Star Formation and Fueling - Gravity vs. Angular Momentum
Often the description of star formation is reduced to the spontaneous, adiabatic (no heat added/removed) collapse of molecular gas cloud by gravity. Not only is this a gross simplification, but thermodynamically unphysical. The truth is much more interesting, even if conceptually more complicated. We can understand star formation by considering it in four states (gas and dust, condensed nucleus, protostar and fusing star) or with three processes that link them condensation, accumulation, and fusion ignition. The accumulation of hydrogen or fusion fuel on a condensed star nucleus is difficult to see because at this stage the star is usually shrouded in dust. Occasionally we see young stellar objects peaking out or emerging both from dark nebula or pillars/elephant trunks in stellar nurseries. Often we see something we call Herbig Haro objects, which act like sign-posts shouting “THERE ARE STARS FORMING HERE”.
Perhaps the most obvious Herbig Haro object that we can image from the northern hemisphere can be found in the Pelican Nebula. In this case two Herbig Haro objects (they often come in pairs) look like a cartoon moustache coming out almost perpendicular from the tip of a dark pillar/elephant trunk – near the centre of the image. If we had a much more powerful telescope that could see through dust, we would see that the two sides of the moustache are emerging from the poles of a protostar. Herbig Haro’s are one of two ways that protostars shed excess angular momentum during their hydrogen accumulation phase – necessary to keep the star together while gravity pulls in more fuel for future fusion.
Pelican Nebula (IC5070) in SHO Narrowband (RGB stars)
Planewave CDK12.5 ; AP 1100 GTOAE ASI6200MM, – Antlia RGB & 3.5nm Narrowband Filters
H,O,S: (38,29,28 x 720s 61 Bin 1, Gain 100); R,G,B: (20,18,20 x 180s, Bin 1, Gain 100)
Total integration time = 21.9 hrs (July 31 & Aug 2-5, 2024) Maple Bay, BC
The Pelican Nebula lies about 1800 ly from us in the constellation of Cygnus, Together with the North America Nebula, they form a prolific stellar nursery that is rich in narrowband light energized by the UV radiation from the young stars. This radiation, coupled with stellar winds, create increased pressure in the molecular clouds being displaced away from the new stars. This pressure increase allows for the abundance of protostar nucleation, particularly in pillars and elephant trunks.
Full resolution, downloadable version on Victoria RASC Zenfolio and Astrobin
Above: a closeup of the Herbig Haro moustache jets coming out from the poles of a protostar near the tip of a dark pillar of molecular cloud. Fortunately these jets are orthogonal to our point of view. Often Herbig Haros present obliquely and less linear. They can even straight on and can easily be confused with small fuzzy stars.
Herbig Haros occur throughout the image (labelled HH above), with some considerably smaller – even too small to be picked out via my imaging train. Some are even so small that we are not sure if we are looking at a Herbig Haro, or a protostar itself.
Viscous Spiral conversion of Angular Momentum to Heat
In ancient times (pre-1961), people thought that angels pushed the planets around the earth. Thanks to Copernicus, we later revised this conception so that the angels pushed them around the sun. Regardless, today we understand that these concepts are both pure mythology – the angels, flapping their wings, actually push the planets directly towards rather than around the sun. Angels are not required to push the planets around, they simply keep moving around the sun because there is nothing to stop them or slow them down. If the angels flap harder, the can move the planets closer to the sun, but they can never actually get there because the planets would just move faster in a smaller orbit due to their angular momentum. In fact, if you actually wanted to get rid of a planet by getting it to collide with the sun, one would have to hire additional angels to push sideways on the plant to slow down its revolution perpendicular to the gravity angels. Only then could the inward pushing angels be successful at pushing the planets to the sun.
