| Lecture |
|
|
The Birth of the Planets
|
==Describe in general terms the interstellar medium: contents, densities, temperatures, etc.
==Outline in general terms the formation of sun-like stars, including explanatory diagrams
==Summarize the observations of our solar system that any valid model must adequately address.
==Summarize the building of the planets themselves, using the vocabulary: accretion, coalescence, planetesimals, bombardment, snowline
==Define what is meant by the "snow line" and why the concept of a snow line is relevant to our understanding the gross characteristics of the inner versus the outer planets.
==Explain what a temperature or condensation sequence of different materials has to do with the basic composition of the planets.
==Discuss what clues to the formation of the solar system are to be found in the minor bodies (comets and asteroids); why are these objects important to our understanding?
==List in general terms the evolution of the terrestrial planets after they had completed most of their building-up of mass.
Take the Quiz == page to the bottom of these lecture notes.
Before we start on how stars form, and how the formation of a planetary system appears to be a natural consequence of the presence of "surplus material," let's do a quick overview of just where we fit into the Galaxy. When we take a look at the pillars of the Eagle Nebula, or discuss the massive stars that are forming in the Orion Nebula, we should have a vague idea of how far away these regions are. They are not "right next door." The following series of images zoom us in, from about 100,000 light years from the center up to about 25 light years from our Sun. Take this brief trip to impress upon yourself just how huge our galaxy is (and it is just one of 40+ billion galaxies in the Universe).
 |
 |
 |
|---|---|---|
 |
 |
 |
Images courtesy of Richard Powell and his Atlas of the Universe.
Our discussion of the formation of our solar systems starts with the interstellar medium (ISM), the stuff in between stars. It's where stars come from, where stars go, where we came from, where we will eventually go.
Gas: individual atoms and molecules
Dust: chalk, soot, smoke, fog, smog
itsy-bitsy particles, but lots and lots of them
Temperature: few Kelvin to thousands of Kelvin (most of space is very, very cold)
Composition:
Gas: H & He (mostly)
Dust: Si, C, Fe, NH3, CH, CN, C60, CH4, H2O, CO, etc.
Opaque to most visible wavelengths
Need radio and infrared telescopes to see "through" the gas and dust.
The Eagle Nebula in
the constellation of Serpens.
More information about the Eagle Nebula
Dark nebulae may represent regions where star formation is just starting. One of the best known dark nebula is the Horsehead Nebula, shown below, which lies just below the bottom-left of the three stars of the belt of Orion.
 |
 |
|---|---|
 |
 |
 |
|---|---|---|
We'll end our overview of the material that lies amongst the stars with
a view of the Trifid Nebula (see your map links above to locate it in our
Milky Way). Here we note
the three kinds of nebula associated with star formation: emission (reddish),
dark, and reflection (bluish).
The Trifid Nebula is also a good lead in for
our study of star formation, as it seems to be in the midst of birthing
stars.
Pointing the Hubble Space Telescope to this region reveals dramatic
events are taking place. The image at the right shows huge dark clouds
of dust, blocking the light from luminous stars (upper left). Also seen
are jets of material, probably produced by young stars, and pillars of
material left as the radiation from massive stars strips the material
away.
[Credit for Trifid Nebula: Todd Boroson/NOAO/AURA/NSF.
Credit for the close-up image of the Trifid:
J. Hester (NASA/STSCI).]
Press Release, November 9, 1999)
Astronomers can be a bit sloppy about the naming and classification of objects in the Galaxy. Nebula has come to mean anything that looks diffuse-places or regions of space where there is a concentration of gas and/or dust. The name "dark nebula" can mean just dust. There is a lot of dust in our galaxy. The name "dark nebula" can mean even more if the material we are discussing is dense, dark, and part of a gigantic structure. Here we are talking about more, we are talking about giant molecular clouds-the birthplaces of stars.
Peeking inside the giant molecular clouds in our galaxy and into the clumps of denser cores, we see some where gravity seems to have a head start over any opposing pressure. These clumps start to collapse, maybe with a push from nearby star formation (or even a supernova explosion). But, gravity soon is checked; the molecules and atoms of the gas and dust all contain gravitational potential energy. During the collapse stage, this potential energy is converted to kinetic energy. The kinetic energy increases the temperature of the clumps and the thermal pressure increases, halting or slowing the collapse for a while, while the heat is radiated away. Once a clump cools a bit, the outward pressure decreases, and the collapse continues, until once again the temperature increases enough to halt the collapse temporarily. This cycle continues: collapsing, heating, pausing, radiating, cooling, collapsing, and so on.
As the collapse continues, the core of a clump is becoming ever more dense, ever hotter. Most of the mass is concentrated in the core of the clump, and collapse there may proceed at a faster rate than in other parts. The core will contain more mass due to the higher density, and therefore more gravitational pull. The higher gravity means that more thermal pressure must be built up before the collapse pauses and some of the heat is radiated away. An interesting period occurs for the collapsing protostar during part of this formative stage: it becomes fully convective. This means it is literally boiling throughout. The star is starting to experience "growing pains."
Figure 8.6 of your text shows schematically the steps a star makes from
being just one of many dense cores in a molecular cloud to a newly formed
star. Why is a disk of material around a star a natural consequence of its
formation? We need to remember that angular momentum is conserved.
Everything in the Universe is moving; the giant molecular cloud and its
associated dense cores had motion. Most of the movement is random; however,
there could be some overall direction to this motion.
