ANSWER KEY
Stars and Sun Unit: Study Guide
1. If Star A and Star B have the same luminosity, but A is closer to earth than B, which one
will look brighter? Why? Star A will look brighter because it is much closer
to Earth than Star B. The closer a star is to Earth, the brighter it will
appear to be.
2. Describe the life cycle of a high-mass star. Nebula → nuclear fusion occurs in a
star → star runs out of hydrogen → heavier elements continue to fuse
→ iron forms in the core → supernova → black hole or neutron star
forms
3. Describe the life cycle of a low-mass star. Nebula → nuclear fusion occurs in a
star → star runs out of hydrogen → heavier elements continue to fuse
→ star cools and expands → becomes a red giant → collapses into a
white dwarf
4. When stars run out of hydrogen, fusion continues to form heavier elements until
__iron_ forms and fusion stops.
5. Based on the HR diagram, circle the
dimmest star.
6. Based on the HR diagram, which color
stars are brightest? blue
7. Which kind of stars are plotted on the HR
diagram? main sequence
8. Which elements are most common in a
young star? hydrogen and helium
9. What will ultimately happen to our sun? When it runs out of hydrogen, it will
cool and become red, expand and become a giant, and finally collapse
into a white dwarf.
10. Which magnitude corresponds to the brightest star? 10, -4, 0.3, -2.8
-4 because the lower the magnitude of a star, the brighter the star
ANSWER KEY
11. What is the difference between apparent magnitude and absolute magnitude?
Apparent magnitude describes how bright a star appears based on our
frame of reference from Earth’s surface. Absolute magnitude describes
how bright a star actually is (as if all of the stars were 10 parsecs away
from Earth).
12. What color are the hottest stars? blue The coolest? red
13. Why are blue stars the hottest? Hotter objects emit EM radiation at higher
frequencies. Blue has a short wavelength on the visible light portion of
the EM spectrum and therefore has a higher frequency.
14. What is a theoretical object that absorbs all of the radiation it receives? A blackbody
15. What kind of stars have the shortest life spans? Hot, bright stars
16. What property of a star determines whether or not it will form a black hole when it
dies? The mass of the star. Stars that are 5 times the mass of our sun
have the potential to form a blackhole.
17. Two stars have the same apparent magnitude as seen from Earth’s surface. Star A is 33
light years away. Star B is 346 light years away. Which star has the smaller absolute
magnitude? Star B must have a smaller absolute magnitude meaning the
star must be brighter than Star A. Star B is more than 10 times farther
away, but has to be brighter in order for both stars to look the same
from Earth’s surface.
18. What two terms refer to the actual brightness of a star? luminosity and absolute
magnitude
19. On what layer of the sun can you find a sunspot? photosphere
20. Describe what happens when nuclear fusion occurs. At high temperatures, atoms
move faster. The temperatures in the cores of stars are around 15
million degrees. The hydrogen atoms slam into each other fusing into
larger helium atoms and releasing enormous amounts of energy in the
process.
ANSWER KEY
21. Name and describe the two types of heat transfer that occur within a star.
Radiation – energy and heat travel from the core towards the star’s
surface.
Convection – plasma rises and sinks as the density changes as it heats
and cools
22. Why does a cooler gas sink when convection occurs? the plasma moves towards
the surface where it cools increasing the density causing it to sink
towards the radiative zone
23. Why don’t main sequence stars collapse under their own gravitation weight? While
gravity from the core is pulling the star inward, the nuclear reactions
occurring within the star push outward.