|
Metaphorically, stars are as much alive as we are. They are born in an environment that nourishes them until they mature enough to start fusion in their core. They may be alone, or part of a twin, triplet, or even multiple stellar system. Their birth is marked by a physical scream of sorts as they go through a stage of powerful outflows of material. Eventually they calm down and settle in for a life determined by what they weighed at birth (the amount of mass they have). This lesson examines the theoretical ideas of how stars are born and the observational evidence to support these theories. Recent results from the Hubble Space Telescope have given us tantalizing evidence that newly formed stars have material around them that may contain planetary systems. These images plus the recent discovery of planets around dozens of nearby, solar-like stars hint that the formation of planets may be a standard by-product of star birth.
|
After completing this lesson, you should be able to
The topic of the birth of stars is truly one of the most fascinating in stellar astronomy. Numerous analogies can be made between the birth of a star and the birth of a human:
This lesson introduces the idea of a star evolving, changing its luminosity and temperature as it ages. Here, we take a look at how these characteristics vary depending on the mass of the star. As Fig. 12.12 in your text shows, a 0.1 solar-mass star may take 100 million years to arrive on the main sequence and start fusion in its core, while a 100 solar mass star takes barely 10,000 years.
Your reading of Chapter 11 is instrumental so that you will have an understanding of where stars come from. In order to understand starbirth, evolution, and star death, you must be familiar with the stuff stars are made of, and where that stuff comes from, and where that stuff goes. A careful reading of those parts of Chapter 11 having to do with starbirth is important. We will return to this topic when we study the Milky Way Galaxy and learn how we know what our home galaxy is like, even though we are "trapped" inside!
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 review 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 refresh your memory of just how huge our cosmos is.
 |
 |
 |
 |
 |
 |
| (Images courtesy of Richard Powell and his Atlas of the Universe.) | ||
Our next task is to look more closely at nearby star-forming regions and the emission nebulae that usually accompany this process. The image at right marks the nearby nebulae and star clusters has been mapped so that you can get a closer look at the objects. As you click through the images, think about the following questions:
You may also note that the Crab and Veil Nebulae are also shown. These are supernovae remnants, gas that has been expelled from the massive stars that once resided in that part of the Galaxy. Recall from the last lesson, that stars of all masses exist in the Galaxy. Even though we do not expect to see many massive stars remaining around star-forming regions, we do expect to see traces of those that have lived and died.
While the emission nebulae seen amongst the spiral arms of the Galaxy usually represent regions where star formation is in its final stages, dark nebulae can represent where star formation is just starting. One of the best known dark nebula is the Horsehead Nebula, shown at the left, which lies just below the bottom-left of the three stars of the belt of Orion.
 |
 |
 |
|---|---|---|
Horsehead NebulaLink
to more information Image Credit: T.A. Rector (NOAO/AURA/NSF) and Hubble Heritage Team (STScI/AURA/NASA) |
Thackeray's GlobulesLink to more information Image Credit: NASA and The Hubble Heritage Team (STScI/AURA) Acknowledgment: Bo Reipurth (University of Hawaii) |
Barnard 68Link
to more information Image Credit: FORS Team at the European Southern Observatory |
 |
 |
|---|---|
Keyhole NebulaLink to more information on the Keyhole Nebula Image Credit: NASA and The Hubble Heritage Team (AURA/STScI) Acknowledgment: N. Walborn (STScI) and R. Barbá (La Plata Observatory, Argentina) |
Pipe NebulaLink to more information on the Pipe Nebula Image Credit: Christopher Picking |
|
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 below 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. We learned during our studies of the Sun that convection is the most effective means of transporting heat, and occurs when there is a large difference between bottom and top of a layer. The star is starting to experience "growing pains."
Figure 12.8 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 (Ch. 2) 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.
From the text:
