Smaller stars can take more than a hundred million years to form. Bate, Ian A. The calculation models the collapse and fragmentation of a molecular cloud with a mass 50 times that of our Sun. The cloud is initially 1. In a newly formed star cluster, there are many more stars with low masses than stars with high masses. For every star with a mass between 10 and solar masses, there are typically 10 stars with masses between 2 and 10 solar masses, 50 stars with masses between 0.
As time passes the balance shifts even more toward smaller stars because the higher mass ones die first. Stars above about solar masses generate power so furiously that gravity cannot contain their internal pressure.
These stars blow themselves apart and do not exist for long if at all. A protostar with less than 0. Rajan Arizona State University , S. Debes and D. Calculations show that the formation of one or more planets could produce such a donut-like distribution of dust. Suppose a planet forms a few AU away from the protostar, presumably due to the gathering together of matter from the disk.
As the planet grows in mass, the process clears out a dust-free region in its immediate neighborhood. Calculations also show that any small dust particles and gas that were initially located in the region between the protostar and the planet, and that are not swept up by the planet, will then fall onto the star very quickly in about 50, years.
If the formation of a planet is indeed what produces and sustains holes in the disks that surround very young stars, then planets must form in 3 to 30 million years.
This is a short period compared with the lifetimes of most stars and shows that the formation of planets may be a quick byproduct of the birth of stars. Calculations show that accretion can drive the rapid growth of planets—small, dust-grain-size particles orbiting in the disk collide and stick together, with the larger collections growing more rapidly as they attract and capture smaller ones. Once these clumps grow to about 10 centimeters in size or so, they enter a perilous stage in their development.
At that size, unless they can grow to larger than about meters in diameter, they are subject to drag forces produced by friction with the gas in the disk—and their orbits can rapidly decay, plunging them into the host star. Therefore, these bodies must rapidly grow to nearly 1 kilometer in size in diameter to avoid a fiery fate.
At this stage, they are considered planetesimals the small chunks of solid matter—ice and dust particles—that you learned about in Other Worlds: An Introduction to the Solar System. Once they survive to those sizes, the largest survivors will continue to grow by accreting smaller planetesimals; ultimately, this process results in a few large planets.
If the growing planets reach a mass bigger than about 10 times the mass of Earth, their gravity is strong enough to capture and hold on to hydrogen gas that remains in the disk.
At that point, they will grow in mass and radius rapidly, reaching giant planet dimensions. From observations, we see that the disk can be blown away within 10 million years, so growth of a giant planet must also be a very fast process, astronomically speaking.
The dust around newly formed stars is gradually either incorporated into the growing planets in the newly forming planetary system or ejected through gravitational interactions with the planets into space.
The dust will disappear after about 30 million years unless the disk is continually supplied with new material. Local comets and asteroids are the most likely sources of new dust. As the planet-size bodies grow, they stir up the orbits of smaller objects in the area. These small bodies collide at high speeds, shatter, and produce tiny particles of silicate dust and ices that can keep the disk supplied with the debris from these collisions.
Over several hundred million years, the comets and asteroids will gradually be reduced in number, the frequency of collisions will go down, and the supply of fresh dust will diminish.
Remember that the heavy bombardment in the early solar system ended when the Sun was only about million years old. It is likely, however, that some small amount of cometary material will remain in orbit, much like our Kuiper belt, a flattened disk of comets outside the orbit of Neptune. In a young planetary system, even if we cannot see the planets directly, the planets can concentrate the dust particles into clumps and arcs that are much larger than the planets themselves and more easily imaged.
The hot gases pile into the surrounding cold molecular cloud, compressing the material in it and increasing its density. If this increase in density is large enough, gravity will overcome pressure, and stars will begin to form in the compressed gas.
There are many molecular clouds that form only or mainly low-mass stars. Because low-mass stars do not have strong winds and do not die by exploding, triggered star formation cannot occur in these clouds. There are also stars that form in relative isolation in small cores.
Therefore, not all star formation is originally triggered by the death of massive stars. However, there are likely to be other possible triggers, such as spiral density waves and other processes we do not yet understand.
Although regions such as Orion give us clues about how star formation begins, the subsequent stages are still shrouded in mystery and a lot of dust. There is an enormous difference between the density of a molecular cloud core and the density of the youngest stars that can be detected.
Direct observations of this collapse to higher density are nearly impossible for two reasons. First, the dust-shrouded interiors of molecular clouds where stellar births take place cannot be observed with visible light. Second, the timescale for the initial collapse—thousands of years—is very short, astronomically speaking.
Since each star spends such a tiny fraction of its life in this stage, relatively few stars are going through the collapse process at any given time. Nevertheless, through a combination of theoretical calculations and the limited observations available, astronomers have pieced together a picture of what the earliest stages of stellar evolution are likely to be. The first step in the process of creating stars is the formation of dense cores within a clump of gas and dust Figure 7 a.
It is generally thought that all the material for the star comes from the core, the larger structure surrounding the forming star. Eventually, the gravitational force of the infalling gas becomes strong enough to overwhelm the pressure exerted by the cold material that forms the dense cores.
The material then undergoes a rapid collapse, and the density of the core increases greatly as a result. During the time a dense core is contracting to become a true star, but before the fusion of protons to produce helium begins, we call the object a protostar.
Figure 7: Formation of a Star. These sketches are not drawn to the same scale. The diameter of a typical envelope that is supplying gas to the newly forming star is about AU. The typical diameter of the disk is about AU or slightly larger than the diameter of the orbit of Pluto.
The natural turbulence inside a clump tends to give any portion of it some initial spinning motion even if it is very slow. As a result, each collapsing core is expected to spin. According to the law of conservation of angular momentum discussed in the chapter on Orbits and Gravity , a rotating body spins more rapidly as it decreases in size.
In other words, if the object can turn its material around a smaller circle, it can move that material more quickly—like a figure skater spinning more rapidly as she brings her arms in tight to her body. This is exactly what happens when a core contracts to form a protostar: as it shrinks, its rate of spin increases.
But all directions on a spinning sphere are not created equal. As the protostar rotates, it is much easier for material to fall right onto the poles which spin most slowly than onto the equator where material moves around most rapidly. As you spin really fast, you are pushed against the wall so strongly that you cannot possibly fall toward the center of the cylinder. The protostar and disk at this stage are embedded in an envelope of dust and gas from which material is still falling onto the protostar.
This dusty envelope blocks visible light, but infrared radiation can get through. As a result, in this phase of its evolution, the protostar itself is emitting infrared radiation and so is observable only in the infrared region of the spectrum.
Once almost all of the available material has been accreted and the central protostar has reached nearly its final mass, it is given a special name: it is called a T Tauri star , named after one of the best studied and brightest members of this class of stars, which was discovered in the constellation of Taurus. Astronomers have a tendency to name types of stars after the first example they discover or come to understand. Only stars with masses less than or similar to the mass of the Sun become T Tauri star s.
Massive stars do not go through this stage, although they do appear to follow the formation scenario illustrated in Figure 7. Recent observations suggest that T Tauri stars may actually be stars in a middle stage between protostars and hydrogen-fusing stars such as the Sun.
High-resolution infrared images have revealed jets of material as well as stellar winds coming from some T Tauri stars, proof of interaction with their environment. A stellar wind consists mainly of protons hydrogen nuclei and electrons streaming away from the star at speeds of a few hundred kilometers per second several hundred thousand miles per hour.
Astronomers have actually seen evidence of these beams of particles shooting out in opposite directions from the popular regions of newly formed stars. In many cases, these beams point back to the location of a protostar that is still so completely shrouded in dust that we cannot yet see it Figure 8.
Figure 8: Gas Jets Flowing away from a Protostar. Here we see the neighborhood of a protostar, known to us as HH 34 because it is a Herbig-Haro object. The star is about light-years away and only about 1 million years old. Light from the star itself is blocked by a disk, which is larger than 60 billion kilometers in diameter and is seen almost edge-on.
Jets are seen emerging perpendicular to the disk. The material in these jets is flowing outward at speeds up to , kilometers per hour. The series of three images shows changes during a period of 5 years. Every few months, a compact clump of gas is ejected, and its motion outward can be followed. The changes in the brightness of the disk may be due to motions of clouds within the disk that alternately block some of the light and then let it through.
This image corresponds to the stage in the life of a protostar shown in part c of Figure 7. On occasion, the jets of high-speed particles streaming away from the protostar collide with a somewhat-denser lump of gas nearby, excite its atoms, and cause them to emit light. These glowing regions, each of which is known as a Herbig-Haro HH object after the two astronomers who first identified them, allow us to trace the progress of the jet to a distance of a light-year or more from the star that produced it.
Figure 9 shows two spectacular images of HH objects. Figure 9: Outflows from Protostars.
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