A more pragmatic example of this problem might involve a proposal to dispose of nuclear waste, by putting it in a rocket ship (the rocket engine takes the place of angels flapping), and crashing it into the sun. Initially, this may sound like a great idea, but practically not so much. Here’s where I recommended any of the countless youtube videos on this topic, but at the end of the day, the amount of fuel required for a rocket to shed its angular momentum to avoid “missing” the sun is too impractical. A workaround would be to use reverse gravity assist on other planets to slow down and shed a lot of the momentum not directly in line with the sun, but it would still require an enormous amount of rocket fuel. That is, even if you could get over putting nuclear waste into a potentially exploding rocket.
The point being that hitting the sun with a planet, rocket, or even the smallest of particles is very very hard. Your aim needs to be perfect, because any angular momentum will result in the object just orbiting the sun. Throwing anything into the sun is too hard since you will almost always miss. Yet this problem encapsulates the problem of accumulating hydrogen molecules (star fuel) on a nucleated protostar.
A protostar consists of condensed hydrogen (in liquid metal form), pressure contained under its own gravity. But how did hydrogen gas get close enough to condense on the nucleus in the first place. Even if we understand how the star was nucleated, why didn’t the molecules just orbit the star nucleus in the same way the planets orbit the sun – kept at a distance by angular momentum? Somehow, either through the hiring of sideways angles, the use of a rocket engines, or some other mechanism, they hydrogen must have shed its angular momentum to get close enough to the star so that it will condense into a liquid (metal).
There are at least two mechanisms at play. The first is the viscous drag (or friction) associated with (real) hydrogen molecules that dissipates angular momentum. The answer is simply that, due to the conservation of angular momentum, hydrogen molecules will increase their angular speed (move faster around the future star) as its radius (or distance from the nucleus) decreases. Again, using the solar system analogy, just as inner planets move faster than outer ones following their Keplerian laws. The second mechanism, Herbig Haro jet formation, takes place later once the hydrogen reaches the fattening protostar.
Without a mechanism or force to shed angular momentum, hydrogen cannot reach a star nucleus to provide it with additional mass and future fusion fuel. As the hydrogen molecules got closer, they would simply move around or away from the proto-star faster and faster.
This is just one of a myriad of reasons why the assumption of ideal gas behavior (inviscid, frictionless) of hydrogen leads to so many star formation theories to go astray.
In reality, hydrogen is a real gas, in the thermodynamic and fluid mechanical sense. As hydrogen molecules attempt to move faster when they get closer to the proto-star, their neighbour molecules to the outside slow them down and take on some of the angular moment. A lot of the angular momentum is converted to heat, but that is for another posting….
In this manner, angular momentum is not conserved by the particle itself, just as a spinning top eventually falls to the table.
We have encountered viscous drag before, in the postings on spiral galactic structure, the effect of viscous forces on hydrogen phase behavior, and why viscosity effect even occur. Similar to a spiral galaxy, viscous drag will likely result in a spiral trajectory taken by hydrogen as it falls toward the star nucleus. It differs, however, from a galactic spiral as the Reynolds number does not get high enough to cause cavitation (phase separation and molecular dissociation) as in galaxies. Instead, the winding problem avoided by galaxies just adds to the viscous forces and heat conversion in star formation. Also and of course, star formation is on a much smaller scale, and shrouded in dust, eludes our direct observation.
A great analogy for the interaction of the battle between viscous drag, centrifugal, and gravitational forces is the emptying of water in a sink by a drain.
Water, being incompressible, manifests the spiral as height waves, while compressible hydrogen would contain waves or pressure/density.
Credit DrainPro Plumbing Services
On its way to joining the nucleus to become a mass expanding protostar, hydrogen goes through phase changes as its pressure (and temperature) increases due to its flow towards higher gravity. The hydrogen will go from gas to super-critical fluid, to potentially a semi-ionized plasma, to join the protostar as a liquid metal. The gravity of the protostar itself provides the containment of its high and building pressure as more hydrogen continues to arrive. The angular momentum is partially dispersed outward to material holding back the spiral, and the remainder is converted to heat – responsible for the phase changes by the protostellar hydrogen and for warming up the protostar itself.
