Discussion Questions - Week 4

4 TOTAL POINTS

1. What are the causes of reflection nebulae? What are the essential ingredients that enable us to see these objects?
When starlight scatters from dust and some molecular gases in a region of high gas/dust concentrations, the blue light scatters more than the red (also called preferential scattering of blue). The physical cause is that light having wavelength comparable to (or less than) the size of the gas molecules and dust tends to scatter the most; this size factor causes blue and shorter wavelength light to scatter preferentially.  The ingredients are then a nearby star to supply light, and dust and/or molecular gas to scatter that light.

2. Find images on the Internet of a "dark nebulae," "reflection nebulae," and "emission nebulae." Pick one example of each category, describe the image.
No explanation needed here, I hope. Your responses will depend on what you picked.

3. Describe how the ultra-cold material in the interstellar medium eventually gets hot enough to cause hydrogen at the center of the protostar to begin to fuse to helium.
As clouds collapse to form stars, gravitational energy gets converted to thermal energy of the collapsing cloud material. The gas/dust heats up via collisions with other cloud particles as a result of this energy transformation. At some point in the thousands of degrees range, enough of the gas ionizes to result in the inner part of the cloud’s become opaque to radiation - i.e., light gets absorbed by material within the cloud. This results in rapid heating in the interior (because the radiation can no longer carry the energy directly to space). Gravity keeps on collapsing the cloud and converting gravitational energy to thermal energy. The collapse continues until fusion begins around 6 million degrees and can ultimately provide enough outward radiation pressure to halt the collapse. At present, all energy conversion in our Sun is from nuclear energy to thermal energy, which is ultimately radiated to space from the Sun’s surface in the form of electro-magnetic radiation.

4. Use the Internet to track down information on M11 (The Wild Duck Cluster).

a.  What kind of object is M11, and where is it in the universe?
b.  Where is M11 in the sky and what time of year is it observable?
c.  How far from Earth is M11, and how old is it?
a.  M11 is the 11th entry in Messier's list of objects, and today we know it as an open cluster of nearly 3000 stars with the popular name 'Wild Duck Cluster.'  It is in the constellation Scutum, about 6000 light years distant from us, in the disk of our own Milky Way Galaxy.

b.It is a magnitude 7 object, so you'd need binoculars or small telescope to be able to find it.   It is viewable from Bellingham and from points farther south.  It transits at midnight in early July, so it would be viewable sometime during night hours for a few months either side of July.

c.  M11 is about 3000-6000 l.y. distant, and about 150-250 million years old,meaning all its thousands of stars formed about 150-250 million years ago from one gas/dust cloud. (note: I saw age estimates all the way up to around 500 million years in my own reading)