A Layperson's Guide to Stars
Recall that the dominant hypothesis for the formation of this and other solar systems (developed by Pierre-Simon Laplace) is the Solar Nebula Hypothesis - that the Sun and planets formed from a collapsing cloud of interstellar gas and dust. The Solar System is part of the Milky Way galaxy, a vast collection of stars and other material roughly 100,000 - 150,000 light years across. The image at right shows the central regions of the Milky Way. It's shape is difficult to discern because we must look at it from inside. Looking into the galaxy, we see evidence of various phases of this process in the form of:
- Giant molecular clouds of gas and dust.
- Proplyds: "Small" (i.e. scale of a few Solar Systems) collapsing dense fragments of giant molecular clouds that conceal bright infrared sources - revealing the earliest stage of star formation. These infrared sources may take the form of......
- Circumstellar disks: (AKA Protoplanetary disks) Rotating disks of gas and dust with dense objects (protostars) at their centers. These may be naked or occupy proplyds. The protostars are bright in infrared and visible light.
Giant molecular clouds of the Milky Way from Pure Insight.org
What would make a giant molecular cloud collapse to form a solar system?
- Mutual gravitational attraction (measured as the density of the cloud), encouraging the cloud to collapse
- Pressure (measured as temperature), encouraging the cloud to expand.
- Compression of gas by the passing shock wave of a supernova explosion.
Stellar wind of Zeta Ophiuchi heats interstellar medium from SciTech Daily
- intense stellar winds from neighboring stars. At right, the effects of Zeta Ophiuchi on surrounding interstellar gas is apparent. (Is Zeta Ophiuchi moving with respect to the gas?)
The antennae galaxies collide from Astronomy Picture of the Day
- Collisions of galaxies, in which the stars whiz past one another but giant molecular clouds collide and compress. The Antennae (NGC 4038 and NGC 4039) are in the midst of this process. Blue regions mark the presence of young massive stars.
Jellyfish Nebula supernova remnant from Astronomy Picture of the Day
The stages of cloud collapse:
- Initial collapse
- Cloud fragmentation: as regions of particular density or low temperature form numerous nuclei around which adjacent material collapses. Stellar winds of the stars that form from this collapse may compress that adjacent giant molecular cloud, stimulating the formation of new proplyds, but eventually eroding the giant cloud away. E.g.: the "pillars of creation" in the Eagle Nebula (right).
The Lagoon Nebula from Astronomy Picture of the Day
The Jewel Box from Astronomy Picture of the Day
Protoplanetary disk of HL Tauri in infrared from Wikipedia.
Dark circles represent the orbits of coalescing exoplanets.
Stupid protostar tricks: Protostars, by definition, are not yet experiencing nuclear fusion at their cores. Instead, their heat comes from gravitational release and friction. That heat can be significant, however.
- Circumstellar disks: As cloud fragments collapse, they tend to form rotating disks of material. Some young stars have been observed to be surrounded by such disks, including:
Bipolar outflow from Hubble Site
- Bipolar outflow: We see young stars that appear to be ejecting plumes of material in opposite directions. This probably results from the heating of material falling onto the protostar from its circumstellar disk. Such material expands fastest along the paths of least resistance perpendicular to the disk.
Artist's view of bipolar outflow transporting
high-temp minerals outward from Astronomy Now
T Tauri from Astronomy Picture of the Day
- T Tauri stage: As gravitational collapse proceeds, the protostar becomes hotter and glows more brightly. Note that the volume of a star (proto or otherwise) volume is a function of two processes:
- Gravitational contraction - will continue to compress material until some balancing process stops it.
- Nuclear fusion - Occurring in the star's denser regions (i.e. its core) generates outward radiative energy that balances gravitational contraction. The star must cross a density threshold before this can happen. What that is depends on what elements are available to fuse.
Called the T Tauri stage after the classic example, the protostar T Tauri (right).
The Sun's life cycle from Wikipedia
For our purposes, stars differ in two ways:
- Life stage: Stars go through a predictable life cycle with distinct steps. As we look into the sky, we see stars at various points in their life-hsitories.
- Mass: The speed with which a star goes through its life cycle, and whether or not certain events occur is a function of mass.
- Very large stars are short-lived burning through their fuel in as few as ten million years.
- The smallest stars should be capable of shining for hundreds of billions of years (Much longer than the estimated age of the universe - 13.7 ga)
- The Sun is expected to shine for roughly ten billion years.
The T Tauri stage ends when the pressure in the protostar's core reaches the threshold at which hydrogen nuclei are fused into helium. It becomes a proper star.
The Hertzsprung-Russell Diagram from Wikipedia
The Hertzsprung-Russell Diagram: Astronomers compare stars and chart their life cycles by plotting their absolute luminosity (y-axis) and color (x-axis) on a standard plot developed by Ejnar Hertzsprung and Henry Norris Russell in 1910. The H-R Diagram encompasses stars of differing mass and at different life-stages. You will note three major groups of stars. During its life, a star will inhabit each of these:
- The Main Sequence: The diagonal stripe in the middle. Stars spend most of their lives in this group. (The Sun is there now.) Note the relationship between mass (indicated by luminosity) and color. Small stars are red. Very massive ones are blue. Color corresponds to temperature, with massive blue stars burning much hotter.
