Basic Definitions

Question 1

What is the definition of one parsec?

Answer 1

The distance at which an object has a parallax angle of one arc second.

Question 2

What is the equation for distance of a star using the trigonometric parallax?

Answer 2

d[pc] = a/pi, where a is 1 parsec and pi is the angle in arc seconds.

Question 3

What is the resolution we can see to from the ground?

Answer 3

~0.01’’ ~ 100pc

Question 4

What is the resolution we can see to from space?

Answer 4

~0.001’’ ~ 1000pc

Question 5

What is GAIA?

Answer 5

A space telescope which is mapping our solar system and has parallaxes of billions of stars.

Question 6

What is the definition of flux?

Answer 6

Amount of energy per second passing through unit area.

Question 7

What is the equation for the flux of a star?

Answer 7

F = integral from 0 to inf of F(λ) dλ

Question 8

What is the definition of luminosity?

Answer 8

Total power output of a star.

Question 9

What is the equation for the luminosity of a star?

Answer 9

L = 4πR^2 F, where R is the radius of the star and F is its flux.

Question 10

What is the magnitude system and who came up with it?

Answer 10

Logarithmic scale to show brightness of stars - smaller number = brighter star. Hipparchus made catalogue of 850 stars position and brightness.

Question 11

What is the magnitude equation?

Answer 11

m1-m2=-2.5*log(f1/f2)

Question 12

What do we do to the magnitude equation to make it possible?

Answer 12

Need a zero-point - Vega has apparent magnitude of 0, so can sub this in to the equation and use log laws.

Question 13

What is the bolometric flux?

Answer 13

The flux over all wavelengths.

Question 14

What do we measure instead of the bolometric flux, since it is too difficult?

Answer 14

The average flux over a well defined band pass, characterised by transmissiob T(λ)

Question 15

What is the equation for the average flux?

Answer 15

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.

Question 16

What does the graph of relative flux against wavelength look like for a blue star and a red star?

Answer 16

Blue means shorter wavelength. So graph for blue is like negative skew but doesn’t come back down, and red is opposite (positive skew)

Question 17

What is a UBVR fliter and why is it useful?

Answer 17

Filters out different wavelengths. Is useful to determine the brightness at different wavelengths.

Question 18

What is the equation for the difference in brightness between two filters?

Answer 18

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.

Question 19

What is the definition of absolute magnitude?

Answer 19

Apparent magnitude of a star if it were located at a distance of 10pc.

Question 20

What is the absolute magnitude equation?

Answer 20

M-m = -5*log(d[pc]) + 5

Question 21

How can we compute the distance to remote stars?

Answer 21

Use a standard candle (object of known brightness M)

Question 22

What are Cepheid stars?

Answer 22

Very luminous and can be seen over large distances, and they pulsate, caused by ignition of a shell which is then extinguished.

Question 23

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?

Answer 23

Larger the period, the larger the luminosity.

M(V) = -2.76*log(P[d]) - 1.4