Lecture 20 Flashcards
What does an H-R diagram compare?
An H-R diagram is a plot of luminosity vs temperature
- hot, bright stars are in the top left corner
- cool, dim stars are in the bottom right corner
What is the problem with apparent brightness?
A star of a given apparent brightness could either be very bright but far or dim but close
How can absolute brightness be determined by parallax distance?
- Measure the apparent movement of stars over a year
- The smaller the parallax angle the greater the distance
How does apparent brightness compare to luminosity?
- Apparent brightness depends on the distance to the observer
- Luminosity is the actual power output of an object and is an intrinsic property
What is the stellar luminosity formula?
Apparent brightness= luminosity/ 4pi*distance^2
What are the two ways to measure the temperature of stars?
- The peak of the thermal spectrum- Wien’s Law
- Spectral lines
How can spectral lines be used to determine temperature?
More complex atoms have more electrons and are more fragile. Therefore, fragile types are easily destroyed by collisions in high temperature.
Signs of fragile atoms (absorption lines)= low temperature star
What is the newly devised star classification?
OBAFGKM
How does temperature affect stars?
- surface temperature is the big factor in star classification
- higher temperatures= more atoms ionised and more molecules dissociated
- cooler temperatures= more atoms and molecules that can withstand the conditions
Where do stars sit on the OBAFGKM
- O,B= hottest, bluest
- A,F,G= middle type, yellow
- K,M= coolest, reddest
What are the 4 main star classifications?
- main sequence- a stripe of dim+cool to bright+hot stars
- giants- moderately bright but cool
- supergiants- extremely luminous and T<10,000K
- white dwarfs- very dim but hot
How can stars be orange but very bright?
They are bigger
What are the two features luminosity depends on?
Temperature and surface area
What is the luminosity, radius, temperature proportionality?
L ∝ R^2 ∝ T^4