- Giants: The cluster at the top right are stars nearing the end of their hydrogen-fusing lives.
- White dwarfs: The diagonal stripe at bottom left. The bare cores of "dead" stars, they shine to release the heat of gravitational contraction but no longer experience sustained fusion.
Solar evolution from Wikipedia
- Main sequence: Eventually, the Sun's core reached the temperature and pressure at which sustained fusion of hydrogen could occur. From this point on, gravity was balanced by outward radiation pressure. As helium is produced by fusion, it "piles up" in the sun's core. This process should continue until the Sun is roughly 8 - 9 ga.
Interior of red giant from The Astronomy Cafe
- Red giant: Eventually, hydrogen in the core will be used up, and the zone of hydrogen fusion will creep outward toward the Sun's surface. As fusion extends into the Sun's outer regions, there is less overlying material above the zone of fusion, so radiation pressure begins to dominate over gravity and the outer layers expand outward. The Sun's surface area will expand faster than the fusion energy that heats it, causing it to cool. The sun will enter its red giant phase. Note: Volumetrically, red giants are very large. The Sun's surface as a red giant will extend past Earth's orbit. The radius of Betelgeuse, a very massive red giant, is roughly 4.5 AU.
The Ring Nebula from Astronomy Picture of the Day
- Planetary nebula: Eventually, the helium in the core begins to fuse, generating energy that causes the core to expand. As the products of helium fusion build up, the Sun will build elements as heavy as carbon and oxygen (the Sun is too puny to make iron). To make things simple, we say that complex interactions between the helium-fusing core and the hydrogen-fusing outer regions cause kinetic instabilities that will ultimate cause the Sun to shed its outer layers in a prodigious solar wind and prolonged interval of expansion and contraction. The result: A Planetary nebula that surrounding what remains of the core. This has nothing to do with planets. The planetary nebula is an expanding cloud of material that will ultimately become part of the interstellar medium.
White dwarf with Earth for scale from Astronomy 162, Ohio State University
- White dwarf: What's left of the Sun is a naked core, now bereft of the the outer layers that had previously compressed it to the point of helium fusion. Fusion ceases and the core, now a white dwarf radiates energy derived form gravitational contraction. With little to halt its gravitational collapse, the white dwarf Sun will be compressed into a volume similar to Earth's. At this point, its contraction is halted by electron degeneracy, the mutual repulsion of negatively charged electrons. Eventually, these will radiate their heat and become cold black dwarfs, but the universe is not old enough for this to have happened.
Interior of main sequence star from In Fact Collaborative
Spectral Classification of stars based on their mass from Wikipedia
- A red dwarf may fuse hydrogen for trillions of years. Although we have never seen it happen, models predict that such stars end their lives more quietly than the Sun will, slowly shedding mass in a stellar wind until they can no longer support hydrogen fusion. Indeed, hydrogen is about all that these stars are able to fuse.
- Large stars age more quickly than small ones. A blue giant might burn out in fewer than 10 ma. (Before it has moved far from the other stars that formed with it.) Large stars, however, achieve the internal pressures needed to fuse heavier elements. They are the ones kicking out everything up to iron. Massive stars die in a nearly instantaneous cataclysm called a supernova, in which the sudden cessation of fusion in their cores cause their sudden collapse. This compression unleashes exotic forces that result in the star's explosion. Depending on the specifics, a remnant of the core may remain or the star may be destroyed utterly. Betelgeuse, a massive red giant, is probably approaching this cataclysm sometime in the next few thousand years.
The Crab Nebula pulsar: Visible wavelengths - red, X-rays - blue (!) from Wikipedia
- If, after the expulsion of a planetary nebula, the stellar remnant has a mass of over 1.44 solar masses, the Chandrasekhar limit, gravitational pressure overwhelms electron degeneracy (the tendency of electrons to repel one another), pushing electrons and protons together to form neutrons. The remnant collapses into a neutron star an object roughly 10 km across, composed of pure neutrons. (At right, the neutron star at the center of the Crab Nebula. Note that it, too, has an accretion disk and bipolar outflow.)
- If the collapsing star exceeds the Tolman-Oppenheimer-Volkoff limit (~20 solar masses), it can become a black hole, an object (?) whose escape velocity is greater than the speed of light. Don't go there.
Key concepts and vocabulary:
- Solar Nebula hypothesis
- Dense clouds
- Circumstellar disks
- Jeans mass
- Stellar wind/supernova shock wave
- Cloud fragmentation
- Protostar / protoSun
- Bipolar outflow
- T Tauri stage
- Hertzsprung-Russell Diagram
- Main sequence
- Red giant
- White dwarf
- Planetary nebula
- Red dwarf
- Chandrasekhar limit
- Neutron star
- Tolman-Oppenheimer-Volkoff limit
- Black hole
- Charles A. Poteet, S. Thomas Megeath, Dan M. Watson, Nuria Calvet, Ian S. Remming, Melissa K. McClure, Benjamin A. Sargent, William J. Fischer, Elise Furlan, Lori E. Allen. 2011. A Spitzer infrared spectrograph detection of crystalline silicates in a protostellar envelope.The Astrophysical Journal Letters 733(2).