exam 2 Flashcards
Luminosity
total amount of energy at all wavelengths that a star emits per
second.– absolute brightness - is a property of the star. It is a measure of the total
power radiated from the star (wattage). Absolute brightness does not change with distance.
apparent brightness (luminosity/distance^2)
the amount of a star’s energy that reaches a given area each
second here on Earth– how bright a star appears to be. This depends on the
distance away and the absolute brightness.
For two stars of the same apparent brightness, the star closer to the Sun will generally have
. a lower luminosity
Magnitude Scale
Magnitude 1 stars are 100 times brighter than magnitude 6 stars.
Thus each division on the magnitude scale changes brightness by about a
factor of 2.5.
Apparent Magnitudes
Use of telescopes to see brighter and fainter
objects has expanded the scale.
Note - negative apparent
magnitudes are brighter!
Changing the distance of a star by a factor of 10
decreases it brightness by a factor of 100 - thus
increases it apparent magnitude by 5 units.
Magnitude equation
apparent magnitude- absolute magnitude = 5log(distance/10 parsecs)
A star has an apparent magnitude 10.0 and an absolute magnitude 2.5.
How far away is it? How does its luminosity compare to the Sun’s?
10-2.5=5log(distance/10)->10^1.5=distance/20-> 316
Which of the following quantities do you need in order to calculate a star’s luminosity?
apparent brightness & distance to the star
Colors of Stars
Stars are not all the same color because they do not all have identical
temperatures. the hottest stars have temperatures of over 40,000 K, and the
coolest stars have temperatures of about 2000 K. Our Sun’s
surface temperature is about 6000 K
Stellar Spectra
more informative than blackbody curves Measuring colors is only one way of analyzing starlight. Another way
is to use a spectrograph to spread out the light into a spectrum
O B A F G K M
Using Stellar Spectra
Analyzing the spectrum, we can infer:
* Star size
* Composition
* Radial velocity
* Rotation
Stellar Spectra - size
A giant star has a large, extended photosphere, thus the gas will have a low
density, therefore, the pressure in the photosphere will be low as well!
A star with a lower-pressure photosphere shows
narrower spectral lines than a star of the same
temperature with a higher-pressure photosphere
More atoms are ionized in a giant star than in a
star like the Sun with the same temperature, and
ionized atoms have different spectra from atoms
that are neutral.
Stellar Spectra - composition
Absorption lines of a majority
of the known chemical
elements have now been
identified in the spectra of
the Sun and stars. If we see
lines of iron in a star’s
spectrum, for example, then
we know immediately that
the star must contain iron.
Stellar Spectra - velocity
Doppler effect of spectral lines yields clues about the relative velocity of the star
We should see all the spectral lines of
moving stars shifted toward the red end of
the spectrum if the star is moving away
from us, or toward the blue (violet) end if it
is moving toward us
proper motion
proper motion, is transverse: that is, across our line of
sight.
Stellar Spectra - rotation
We can also use the Doppler effect to measure how fast a star rotates
If an object is rotating, then one of its sides is approaching us while the
other is receding rotating toward us are shifted to shorter
wavelengths and the lines in the light from the
opposite edge of the star are shifted to longer
wavelengths.
A stellar census
there are
many more lowluminosity (and hence low
mass) stars than highluminosity ones. Only three of
the stars in our local
neighborhood
Measuring stellar masses
The mass of a star - how much material it contains - is one of its most important
characteristics
binary stars
The first binary star was discovered in 1650. About half the stars are binary
stars, two stars that orbit each other,
bound together by gravity. Masses of
binary stars can be calculated from
measurements of their orbits can be seen with
a telescope is called a visual
binary. but which spectroscopy
shows really to be a double star, is
called a spectroscopic binary.
Masses of binaries
We can estimate the masses of binary star systems using Newton’s
reformulation of Kepler’s third law
Mass-Luminosity relation
The more massive stars are generally also the more luminous The mass-luminosity
relation reads:
L ∝ M^3.9
A binary star system:
Is composed by two stars
Is held together by gravity
Is useful to find the mass of the component stars
Is not always visually detectable
Stellar size
observe the dimming of light that occurs
when the Moon passes in front of a star Accurate sizes for a large number of stars come
from measurements of eclipsing binary star
systems: stars that are lined up in such a way
that, when viewed from Earth, each star passes
in front of the other during every revolution
Radiation law:
Another method for measuring star diameters makes use of the StefanBoltzmann law for the relationship between energy radiated and temperature
Most nearby stars are roughly the size of the Sun, with typical diameters of
a million kilometers or so. Faint stars are generally smaller than more
luminous stars. However, there are some dramatic exceptions to this
simple generalization.
F = σT^4
