Birth of Stars
Step #1. Nebula
: A
cloud of
low-temperature interstellar material
• Attracts more particles and more mass
through growing gravitational pull
A Star is Born
Step #2. Protostar
: the density at the center
greatly increases and pressure &
temperature begin to rise (10,000 deg C)
• Hydrogen nuclei fuse to form helium
nuclei, a thermonuclear reaction
– Large amounts of radiant and thermal energy are released
• Outward radiation & gas pressure =
inward gravitational pressure
Step #3 Main Sequence Stars
• Star stays balanced between fusion and
gravity
– Larger stars use up H2 faster = shorter life span – Smaller stars use up H2 slower = longer life span
Step #4 The Beginning of the end
• As helium increases the star becomes
hotter
– Harder to fuse helium
– Needs more gravity and heat
Step #5 The End Most the Time
• Main sequence stars
(97% of time in milky way)– Red giant core explodes in nova – Planetary nebula forms
The End Sometimes
• Large stars (~9-20x solar masses in main sequence)
– Red Supergiant Core explodes in supernova – Planetary nebula forms
– Gasses collapse into very dense neutron star (1.5x mass of sun with 12mi diameter)
• Extremely large stars (>20x solar masses in main sequence)
– Red Supergiant Core explodes in supernova – Planetary nebula forms
Videos of Life Cycle:
•
http://www.nasa.gov/mov/196759main_04
8 to 19 solar mass
More than 20 times solar mass
.5 to 8
Star Brightness
• How bright a star is called its luminosity
• Star brightness as it appears from Earth is
called “apparent magnitude”
• Three factors control brightness:
– Size of star – How HOT it is
Measuring Distance to Stars
• Parallax is most basic way to measure,
– Uses slight shifting in the position of a star due to orbital motion of Earth
– Nearest stars have largest parallax angles, but angles are extremely small
– Distances are measured in