Herbig Haro Electromagnitic conversion of Angular Momentum to Linear Jets
The hydrogen has indeed shed some of its angular momentum by the time of its arrival at the protostar, but what remains is added to the protostar causing it to spin faster and faster. Spin is just another way a body can hold angular momentum – a spinning object is essentially revolving about itself. The move orbiting hydrogen arrives, the faster the star will spin and ultimately this angular momentum must be converted and shed. Some of the arriving hydrogen at the protostar’s likely oblate equator and will be transported to the poles of the protostar (off of the spiral plane) where it may speed up further as its radius further decreases.
The hydrogen in liquid metal that is spinning creates a stellar dynamo/magneto, and now it is electromagnetism’s turn to provide our second mechanism of shedding angular momentum – the creation of Herbig Haro jets.
The arrival of any ionized or polar magnetic material into the atmosphere of a star, will set up a magneto-hydrodynamic system with the spinning metallic, conductive hydrogen core. In other words, the protostar becomes a giant electromagnet powered by the “solenoid” of orbiting material – similar to that created within the sun, earth, and gas giant planets. (Jupiter and the rest are really failed stars – prematurely robbed of hydrogen before growing big enough). The magnetic lines are strongest at the protostar’s magnetic poles and the flow of electromagnetic material will be along these magnetic line.
At right is illustrated the magneto that powers earth’s magnetic poles. The core of the earth is conductive material spinning with polar material within the crust. Such a magneto (or dynamo – I can never remember the difference) acts like a electric motor in reverse – mechanical energy, in the form of spin, is converted into electromagnetic potential. It’s a good thing too, as this prevents solar winds from reaching our atmosphere. The magnetic field also is also an ingredient in the creation of Aurora’s.
Credit Klaus Kolrusch – pinterest
If you are interested in how magneto-hydrodynamic systems operate, it is somewhat analogous to normal hydrodynamics/fluid mechanics with electromagnetism rather than pressure being the chief driver of fluid movement. It even has its own version of the Reynolds number.
In essence the protostellar dynamo converts angular momentum into concentrated linear momentum from the poles and blasts it away ultimately reducing the protostars’ spin. The jets contain partially ionized hydrogen and dust that is sent back into the molecular cloud at enormous velocities simultaneously at both poles. The jets likely form at some limit of spin and can form sporadically – like a check valve relieving the angular momentum.
When the jets form they collide with material outside of the protostar, heating the material up. Often these temperatures are modest and we can observe IR from hydrogen molecules – normally invisible at colder temperatures. Sometimes, the material will become hot enough to emit visible light that we can capture in astrophotography. Sometimes they are linear, but often curvy and swirly. The heated Herbig Haro object look like they detach from the stellar poles, but this reflects a pause in the jet formation. In terms of hydrogen accumulation on the star, the jets are a form of two steps forward as the future fuel arrives, followed by one step back as some of this material is ejected to control angular momentum.
We call these jets of emissions “Herbig Haro” objects after Herbig and Haro – two astronomers that discovered them. It is these jets that provide the smoking gun evidence of star formation, in this particular elephant trunk/pillar of creation within this stellar nursery. But as we will see in future posts, Herbig Haros aren’t restricted to stellar nurseries, they also occur in dark nebula – wherever protostars are accumulating hydrogen to form new stars. This brings us full circle to our image of the Pelican Nebula’s Herbig Haro moustache, and how a protostar sheds angular momentum in order to fuel up its future fusion.
In this stunning image, the Hubble telescope had captured both methods of a protostar’s shedding of angular momentum.
Viscous forces allow hydrogen to spiral down to the star, while remaining angular momentum is transformed into jets leaving at the poles.
This image was made possible by combining visible light wavelengths for the Herbig Haro and IR for the swirling molecular cloud.
Credit NASA, B. Nisini and SciTechDaily
