Chelsea Esposito's Quiz

Chelsea Esposito

1. How do stars form?
Stars form in dark clouds of dusty gas in interstellar space. The gas between the stars is called the interstellar
medium. Gravity can create stars only if it can overcome the force of thermal pressure in a cloud. Within a
contracting gas cloud becomes stronger as the gas becomes denser.

2. How massive are newborn stars?
Models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart.
Observations have not found stars more massive than about 150MSun.

3. What are the life stages of a low-mass star?
A star remains on the main sequence as long as it can fuse hydrogen into helium in its core.

4. How does a low-mass star die?
After core helium fusion stops, He fuses into carbon in a shell around the carbon core, and H fuses to He in
a shell around the helium layer. This double-shell-burning stage never reaches equilibrium—the fusion rate
periodically spikes upward in a series of thermal pulses. With each spike, convection dredges carbon up from the
core and transports it to the surface.

5. What are the life stages of a high-mass star?
Late life stages of high-mass stars are similar to those of low-mass stars: Hydrogen core fusion (main sequence)
— Hydrogen shell burning (supergiant) — Helium core fusion (supergiant)

6. How do high-mass stars make the elements necessary for life?
Big Bang made 75% H, 25% He—stars make everything else. High-mass main- sequence stars fuse H to He
at a higher rate using carbon, nitrogen, and oxygen as catalysts. A greater core temperature enables H nuclei to
overcome greater repulsion. Helium fusion can make carbon in low-mass stars. High core temperatures allow
helium to fuse with heavier elements.

7. How does a high-mass star die?
Iron builds up in the core until degeneracy pressure can no longer resist gravity. The core then suddenly
collapses, creating a supernova explosion.

8. How does a star's mass determine the life story?
A star’s mass determines its entire life story because it determines its core temperature. High-mass stars have
short lives, eventually becoming hot enough to make iron, and end in supernova explosions. Low-mass stars have
long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs.

9. How are the lives of stars with close companions different?
Stars in Algol are close enough that matter can flow from the subgiant onto the main-sequence star.

10. Choose a supernova from Wikipedia, and describe it.
Sagittarius is a constellation of the zodiac, the one containing the galactic center. Its name is Latin for the archer,
and its symbol is a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow. It lies
between Ophiuchus to the west and Capricornus to the east.