Formation Of Stars

Introduction

The story of the life cycle of stars is something that astronomers have been trying to piece together for quite some time, and this is huge indeed. The stars themselves are shrouded in secrecy. For example, those stars are points of light and they’re 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 just 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.

Formation of protostars

If we understand from ground zero, from the formation of protostars we would be able to understand the reason for the stages of star formation. Protostars are just the course 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 amount of infrared radiation. Now the gas themselves are hot, but they’re not yet hot enough to cause hydrogen to diffuse 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. 

Outlook of star formation

• So let’s take a look at the broad steps of star formation. We initially begin with a cloud, and the cloud is consistent with moving molecules of gas, so that means that there’s pressure in the cloud. and even though the cloud has some mass to it rather than just collapsing under its gravity
• It’s kind of repelled from doing that by 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 a certain mass, and therefore they have gravity now and gravity starts to overtake the pressure. 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 the effect of accelerating the collapse, but as the cloud collapses its rotation, and this causes the disk to flatten out, so now we’ve formed what’s called a circumstellar disk
• We can think of the disk as an inner disk that directly feeds the protostars, so this sometimes is called an “accretion disk” or also called a protostele or disk, and the remaining outer disk flattens eventually becomes what’s called a “protoplanetary disk”. So eventually we would expect planets to form inside this disk and at some 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 help sweep out the disk as well as the plants themselves. They collect all the debris in their path and the planetary system is now born, the star in a proper planetary system

Angular momentum factor

• So the protagonist 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 accurately angular momentum
• 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 its mass is being spread out
• Now angular momentum has a certain property, 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

Life cycle and stages of star formation

So you might ask what does this have to do with the life cycle of a star? Again, we’ll 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 a mass of anywhere from a couple of thousands to up to a million sons. And they are spread out over hundreds to thousands of light-years.

So even though these clouds are rotating very slowly, each of them contains 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 cause a couple of rotating clouds to combine, forming clumps and increasing the rotational velocity.Massive stars can explode as a supernova, and these can send shock waves through interstellar space if these shock waves collide with a giant molecular cloud.

That can cause that cloud to fragment and collapse into multiple stars or in a kind of chicken and egg scenario. We can have what we see here, the presence of these very young massive stars at the centre of this nebula can trigger star formation by flooding the entire region with ultraviolet radiation and superfast stellar winds that kind of slam into the denser, cooler gas around it, forming these tightened knots and clumps, and each of the clumps that you see here in this image is the cocoons of a young forming protostar within and when stars are reaching the end of their stages of star formation. So we’re talking about stars that are about the same mass as the sun, and they aren’t fusing hydrogen in their cores yet, but they’re about to.The circumstellar disk is seen more or less phase on, what’s happening is that the protostars are emitting a very fast stellar wind, but the disk itself is made of such cool, dense gas and dust that it has the effect of confining the outflow of that stellar wind.Moderately much along the axis of its rotation. In other words, the outflow is being channelled out through the polls.

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 word is somehow kept all of its angular momenta.It would have to rotate so fast that it would tear itself apart, and when this material slams into the surrounding dust and nebula city.It ionizes that gas and it causes it to glow, so these jets, these glowing jets are known as her big hero objects. And they’re just high-velocity jets slamming in hundreds of kilometres per second and we think these are relatively short-duration events. In other words, we don’t think that these last very long, maybe on the order of a couple of thousand, maybe two hundred thousand years or so, but remember the formation of these stars takes place over tens of millions of years. Now we’re lucky here. We get to 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. 

So these heroic objects seem to herald the arrival of a newly forming star, so this is a very well studied object. 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, We have the jet, which is being emitted at a rate of about three hundred kilometres per second. There’s also a lot of light from the star itself being reflected or scattered.the disk now the disk itself is in silhouette, and that’s because deep at the centre is the protostar itself, hidden from our view, and to keep the scale.Let’s just consider the radius of this disk. It’s about four hundred and thirty astronomical units. Remember the semi-major axis of Neptune is about thirty astronomical.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 on and there are presumably planets inside this disk that are forming from the disc material itself. Best of all we can take images of these phenomena over and over again and over fourteen years, this particular jet was captured by the Hubble Space Telescope and so we can 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. Not only that, but 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 protostellar or protoplanetary disks that surround newly forming stars.

Conclusion

It seems that every star that forms starts with a disk-like this, but not all these discs have jets associated with them, so the jets themselves are probably transient events. It’s almost as if there were 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 star’s formation.