Sorry we’re late, we just had to take the time to evolve this post. That’s right, we are here to discuss the first step in a star’s evolution! In one of my many twisted analogies I said that stars are like people. So, we have the early stages of a star’s life! Just like how people have baby and teenage years we have the topic of star formation and nebulae (again, twisted analogies…but I think we can all deal with the puns and any bad humor presented here).
Interstellar medium (ISM) is quite literally “the stuff between the stars”. And by stuff, we mean the gas and dust that stars form out of, and often, spew back into space during their lifetimes through stellar winds or supernovae. Unsurprisingly, hydrogen makes up most of the ISM as HI (neutral), HII (ionized), or H2 (molecular), and helium makes up most of the remainder. It’s hard to detect neutral hydrogen because its single electron is in the ground state, so it cannot jump down an energy level to release radiation, and only rarely does a photon of the right energy come along to boost the electron to a higher energy level. However, astronomers can still detect HI using the 21 cm line. This relies on the fact that electrons and protons both have a quantum “spin”. A hydrogen atom has slightly less energy when they are spinning in opposite directions as compared to when they are spinning in the same direction – the photon corresponding to this difference in energy has a wavelength of, you guessed it, 21 cm.
While it doesn’t make up a large percentage of the ISM, dust blocks light and also reddens it, since longer wavelengths are less likely to interact with dust grains. It is sometimes found in the form of Polycyclic Aromatic Hydrocarbons (PAHs), which are relatively complicated hydrocarbons with ring structures. Why do we mention PAHs? Not for any important reason, they’re just thought to be necessary for life (even though we consider them scary carcinogens down here on Earth).
Thus, we start our evolution with stuff. What is this stuff? Oh, various things. Hydrogen, helium, lithium… Yes, lithium in fact would exist, but we will ignore this until a bit later into the post. But we just have stuff, what do we need to make a star? Well, this stuff, at this point forming a nebula, literally meaning cloud, is not condensed.
Before we go further, we should note that there are several kinds of nebulae. Reflection, emission, absorption/dark, planetary, supernova remnants… heck, people used to call galaxies and star clusters “nebulae” as well. Note that this post will deal with star forming regions or cloud complexes, so there won’t be SNRs or planetary nebulae because they don’t really apply to the start of a star. Emission nebulae are formed when gas molecules are excited (yes, you be excited as well) by radiation from a nearby star and release radiation; they are typically red in color because of their hydrogen content. Reflection nebulae occur when starlight doesn’t have enough energy to excite the electrons and just reflects off dust particles instead (typically blue because shorter wavelength light is easier to scatter). Both these types of nebulae signal regions of star formation. Absorption nebulae, or dark nebulae, appear “dark” because they’re made of relatively dense clouds of dust that simply block light from behind them. And why study these? Well…they’re dark for one. So yeah, they block out light, how annoying! Luckily we have some ideas of how they block out light, but if we didn’t it would be quite the nuisance. But then what else could they involve (you know, since astronomy just can’t leave something as is)? Well, there are these things known as Bok globules. They are smaller dark nebula that are also regions of star formation in HII regions, and they can also hint at some star formation in general since they can be in molecular clouds.
But back to stars. For a star to form, a chunk of the gas cloud has to increase in density to such a point that it collapses under its own gravity. Either random turbulence within the gas cloud itself may achieve this critical density, or an outside source, such as collision with another gas cloud or shock waves from a nearby supernova, may be involved. Astronomer James Jeans derived an equation to show the minimum mass necessary for a cloud of a certain radius to collapse; we of course know this as the Jeans mass. However, the Jeans mass neglects external gas pressure (which is factored in by the Bonner-Ebert mass) as well as several other factors that may influence collapse. The initial gas cloud fragments into several pieces, for reasons that we don’t quite understand yet, and these pieces may fragment further until their density is so great that they just keep collapsing. And you guessed it, this is called fragmentation! These are important because they can form binary stars and relate clusters of many stars, both of which are quite useful. Fragmentation itself stops and changes due to differences in density and energy radiated as the collapse occurs. Aside from this, factors influencing protostars are mainly motions (rotational or angular) and magnetic aside from atoms.
And to further the fascination let’s add some more factors. As we said there is rotation and magnetic fields. There are also differences in density. Another factor? Well, a commonly seen occurrence is to see stellar winds from massive stars blowing at or ionizing the protostar such that they are eroded away. So, what do we get from these complications? Birth lines! Yes, they really are just like humans, well not at all actually, since these are actually lines for the beginning of protostars’ evolution.
Also, massive stars may not form from large amounts of mass collapsing, but from multiple smaller stars coming together because the high temperature, luminosity, and radiation associated with large stars are thought to be hard to achieve by collapse. But this may not be needed because mass can fall into an accretion disk (an accumulated disk of material) around the star. This could then make a massive star grow, and it wouldn’t be fully ionized, which could prevent collapse.
So, are we at a star yet? Nope, we still have a ways to go. You see, like we said stars evolve, like life theoretically, but we can track them much better (see, stars are so much more stellar than other things). Tracks we say? Yes, evolutionary tracks in fact! These curves show major tracks for different mass stars. The energy produced during this time is created from falling material going so fast that it goes supersonic and has what’s called a shock front, which basically just means that we get this whole mix of really fast moving stuff slowing it down to the point where energy is released. Another note is that material is accumulating, or accreting (yes, it’s a word that will be used many many times) around the star as it collapses. In fact, the collapse that occurs appears to be of infrared sources which appear in Bok globules.
Alright, but now what do we have? Well, we’ll just say by this point we’ve gotten a protostar. This collapses through the Kelvin-Helmholtz mechanism of gravitational contraction which can release potential energy as heat, explaining the light we see from a protostar. Moving on we have the oh so important Hayashi Track! In fact, Hayashi did many things with these early stars, but let’s just start with this. The Hayashi track shows that for a collapsing protostar the opacity of a star increases from slight ionization of hydrogen. This results in convection in the envelope, which he managed to show as a vertical line on the H-R diagram. This line shows that the collapse eventually decreases luminosity and increases temperature. The best part about this track? It actually forbids certain types of stars from forming, greatly helping our search for star formation.
After all this and some modelling we get what’s called pre-main sequence evolutionary tracks. These are basically more tracks to see the evolution of stars. Interestingly, a core and convective zone begins to form at this point, which can allow some slight fusion through the PP chain and the CNO cycle, but not enough to really stop collapse. This can produce a slight expansion such that luminosity would actually slightly decrease. In lower mass stars carbon generally can’t be burned, so later in life the CNO cycle wouldn’t really occur as much, but in massive protostars it’s slightly different since the CNO cycle would be more dominant. So, what does this show? Basically that mass matters.
We’ll cover types of Pre-Main Sequence objects and more star formation in Part II.
TL;DR – Stars form out of interstellar medium, massive clouds of gas and dust. A gas cloud becomes denser in one area, which collapses under its own gravity and also fragments to form several protostars. These not-quite-stars follow evolutionary tracks as they evolve towards the main sequence on the H-R diagram. And what is basically the most important factor? Mass. This beginning stage is important to understand where to look for types of stars and events, to understand the processes in stars, and to help make better interpretations of how the universe works.
Star Formation (these sites cover most of the star formation post in general, which is why this isn’t split up so much)
Carroll and Ostlie, An Introduction to Modern Astrophysics, 2nd edition (pg 398-445)