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.
Study Guide for Test #3
Dr. J. R. Webb
17. The Nature of Stars.
magnitude (m) How bright an object appears to be to an observer.
the change in magnitude, DM
, is 1, then the difference in brightness DB,
is 2.512. In general, DB
= (2.512)DM (e.g. DM = 5,
DB = (2.512)5 =
Magnitude (M) - The magnitude
a star would be if it were exactly 10 parsecs away. M = m 5log(d)
+5, where d is the distance in parsecs.
Suns 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.
parallax -The distance d(parsecs) = 1./parallax angle in arc seconds.
Closest other star proxima
Centauri p = 0.75 ~ 20 parsecs.
The energy output per unit time.
Indices U B = mu mv, and is called the U-B
color index. It measures the
amount of ultraviolet light relative to the amount of Blue light emitted
by a star. B-V = mb mv
is the B-V color index. (Note:
you can use either apparent magnitudes m, or absolute magnitudes M to
compute the color index, since they are differences, the distance
cancels out.) A graph of B-V
versus temperature allows one to determine the temperature of a star by
measuring just the mb and mv.
Types Originally Harvard astronomers classified stellar spectra into
categories A through Q, but upon further study, realized that when they
spectra arranged according to temperature, the classes became:
A F G K M
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)
of Absorption lines: See
Figure 19-11. You MUST
understand this table. Line
strength means, how visible or how dark the lines are compared to the rest
of the spectrum. Spectral type is
Hertzsprung-Russell Diagram (H-R diagram)
IMPORTANT!!! Plot absolute magnitude versus
color index (or temperature or spectral type). Every phase of a stars evolution can be traced on this
Key Parts of the H-R Diagram
- Main sequence stars converting H
to He, in hydrostatic equilibrium.
Middle age of star.
Luminosity class V, also called dwarfs.
- White Dwarf region cores of dying
stars. (no luminosity class)
- Sub Giants highly evolved low
mass stars. Luminosity class IV.
- Giants highly evolved stars. Luminosity Class III.
Giants Highly evolved massive stars. Luminosity Class II.
Giants Highly evolved massive stars. Luminosity class I.
Sequence is a Mass relationship massive stars, upper left, less
massive stars as you go toward lowert right-hand side of HR diagram.
Radius Larger stars are brighter for a given temperature, thus occur
higher on the H-R diagram than smaller stars. (e.g. White Dwarfs versus
Stars used to determine stellar masses by applying Keplers 3rd Law.
(refer to chapter 3, P2
= [ (4p2)/G(M1+M2)]a3
where P is sidereal period and a is the semimajor axis.).
Doubles two stars that appear to be physically connected but are
really far apart.
binaries can be seen as two distinct stars from Earth.
binaries star systems where stars are to close together to be
resolved from Earth, but can be detected by their spectral lines that
shift as the stars orbit each other (Doppler shifts).
binaries When the stars orbital plane is parallel to the line of
sight, thus we see the stars eclipse each other.
we get from Binaries: relative masses of stars from radial
velocity curve, relative sizes of stars and relative brightnesses
Sequence Lifetimes Low mass. Later spectral types, G, K M, live
longer than more massive O,B or A stars.
Hot stars have more fuel, but burn hotter and use their fuel much
faster than low mass stars.
18. The Birth of Stars.
GRAVITY VERSUS INTERNAL
depends solely on the mass of the star.
(gas and radiation pressure)
of Star Formation
regions in interstellar space HI regions l
= 21-cm line due to the spin-flip of the electron in the hydrogen atom.
regions , molecular hydrogen only forms in very cold regions but radiates
very little. Fortunately, tracer
CO radiates at l = 2.6 mm and
ratio is: H2/CO =
- Dark Nebula cold dark regions usually
embedded in HII regions.
Globules small, round nebula r ~ 1-2 ly, M ~ 20-200 Mo, T ~
Big Picture of Star Formation
A cloud of gas collapses by mutual gravitational attraction.
Inner regions cool and further collapse Dark nebula
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..
Since cooling can never be efficient enough to reduce the
internal pressure enough according to current theories, Something causes
waves from other, newly formed stars
waves from supernova explosions
waves from galactic spiral arms.
Further collapse forms small, round Bok Globules.
Globules collapse further and form proto-stars.
After 1000s of years, Tsurface = 2000-3000K, M ~
1Msolar, R ~ 20 Rsolar, L ~ 100Lsolar. Star now appears on the Birthline
on the H-R diagram.
Masses of stars: 80 Msolar
> Mstar > 0.08Msolar.
Problems with Star formation- How does a single, isolated star
not in a spiral galaxy form? No shocks
19 & 20 Stellar Evolution: Stellar Evolution: The Death of Stars.
Low mass Stars (3-4 Msolar)
Core composition changes from H to He
He fuses to Carbon at 100,000,000 K.
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.
Core continues to collapse until T ~ 100,000,000K. Helium burns explosive (Helium flash)
because of quantum degeneracy (all He is at same temp).
Outer layers are blown off into interstellar space (planetary
nebula) and hot, solid carbon core becomes a White Dwarf.
- High Mass Stars (over
Core composition changes from H to He
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.
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
At this point Iron burning is an endothermic reaction,
it doesnt 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!
If core mass is less than 1.4 Msolar the the core
remnant is a white dwarf.
If core mass is greater than 1.4 Msolar, neutron
If core mass exceeds abut 3 Msolar, then core
remnant is a black hole!