We know how stars form. But HOW do we know?
Astronomers are confident that we know how stars form, but where does that confidence come from? The story of how we know is as important and awe inspiring as the story of what we know.
A self-consistent explanation of the our sun’s origin as a physical process has been a hot topic for a while now. In the 18th century Emmanuel Kant and Pierre Laplace proposed a process they developed using Isaac Newton’s law of universal gravitation. In principle this could explain the formation of all stars. They called it the Nebular Hypothesis.
It goes like this: Stars form in clouds of gas in interstellar space, often called nebulae because they are fuzzy, clumpy, wispy, and … well … nebulous. Gravity condenses the cold, dark core of a swirling nebula transforming it into a luminous sphere. A star is born. That’s the thumbnail version of the story and it’s still our story. The difference now is that we have filled in many more details and we can back them up with observational evidence.
Telescopes had made direct observations of nebulae possible by the time Newton was working on his laws of motion. Those observations — like ours today — were images of luminous nebulae, glowing gas clouds. Until the mid 19th century people spent hours looking through telescopes and making detailed drawings of what they saw. Then film photography enabled much more accurate and detailed images. These days we strap a digital camera on the telescope and record images more quickly and with even more detail.
Images are direct observations of the apparent brightness, apparent color, apparent position, and apparent shape of celestial objects. They enable us to apply what we know about physics and chemistry here on Earth to astronomical studies with the goal of describing the properties of stars and their ancestral nebulae.
To begin with, how do we know stars form inside gas clouds? Using telescope and imaging systems that are sensitive to visible and infrared light we observe groups of bright young stars within nebulae and we rarely see such young stars outside of nebulae. How do we know the stars are young? The combination of their color and brightness is distinctive compared to other stars in the vicinity that resemble older stars like our sun. Here’s a more detailed explanation of this process:
We know from the same observations that star-forming clouds themselves are composed mostly of hydrogen molecules and helium atoms. Dense pockets in these molecular clouds are cold, between ten and a few hundred Kelvins, well below zero on either the Fahrenheit or Celsius scale. These dense clouds are exceedingly dim, emitting almost no light in the visible part of the spectrum or even in the infrared.
Mostly hydrogen? Yep, about 75%. We can determine the relative abundance of hydrogen and the other elements by precisely measuring the color of the glowing gas in interstellar clouds and of the stars themselves. By separating very narrow ranges in the spectrum of light wavelengths we can distinguish the glow from the different chemical elements since each atom of a particular element emits light in a unique mixture of wavelengths that depends on the number and organization of its electrons. Generally speaking human eyes and brains perceive different wavelengths of visible light as different colors.
Giant clouds of cold gas? We observe cold molecular clouds by measuring their apparent size, shape, and brightness in the radio part of the spectrum. Our radio telescopes and detector systems are sensitive to light from the lower-energy cold gas and dust, enabling us to determine densities, temperatures, and chemical composition.
So how do stars, which are intensely hot and bright, form in such dark and cold environments? Where do stars get their energy? The short answer is: gravity.
Atoms and molecules, despite their diminutive size, exert gravitational forces because they have mass. That gravitational attraction is weak since they each have so little mass, but there are so many in a star-forming cloud that eventually — typically after a few million years — gravity pulls the molecules closer together into concentrated cores. This happens faster in the parts of the cloud that are more dense to begin with.
How do we know the cores are collapsing if the process takes millions of years? The unique light spectral mixture of colors emitted by each type of atom and molecule can also help us detect bulk motion within a gas cloud. Slight shifts in those color patterns can indicate to us that the molecules are moving. For more information prompt your favorite AI chatbot to tell you about the Doppler Effect for light.
We can tell from shifts of color to red that some parts of the gas cloud are moving away from us. At the same time some of the light from the same parts of the cloud appear blue shifted, indicating gas moving towards us.
Huh? How can gas move both towards and away? It can if we’re observing gas from both the near and far sides of the cloud simultaneously; both moving towards the center of the cloud!
But is it really gravity pulling on individual molecules and atoms? Gravity between two objects depends on their masses, regarless of their size. So just as the gravity from all matter in planet Earth confines the molecules of gas in our atmosphere, the mass of all the molecules in a nebula attract each of the other molecules. Since there is no solid surface to hold the gas up, the cloud collapses. And these self-gravitating spheres of gas will be denser in their cores than in their outer layers. Gravity really is doing the work of star formation.
