Chapter 5: The Formation of Stars and PlanetsPersonal E-Book Notes5.1 Molecular Clouds are the Cradles of Star Formation- The space between the stars is filled with giant clouds of cool gas and dust- Hot gas fills the space between these clouds, pressing on them and helping to keep them together- Each of the atoms and molecules in a cloud is gravitationally attracted to every other par-ticle, some clouds collapse under this, forming multiple stars, sometimes further fragmenting toform planets- Pressure: in astronomy, the force that atoms or molecules exert on each other over an area when they are zooming around quickly and colliding with each other- Dense: in astronomy, the amount of mass packed into a volume- molecular clouds are the cradles of star formation - a system of planets surrounding a star is a planetary system, there are many inside the Milky Way- self-gravity holds planets and stars together, it’s the gravitational attraction between the parts of planet or star that pulls all the parts towards its center - this can be opposed by structural strength, the rocks that make ip terrestrial planets, or theoutward force resulting from gas pressure within a star- if the outward force is weaker than self-gravity, the object contracts, if it is stronger, the object expands, it is a stable object if the forces are balanced- interstellar medium: this concept of the opposing forces of self-gravity and internal pres-sure- interstellar cloud- relatively dense region of the interstellar medium, has self-gravity, in most interstellar clouds the internal pressure is much stronger than self-gravity so the clouds should expand, but the opposing pressure of the less dense, hotter gas surrounding the clouds holds them together: pressure is proportional to density and temperature- the densest, coolest clouds are called molecular clouds because the conditions within them allow hydrogen atoms to combine to form hydrogen molecules, some are overwhelmed by self-gravity and collapse, this collapse happens slowly due to conservation of angular mo-mentum, turbulence---random motion of pockets of gas within the cloud, and magnetic fields- NOT DENSE = Hot- DENSE = cool- as a clouds gets smaller, gravitational attraction increases even more because the objects within it are closer, speeding up the process- molecular clouds are never uniform, some regions are denser and collapse more rapidly than surrounding regions, causing dense concentrations of gas called molecular-cloud cores,one cloud may form hundreds or thousands of these, which are each a few light months in size, and some of these will form stars- each layer of the cloud core falls inward in turn, removing support from the layers still farther out5.2 The Protostar Becomes a Star- the core is called a protostar, an object that is about to be a star, the surgace is heated to a temp of thousands of degrees as the cloud collapses, and particles are pulled toward the center by gravity- as they fall, they move faster and faster, more dense, they crash into each other, raising the temp of the core, thermal energy is created from gravitational energy- protostar is tens of thousands of times larger than the surface of the Sun, so its also that much more luminous, but we can only see it through infrared light because the protostar is buried deep in the heart of a dense and dusty molecular cloud, and dust blocks visible light, longer-wavelengths of infrared light can see through the cloud and this is made easier by radia-tion heating the dust- at any given moment the protostar is in balance between outward pressure and inward gravity, but the balance is always changing, like a spring balance- the protostar loses internal thermal energy by radiating it away, but the material that has fallen on the protostar also compresses and heats it, interior becomes denser, hotter, pressure rises, balance is always maintained- dynamic balance persists: energy is radiated away and the protostar slowly contracts, thermal energy heats the core, raising the energy to oppose gravity, smaller and smaller, hotter and hotter, until the center is hot enough to ignite, or begin turning hydrogen into helium, the main energy source for most stars- the protostar’s mass determines whether it will actually become a star, if the mass is greater than about .08 times the mass of the Sun, the temp will reach 10 million K and the nu-clear reaction that converts hydrogen into helium will begin- if the mass of the protostar is just under .08 times the mass of the sun, it will never be hot enough to become a star, these failed stars are called “brown dwarfs”, they are neither stars nor planets but something in between, - a protostar’s luminosity comes from gravitational collapse: thermal energy escapes from the interior of a protostar and is radiated into space, the protostar contracts; gravity is stronger in the smaller protostar, pushing up pressure and temperature; the cycle continues, it gets smaller and smaller, gravitational energy is converted into thermal energy, until the core gets sohot that hydrogen begins combining to form helium; the new star settles down into its main-se-quence life, burning hydrogen in its core5.3 Planets Are Born- how planets are formed: disks of gas and dust have been found surrounding young stellar objects - cloud collapses into a disk like a spinning ball of pizza dough forms a flat, circular crust- a piece of dust or molecule of gas will either - 1) travel inward onto the protostar at its center,- 2) remain in the disk to form planets and other objects, or - 3) be thrown back into interstellar space- the orbits of all the planets lie very close to a single plane, so the early Solar System musthave been flat- additionally, all the planets orbit in the same direction so the material which became planets must have been orbiting in the same direction as well- meteorites: pieces of material left over from the Solar System’s youth, many resemble a piece of concrete (pebbles and sand are mixed with a much finer filler), suggesting larger bod-ies of S.S. grew from the aggregation of smaller bodies- the disk surrounding the young Sun was filled with gaseous and solid material which swirled to form our S.S.- roughly 5 billion years ago: the disk surrounding the protostellar Sun was a flat, orbiting disk of gas and dust like those seen around protostars today, this disk is a protoplanetary disk which was 1/100 as massive as the protostar, still