Download Lecture 5

Final part of The Nature of the Stars
Chapter 19
• The iClickers are in the bookstore…please
get them if you want extra credit. We will
start using them next week!
• You will need to register them on the web:
http://www.iclicker.com/registration/index.html
Questions for the class (iClicker)
1.
Consider the spectral classification of stars
using only the letters A B F G K M O. When these
are arranged in order from coolest to hottest they
would be:
A.
B.
C.
D.
E.
O-B-A-F-G-K-M
M-K-G-F-A-B-O
A-B-O-F-G-K-M
M-K-G-F-O-B-A
B-A-G-K-O-M-F
Questions for the class (iClicker)
2. Using the Hertzsprung-Russell diagram above, determine which type of
star has the following characteristics: surface temperature of 40,000 K and
luminosity 100,000 times that of the Sun.
A. white dwarf
B. main sequence red dwarf
C. red giant
D. blue main sequence star
Questions for the class (iClicker)
3. The ratio of the brightness of a star at two different colors,
blue and visual –bv/bB- is a direct measure of what property of
the star?
A. distance from Earth
B. surface temperature
C. luminosity
D. radius
By carefully examining a star’s spectral lines,
astronomers can determine whether that star is a
main-sequence star, giant, supergiant, or white
dwarf
Using the H-R diagram
and the inverse square
law, the star’s luminosity
and distance can be
found without
measuring its stellar
parallax
Just from the spectra->luminosity
Classes
(the higher the number the lower
The luminosity)
Spectroscopic Parallax (no parallax involved though!)
NO MATTER
HOW
DISTANTE
A STAR IS!
Example:
Star B8 V
How much is its
Luminosity?????
Why stars have different spectral types and
luminosities?
Stars Masses
How do we determine their masses? No direct way for an Isolated Star
BUT: double stars….
Optical Double Stars (not at the same position but along the line of sight)
We are looking for binaries!
Visual Binaries
A Binary Star System
Binary Stars
• Binary stars, in which two stars are held in orbit
• around each other by their mutual gravitational
attraction, are surprisingly common
• Those that can be resolved into two distinct star images
by an Earth-based telescope are called visual binaries
• Each of the two stars in a binary system moves in an
elliptical orbit about the center of mass of the system
Some periods can
be very long!
Binary star systems provide crucial information
about stellar masses
• Binary stars are important because they allow
astronomers to determine the masses of the two stars
in a binary system
• The masses can be computed from measurements of
the orbital period and orbital dimensions of the system
How do I determine a (semimajor axis)?
Angular separation (convert to physical
distance)
For that I need to know the distance to the
binary (parallax measurements or
spectroscopic parallax)
But still this only give me M1+M2
Center of Mass
Movie…..
The center of mass lies along the line
connecting the two stars
It can be found by plotting the separate
orbits of the two stars using the background
stars as reference.
Comparing the relative sizes of the orbits
around the center of mass yields M1/M2
We know M1+M2
M1, M2
Mass-Luminosity Relation for MainSequence Stars
• Main sequence stars are stars like the Sun
but with different masses
• The mass-luminosity relation expresses a
direct correlation between mass and
luminosity for main-sequence stars
• The greater the mass of a main-sequence
star, the greater its luminosity (and also
the greater its radius and surface
temperature) (for radius: look at H-R
diagram)
Mass-Luminosity Relation
For radius: look at H-R diagram
Why Mass Control the Properties of a MainSequence Star?
• Main sequence stars are same as Sun>thermonuclear reactions
• Greater the Mass, greater the pressure and
temperature at the Core->FASTER
thermonuclear reaction->more LUMINOUS
• Models of Main-Sequence Stars: for the stars to
maintain equilibrium, more massive->larger
radius, higher surface T
(look at the HR diagram)
Spectroscopy makes it possible to study binary
systems in which the two stars are close together
•
•
•
•
Some binaries can be
detected and analyzed,
even though the system
may be so distant or the two
stars so close together that
the two star images cannot
be resolved
A spectrum binary appears
to be a single star but has a
spectrum with the
absorption lines for two
distinctly different spectral
types
A spectroscopic binary has
spectral lines that shift back
and forth in wavelength
This is caused by the
Doppler effect, as the orbits
of the stars carry them first
toward then away from the
Earth
From the Doppler Shift -> Radial Velocity
Orbital Period
Light curves of eclipsing binaries provide detailed
information about the two stars
•
•
An eclipsing binary is a
system whose orbits are
viewed nearly edge-on from
the Earth, so that one star
periodically eclipses the
other
Detailed information about
the stars in an eclipsing
binary can be obtained from
a study of the binary’s radial
velocity curve and its light
curve
Questions (iClicker)
• 4.
Absorption lines in the spectra of some binary
stars are seen to change periodically from single to
double lines and back again. Why is this?
– A. Oscillations on the surfaces of the stars leads to
Doppler-shifted lines.
– B. Periodically, the magnetic field of one star produces
Zeeman splitting of spectral lines in atoms of the second
star.
– C. The effect of the gravitational field of one star on the
atoms of the second star produces spectral line shifts
periodically.
– D. Motion toward and away from Earth during their
orbital motion results in Doppler shift of light from these
stars at times and no shift when the stars are moving
perpendicular to the line of sight.
Light Curves of Eclipsing Binaries
Light curve has a different shape due to tidal interactions
Light curves can yield information about ratio of surface temperatures,
The duration of an eclipse can tell astronomers about the relative sizes and their
Orbits.