Study Guide for Test #3

Dr. J. R. Webb

Stellar Astronomy



Chapter 17.  The Nature of Stars.


(Note:  The Sun’s apparent magnitude from Earth is –26.5, its apparent brightness would be +4.8 if it were 10 parsecs from Earth, thus its absolute magnitude is +4.8.

Closest other star – proxima Centauri – p = 0”.75 ~ 20 parsecs.



O             B                 A              F                G                     K                    M

Hot                                                                                                             Cool

A stars spectrum is included in one of these categories based on the presence, absence, and strength of various spectral lines.  (example:  The Balmer lines of hydrogen or the H and K lines of Calcuim). (See Figure 19-10 and Box 19-4)




Key Parts of the H-R Diagram




Chapter 18.  The Birth of Stars.





1.      Cold regions in interstellar space – HI regions – l = 21-cm line due to the spin-flip of the electron in the hydrogen atom.


1.      A cloud of gas collapses by mutual gravitational attraction.

2.      Inner regions cool and further collapse –Dark nebula

3.      Internal temperatures and pressures rise and fight collapse.  This process is a balance between gravity, cooling by radiation which tends to keep internal  temp and pressure low, and heating due to gravitational collapse.  Cloud fragments into smaller pieces..

4.      Since cooling can never be efficient enough to reduce the internal pressure enough according to current theories, “Something” causes further collapse.


5.      Further collapse forms small, round Bok Globules.


6.      Globules collapse further and form proto-stars.


7.      After 1000’s of years, Tsurface = 2000-3000K, M ~ 1Msolar, R ~ 20 Rsolar, L ~ 100Lsolar.  Star now appears on the Birthline on the H-R diagram.


8.      Cocoon nebula.


9.      Masses of stars:  80 Msolar > Mstar > 0.08Msolar.


10.  Problems with Star formation- How does a single, isolated star not in a spiral galaxy form?  No shocks around?



Chapter 19 & 20  Stellar Evolution:    Stellar Evolution: The Death of Stars.

·        Low mass Stars (3-4 Msolar)


a.       Core composition changes from H to He

b.      He fuses to Carbon at 100,000,000 K.

c.       As H is depleted in core, core collapses and heats up a H shell around core begins to burn and envelope expands – Red Giant stage.

d.      Core continues to collapse until T ~ 100,000,000K.  Helium burns explosive (Helium flash) because of quantum degeneracy (all He is at same temp).

e.       Outer layers are blown off into interstellar space (planetary nebula) and hot, solid carbon core becomes a White Dwarf.



a.       Core composition changes from H to He

b.      He burning takes off smoothly due to high gravitational pressure.  So star has a core burning He to Carbon, and a shell buring H to He, and a H envelope.

c.       Onion Skin Model

·        H to He

·        He to Carbon

·        Carbon to Magnesium and Neon

·        Neon to Magnesium and oxygen

·        Oxygen to silicon

·        Silicon to iron

d.      At this point Iron burning is an endothermic reaction, it doesn’t release energy as the previous reaction, but saps the energy of the core!  This causes immediate core collapse.  As outer layers crash onto the core, the envelope material is blown off and a supernova results!

e.       If core mass is less than 1.4 Msolar the the core remnant is a white dwarf.

f.        If core mass is greater than 1.4 Msolar, neutron star.

g.       If core mass exceeds abut 3 Msolar, then core remnant is a black hole!



Chapters 21 and 22 will also be on the test, that information that we covered in class. I will work on getting mroe info on here about those chapters, but make sure you READ the chapters adn study the class notes and you will be fine.