Chapter 13- Taking the Measure of Stars Flashcards
Parallax
Apparent motion of a relatively nearby object seen against a background of distant objects and caused by the motion of the observer
Refers to the motion of a relatively nearby star caused by Earth’s motion around the sun
Figure 13.3
Measured in parsecs
Inverse Square Law
Luminosity- total energy produced by each star
Brightness- energy per second recieved by each unit area of your detector
Brightness decreases like 1/d2 as it travels across a large area, because the luminosity is spread further apart
Magnitude Scale
Hipparchus
Perceived intensity (what your eyes see) correspond to a factor of 100 in brightness
Equal intervals of energy correspond to equal ratios of energy. A single step in magnitude increases by 2.512 (5th root of 100).
A fifth magnitude star is thus 2.512 times brighter than a 6th magnitude star, etc.
Magnitude only measures what you see, not the actual energy output of the star (luminosity)
Magnitudes and Distance
Absolute magnitude- magnitude a star would have if it were 10 pc away
d 10= 10m-M/5 +1
Binary stars
We get most of our information on stars by observing binaries. Most stars besides the Sun are binaries
Optical double
Stars that appear close together because they are in the same line of sight. Not true binaries
Visual binaries
Both orbits are visible.
Both stars orbit in ellipses
Because m1a1=m2a2 where the a’s are the semi major axes of the 2 ellipses, observation of the two orbits gives the ratio of the 2 star masses.
The sum of the 2 masses can be obtained from Newton’s version of Kepler’s 3rd law.
These 2 equations allow for a calculation of stellar mass, the only direct way we have. We have found that stars have masses ranging .8 times the mass of the Sun (80 times the mass of Jupiter) to perhaps 150 times the mass of the Sun.
Eclipsing binaries
One star passes in front of the other from Earth’s vantage point.
Allows us to calculate a star’s diameter.
If you plot brightness as a function of time, you will get a light curve. If you measure the time for one star to disappear behind the other, you can calculate the diameter.
Only direct means to calculate the size of a star. Star sizes run from about a hundreth the size of the Sun (about the size of Earth) to perhaps a thousand times the Sun
Astrometric binaries
One star’s orbit appears to wobble because of the unseen companion
Spectroscopic binaries
Both stars are invisible, but the spectrum shows 2 types of stars which show Doppler motion back and forth
Hertzprung Russell Diagram
Brightness: b = L / (4 pi d 2). If we know the distance to the star we can calculate L.
Temp can be calculated by Wein’s Law. As the surface temp of a star increases, the peak wavelength in its spectrum shifts to shorter wavelengths.
HR Diagram shows that temp and stellar radius are related
Spectral classes of stars
Classification of stars by temperature, strength of hydrogen lines. Higher letters have higher temperatures, stronger hydrogen lines
