Astrophysics Part 2 : Classification Of Stars ( With Distances And Magnitude) Flashcards
What is apparent magnitude and absolute magnitude?
Also what is the equation associated with it?
Apparent magnitude (m) - how bright the star appears from Earth
Absolute magnitude (M) - how bright the star would appear if it were placed 10 parsecs from earth.
m - M = 5log(d/10)
What is the Hipparcos scale? Also what is the difference in intensity of one magnitude?
The Greek astronomer Hipparchus catalogued stars, defining their brightness in terms of apparent magnitude (m), with the brightest stars a magnitude of 1 and the faintest a magnitude of 6.
It has since been extended to include brighter objects like the sun, with an m of -26.47.
Magnitude 1 star is about 2.51 times brighter than a magnitude 2 star.
Remember that brightness is a subjective scale of measurement.
Define parsec
The distance to an object that subtends an angle of one arc second (1/3600th of a degree) to 1 AU. (Image in a triangle where adjacent side is 1 parsecs away and the opposite side is 1AU).
Define light year
Distance travelled by light in a vacuum in one year.
What is the luminosity of a star?
The total energy emitted in the form of EM radiation each second.
What is the intensity of an object we observe?
The power received from it per unit area at earth. This is the effective brightness of an object.
What does the brightness of a star depend on?
It’s luminosity and distance from us. So the brightest stars will have a high luminosity and closest to us.
How do I calculate the brightness (intensity) ratio between two stars?
IA / IB = 2.51^(mB - mA)
m = apparent magnitude oeither star 1 or 2
What is astronomical unit defined as?
mean distance between earth and the sun
What is stefans law and state what the values are?
The power output of a star (luminosity) is directly proportional to its surface area and its (absolute temperature)^4.
P = σAT^4
A = surface area
T = temperature in kelvin
σ = stefan constant
Used to compare the power output, temperature and size of stars.
State Wien’s displacement law
The wavelength of a stars’s emission at peak intensity is inversely proportional to is absolute temperature.
λmaxT = constant = 2.9. 10^-3 mk
mk = metre-kelvin not millikelvin
Equation can be used to calculate either max wavelength of temperature on black body radiation curves.
How do we calculate intensity of the radiation received from a star’s emission from earth?
I = P/4πd^2
P = power output of star in watts
d = distance from the star in metres
What is a black body?
A black body absorbs electromagnetic radiation of all wavelengths and can emit electromagnetic radiation of all wavelengths.
It does not reflect any radiation - it absorbs all radiation incident on it.
A star should assumed to be a black body
Draw 3 black body radiation curves for 3 black bodies of decreasing temperature.
Graph of intensity (power radiated) against wavelength. Check cgp or pmt
Where is the sun located on the Hertzsprung-Russell diagram?
It’s a main sequence star and its spectral class is G, absolute magnitude is 4.83. Therefore it can be located on the diagram.
What is the evolutionary path of a sun-like star (a main sequence star) on a HR diagram?
Draw it
Once main sequence star uses up all the hydrogen in its core, it will up to the red giant section. (Brighter and cooler)
Once red giant uses up all the helium in its core. It will eject its outer layers and will move down to the left on the HR diagram as it becomes a white dwarf. (Hotter and dimmer than a main sequence star).
see a diagram on pmt
Temp is logarithmic
Draw an HR diagram with axis
Check cap or pmt
Axis:
absolute magnitude 15 - -10
Temperature 50000k - 2500k or OBAFGKM
For a hydrogen absorption line to occur in the visible part of the star;s spectrum what energy level must the hydrogen atom electrons need to be in?
n = 2
Whois temperature related to absorption spectra?
High temperatures, where collisions between atoms give the electrons extra energy.
If the temperature is too high, though the majority of the electrons will reach n = 3 level or above instead os not so many balmer transitions
A star has an intrinsic colour of yellow white. What is its spectral class, temperature and prominent absorption line?
G
5000 - 6000
Ionized and neutral metals
A star has a temperature of 7500 - 11000. What is its spectral class, intrinsic colour and prominent absorption line?
A
Blue - white
H (strongest ionized metals)
A star has a spectral class of O. What is its intrinsic colour, temperature and prominent absorption line?
Blue
25000 - 50000
He+, He, H
