Star Formation

the story of the formation of stars is something that astronomers have been trying to piece together for quite some time and this is largely because the stars themselves are shrouded in secrecy I mean literally and shrouded in secrecy we know for example that stars are points of light and they are composed of hydrogen and helium gas however when we look around the night sky we see mostly the points of light we seldom see the gas itself well that's because the gas is quite cool if you look at the constellation of Orion you'll see there's a relatively small patch of that gas that is glowing kind of like a fluorescent lamp but if we could look into the same region with infrared eyes we would see all of that cool gas known as the Orion molecular cloud let's take a closer look however at the Orion Nebula let's take a closer look at the near-infrared so you can see those very hot bright stars at the center the-- these stars are called the trapezium and surrounding the trapezium are these tinier red little points of light well these points of light look like stars but they're not quite stars just yet they are in fact proto stars so proto stars are really just the cores of collapsing molecular clouds we have clouds within the clouds within the clouds and tiny clumps form and they collapse as they collapse they heat up so they release a great deal of infrared radiation now the cores themselves are hot but they're not yet hot enough to cause hydrogen to fuse into helium through thermonuclear fusion and this is just because the temperatures and the pressures are building but they haven't yet reached that critical mass or that critical pressure to trigger hydrogen fusion so let's take a look at the broad steps of star formation we initially begin with a cloud and the cloud is consisting of moving molecules of gas so that means that there's a pressure in the cloud and despite the fact that the cloud has some mass to it rather than just collapsing under its own gravity it's kind of repelled from doing that by virtue of the motions of the particles themselves so pressure overcomes its mutual gravity but something happens something triggers the formation of little clumps of gas and dust somewhere in the middle of these clouds now these dense clumps have mass and therefore they have gravity now gravity starts to overtake the pressure right it starts to overtake the motions of the molecules and so as clumps form then little cores form inside the clumps these cores are essentially the protostars they gain mass and therefore the gravitational pull of these cores increases so there now becomes a bit of a snowball effect and this has effect of accelerating the collapse but as the cloud collapses it's rotating and this causes the disc to flatten out so now we've formed what's called a circumstellar disk and we can think of the disk as an inner disc which directly feeds the protostar so this sometimes is called an accretion disk or also called a proto stellar disk and the remaining outer disk flattens eventually and becomes what's called a proto planetary disk so eventually we would expect planets to form inside this disk and at some point the star itself the protostar turns on in other words the pressures and the temperatures are now high enough that suddenly we can have thermonuclear fusion in the core and this produces tremendous fast winds that's helped sweep out the disk as well as the planets themselves they collect all the debris in their path and the planetary system is now born the star is a proper planetary system so a main driver of the story is the fact that the entire cloud and subsequent disk and the stars themselves are all rotating and whenever we think of something that's rotating it's important to consider a quantity called angular momentum all objects in motion have momentum and if the object is rotating we call that rotational momentum or more properly angular momentum and angular momentum depends on three things it's a product of the mass of the rotating body the rotational velocity or the angular velocity the rate at which it's rotating and also how spread out the masses now angular momentum has a certain property and namely that it's a conserved quantity in other words it's always going to remain constant even if we were to somehow change one of the three parameters so when a figure skater is spinning if she has her arms spread out she rotates relatively slowly but as she brings her arms in she rotates faster this is how the angular momentum is conserved her mass doesn't change so what has a change it's her rotational velocity so what does this have to do with stars again well remember stars are forming out of these giant molecular clouds and there are thousands of them spread out just within our galaxy alone each of these molecular clouds has the mass of anywhere from a couple thousand to up to a million Suns and they are spread out over hundreds to thousands of light year so even though these clouds are rotating very slowly each of them contain a tremendous amount of angular momentum so there needs to be something to get these giant molecular clouds to begin collapsing for example clouds can just randomly collide and that can get a couple of rotating clouds to combine together forming clumps and increasing the rotational velocity massive stars can explode as supernovae and these can send shockwaves through interstellar space if these shockwaves collide with a giant molecular cloud that can cause that cloud to fragment and collapse into multiple stars or in a kind of a chicken and egg scenario we can have what we see here the presence of these very young massive stars at the center of this nebula can trigger star formation by flooding the entire region with ultraviolet radiation and superfast stellar winds that kind of slam in to the denser cooler gas around it forming these tight knots and clumps and each of the clumps that you see here in this image are the cocoons of a young forming protostar within and when stars are reaching the end of their formation stages they often go into what's called a teatari star phase so we're talking about stars that are about the same mass of the Sun and they aren't yet fusing hydrogen in their cores yet but they're about to and T star II stars are often characterized by the presence of circumstellar disks and by these very fast stellar winds so in this image we're looking at v1 3:31 signe the circumstellar disk is seen more or less face-on and if we could take an imaginary view of this same disc kind of at an angle you'll see what's going on what's happening is that the protostar is emitting a very fast stellar wind but the disc itself is made of such cool dense gas and dust that it has the effect of confining the outflow