ASTR 101 1st Edition Lecture 24 Outline of Last Lecture I HR diagram II stars outside of main sequence III star clusters Outline of Current Lecture I Clicker questions II Star birth Current Lecture I II Clicker questions a What is the first step in the formation of a protostar gravity causes a cloud of cold gas and dust to begin to contract It will be difficult for the gas to contract if the gas is warm needs to be cold b Why do we think that clouds of gas and dust form stars we see young star clusters with gas and dust around them we can see it through telescopes c What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy its internal pressure would increase slowing down the collapsing process Radiating away its thermal energy would accelerate the collapsing process 13 1 Star birth a How do stars form In dark clouds of dusty gas in interstellar space this diffused gas between the stars in interstellar medium i To form a star we need to balance gravity with pressure Gravity has to overcome the force of pressure in the cloud When a gas cloud starts to contract it becomes denser heats up and gravity becomes stronger conservation of energy Contraction can continue only if thermal energy is radiated away Collapse and collapse until it triggers nuclear fusion ii As stars begin to form dust grains that absorb visible light heat up and emit infrared light Infrared appear as emissions and appear brighter The longer the infrared the colder the cloud and the more efficient it is to form stars Infrared is also brightest where the most stars are forming ex the Orion nebula constellation is very easy to see in our night sky because this region is producing lots of stars Orion is a dense cloud producing many new stars iii Conservation of angular momentum causes the cloud to spin faster and faster as gravity forces the cloud to become smaller iv Collision between particles is a reason the gas flattens out into a disc as it rotates b How massive are new stars i A star forming molecular cloud is typically about a few hundred particles per cubic centimeter These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute ii iii It needs at least a few hundred solar masses of material of this cold gas for gravity to overcome pressure In the end we want a configuration that is hot and dense enough to form a star This pull between the force of gravity and the star s pressure works until that is reached It s actually quite unnatural to form a single star without any companions each lump of the cloud in which gravity can overcome pressure can go on to become a star As a cloud fragments it generally makes many stars or a cluster of stars