The Formation of Stars Chapter 14

Where Do the Stars Form? Most stars form within giant clouds of molecular gas. These clouds typically contain enough mass to form 100,000 suns spread out over a diameter of 50 pc. Their temperatures begin just a few degrees Kelvin (very cold!). But with mass comes gravity. However, there are 4 factors that gravity must overcome in order to compress the gas into a star.

The Four Factors 1. Thermal energy: molecules are still moving fast enough (~800 mph) to drift apart even in the cold. 2. Interstellar magnetic field: though weak, this magnetic field works like a spring trying to prevent compression. 3. Cloud rotation: as the cloud begins to contract, its rotational speed increases due to conservation of angular momentum. Faster rotation makes it harder to collapse. 4. Turbulence: “winds” in the ISM distort the clouds and make them more difficult to contract.

Gravity Finally Wins! One way that gravity might win out is with the help of a shock wave, such as from a supernova explosion. This has the effect of destabilizing regions of the cloud forming dense pockets Once the cloud begins collapsing, the core temperature increases. Gravitational potential energy is transformed into thermal energy. Some of these pockets become hot enough so that they emit IR. This core, that now emits IR but has not yet begun nuclear fusion, is called a protostar (proto = “before”).

Stellar Nurseries M 8 (left) and M 42 (right) [note how the stars are clearer in the right image in IR].

Stellar Nurseries A cocoon is the cloud of gas and dust surrounding a contracting protostar and blocks our view in visible light. Note: this is an artist’s concept. But note the rapidly-spinning disk of dust and gas around the hot core!

Eagle Nebula Wide View Stellar nurseries are located in these pillars, called the “Pillars of Creation.” New stars are most likely located at the tops of these pillars, where dust is most dense.

Close ups of the stellar “cocoons.”

More Examples of Cocoons

Birth Timelines Massive protostars can collapse more rapidly and begin nuclear fusion sooner than less massive ones. As protostars collapse their position on the HR Diagram moves toward the main sequence line (MS runs diagonal from upper left to lower right). The exact path depends on mass. The sun took about 30 million years to reach main sequence, but one 30 times more massive only takes about 30 thousand years! Star formation 102/lectures/starform.htm /lectures/starform.htm /lectures/starform.htm /lectures/starform.htm -

Star Formation Paths on the HR Diagram Different paths take different amounts of time and depend on the starting mass.

Fusion Review Stars like the sun, having core temperatures of at least 10 million K, undergo nuclear fusion via the proton-proton chain. Essentially, this process fuses individual protons (H) into helium (He) nuclei and releases much energy (E = mc 2 ). Heavier stars, with higher core temperatures (16 million K or above), often use another process. The CNO cycle uses carbon (C), nitrogen (N), and oxygen (O) as steppingstones in the process. The outcome (production of He) is the same for both processes, except in the amount of energy released.

The Crossover Point between PP and CNO

Structures of Stars Many stars have a structure similar to the sun, which we have already studied in chapter 12. Core: At the center, where nuclear fusion takes place. Must be at least 10 million K. Radiative Zone: Energy flow in the form of photons that are absorbed and reemitted in random directions. Each super high energy photon get broken down into 2500 photons of visible light. Convective Zone: Begins where photons can no longer penetrate the gas (cooler, more opaque) so energy builds up, begins churning. Hotter gas rises while cooler gas sinks.

Dance of the Forces Two forces must be balanced in order for a star to remain stable: the inward pull of gravity and the outward push of thermal pressure (because gases expand when heated). The gravity-pressure balance that supports the sun is a fundamental part of stellar structure known as the law of hydrostatic equilibrium (hydro = fluid, static = stable). A star can be thought of as having multiple layers, like an onion, each of which must be in hydrostatic equilibrium. The interior of the sun must be very hot, in order to support the great weight of the above layers.

What’s Mass Got to Do With It?? It turns out that the model we used for the sun applies only to stars between 0.4 and 1.1 times the sun’s mass. Why? Mass largely determines core temperature, which determines which nuclear process dominates at the core! Main Sequence “Tweener” Stars: These stars generate most of their energy via the proton-proton chain, so the core is not as concentrated in dead center. Ex. Sun: 50% of its energy comes from 11% of its volume So, core  radiative zone  convective zone (little core mixing)

Sun-like stars: radiative zone inside of convective zone.

More Massive Stars—Role Reversal? Stars that are heavier than 1.1 solar masses have core temperatures that are hot enough to fuse much of their hydrogen using the CNO cycle, which is extremely sensitive to temperature. Ex. 10-Msun stars generate 50% of their energy from only 2% of their volume! Therefore, a “log jam” occurs as energy tries to leave the center. Therefore, convection occurs around the core instead of radiation!! So, core  convective zone  radiative zone

Comparing 3 Types of Stellar Interiors

Mini Me Stars—Missing Something? Stars that are lighter than 0.4 suns have relatively cool interiors, which makes them more opaque to photons. As a result, radiation cannot easily flow through any part of them, so they only have convective zones. No radiative zones. Core  convective zone “Stars” between 0.08 and solar masses are not really stars at all; called “brown dwarfs” (failed stars). No nuclear fusion is ever sustained. For comparison, Jupiter is ~0.001 solar masses.

Brown Dwarf Candidate Gliese 229b

Star Regulation How does the star maintain the right amount of energy so that it neither expands or contracts too much? If the star begins producing more energy, then it expands. This results in a lower central temperature and density and slows nuclear fusion until the star regained stability. The same process works in reverse: if less energy were generated, then the star would contract slightly and cause central temperature and density (and energy production) to increase.A stellar thermostat!

Star Families Most emission nebulae are massive enough to form clusters of stars that tend to disperse over a few hundred million years. These dispersed stars become isolated individuals or small groups (such as binaries) after enough time. Our sun must have been a member of a cluster at one time, but has since drifted away. A good thing!