Are the stellar remnants left by low mass and high mass stars different? How do high mass and low mass stars end their normal phases of stellar evolution? What are the masses which separate planets from brown dwarfs, brown dwarfs from low mass stars, and low mass stars from high mass stars?
Draw the evolutionary track on an Hertzsprung-Russell diagram for the Sun. Be sure to label each phase. Pay attention to the nuclear burning and the various stages each star passes through. For massive stars, describe the different nuclear burning phases and how long each lasts. After which burning stage does neutrino emission dominate the production of radiation in a star? Explain in words why and how a star like the Sun evolves.
Be careful to explain clearly the difference between the evolution of the core and the outer layers. What is a white dwarf? How are white dwarfs used to test our theories for the evolution of the Universe? Discuss the effects on the Earth produced by the evolution of the Sun. What will be the fate of the Earth as the Sun evolves? Does the Earth sruvive? Does life on Earth survive?
Suppose that the Sun started from an ISM cloud which was pure helium rather than hydrogen, helium, and small amounts of other stuff. What would be different about this other Sun compared to our Sun? What is shell burning? Stellar Clusters: Why is the observational study of stellar evolution difficult? How are stellar clusters used to test our understanding of stellar evolution? Why are they such useful tests of stellar evolution?
What are the properties of a cluster of stars that makes the cluster a valuable test of our understanding of stellar evolution? Sketch the Hertzsprung-Russell diagrams for clusters of various ages? What is the Main Sequence Turn-off for a cluster of stars? What does the Main Sequence Turn-off tell us? Will a cluster show a fully occupied Main Sequence stars all the way from O stars to M stars?
Why not if not? Explain how stellar clusters can be used to place lower limits on the age of the galaxy. What is the largest age found for a stellar cluster? Does this age have implications for our understanding of cosmology the evolution of the Universe? What are globular clusters? What are galactic clusters? How old are the oldest globular clusters? How old is the Universe estimated to be? How is this age estimate made?
What is Hubble's Law? What are the ages of the oldest white dwarfs in our Galaxy? How do they compare to the age of the estimated age of the Universe? Supernovas: What are Type I Supernovas? What are Type II Supernovas? What are SN remnants? What is the Crab Pulsar? List several reasons why astronomers find SN so interesting. How is the energy generated in a Type II SN divided among neutrinos, the explosion, the electromagnetic radiation.
What role do SN play in nucleosynthesis? What is the r-process? What is the s-process? Collapse into a star like our Sun takes about 50 million years. The collapse of a very high mass protostar might take only a million years. 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 clouds turn out to be the birthplaces of most stars in our Galaxy.
The masses of molecular clouds range from a thousand times the mass of the Sun to about 3 million solar masses. The molecular cloud filaments can be up to light-years long. Within the clouds are cold, dense regions with typical masses of 50 to times the mass of the Sun; we give these regions the highly technical name clumps.
Within these clumps, there are even denser, smaller regions called cores. The cores are the embryos of stars. The conditions in these cores—low temperature and high density—are just what is required to make stars. Remember that the essence of the life story of any star is the ongoing competition between two forces: gravity and pressure. The force of gravity, pulling inward, tries to make a star collapse.
Internal pressure produced by the motions of the gas atoms, pushing outward, tries to force the star to expand. When a star is first forming, low temperature and hence, low pressure and high density hence, greater gravitational attraction both work to give gravity the advantage. In order to form a star—that is, a dense, hot ball of matter capable of starting nuclear reactions deep within—we need a typical core of interstellar atoms and molecules to shrink in radius and increase in density by a factor of nearly 10 It is the force of gravity that produces this drastic collapse.
One of the best-studied stellar nurseries is in the constellation of Orion, The Hunter, about light-years away Figure 2. The Orion molecular cloud is much larger than the star pattern and is truly an impressive structure. In its long dimension, it stretches over a distance of about light-years.
The total quantity of molecular gas is about , times the mass of the Sun. Most of the cloud does not glow with visible light but betrays its presence by the radiation that the dusty gas gives off at infrared and radio wavelengths. Figure 2: Orion in Visible and Infrared.
The ancients imagined a sword hanging from the belt; the object at the end of the blue line in this sword is the Orion Nebula. Heated dust clouds dominate in this false-color image, and many of the stars that stood out on part a are now invisible.
The large, yellow ring to the right of Betelgeuse is the remnant of an exploded star. The infrared image lets us see how large and full of cooler material the Orion molecular cloud really is. The lower one is the Orion Nebula and the higher one is the region of the Horsehead Nebula. The region about halfway down the sword where star formation is still taking place is called the Orion Nebula.
About young stars are found in this region, which is only slightly larger than a dozen light-years in diameter. The Orion Nebula also contains a tight cluster of stars called the Trapezium Figure 4. The brightest Trapezium stars can be seen easily with a small telescope. Figure 3: Orion Nebula. Megeath University of Toledo, Ohio.
Compare this with our own solar neighborhood, where the typical spacing between stars is about 3 light-years. Only a small number of stars in the Orion cluster can be seen with visible light, but infrared images—which penetrate the dust better—detect the more than stars that are part of the group Figure 4.
Figure 4: Central Region of the Orion Nebula. The Orion Nebula harbors some of the youngest stars in the solar neighborhood. At the heart of the nebula is the Trapezium cluster, which includes four very bright stars that provide much of the energy that causes the nebula to glow so brightly. In these images, we see a section of the nebula in a visible light and b infrared.
The four bright stars in the center of the visible-light image are the Trapezium stars. Notice that most of the stars seen in the infrared are completely hidden by dust in the visible-light image. Schneider, E. Young, G. Rieke, A. Cotera, H. Chen, M. Rieke, R. Thompson Steward Observatory, University of Arizona.
Studies of Orion and other star-forming regions show that star formation is not a very efficient process. That is why we still see a substantial amount of gas and dust near the Trapezium stars. The leftover material is eventually heated, either by the radiation and winds from the hot stars that form or by explosions of the most massive stars.
We will see in later chapters that the most massive stars go through their lives very quickly and end by exploding. Whether gently or explosively, the material in the neighborhood of the new stars is blown away into interstellar space. Older groups or clusters of stars can now be easily observed in visible light because they are no longer shrouded in dust and gas Figure 5. Figure 5: Westerlund 2. This young cluster of stars known as Westerlund 2 formed within the Carina star-forming region about 2 million years ago.
Stellar winds and pressure produced by the radiation from the hot stars within the cluster are blowing and sculpting the surrounding gas and dust. The nebula still contains many globules of dust. Stars are continuing to form within the denser globules and pillars of the nebula. This Hubble Space Telescope image includes near-infrared exposures of the star cluster and visible-light observations of the surrounding nebula.
0コメント