"The Clues:"
We had the sky up there, all speckled with stars, and we used to lay on
our backs and look up at them, and discuss about whether they was made or
only just happened. Jim he allowed they was made, but I allowed they happened;
I judged it would have took too long to make so many. Jim said the moon could
a laid them; well, that looked kind of reasonable, so I didn't say nothing
against it, because I've seen a frog lay most as many, so of course it could
be done. We used to watch the stars that fell, too, and see them streak down.
Jim allowed they'd got spoiled and was hove out of the nest.
Huckleberry Finn, Mark Twain
Where does this myriad of stars come from? Let's consider what a careful investigation
of the night sky will reveal by looking at the clues...
1. The Interstellar Medium (ISM)
With the refinement of the telescope and especially with the skillful observations
of William and Caroline Herschel in the late 18th century we began to see a richness
in the universe that was hitherto unknown. 
Famed for their discovery of many nebulae and clusters the Herschels also noticed,
especially when looking in the crowded chaos of the milky way, numerous "empty" patches.
They thought that this was just what it seemed - holes in the galaxy - no stars.
By the early part of this century (circa 1930), however, the view
was much different. Robert Trumpler succeeded in showing that these patches were
in fact very dusty regions that were obscuring our view. Evidence for this came
primarily by noting that around the periphery of such regions it is not uncommon
to find stars that are reddened. That means that while their spectra say they
should be blue (or example) their colour indices say they should be red (big
B-V
for example). This is not unlike what you see as stars approach the horizon.
The explanation is the same - scattering from interstellar dust. We now recognize
that parts of the galaxy are very dusty - important clue #1
2. Nebulae
the name derives from the Latin name for cloud. Nebulae come in different types:
- Emission Nebulae: hot glowing gases, characterized by emission spectra.
HII regions are emissions of ionized hydrogen and are often seen around extremely
bright stars. What powers the nebula? Is it just a coincidence that we see
bright (and as we shall soon see YOUNG) stars in the presence of hot regions
of gas? Clue #2!
- Reflection Nebulae: light reflecting from dust grains surrounding bright
stars. This light is often polarized which gives indirect evidence for the
dust grains as well as the presence of interstellar magnetic fields. Clue
#3!
3. Dark Nebulae and Bok globules:
dark, dusty regions. Bok globules are
several
pcs in diameter and can have masses ranging from 10's to 1000's of solar masses.
Their compactness is very suggestive. What observation would you consider
to be the "smoking gun" that would implicate Bok globules as significant players
in star formation? Clue #4!
These images were produced by the Anglo-Australian Observatories.
To see more exquisite nebulae follow this link......The
Web Nebulae
The Stakeout: Our Methods of Surveillance
Weighing the Evidence
A picture is beginning to emerge. We think that new stars form (and are forming)
from the vast supply of dust and gas that we find spread throughout the galaxy.
This seems to be a ubiquitous mechanism for, as we shall learn next term, we find
similar structures in other (spiral) galaxies. In very simple terms, then, we
believe that stars form by the condensation and collapse of huge interstellar
clouds. The details of this mechanism are just now being worked out and this will
form the content of our next lecture.
Seeds:
Chp 11; pgs 207-222
Kaufmann:
Chp 20; pgs364-370
