Stellar Classification Flashcards
What is apparent magnitude (m)?
A measure of how bright a star appears in the sky
What is absolute magnitude (M)?
The apparent magnitude a star would have if it was 10 parsecs away
What is the Hipparcos scale?
Scale of apparent magnitude of 1 → 6
- 6 → naked eye limit
- Each step = x2.51
How does a 4th magnitude star compare to a 7th magnitude star?
3 magnitudes difference
So apparent magnitude (m) = 2.513 = 15.8 x brighter
(4th magnitude is brighter)
How could star B be bigger than star A?
Star B could be much further away
So appear dimmer from Earth
What does apparent magnitude correspond to?
The intensity of the star from the surface of the Earth
What does absolute magnitude correspond to?
The power output of the star
Define the lightyear
The distance light travels in a year
Define the parsec
Distance to star with parallax angle of 1”
A distant star has a parallax angle of 3”
Calculate how far away it is?
Make a right angled triangle
What information does m-M provide? (magnitude difference)
The smaller m-M → the closer the object
If m-M < 0 …
If m-M < 0 → star less than 10pc
If m-M = 0 …
If m-M = 0 → star at 10pc
If m-M > 0 …
If m-M > 0 → star further than 10pc
What’s wrong with this calculation?
d must be in parsecs!!!
Which star appears brighter?
Betelgeuse → has a smaller apparent magnitude (m)
Which star has a greater power output?
Betelgeuse → has a smaller absolute magnitude (M)
Which star is closer?
Bellatrix → m-M is smaller
What is the Inverse Square Law?
What does the Hertzsprung-Russel Diagram show?
Luminosity (or Absolute Magnitude) of star
vs
Surface Temperature (or Spectral class)
What are the 3 main regions of the Hertzsprung-Russel Diagram?
As the Sun runs out of Hyrdogen in the core how does it progress along the Hertzsprung-Russel Diagram?
- Becomes a Red Giant
- Becomes a White Dwarf (after Plaetary Nebula)
How can Red Giants be incredibly luminous but have a Low surface temperature?
They have a massive Surface Area
(Stefan’s Law)
How can White Dwarfs be not very luminous but have a very high surface temperature?
They have a tiny Surface Area
(Stefan’s Law)
What is a perfect Blackbody?
An object that absorbs and re-emits all incident radiation (eg radiators, filament bulbs and stars)
How is the** Blackbody Spectrum** different for a star with a greater surface temperature?
- Peak moves up and to left (shorter wavelength)
- Greater Intensity across all Wavelengths
- Shorter starting wavelength (x-intercept)
What are the 7 Stellar Spectral Classes (in Temperature order)?
What are the surface temperature ranges for the 7 Stellar Spectral Classes?
How does a star produce an absorption spectra?
Colder outer layers of star absorb some wavelengths of radiation produced in the core
Why is the Balmer Series most visible in the Absorption Spectra of A type stars?
Surface temperature hot enough for hydrogen electrons to be in n=2 state
But not too hot that electrons excite to higher states or for hydrogen to be ionised
Describe the strength of the Balmer Series Absorption Lines for each of the Spectral Classes
What is the lifecycle of a regular Star?
(<1.4xMsun)
What is the lifecycle of a bigger Star?
(>1.4xMsun)
When and how does a Red Giant form?
- At end of main sequence hydrogen in core runs out
- Gravitational pressure > Radiation Pressure
- Core shrinks heating up (GPE -> Thermal)
- Outer layers expand
- Core gets hot enough for He to fuse
