Lecture 12: Stars Flashcards
Stellar Parallax
- The motion of earth around the Sun causes an apparent motion of nearby stars relative to more distant background stars
- stars appear to move in a loop around the sky once per year
Trigonometric Parallax
- nearby stars have larger parallax
- 1 parsec is 206,265 A.U or 3.26 light years
Brightness vs Luminosity
- brightness is where total light energy is collected from an object
- brightness depends on distance
- can also be affected by something that blocks the line of sight
- Luminosity is total light emitted by the object
- depends only on the properties of the emitting object
The Magnitude System
- Larger number means fainter
- first magnitude star is 100 times brighter than a sixth magnitude star
- each magnitude is a factor 2.5 times fainter
- scale reflects how the eye responds
The colours of stars
- A star’s colour is a direct measure of its temperature
- Processes that can change the colour of the light
- The sun is green at its brightest spot
Stars and Blackbodies
- Stars are nearly blackbodies but not quite
- spectral lines cause a star’s spectral shape to deviate from blackbody curve
- a star’s spectrum tells us more about the star then just surface temp
Stellar Spectra
- Stars can produce all three kinds of spectra (continuous, emission line, absorption line)
- Most produce absorption spectrum
- strength of lines tells us about the density and temperature where lines are formed and also amount of elements present
- Hot stars dominated by H and He lines while cool stars dominated by metals and molecules
- Classification system for spectra include a letter followed by a number and a roman numeral (Sun is G2V)
- Brown dwarfs have been found to not fit the classification system
Spectra vs. Photometry
- Photometry is better for large scale surveys, spectra is better for more detailed information
- both used in astornomy
Hertzsprung-Russell Diagram
- Spectral type or temp or colour on x-axis
- Luminosity on y-axis
- most stars either in the main sequence or a branch off it
- Luminosity, Temperature, and Radius are all related
- If temp is constant and 2 stars have different luminosities, they have to have different radii’s
- Nearest stars are mostly on the main sequence and are faint red and cool
Binary Stars
- can obtain stellar masses and radii from binary stars
- detected visually, spectroscopically, and via eclipses
Possible Binary Star Cases
Visual Binary
- both stars visibly orbiting each other
- Need to know distance to determine true masses
Astrometric binary
- only one star is visible and moves around in a loop or a wave
Spectroscopic Binary
- Only one star and does not display motion, but a regular periodic motion can be detected from doppler shifts from it’s spectrum
- periodic changes in spectral line wavelengths
Eclipsing Binary
- One star passes in front of the other can blocks light causing a change in brightness we detect
Mass-luminosity relationship of stars
- More than 90% of all stars have mass smaller than the mass of the sun
- Luminosity is proportional to mass to the power of 3.5
- More mass, more luminous
Stellar Motions
- measurable components are radial velocity and transverse velocity
- transverse velocity only measurable if moving fast and close to us
- more motion means stronger gravity which means more mass
