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Other HST images used during
THE LIFE AND DEATH OF
BOB (a.k.a. NGC 6397) IN
AN INTRODUCTORY
COLLEGE-LEVEL
ASTRONOMY COURSE
Dr. C. Renee James NASA Top Stars
2010
• In a course as rich as a college-level stars
and galaxies course, it would be unthinkable
to feature only one image.
• Bob is simply the anchor – a reference point
for the students.
• But other HST images are used to illustrate
concepts along the way.
• The following are slides from my Fall 2009
class that show the use of HST imagery
throughout our exploration of Bob.
Dr. C. Renee James NASA Top Stars 2010
LIGHT
• The initial topic that students choose to explore.
• Electromagnetic spectrum is introduced.
• Emission and absorption of light, atomic energy
levels are next.
• In class, students get to witness neon, helium,
hydrogen discharge tubes, learning that
“energized, low-density gases produce emission
spectra.”
Dr. C. Renee James NASA Top Stars 2010
Emission Spectra In Space…
• Need VERY low density
gas so atoms don’t bump
into each other much.
• Need energy source
• Planetary Nebulae are
good examples
• VERY hot, dense star in
center energizes lowdensity gas
• Eskimo Nebula shown
here.
http://hubblesite.org/gallery/album/nebula/pr2000007a/
Dr. C. Renee James NASA Top Stars 2010
Orion
Nebula
• Jillions of
new, hot stars
being born,
flooding the
gas with UV
(high-energy)
light that kicks
those little
electrons up
http://hubblesite.org/gallery/album/nebula/pr2006001q/
the steps.
Dr. C. Renee James NASA Top Stars 2010
Butterfly Nebula
(http://hubblesite.org/gallery/album/pr2009025b)
Dr. C. Renee James NASA Top Stars 2010
Spectrum Representations
• Students freehand an intensity – color graph
for the spectra they witness in class.
• A spectroscope that displays the graph on
the screen verifies their observations.
• Then I show them the following nebular
spectrum.
Dr. C. Renee James NASA Top Stars 2010
What a nebular spectrum looks like:
Dr. C. Renee James NASA Top Stars 2010
CONNECTIONS
• Thus the connection between the emission
spectra they OBSERVE IN CLASS and
actual objects in the universe is made
• Class is built on connections, which is why
Bob keeps coming back to haunt them.
• Moving on to additional images used to
illustrate the concepts in class…
Dr. C. Renee James NASA Top Stars 2010
Star formation
• Around week 8 of 15, after digesting light,
magnitudes, parallax, determining stellar masses
(from binary observations), and cluster HR diagrams,
students find that Bob’s main sequence doesn’t span
the entire range of masses, and they ask WHY NOT?!
(see main Bob presentation for more on this)
• At this point, we look at how stars form so that we can
determine how they shine and why Bob’s main
sequence is so limited.
• But since there aren’t examples of star formation in
Bob, I look elsewhere…
Dr. C. Renee James NASA Top Stars 2010
Eagle Nebula (also
known as the Pillars
of Creation) About
6500ly from earth;
About 10,000 solar
systems would fit into
the length of the
tallest column. Huge
cloud currently
experiencing multiple
collapses into stars &
solar systems.
(interesting side note: P.
Scowen was the labbie for my
first undergrad astronomy
class)
Dr. C. Renee James NASA Top Stars 2010
COLLAPSING FURTHER
• If we had the time to wait around, regions
of the Eagle Nebula might look like this
piece of the Orion Nebula, whose stars are
farther along in the formation process.
• Many “proplyds” visible – protoplanetary
disks, which are the ‘pancakes’ from which
planetary systems are formed.
Dr. C. Renee James NASA Top Stars 2010
http://hubblesite.org/newscenter/archive/releases/1994/24/
Dr. C. Renee James NASA Top Stars 2010
STILL FURTHER
• Eventually the stars will “turn on” and blow
the leftover gas and dust away.
• The Pleiades are a prime example of a
young cluster. Note how blue the stars
appear! And how tenuous the leftover gas
and dust appears.
[Note: the youthfulness of the Pleiades comes back when we explore HR
diagrams. Its main sequence spans a much greater range of
temperatures, unlike Bob’s, which ends a bit hotter than the Sun]
Dr. C. Renee James NASA Top Stars 2010
http://hubblesite.org/newscenter/archive/releases/2004/20/image/a/format/web/
Dr. C. Renee James NASA Top Stars 2010
So where are star-forming
regions?
• Inside the disks of galaxies, mostly, where
there’s enough gas and dust.
• Bob is currently not forming new stars, but
here’s a place that IS:
Dr. C. Renee James NASA Top Stars 2010
Brand spankin’ new photo (11/5/2009) of starforming region in nearby galaxy M83:
Dr. C. Renee James NASA Top Stars 2010
Then we explore a couple of
chapters on stellar lives and
deaths to find out what’s
happening with Bob’s stars. This
exploration includes such HST
favorites as…
Dr. C. Renee James NASA Top Stars 2010
• The “cat’s eye”
nebula.
• A sun-like star
that has shed its
outer layers.
• WHAT KIND
OF SPECTRUM
PRODUCED?
WHY?
• WD in center
http://hubblesite.org/gallery/album/nebula/pr1995001a/large_web/
Dr. C. Renee James NASA Top Stars 2010
Supernova 1994D in Distant Galaxy
http://hubblesite.org/gallery/album/star/supernova/pr1999019i/
Dr. C. Renee James NASA Top Stars 2010
Hubble
Space
Telescope
view of a
piece of the
Crab Nebula
Dr. C. Renee James NASA Top Stars 2010
At this point, we kill off Bob…
• With the deaths of stars, we have to say goodbye
to NGC 6397, complete with funeral (see main
presentation)
• For the remainder of the semester, we focus on
galaxies and cosmology, for which there are
numerous HST images to exemplify the concepts.
• My particular favorites are the Hubble Deep Field
and Ultra Deep Field.
• But since we’ve dispensed with Bob, I leave
incorporating these into your class as an exercise
for the reader. 
Dr. C. Renee James NASA Top Stars 2010