Chapter 18 Responses
"Draw a simple sketch of our galaxy as it would appear face-on and edge-on, identifying the disk, bulge, halo, and spiral arms."
"What do we mean by the interstellar medium? How does it affect our view of the galaxy?"
Interstellar medium is the collective term for all the clouds of interstellar gas and dust. Since it fills the galaxy it covers up our view of some objects or causes them to appear dimmer.
"What do we mean by heavy elements? How much of the Milky Way's gas is in the form of heavy elements? Where are heavy elements made?"
Heavy elements are those which are heavier than helium. They're made in the stars when fusion goes past the hydrogen burning stage, and released into the galaxy when the star dies. Only 2% of the Milky Way's gas is made up of heavy elements.
"What is the star-gas-star cycle, and how does it lead to chemical enrichment? Use the idea of chemical enrichment to explain why stars that formed early in the history of the galaxy contain a smaller proportion of heavy elements than stars that formed more recently."
The star-gas-star cycle refers to the process of stars being born from interstellar gas, eventually returning some of that gas when they die, then new stars being born from this same material, and so on. High mass stars create heavy elements in their different stages of fusion which are contributed to the interstellar matter when the stars go supernova. This is chemical enrichment. Then the interstellar matter must cool to form atomic hydrogen clouds followed by clouds of molecular hydrogen. From these molecular clouds new stars form and the cycle continues. This is why older stars have a smaller amount of heavy elements: they were formed from material that was less "recycled."
"What is the most common form of gas in the interstellar medium? What is the wavelength of the radio emission line characteristic of the gas?"
The most common form of gas in the interstellar medium is hydrogen. Its wavelength is 21-cm.
"Will the star-gas-star cycle continue forever? Why or why not?"
The star-gas-star cycle won't continue forever because with every cycle some material becomes either a brown dwarf, white dwarf, neutron star, or black hole. So with each cycle a little less is returned to the interstellar medium.
"Describe the stars that we find in the halo in terms of mass, luminosity, color, and proportion of heavy elements. Why do halo stars have these characteristics?"
Halo stars are less massive than our Sun, dim, red, and have only about .02% heavy elements. These stars are old and thus were formed before there had been as many supernovae, so they're not as enriched with the heavy elements.
"What is an ionization nebula? Why are such nebulae found near massive young stars? What produces their striking color?"
An ionization nebula is a glowing cloud of gas in the galactic disk, commonly found in the spiral arms. It glows due to irradiation by the ultraviolet photons of nearby hot stars which cause the nebula's atoms to ionize and emit light. Most of the nebula's colour comes from spectral lines produced by atomic transitions. Colour also comes from starlight reflected by dust grains (blue) and dusty gas clouds (black).
"Contrast the general patterns of the orbits of stars in the disk with the orbits of stars in the halo."
Stars in the disk orbit the center of the galaxy in the same direction, bobbing up and down as gravity pulls them in a sort of ongoing equalizing maneuver. Stars in the halo have elliptical obits around the galaxy's center and don't all travel in the same direction. Their velocity is so much higher than a disk star's that the gravity causing disk stars to bob barely affects them.
"At what speed does the Sun travel around the solar circle? About how long does one orbit take? Briefly explain how we can use the characteristics of the Sun's orbit to determine the mass of the Milky Way contained within the solar circle. What is the result?"
The Sun travels around the solar circle at 220 km/s and one orbit takes 230 million years. The Sun's orbit can be used to determine the Milky Way's mass inside the solar circle by applying the orbital velocity law. The solar circle's mass is equal to the Sun's radius from galactic center (28,000 ly) multiplied by the Sun's orbital velocity (220 km/s) squared, divided by the gravitational constant. The result is 2 x 10^41 kilograms or about 100 billion solar masses.
"What is a rotation curve? Describe the rotation curve of the Milky Way and explain how it indicates that most of the Milky Way's mass lies outside the solar circle. What do we mean by dark matter?"
A rotation curve plots rotational velocity against distance from the center. The rotation curve for the Milky Way shows an increase in speed as objects are further away from the center. There is a very sharp increase in the first 5000 ly, then some bobbing, then a very gradual increase beginning around 40,000 ly from the center. Since the objects further away are moving faster than those closer to the center, most of the galaxy's mass must be beyond the solar circle.
Dark matter is mass that emits no radiation and thus can't be seen, yet we still detect it. Studies point to a great amount of mass in the Milky Way's halo where there is very little radiation. The nature of dark matter remains a huge mystery.
"What is Sgr A*? What evidence suggests that it contains a massive black hole?"
Sgr A* is a source of bright radio emission at the very center of our galaxy. It's called Sagittarius A* because the Milky Way's center is in the direction of that constellation. The evidence for Sgr A* containing a massive black hole includes the detection of an extremely massive but relatively very small object: 2.5 million solar masses measuring 3 light years across. This is too massive to be explained by the star cluster in Sgr A*.
ASIF BUREWALA
Posted by: ASIF | 21 December 2009 at 07:09 AM