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Galaxy structure and evidence for active nuclei

Science · Astronomy & Astrophysics · ages 21-22

Name ______________________ Date ____________

In the Milky Way, where are young hot stars from recent star formation concentrated?
- In the sparse stellar halo
- In the central stellar bulge
- In the disc and spiral arms
A galaxy shows a flat blue disc with arms plus a yellow central bulge. What type is it?
- Spiral
- Elliptical
- Irregular
Typical elliptical galaxies contain abundant cold gas and form many new stars.
Circle one: True False
A smooth, rounded galaxy has mostly older stars and very little cold gas. What should we expect while that gas shortage continues?
- Continued rapid star formation despite its shortage of cold gas
- An immediate collision determined solely by its rounded shape
- Little new star formation unless fresh cold gas arrives
An astronomer has distance estimates for tracers in the Milky Way's dusty disc. Which observations best help map hidden gas and stars?
- Visible photos and infrared star surveys
- Radio gas maps and infrared star surveys
- Radio gas maps and visible-light surveys
A nucleus persistently outshines its host's stars. Much of its light changes coherently and intrinsically within days in the source's frame. Ignoring relativistic beaming, which engine best explains both facts?
- Fusion in a dense cluster of ordinary stars
- Gas accreting onto a central supermassive black hole
- Many unrelated supernovae whose explosions sustain the nucleus
In a density-wave model, bright spiral arms consist of the same stars staying in the arms permanently.
Circle one: True False
Much of a quasar's light varies coherently and intrinsically in four days in its own frame. Ignore relativistic beaming. Using the light-travel estimate, which choice gives the tightest approximate upper size limit for the varying region?
- About four light-days or less
- About four light-years or less
- About four light-days or more
- Exactly four light-years across

Answer key

For grown-ups. Fold this page away before handing over the rest.

Galaxy structure and evidence for active nuclei W1-mt_Eps5CXEB5C-s1

In the disc and spiral arms · Recent star formation is concentrated where gas is available, especially near the disc plane and in the arms. Older stars also occur in the disc.
Spiral · The disc and spiral arms identify a spiral galaxy. Colour gives clues to its stars, but colour alone does not determine its type.
False · Typical ellipticals have little cold star-forming gas, and their light is dominated by older stars. Not every elliptical is gas-free.
Little new star formation unless fresh cold gas arrives · Cold gas supplies the material for new stars. A continuing shortage limits new star formation; a rounded shape does not predict an immediate collision.
Radio gas maps and infrared star surveys · Radio emission traces gas through dust, and infrared reveals more stars than visible light. Directions combined with distance estimates locate the tracers.
Gas accreting onto a central supermassive black hole · High luminosity and rapid coherent variation require a bright, compact engine. Accretion releases gravitational energy in hot gas outside a supermassive black hole's event horizon.
False · In this model, stars and gas pass through a higher-density pattern. Gas compression can produce new stars, so an arm is not a permanent collection of the same stars.
About four light-days or less · Light travels four light-days in four days, giving an approximate upper size limit of four light-days. Four light-years is a larger, weaker upper limit. Neither a lower size limit nor an exact size follows.