Basic Definitions Flashcards
What is the definition of one parsec?
The distance at which an object has a parallax angle of one arc second.
What is the equation for distance of a star using the trigonometric parallax?
d[pc] = a/pi, where a is 1 parsec and pi is the angle in arc seconds.
What is the resolution we can see to from the ground?
~0.01’’ ~ 100pc
What is the resolution we can see to from space?
~0.001’’ ~ 1000pc
What is GAIA?
A space telescope which is mapping our solar system and has parallaxes of billions of stars.
What is the definition of flux?
Amount of energy per second passing through unit area.
What is the equation for the flux of a star?
F = integral from 0 to inf of F(λ) dλ
What is the definition of luminosity?
Total power output of a star.
What is the equation for the luminosity of a star?
L = 4πR^2 F, where R is the radius of the star and F is its flux.
What is the magnitude system and who came up with it?
Logarithmic scale to show brightness of stars - smaller number = brighter star. Hipparchus made catalogue of 850 stars position and brightness.
What is the magnitude equation?
m1-m2=-2.5*log(f1/f2)
What do we do to the magnitude equation to make it possible?
Need a zero-point - Vega has apparent magnitude of 0, so can sub this in to the equation and use log laws.
What is the bolometric flux?
The flux over all wavelengths.
What do we measure instead of the bolometric flux, since it is too difficult?
The average flux over a well defined band pass, characterised by transmissiob T(λ)
What is the equation for the average flux?
integral from 0 to inf of f(λ)*T(λ) dλ / integral from 0 to inf of T(λ) dλ, where T(λ) is the transmission.
What does the graph of relative flux against wavelength look like for a blue star and a red star?
Blue means shorter wavelength. So graph for blue is like negative skew but doesn’t come back down, and red is opposite (positive skew)
What is a UBVR fliter and why is it useful?
Filters out different wavelengths. Is useful to determine the brightness at different wavelengths.
What is the equation for the difference in brightness between two filters?
e.g. U-B = m(U)-m(B)=M(U)-M(B), so if B-V > 0, its redder than Vega, and if B-V > 0, is bluer than Vega.
What is the definition of absolute magnitude?
Apparent magnitude of a star if it were located at a distance of 10pc.
What is the absolute magnitude equation?
M-m = -5*log(d[pc]) + 5
How can we compute the distance to remote stars?
Use a standard candle (object of known brightness M)
What are Cepheid stars?
Very luminous and can be seen over large distances, and they pulsate, caused by ignition of a shell which is then extinguished.
What is the relationship between period of pulsation and absolute brightness for Cepheid stars? What is the equation for the magnitude of a cepheid star?
Larger the period, the larger the luminosity.
M(V) = -2.76*log(P[d]) - 1.4
