nebula, (2) Supernova, (3) Nova?
[Source: Open Stax Astronomy 22.4 Further Evolution of Stars, 22.5 The Evolution of More Massive Stars, 23.2 Evolution of Massive Stars An Explosive Finish; Encyclopedia Britannica Nova Astronomy]
Condition at which the internal pressure of a gaseous body, such as a star, balances its gravitational pressure by a pressure-gradient force. Stable stars are in _________.
What is Hydrostatic Equillibrium?
[Source: OpenStax Astronomy 16.3 The Solar Interior: Theory]
States that the structure of a star, in hydrostatic and thermal equilibrium with all energy derived from nuclear reactions, is uniquely determined by its mass and the distribution of chemical elements throughout its interior.
MYTH: a more massive star, having more fuel, would last longer
FACT: The lifetime of a star in a particular stage of evolution depends on how much nuclear fuel it has and on how quickly it uses up that fuel. More massive ones use up their fuel much more quickly than stars of low mass. Massive stars use up fuel much quicker because the rate of fusion depends very strongly on the star’s core temperature. How hot a star’s central region gets is determined by its mass. The weight of the overlying layers determines how high the pressure in the core must be and higher mass requires higher pressure to balance it, which is produced by higher temperature. The higher the temperature in the central regions, the faster the star races through its storehouse of central hydrogen. Although massive stars have more fuel, they burn it so prodigiously that their lifetimes are much shorter than those of their low-mass counterparts.
What is the Vogt-Russell Theorem?
[Source: OpenStax Astronomy 22.1 Evolution From the Main Sequence to Red Giants]
(1) Named for the primary mode of transporting energy across it, this region starts at about 25% of the distance to the solar surface and extends up to about 70% of the way to the surface. The light generated in the core is transported through this zone very slowly. The high density of matter in this region means a photon cannot travel too far without encountering a particle, causing it to change direction and lose some energy.
(2) The outermost layer of the solar interior - a thick layer approximately 200,000 km deep that transports energy from the edge of the radiation zone to the surface through giant convection cells. Plasma at the bottom of the this zone is extremely hot - it bubbles to the surface where it loses its heat to space and once the plasma cools, it sinks back to the bottom of the this zone.
What is (1) Radiation Zone and (2) Convection Zone?
[Source: OpenStax Astronomy 15.1 The Structure and Composition of the Sun]
(1) - Stable, compact objects with electron-degenerate cores that cannot contract any further. Calculations showing that this is the end state of low-mass stars were first carried out by the Indian-American astrophysicist Subrahmanyan Chandrasekhar. They are far denser than any substance on Earth, so the matter inside them behaves in a very unusual way. At this high density, gravity is incredibly strong and tries to shrink the star still further, but all the electrons resist being pushed closer together and set up a powerful pressure inside the core. The temperature in the interior of a star is always so high that the atoms are stripped of virtually all their electrons. For most of a star’s life, the density of matter is also relatively low, and the electrons in the star are moving rapidly. This means that no two of them will be in the same place moving in exactly the same way at the same time. But this changes when a star exhausts its store of nuclear energy and begins its final collapse - as the star’s core contracts, electrons are squeezed closer and closer together and eventually, a Sun-like Star becomes so dense that further contraction would require two or more electrons to violate the rule against occupying the same place and moving in the same way - this is known as electron degeneracy.
(2) When nuclear reactions stop, the core of a massive star is supported by degenerate electrons, similar to a white dwarf. No energy is being generated within the core of the star, but fusion still occurs in the shells that surround the core. As the shells finish their fusion reactions and stop producing energy, the ashes of the last reaction fall onto the core, increasing its mass and the collapsing core can reach a stable state as a crushed ball made mainly of neutrons. If the mass of the core is greater than a certain number, neutron degeneracy cannot stop the core from collapsing further. The neutron degenerate core strongly resists further compression, abruptly halting the collapse and the shock of the sudden jolt initiates a shock wave that starts to propagate outward. This shock is not enough to create a star explosion.
(3) A dying star, a result that is even more extremely compressed from which nothing can escape.
What is (1) White Dwarf, (2) Neutron Star, (3) Black Hole?
[Source: OpenStax Astronomy 23.1 The Death of Low Mass Stars, 23.2 Evolution of Massive Stars An Explosive Finish]
British physicist Lord Kelvin and German scientist Hermann von Helmholtz in the nineteenth century, proposed that the Sun might produce energy by the conversion of gravitational energy into heat. They suggested that the outer layers of the Sun might be “falling” inward because of the force of gravity. They proposed that the Sun could be shrinking in size, staying hot and bright as a result. This is known as a star maintaining _____________.
What is Thermo-gravitational Equilibrium?
[Source: OpenStax Astronomy 16.1 Sources of Sunshine: Thermal and Gravitational Energy]
How long does it take for energy to get from the core of the Sun to Earth?
What is 100,000 years to 1,000,000 years?
[Source: OpenStax Astronomy 15.2 The Solar Cycle]