Chapter 14
Describe, on the HR diagram, the evolution on the main sequence up until it is getting ready to move off the MS
A star is considered to be a main-sequence star once it sustains continuous hydrogen fusion. The Relation among stellar Mass, Luminosity, and Lifetime The star's mass determines its starting luminosity (absolute magnitude) and lifetime. The Life of a Main-Sequence Star Stars spend 90% of their lifetime on the main-sequence. While on the Main Sequence the star undergoes hydrogen fusion. The time a star spends on the main sequence is dependent upon its mass. The smaller the mass the more time spent on the main sequence. A star having a mass 40 times that of the Sun would only spend about 1 million years on the MS A star having a mass 0.4 that of the Sun would spend about 200 billion years on the MS Sun will run out of hydrogen eventually gravity will start to shrink the core it, it changes gravitational energy.
Making Stars from the Interstellar Medium
All thins in this universe have four states of existence: birth, growth or maturing, decay, and death. Stars are no exception. The study of the life cycles of stars is called stellar evolution. is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", fuse to form stars.
Young Stellar Objects and Protostellar Disks
As mentioned above, as the interstellar cloud collapses it creates a protostellar disk The youngest stars are found in the stellar associations. The changes in energy output and surface temperature can be plotted on an H-R diagram. These evolutionary tracks give astronomers insight into stellar life cycles. The source of energy used to heat this cloud is gravitational. The rate of collapse s determined by the mass of the protostar, the more massive ones collapsing faster. For a star like the Sun, the initial gravitational collapse takes millions. Massive stars like O and B stars go through their protostar stage in only 10,000 years.
Describe the production of 21-cm emission and how astronomers "see" HI regions
Besides producing absorption lines, cool neutral Hydrogen produces an emission line. This emission line is found at 21-cm and is in the radio region of the electromagnetic spectrum. This 21-cm line was predicted n 1944 by van de Hulst and was confirmed by observation in 1951. The reason why the hydrogen atom emits this wave is that the atom finds itself at a lower energy level.
List and describe the different types of nebulae
Emission Nebulae (HII regions): Most of the interstellar gas is in a neutral state but in some regions astronomers find what are called HII regions these regions are produced when a hot star (10,000 K or hotter) is located in a gas cloud. The surrounding gas is ionize by the star and produces glowing cloud HII region) HII regions are often called emission nebulae. Reflection Nebulae: Reflection nebulae are illuminated by reflected starlight off the interstellar dust. The most famous reflection nebula is the Pleiades. It reflects the light of nearby stars. Their spectra are the same as the stars surrounding them, though the light is bluer, shorter wavelengths scatter better than longer waves. Dark Nebulae (often called absorption nebulae) : When the concentration of dust gets great enough it can block out light and form a dark (absorption) nebula. Great concentrations of dust can be seen along the dark rift that runs along the Milky Way.
Describe the conditions prevailing in HI and HII
HII is a region of interstellar hydrogen that is ionized. It is typically a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived blue stars forged in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas. HI is a cloud in the interstellar medium composed of neutral atomic hydrogen (HI), in addition to the local abundance of helium and other elements. These regions do not emit detectable visible light (except in spectral lines from elements other than hydrogen) but are observed by the 21-cm (1,420 MHz) region spectral line.
Describe the effects of dust on light passing through it
Interstellar Reddening: The interstellar dust found in the Galaxy dims and reddens starlight passing through it. By studying the amount of reddening astronomers learn about the amount on interstellar dust that is along the line of sight.
The Orion Nebula: Evidence of Star Formation
One of these interstellar nurseries is known as the Orion Molecular Cloud. Within this cloud, star formation is revealed from infrared observations.
Describe the process of star birth including the source of energy that heats it up.
Prostars: Before a star was a star it was an interstellar cloud of dust and gas. This cloud was squeezed in some way (supernova, density wave, or perhaps a cloud collision) an began to collapses under gravity. As it collapsed, its temperature and density increased. This warming cloud can be detected by the infrared radiation that it emits. The collapsing cloud, which is on the way to becoming a star, s known as a prostar. The collapsing cloud is heated by changing gravitation energy into heat, and its rotation increased due to the conservation of angular momentum. As the spin rate increases, the prostar begins to take on a disk shape. At this stage the prostar and disk is within a cloud of dust and gas.
Describe where astronomers find interstellar molecules
Since 1968 over 150 different types of molecules have been discovered in the interstellar medium. Examples: Methyl cyanide, Methyl alcohol, Ethyl alcohol. These molecules are found in great interstellar clouds that are rich in dust. These great masses of material are called molecular clouds. Much of this material is concentrated in the spiral arms of the Galaxy.
Discuss where astronomers find star formation occurring in our Galaxy.
Star Birth in Molecular Clouds: Molecular clouds: Stellar Nurseries. Stars form from great molecular clouds in the disk of the Galaxy. At present , over 150 different types of molecules are known to exist in these clouds. These giant molecular clouds are not smooth, homogeneous objects, but are considered to be more or less in a "lumpy" states. These lumpy, high density, areas are thought to sometimes collapse under gravity and form stars.
Describe what astronomers mean when they speak of the interstellar medium
The interstellar medium is the material found between the stars and is primarily gas (hydrogen and helium) mixed with small amounts of dust. About 1% of the interstellar medium is made up small solid particles. These particles are also referred to as dust or interstellar grains. This dust produces whats called interstellar extinction.
Describe the CNO cycle and the type of stars this form of energy production is important
The primary elements in stars are hydrogen and helium, but smaller amounts of heavier elements are present. In particular there can be carbon, nitrogen, and oxygen. In higher-mass stars, there's greater gravitational pull and pressure and therefore higher central temperatures. At these higher temperatures (16 million K or higher) , nuclear reactions other than those of the proton-proton cycle are important. For stars having a mass greater than about 2 solar masses, there is a different cycle of nuclear reactions. This is the CNO cycle.
Energy Transport
The three possible modes of heat transfer in a star are conduction (in most cases-not very important astronomically): is the process by which heat energy is transmitted through collisions between neighboring heat sources. convection: transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. radiation (radiation from Sun to Earth): the emission of energy as electromagnetic waves or a moving subatomic particles
Describe the basic equilibrium conditions involved in stellar models
To determine the internal structure of stars astronomers assume that the star is in equilibrium This assumption allows astronomers to create mathematical models of stars A set of computer solutions to these equations results in a theoretical model of a star.
Stellar Structures and Nuclear Fusion
What keeps a star stable: since the star is stable (it does not shrink or expand) it can be assumed that it is in hydrostatic equilibrium. Hydrostatic equilibrium occurs when the outward forces resulting from gas and radiation pressure are balanced by the inward force of gravity.
