Stars and Stellar Structure Flashcards
Name 4 ways we name stars
- constellations (groups of stars)
- full names (bright stars have proper names)
- Labelled within their constellations by Bayer letters or Flamsteed numbers
- Catalogue numbers
What is a bayer letter and what is Flamsteed number?
- Bayer letter = order of brightness
- Flamsteed number = west to east
How do we describe the brightness of stars?
By using the magnitude system
What are the main features of the modern magnitude system?
- lower numbers mean brighter stars
- dimmest stars visible to naked eye are around magnitude 6
- Stars can have non-integer magnitudes
- A logarithmic system
- A difference of 5 magnitudes means a brightness ratio of exactly 100
- A difference of 1 magnitude means a brightness ratio of ~ 2.5
What is the inverse square law?
Light from stars spreads out as it travels towards us
What is the annual parallax
- a way to calculate the distance to a star
- as the earth orbits the sun, close stars appear at different positions in the sky
- the change in the stars is measured by the parallax angle
- we can then use trig to find the distance
- distance (parsecs) = 1/parallax (arc seconds)
What is proper motion?
Change in star’s position due to this relative motion
- close star would have higher proper motion compared to a distant star if had same velocity
What is the star with the greatest proper motion?
Barnards Star
What do we use to measure how bright a star is more quantitatively?
Luminosity - rate of energy production
What is luminosity measured in?
Watts
Measuring brightness: Star a and star b are identical. Star A is 3pc away and star B is 6pc away. How much brighter does star A appear than star B?
The distance of Star B is twice of star A. Since the intensity changes with distance squared the factor of difference is 2^2 = 4 times.
How do we use the inverse square law to measure brightness?
- luminosity of a star spreads out in space as distance of the star increases
- intensity is inversely proportional to 1/area
- 1/area is inversely proportional to 1/R^2
What is the difference between Absolute magnitude and apparent magnitude?
Absolute = magnitude of a star if we observed it at a distance of 10 parsecs
Apparent = magnitude as observed from earths
What is the absolute magnitude equivalent to?
the luminosity of stars
What is distance modulus?
the difference in a star’s apparent and absolute magnitudes
A star has an apparent magnitude of m=8 and a distance modulus of 5. What is its absolute magnitude
3
What wavelength gives blue colour? what wavelength gives a red colour?
blue = 400nm
red = 700nm
How is white light shown?
White light is a combo of different wavelengths
how do we measure the brightness of light at different wavelength?
use detectors called spectrometers
What is a black body?
- object which perfectly absorb all radiation (light)
- but also re-radiates at the same rate, keeping its temp constant
As the temp of a star increases….
- the intensity grows
How hot is a hot star? how hot is the sun? a cool star?
Hot star = 10000 k
Sun = 5800 k
cool star = 3000k
What is filter photometry?
- technique that isolates specific wavelengths of light and measures their intensity
How can the temp of a star be found?
by examinining their black body spectra
what colour do hot stars emit? and cool stars?
hot = blue
cool = red
what is the colour index of a star related to?
its temperature
How do we determine the physical size of a star?
By finding its radius via the Stefan-Boltzmann Law
what is the Stefan-Boltzman Law
(constant) x (temperature)^4
𝐿 = 𝐴𝜎𝑇^4
Example of Stefan-Boltzman Law: An increase in temperature of 20% results in an increase luminosity of….
2.07 - 1.2 x 1.2 x 1.2 x 1.2
This is because 20% increase = 1.2
How do we use the Stefan-Boltzman law to find the radius?
If we know T and L, then we can figure out the A in the law. From hear using the formula for the surface area for a sphere, we can find the radius ( 𝐴 = 4𝜋𝑅^2)
We want to find out how big a star’s radius is. What measurements would we need to do this?
The star’s annual parallax, apparent magnitude, and B – V colour
index
What are binary stars
Two stars in are orbiting each other around a center of mass. around 50% of Stars are binary Stars
Name the four types of binary stars
- Astrometric
- Eclipsing
- Visual
- Spectroscopic
What is Visual binary?
Were we can see the two stars orbiting each other, some can be seen with he naked eye but most with binoculars/telescopes
What is Astrometric binary?
Like visual binary but one star is far too faint to see, but you can still see the wobble of the bright star
What is an eclipsing binary?
From “side view” one star passes between the other and the observer. Like when the moon and sun eclipse.
What is a spectroscopic star?
Doppler shifts in the spectrum of a star reveal a companion
Measure the spectrum of
the bright star over time
The Doppler shift changes
periodically
Indicates orbiting
companion
