Small Mass Newborn Stars
Fusion and Contraction
● Fusion will not begin in a contracting cloud if a force stops contraction before the core
temperature rises to about 10 million K.
● Thermal pressure cannot stop contraction because the star is constantly losing thermal
energy from its surface through radiation.
● Degeneracy Pressure: the laws of quantum mechanics prohibit two elections from
occupying the same state in same place.
● Thermal Pressure:
○ Depends on heat content
○ Is the main form of pressure in most stars
● Degeneracy Pressure
○ Particles can’t be in same state in the same place.
○ Doesn’t depend on heat content.
● Degeneracy pressure halts the contraction of objects with <0.08Msun before core
temperature becomes hot enough for fusion.
● Starlike objects not massive enough to star fusion are brown dwarfs.
● Abrown dwarf emits infrared light because of heat left over from contraction.
● Its luminosity gradually declines with time as it loses thermal energy.
Brown Dwarfs in Orion
● Infrared observations reveal recently formed brown dwarfs because they are still
relatively warm and luminous.
Large Mass Newborn Stars
● Photons exert a slight amount of pressure when they strike matter.
● Very massive stars are so luminous that the collective pressure of photons drives their
matter into space.
Upper Limit on a Star’s Mass
● Models of stars suggest that radiation pressure limits how massive a star can be without
blowing itself apart.
● Maximum thought to be around 150Msun, but new observations indicate some may be
● Stars more massive than 150Msun would blow apart.
● Stars less massive than 0.08Msun can’t sustain fusion.
Demographics of Stars
● Observations of star clusters show that star formation makes many more low-mass stars
than high-mass stars.
REVIEW ● Degeneracy pressure stops the contraction of objects <0.08Msun before fusion starts.
● Stars greater than 150Msun would be so luminous that radiation pressure would blow
● Star formation makes many more low-mass stars than high mass stars.
Chapter 17: Star Stuff
Stellar Mass and Fusion
● The mass of a main-sequence star determines its core pressure and temperature.
● Stars of higher mass have a higher core temperature and more rapid fusion, making those
stars both more luminous and shorter-lived.
● Stars of lower mass have cooler cores and slower fusion rates, giving them smaller
luminosities and longer lifetimes.
Types of Stars
● High mass stars > 8Msun
● Intermediate Mass Stars
● Low-Mass Stars 0.08 - 2 Msun
● Brown Dwarfs
Star Clusters and Stellar Lives
● Our knowledge of life stories of stars comes from comparing mathematical models of
stars with observations.
● Star clusters are particularly useful because they contain stars of different mass that were