Gravity isn’t the only fundamental force in town, though. Each atom or molecule in the collapsing cloud core is surrounded by its electrons. As they get close to one another the negatively-charged electrons tend to push the atoms (and the molecules they form) apart. This repulsive electric force resists the gravitational pull between atoms and molecules. Bill Bryson summarized it perfectly in A Short History of Nearly Everything: electrons give atoms their personality; how they interact with other atoms. Eventually stability of the new stars will depend on a balance between the gravitational force and the opposing electric force. A balance between attraction and personality.
Confined by gravity to a shrinking volume of gas, the molecules push on each other so the pressure within a collapsing cloud core increases. Because gravity and the electric force act equally in all directions the system eventually becomes opaque and takes a roughly spherical shape.
The outer layers of of the sphere insulate the collapsing core so that the temperature is hottest in the center and decreases outwards. Since all warm things glow, we now have a huge, self-gravitating and glowing sphere of gas in space. But it’s still not quite a star; it’s almost a star — it’s a protostar.
The last thing a protostar needs to become a star is a source of energy to replace what’s lost as the star shines, radiating its internal energy into space as light. Like us, healthy stars need equilibrium.
Protostellar cores eventually reach temperatures high enough that the fast-moving hydrogen nuclei (positively-charged protons) collide with enough energy to overcome their electrical repulsion and combine. Some protons then transform into neutrons. This series of collisions and transformations — nuclear reactions — eventually results in the synthesis of helium nuclei. Now two protons and two neutrons are bound together. Protons give atoms their identity (Bryson again).
Hydrogen fusion into helium releases energy. A teeny, tiny amount per helium atom, but stop for a moment and meditate on how many atoms there are in a protostar! (About a trillion, trillion, trillion). Added up they release enough energy to power the luminosity of a star. For the sun this is 400 million trillion Watts.
How do we know that it’s hydrogen fusion if we can’t see into the core? We know that hydrogen fusion is the source of stars’ energy because we observe streams of tiny particles, called neutrinos, coming from the sun. Neutrinos are only produced in large numbers at the observed energies when they result from nuclear fusion reactions. Nuclear fusion is also the only source of energy that would occur spontaneously in the hot core of a massive and dense gaseous sphere and that could maintain a star in equilibrium at such high temperatures for millions or even billions of years.
The protostar reaches stardom when it settles into equilibrium between its internal pressure pushing outward — fueled by hydrogen-to-helium fusion — and its gravitational force pulling inward.
In this state of equilibrium the star can maintain a remarkably constant size and temperature, therefore brightness, for as long as its core hydrogen fusion lasts. Most of the stars that we see in the night sky are in this equlibrium state. It’s called hydrostatic equilibrium from hydro (fluid) and static (not moving). Jargon, yes, but what a fun and satisfying phrase. Hydrostatic equilibrium.
Energy from nuclear fusion in stellar cores slowly percolates out to the cooler surface layers, which shine brightly but aren’t hot enough to sustain nuclear fusion. The surface temperatures of stars range from a few thousand degrees to a few tens of thousands depending on the star’s mass; more massive stars are hotter.
How do we know the temperatures of stellar surfaces? Hotter things glow more brightly, but they also glow more in blue light than in red light. We can therefore calculate the temperature of a star from precise measurements of its color: by comparing apparent brightness in both blue and red light. We can test this method in a laboratory and it works remarkably well. I do this with my students by measuring the color of a glowing lightbulb filament, using the color to determine the temperature, and then comparing the result to an independent measurement of the temperature. They match. Of course they match.
Our story of star formation is still unfolding. We’re gathering observations and building in more details as we go. This brings us to a parallel method for learning the details of star formation; one that is complementary to making observations and measurements. It’s powerful and wicked fun: model building.
How well do you understand a physical system or process? Build a model of the system based on your current understanding. Make your model as detailed as you can. Use the model to predict observable properties of the system and then observe the system to see if the actual properties match your model. Now iterate the modeling process, revising the model at each cycle. These days astrophysical models are computer models; simulations of the system and process we study.
When our models and our observations tell us the same story it builds confidece. Confidence that the emerging story is an accurate description of nature. I’ve often heard this called ‘predictive power.’ The model gives us the superpower to predict the results of future observations.
The Nebular Hypothesis has become the scientific theory of star formation.