of that stellar wind pretty much along the axis of its own rotation in other words the outflow is basically being channeled out through the poles now sometimes this outflow is very intense and when we look in places like the Carina Nebula remember every one of these knots every one of these clumps is a forming star system in and of itself however when we look into the upper right corner of this image you'll see that there's a very pronounced jet of material that's blasting out in a bipolar fashion now when this stuff is being ejected out at very high speeds this has the effect of getting rid of some of that angular momentum and that's important because if the star were to somehow keep all of its angular momentum it would have to rotate so fast that it would literally tear itself part and when this material slams into the surrounding dust and nebulosity it ionizing that gas and it causes it to glow so these jets these glowing jets are known as herbig-haro objects and they're really just the high velocity jet slamming in at hundreds of kilometers per second now we think these are relatively short duration events in other words we don't think that these last very long may be on the order of a couple thousand maybe to a hundred thousand years or so but remember that the formation of these stars take place over tens of millions of years now we're lucky here we get to actually see both lobes of the jet often times however we have a very dense pillar of gas in this particular case that's blocking one of those jets from view luckily though we can explore this region at infrared and submillimetre wavelengths that allows us to see through much of that blocking gas and dust revealing both lobes in the process so these herbig-haro objects really seem to herald the arrival of a newly forming star so this is a very well-studied object HH 30 it may not look very impressive because we are looking at the individual pixels on the Hubble Space Telescope's main camera but there are still some very familiar features first of all we have the jet which is being emitted at a rate of about 300 kilometers per second there's also a lot of light from the star itself being reflected or scattered from the disk now the disk itself is in silhouette and that's because deep at the center is the protostar itself hidden from our view and to keep the scale in mind let's just consider the radius of this disk it's about 430 astronomical units remember the semi-major axis of neptune is about 30 astronomical units so this is an extremely still an extremely very very large system that is still in the process of collapsing so what we think is happening is that the star itself is about to switch John and there are presumably planets inside this disk that are forming from the disk material itself best of all we can take images of these phenomena over and over again and over a 14-year period this particular jet was captured by the Hubble Space Telescope so we can actually watch how it moves through space we don't have to pretend or assume that this is happening we can see it happening over time now we only have one of the two Jets the other jet would be moving toward the left of your screen however it's concealed behind a dense molecular cloud so we have a story that we're starting to fit together about how stars form there are these proto stellar or protoplanetary disks that surround newly forming stars and it seems that every star that forms starts out with a disc like this but not all these disks have jets associated with them so the Jets themselves are probably transient events in fact if you look at this object on the right side of your screen you're gonna see little knots it's almost as if there were like bullets of material that were being ejected from the star so this tells us that the Jets themselves probably turn on and turn off and turn back on again at various times throughout the stars formation so quick anatomy and recap we have the circumstellar disk all this is the rotating gas and dust and it's flattening out due to the conservation of angular momentum the inner disk is rotating the fastest and therefore it's the flattest part of the system this inner disk is what directly feeds the protostar and that's why it's often called an accretion or a proto stellar disk the protostar itself is hidden from view it's directly at the center and it's this disk that confines the outflow of the proto stellar wind along the poles and sometimes the outflow is intense enough to create a jet so far we've been talking about the formation of stars that are approximately the mass of the Sun but it turns out the same phenomena is true even of massive stars like this one this is a 15 solar mass star called Sharples 2 - 106 and as the picture may indicate it's the same process just happening on a much much larger scale so we have the protostar itself which is basically hidden from view we don't really even see the protostar but this massive bipolar outflow and you could just see all the different knots that suggest how the stuff has been colliding with the interstellar medium so this whole system is about two light-years from one end to the next so this is just the same thing that happened to our solar system except obviously on a much larger scale now most of the stars in the sky are actually members of binary systems so here is an example of a forming binary system this is HK tari and we have two separate discs so we have two stars each forming from within their own discs and since these two discs and the 2 proto stars within are gravitationally bound they will undoubtedly go on to become a proper binary star system these discs and the stars forming within them will continue to orbit one another but sometimes a binary or even in this case a triple star system can form from within the same disk what we have is a single disk and it looks like what's happened is that the disk has fragmented and it's this fragmentation of the disk that gave rise to additional clumps or additional cores so here's an artist's impression of a similar system GG tari a it looks like we had the primary star at the centre and then from somewhere within the disk some instability formed and rather than accreting onto the main star at the center it formed its own accretion disk around a second protostar as the protostar continues to develop the remaining disk is flattening and then little clumps form inside that disk and go on to become planets so we're going to learn more about the formation of planetary systems next

Channel: Launch Pad Astronomy

Views: 50,468

Rating: 4.8903508 out of 5

Keywords: Astronomy, Stars, Star Formation, Protoplanetary Disks, Circumstellar Disks

Id: lI57XIZ17hE

Length: 15min 41sec (941 seconds)

Published: Fri Apr 06 2018