Chapter 5 The Formation of Stars and Planets Personal E Book Notes 5 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 to form 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 the outward 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 out 5 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 so hot that hydrogen begins combining to form helium the new star settles down into its main se quence life burning hydrogen in its core 5 3 Planets Are Born objects how planets are formed disks of gas and dust have been found surrounding young stellar 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 must have 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 more than enough to account for bodies that make up the solar system today the reason that star formation leads not only to a star but a flat orbiting collection