As the clumps
collapse, their sizes decrease--their radii shrink. As the distance to
the center of the spin axis of a clump decreases, its rotation must
increase to conserve angular momentum. The material that is falling down
upon the poles of the spin axis does not have much resistance, and can
fall directly onto the star. The
material that is trying to collapse onto the equatorial region, however,
finds itself spinning too rapidly.
Much like a child on a spinning playground
merry-go-round, "crawling" to the center is very difficult. The material
thus falls onto a disk. Since the disk has the same rotation direction as
the star, the orbits of any planets that eventually form from that disk
will be in the same direction as the star's rotation. In our solar system,
all of the planets orbit the Sun in a counter-clockwise direction--the same
direction that the Sun rotates. This did not happen just by chance.
Disks orbiting central objects are common in the universe, and are usually
associated with violent reactions. Newly forming
stars have them, close-binary stars that are undergoing mass exchange
have them, even some black holes have them if they have a close companion.
Material from these disks flows onto the central object and transfers some
of its energy to it. In the case of the newly forming star, it is already
suffering pre-birth disruptions in its core as the core reaches the
pressure and temperature necessary for nuclear fusion to start. (As one
might imagine, this process does not proceed smoothly.) Its surface
regions are adding material from the disk and there is just way too much
energy for the star to dissipate in an orderly fashion. Winds and jets
form as a result, essentially halting the collapse and clearing away the
disk of material. As the Hubble Space Telescope images in your text, these
jets of protons and electrons speed away from the baby star at high
velocities and can extend for billions of kilometers.
The Formation of Sun-like Stars -- in Outline Form
Molecular cloud must lose enough heat to collapse under own gravity
Or cloud must be nudged by an external force into collapsing
A nearby supernova can do the trick
Radiation from hot, massive stars can also push the ISM
Collapse starts
Cloud fragments
Fragments continue to radiate, cool, and collapse (from the inside out!)
Central temperatures reach 10,000 - 100,000 K
Star becomes opaque to its own radiation and continues to heat up.
Protostar is convective throughout
Shrinks, heats up, radiates
Heat comes from gravitational energy
Star is not in equilibrium; undergoes numerous gigantic "burps"
Star exhibits violent surface activity and bi-polar stellar winds of a T Tauri star
Star starts out rotating rapidly
Material forms into disk around star
Planets are born along with the star
10,000,000 years, a star, our Sun, is born
Stars move from cool temperatures, high luminosity to low luminosity and hot temperatures.
The solar system is encased in a "bubble" about 100 parsecs in diameter. "Blown out" by an ancient supernova? Evidenced by the enriched composition of our neighborhood?
The planets orbit the Sun on nearly the same plane.
The planets all orbit the Sun in the same direction, and that direction is the same as the rotation of the Sun.
Most planets rotate in the same direction as their orbit
The orbits of the planets are all nearly circular
The planets close to the Sun are small, rocky, and have high densities.
The planets far from the Sun are large, gaseous, and have low densities.
There is a spacing pattern to the orbits of the planets.
The solar system is, in general, enriched with heavy elements.
The terrestrial planets show evidence of past heavy bombardment.
The Sun dominates the mass of the solar system
The planets and some moons show evidence of extensive melting and differentiation.
There remains ample remnant debris in the solar system.
(gif 110 Kb) |
|
| Review the condensation temperatures of the various molecules and metals, and compare them to the temperatures of the solar nebula as the planets were forming. (The temperatures and fall-off with distance are approximate.) |
|---|
 |
 |
What this graph is telling us, if we think about it a bit deeper, is that inside the frost line (on the side towards the Sun) we have small, rocky, dense planets (we know because we live on one). Outside the snowline (farther away from the Sun), we have gigantic gaseous planets, made mostly of hydrogen and helium and all having low densities. Why? Far enough from the Sun, the temperature was below the freezing level of water (in other words, water solidifies--we could just as easily say iron freezes at temperatures between 1000 and 1500 Kelvin). Farther out yet, ammonia and methane freeze. Ices have mass. There WERE LOTS OF ICES beyond the snowline in the protoplanetary disk (the disk from which the planets were made). So, the giant planets had more material to "grab" and therefore more "mass" and therefore more "gravity" and so they just kept on growing and growing -- runaway growth that may have taken less than 10,000 years to fully form Jupiter. When the Sun went through its T-Tauri stage and the solar wind "blew," the inner planets were way too small to hang onto any H and He, but the giant planets were able to hang onto all of their gas, even hydrogen. Thus, the distinctive dividing line between inner and outer planets.
Visit another discussion of the Origin of our Solar System
[OPTIONAL READING]
Until the HST, no direct evidence. HST has given us some fantastic views of other worlds(?) being born, or at least evidence of the material needed to form planets.
(65 Kb)
|
Proplyds in the Orion Nebula |
(90 Kb)
|
Close-up of a proplyds in the Orion Nebula |
(180 Kb)
|
Disks around T Tauri stars |
There are numerous links to information regarding the formation of stars and stellar planetary systems.
Stars strongly suspected of having one or more planets:
Known planetary systems in nice tabular format.
SEDS Other Planetary Systems (A MUST-VISIT SITE)
Find these stars at the PBS worlds
A whole site on exoplanets
A binary star suspected of losing one of its planets
An overall protoplanetary disk resource page
Last, a bit of fantasy
And a special one on extrasolar visions
Last updated on: