21 (2) Flashcards
The difference between the density of a molecular cloud core and the density of the youngest stars that can be detected is huge. why can’t we observe this stage? 2
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.
Birth of a star (1)
- Forming …………..
- ………… ………….. 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 ……………, and the density of the core increases greatly as a result.
- core
- gravitational force
- collapse
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
The natural turbulence inside a clump tends to give any portion of it some initial ……….. ……… (even if it is very slow). As a result, each collapsing core is expected to ……….
spinning motion
spin
According to the law of conservation of angular momentum, a rotating body spins more rapidly as it ………….. in size.
decreases
The protostar and disk at this stage (collapsing, spinning, equator spinning so fast so gas coming to the center from different directions) 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.
R 2
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
Only stars with masses ……….. than or similar to the mass of the Sun become T Tauri stars.
Massive stars do not go through this stage, although they do appear to follow the formation scenario
less
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, Why?
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 (………….. ………..) and ……….. streaming away from the star at speeds of a few hundred kilometers per second
hydrogen nuclei / electrons
When the wind first starts up, the disk of material around the star’s equator blocks the wind in its direction. Where the wind particles can escape most effectively is in the direction of the star’s …………..
poles
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
The wind from a forming star will ultimately sweep away the material that remains in the obscuring envelope of dust and gas, leaving behind the naked disk and protostar, which can then be seen with visible light.
We should note that at this point, the protostar itself is still contracting slowly and has not yet reached the main-sequence stage on the H–R diagram
As a star goes through the stages of its life, its luminosity and temperature change. Thus, its position on the H–R diagram, in which luminosity is plotted against temperature, also changes. As a star ages, we must replot it in different places on the diagram.
Therefore, astronomers often speak of a star moving on the H–R diagram, or of its evolution tracing out a path on the diagram. In this context, “tracing out a path” has nothing to do with the star’s motion through space; this is just a shorthand way of saying that its temperature and luminosity change as it evolves.
Given a model that represents a star at one stage of its evolution, we can calculate what it will be like at a slightly later time. At each step, the model predicts the …………. and ……….. of the star, and from these values, we can figure out its surface temperature.
luminosity / size
A series of points on an H–R diagram, calculated by creating mathematical model, allows us to follow the life changes of a star and hence is called its ……… ………
evolutionary track.
These young stellar objects are not yet producing energy by nuclear reactions, but they derive energy from ………… ………..—through
gravitational contraction
Prostart stages (1)
Initially, a protostar remains fairly cool with a very large radius and a very low density. It is transparent to ……… ………, and the heat generated by gravitational contraction can be radiated away freely into space.
infrared radiation
Prostart stages (2)
Because heat builds up slowly inside the protostar, the gas pressure ……….. ……….., and the outer layers fall almost unhindered toward the center. Thus, the protostar undergoes very rapid collapse, a stage that corresponds to the roughly vertical lines at the right of Figure 21.12.
remains low
Prostart stages (3)
As the star shrinks, its surface area gets smaller, and so its total luminosity ………… The rapid contraction stops only when the protostar becomes ………. and ……….. enough to trap the heat released by gravitational contraction.
decreases / dense / opaque
Prostart stages (4)
When the star begins to retain its heat, the contraction becomes much slower, and changes inside the contracting star keep the ………….. of stars like our Sun roughly constant.
luminosity
Prostart stages (4)
The surface temperatures start to build up, and the star “moves” to the left in the H–R diagram. Stars first become visible only after the stellar wind described earlier clears away the surrounding dust and gas.
This can happen during the rapid-contraction phase for low-mass stars, but high-mass stars remain shrouded in dust until they end their early phase of gravitational contraction
astronomers say that a star is ……….. when it is able to sustain itself through nuclear reactions (by making its own energy.)
born
When the star’s central temperature becomes high enough (about 12 million K) to fuse hydrogen into helium, we say that the star has reached the ……….. ……..
main sequence
It is now a full-fledged star, more or less in equilibrium, and its rate of change ………… ………… Only the gradual depletion of hydrogen as it is transformed into helium in the core slowly changes the star’s properties.
slows dramatically
