Chapter 19 Responses
"Distinguish between spiral galaxies and elliptical galaxies in terms of their shapes and colors. What are irregular galaxies?"
Spiral galaxies appear to be white disks with yellowish center bulges while elliptical galaxies are rounder, more football-shaped, and are redder. Irregular galaxies are those that are neither disklike or rounded in appearance.
"Distinguish between the disk component and the spheroidal component of a spiral galaxy. Which component includes the galaxy's spiral arms? Which includes its bulge? Which includes its halo?"
The spheroidal component of a spiral galaxy is composed of the center bulge and the halo. The disk component is the thin disk which slices through the spheroidal component. The galaxy's spiral arms are part of the disk component.
"What is the major difference between an elliptical galaxy and a spiral galaxy? How is an elliptical galaxy similar to the halo of the Milky Way?"
The lack of a significant disk component is the major factor differentiating an elliptical from a spiral galaxy. Therefore an elliptical galaxy is similar to the halo of the Milky Way because it resembles what the Milky Way would look like without a disk and with just the bulge and halo instead. Elliptical galaxies also have very little dust or cool gas, just as the Milky Way's halo does compared to its disk.
"What do we mean by a standard candle? Briefly explain how, once we identify an object as a standard candle, we can use the luminosity–distance formula to find its distance."
A standard candle is any light source with a standard, known luminosity. Once an object is identified as a standard candle, we can plug its luminosity and apparent brightness into the luminosity–distance formula to discover its distance.
"Why are Cepheid variables good standard candles? Briefly explain how Edwin Hubble used his discovery of a Cepheid in Andromeda to prove that the 'spiral nebulae' were actually entire galaxies."
Cepheid variables are good standard candles because they have a known period-luminosity relationship that is consistent enough for us to be able to determine their luminosity within 10% when their pulsation period is measured.
Edwin Hubble compared photos of the Andromeda Galaxy taken days apart and discovered the galaxy had Cepheid variables. After he determined their luminosities by observing their periods, he then used the luminosity-distance formula to determine their distance. The distance was so much further than the most distant stars in the Milky Way that 'spiral nebulae' like Andromeda were determined to be galaxies in their own right.
"Describe how we can use Hubble's law to determine the distance to a distant galaxy. What practical difficulties limit the use of Hubble's law for measuring distances?"
Hubble's law says the velocity of a galaxy (the speed at which it's moving away) divided by Hubble's constant is equal to the galaxy's distance from us. One of the difficulties with Hubble's law is that Hubble's constant hasn't been precisely pinned down yet, per question eleven. Also, the law doesn't take into account gravitational tugs that affect galaxies, such as the gravitational influences within our Local Group.
"What makes white dwarf supernovae good standard candles? Briefly describe how we have learned the true luminosities of white dwarf supernovae."
White dwarf supernovae make good standard candles because a white dwarf supernova always takes place when the star reaches the 1.4 solar mass limit, so all white dwarf supernovae have about the same luminosity. This was confirmed by observing Cepheid variables in the same galaxies as the few known white dwarf supernovae. The Cepheid variables were used to determine the galaxy's distance, which meant the luminosity-distance formula could be used to check each of the white dwarf supernovae. As expected, they all had about the same luminosity.
"What is our current best estimate of Hubble's constant? Summarize all the links in the distance chain that allow us to estimate distances to the farthest reaches of the universe."
The current best estimate of Hubble's constant is between 55-75 kilometers per second per megaparsec (km/s/Mpc). One of the projects of the Hubble Space Telescope is to try to narrow it down further.
Distance measurement begins with bouncing radar off the planets in our own solar system. Once the Earth-Sun distance is understood, it can be used to measure stellar parallax and compute the distances of nearby stars. The main sequence intensities of clusters elsewhere in the Milky Way can then be compared to intensities in the Hyades Cluster, whose distance is already known through parallax, in a process called main-sequence fitting.
Beyond the galaxy, measuring the periods of Cepheid variables in other galaxies then using the luminosities with the luminosity-distance formula provides the distances to other galaxies. Measuring Cepheids in nearby galaxies confirmed the luminosity of white dwarf supernovae, which can be used to measure more distant galaxies. For spiral galaxies without white dwarf supernovae, the Tully-Fisher relation can be used. (The faster a galaxy's rotation, the more luminous it is.)
The data collected through measuring distances with white dwarf supernovae and the Tully-Fisher relation can then be used to continue refining the definition of Hubble's constant. Hubble's constant allows a galaxy's distance to be calculated based on its redshift.
"In what ways is the surface of an expanding balloon a good analogy to the universe? In what ways is this analogy limited? Explain why a miniature scientist living in a polka dot on the balloon would observe all other dots to be moving away, with more distant dots moving away faster."
An expanding balloon is a good analogy to the universe because it simply expands, without expanding into something else, because it doesn't have a center, because it doesn't have edges, and because polka dots attached to the balloon stay the same size (as galaxy clusters do since gravity holds then together) and expand uniformly away from all other polka dots when the balloon is inflated (as galaxies do as the universe expands).
The analogy is limited because just the balloon's surface has to represent all three dimensions of space and the space inside and outside the balloon must be ignored.
The miniature scientist living in a polka dot would observe the more distant dots moving faster because the universe's expansion is uniform. If the scientist is on Dot A and Dot B is 1cm away from Dot A and Dot C is 1cm away from Dot B, and the air going into the balloon pushes each dot a further 1cm away from each other, Dot B will then be 2cm from Dot A and Dot C will be 2cm from Dot B. However, whereas before Dot C was 2cm from Dot A, now it will be 4cm from Dot A. It appears to be moving faster, but it is just the uniformity of the expansion that creates this effect. This uniformity is also why the scientist could live on any polka dot and see all the others moving away.
Again, this site is very reliable for students taking astronomy courses!!!!!!!!
Posted by: Brittany | 24 April 2007 at 03:27 PM
BEST SITE EVER!
Posted by: Breeze Batal | 26 July 2009 at 03:46 PM