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Lecture 14

Astronomy 1021 Lecture Notes - Lecture 14: Brown Dwarf, Degenerate Matter, Protostar


Department
Astronomy
Course Code
ASTR 1021
Professor
Sarah Gallagher
Lecture
14

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Jan 28th
Astronomy- Last before 2nd test
Q. During which stage would a planet be forming around star?
A. After fusion starts, but before the protostellar disk is gone
Q. Compared to stars like the sun, how common are massive (10, 20, 30 solar mass
stars)?
A. Much Less common
- Stars more massive than 150Msun would blow apart
- Stars less massive than 0.08Msun cant sustain fusion
-Observations show that star formation makes many more low-mass stars than high-
mass stars
-Brown dwarfs are starlike objects not massive enough to start fusion
- Masses < 0.08Msun
-Degeneracy pressure halts contraction before core temperature becomes hot
enough for fusion
-If Jupiter were not a planet orbiting the sun it would be considered a Brown Dwarf
- In very small stars, degeneracy pressure halts contraction before fusion stars
-Laws of quantum mechanics prohibit two electrons from occupying the
same state in same place
-Brown Dwarfs shine faintly: they emit infrared light because of heat left over from
contraction
-Luminosity gradually declines with time as the brown dwarf loses thermal
energy
- Young brown dwarfs are easier to see: still relatively warm and luminous
-Find them with infrared observations: brown dwarfs in Orion
-Radiation pressure limits how massive a star can be without blowing itself apart
How massive is too massive?
-Radiation from fusion can overcome gravity
-Eta Car is literally blowing itself apart
-100,000x lum of the sun
-100,000x the mass of the sun
-8000LY away
Stellar Evolution
Key points
- Mass is density
- Pressure vs. gravity (again and again)
- Energy from fusion luminosity (as long as fusion is occurring)
- Motion in HR diagram (changes in luminosity and temperature) over the
lifetime of a star
The end of a solar mass star
- All the He that gets hot enough fuses to C, and that the end of the fusion story
oGravity wins (again)
-Core: Carbon supported by degeneracy pressure
-Outer Layer: Drift Away (a.k.a planetary nebula)
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