Exam 3 - Stars and Galaxies

Where do stars form and why?

• Stars form in hot gas clouds because stars have to be hot to start fusion reactions.

  
  

• Stars form around giant planets because planets provide the "seeds" of star formation.

  
  

• Stars form in cold gas clouds because gravity has to be stronger than gas pressure.

  
  

The buildup of thermal energy in a collapsing gas cloud leads to thermal pressure that can halt the gravitational collapse. What must happen for the gravitational collapse to continue?

• The cloud must have a very strong magnetic field.

  
  

• The cloud must get a push from the radiation of nearby hot, young stars.

  
  

• The cloud must radiate away some of its thermal energy.

  
  

Which stars spend the shortest amount of time as protostars?

• the most massive stars

  
  

• the least massive stars

  
  

• mass doesn't matter

What key event marks the transition of a protostar into a true (main-sequence) star?

• the onset of sustained nuclear fusion in its core

  
  

• the onset of a strong stellar wind

  
  

• the clearing of the gas from the surrounding gas cloud into interstellar space

  
  

Tags

NGSS.HS-ESS1-1

What is the primary source of energy for protostars (that have not yet become hot enough for fusion in their cores)?

• the conversion of mass into energy in accord with E = mc²

  
  

• gravitational contraction

  
  

• fission from concentrated radioactive elements

  
  

Why do disks form around young stars?

• Intense winds from nearby massive stars flatten the gas cloud.

  
  

• Collisions between gas particles flatten the rotating gas cloud.

  
  

• The rotation of the cloud causes gas to be spun outward from the central star.

  
  

Generally speaking, a main-sequence star is __________ than it was during the time it was a protostar

• cooler and dimmer

  
  

• hotter and brighter

  
  

• hotter and dimmer

  
  

Why are main-sequence lifetimes shorter for more massive stars?

• They aren't; higher mass stars have longer lifetimes.

  
  

• Strong stellar winds cause higher mass stars to lose mass quickly.

  
  

• Higher core temperatures allow fusion to proceed much more rapidly.

  
  

Tags

NGSS.HS-ESS1-1

Why does a star grow larger in radius immediately after it exhausts its core hydrogen?

• Helium fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward.

  
  

• Helium fusion in the core generates enough thermal pressure to push the upper layers outward.

  
  

• Hydrogen fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward.

  
  

What happens to the core of a star after a planetary nebula occurs?

• It contracts from a protostar to a main-sequence star.

  
  

  
  

• It becomes a white dwarf.

  
  

• It can become either a neutron star or a black hole.

  
  

• It breaks apart in a violent explosion.