Classification Of Stars Flashcards
Luminosity
rate of light energy released / power output of a star
Intensity
this is the power received from a star (luminosity) per unit area (W m-2).
intensity of a star follows the inverse square law
Apparent magnitude
how bright the object appears in the sky, therefore this will depend on a star’s luminosity and distance from the Earth.
Hipparcos scale
classifies astronomical objects by their apparent magnitudes, with brightest stars given apparent magnitude of 1, and faintest stars given apparent magnitude of 6.
intensity of a magnitude 1 star is 100 greater than a magnitude 6 star.
The Hipparcos scale is logarithmic, as the magnitude changes by 1, the intensity changes with a ratio of 2.51.
Absolute magnitude
what its apparent magnitude would be if it were placed 10 parsecs away from the Earth.
Equation for magnitudes
m − M = 5log( d/10)
d is the distance in parsecs
Parallax error
apparent change of position of a nearer star in comparison to distant stars in the background, as a result of the orbit of the Earth around the Sun.
property is measured by the angle of parallax (θ)
The greater the angle of parallax, the closer the star is to the Earth.
Astronomical Unit (AU)
average distance between the centre of the Earth and the centre of the Sun.
1 AU = 1.50 × 10^11 m
Parsec (pc)
The distance at which the angle of parallax is 1 arcsecond (1/3600th of a degree).
This distance can also be described as the distance at which 1 AU subtends an angle of 1 arcsecond.
1 pc = 2.06 × 105 AU = 3.08 × 1016 m = 3.26 ly
Light year (ly)
The distance that an EM waves travels in a year in a vacuum. 1 ly = 9.46 × 1015 m
To find d in magnitude equation
To find d use trigonometry. tan θ = opp → tan θ = r → d = θr As tan θ ≈ θ for small θ
Where d and r are in metres and θ is in radians.
Using the equation above you can find d in parsecs,.
d = 1/θ
Where d is in parsecs and θ is in arcseconds.
Parallax diagram
Black body radiator
is a perfect emitter and absorber of all possible wavelengths of radiation.
➔ Stars can be approximated as black bodies.
Stefans law
the power output (luminosity/P) of a black body radiator is directly proportional to its surface area (A) and its (absolute temperature)4 .
P =σAT^4
Where T is the absolute temperature and σ is the Stefan constant (5.67 x 10- 8 W m- 2 K- 4) .
Weins displacement law
the peak wavelength (λmax) of emitted radiation is inversely proportional to the absolute temperature (T) of the object.
The peak wavelength (λm ax) is the wavelength of light released at maximum intensity.
λmaxT = constant = 2.9 × 10−3m K Where the unit mk is metres-Kelvins
Weins displacement law explanation
Wein’s law shows that the peak wavelength of a black body decreases as it gets hotter meaning frequency increases so energy of wave increases
law can be used to estimate temperature of black-body sources.
Black body curves graph
follows Wein’s law, as the temperature of the body increases, the peak wavelength decreases.
intensity of light emitted by a star follows the inverse square law, meaning intensity is inversely proportional to the distance between the star and the observer.
shown in the following formula:
I=P /4πd^2
P is the power output by the star and d is the distance from the star.
Stellar spectral classes
Stars classified into spectral classes based on the strength of absorption lines.
absorption lines dependent on the temperature of the star.
This is because energy of particles which make up star is dependent on its temperature.
Hydrogen balmer lines
absorption lines that are found in the spectra of O, B and A type stars.
-caused by excitation of hydrogen atoms from the n = 2 state to higher/lower energy levels.
-if temperature of a star is too high, majority of hydrogen atoms will become excited to higher levels than n=2 or electrons might even become ionised, so hydrogen balmer lines will not be present.
-if temperature of star too low, hydrogen atoms unlikely to become excited, or may not be present at all, so hydrogen balmer lines will not be present.
-therefore intensity of hydrogen balmer lines is dependent on temperature.
Spectral class table
