NATS1745 6.0 History ofAstronomyChapter 10 Exam Review
What do we call this strip of light today? Galaxy.
What part of our galaxy does this strip represent? Milky Way.
3. What did William, Caroline and John Herschel produce together? Interested
in figuring out the organization of the universe. Used their great telescopes to
catalogue over 5000 “nebulae”
What was Herschel attempting to prove from this research? They attempted
to identify whether nebulae are distant star systems or nearby glowing gas clouds.
He hoped to figure out whether nebulae are like all nearby gas clouds, which
suggests our galaxy is the only galaxy in the universe or whether the nebulae are
distant star systems beyond our galaxy.
Why was he not able to prove this? Due to the large variety of nebulae, he
couldn’t come to a general conclusion about their composition.
4. What did Herschel's map of the visible stars reveal about the shape of our
galaxy? Our galaxy is flattened disk like.
5. What did Lord Rosse discover when he looked at the elliptical-shaped
nebulae through his Leviathan telescope? He discovered that many elliptical
nebulae are “spiral nebulae”
6. What is a variable star? Variable star – stars which fluctuate in brightness at
regular time intervals.
What characterizes the Cepheid variables? Luminous variable stars which
pulsate rapidly @ intervals of days to weeks.
Explain briefly why Cepheids pulsate. Cepheids pulsate due to unusual high
amounts of twice ionized helium.
7. What are the SMC and LMC? SMC – the brightest nebulae seen in the
What did Henrietta Leavitt discover when she observed Cepheids in the
SMC? She founds that the brighter the Cepheid, the longer its period.
Why was this discovery so valuable? If the Cepheid is in another system then
once you have the Cepheid’s distance, you have the distance to another star
system. Distances to other star systems were needed in order to figure our whether
other star systems are within our galaxy or whether they are island universes
beyond our galaxy.
Who used her discovery to determine the boundaries of our galaxy? Harlow
8. What are the 3 components of our galaxy, according to Shapley's model? (1)
1 Its disk is 300000 ly in diameter, this large size implied all nebulae are systems
within our galaxy. (2) Our sun resides in the outskirts of its disk, the center of our
galaxy, our stars get denser forming a large spherical bulge of stars and we are
near the outskirts. Found come Globular clusters below and above the disk,
concluded that our galaxy is surrounded by a spherical halo of star clusters and
spiral nebulae. (3) The Milky Way is in Sagittarius. Where is the Sun located in
this model? Located in the outskirts of the disk.
Where is the Galaxy's centre? Stars.
In what 2 ways is this model incorrect? The diameter of the disk was not
300000 ly but 100000 ly. And it was not surrounded by spherical halo of spiral
9. Why was Shapley convinced that the spiral nebulae are within our galaxy's
boundaries? There were no spiral nebulae in the zone of avoidance, which
convinced him they must be contained within our galaxy.
10. What is the Zone ofAvoidance? Spirals are pushed from our disk by radiation
pressure from its high density of stars, therefore there are no spiral nebulae in this
What was Shapley's explanation for it? Since the disk is a very dense measure
of stars, perhaps all the stars combined energy output, radiation pressure is
pushing the spirals away from the disk.
What was Curtis'? He proposed that if the Milky Way’s disk contains a band of
dust, then if spirals are outside our Galaxy, they aren’t seen in the “zone of
avoidance” because they are obscured by our dust.
Who was correct? Heber Curtis.
11. What observation led Curtis to suspect that the disks of galaxies are filled
with interstellar dust? He completed a high-resolution survey of spiral nebulae
revealing obscuring matter in their disks.
Where does this dust come from? Dust comes from the ejecta of dying stars.
12. What did Vesto Slipher observe about the spectra of spirals? Measured large
redshifts in the absorption lines of spirals, suggesting recessional speeds as high
as 1000s of km/sec.
What did Curtis argue? Distant galaxies are outside our galaxy. Belief that the
spirals are not intragalactic objects but island universes, like our own galaxy