Parallax, Parsecs and Magnitude

Question 1

What is meant by parallax?

Answer 1

The apparent change in position of something stationary due to the observer moving.

Question 2

How can you work out the distance to a star using parallax?

Answer 2

By using trigonometry in a triangle linking the radius of Earth’s orbit, the distance from the sun to the star and the angle of parallax (due to the Earth being offset from the Sun).

Question 3

What does tan(theta) equal for small angles, and what equation does this make?

Answer 3

For small angles tan(theta) = theta.

This means d = r / theta(in radians)

Question 4

What is the definition of one parsec?

Answer 4

The distance at which a star gives a parallax angle of one arc second.

Question 5

What is one arcsecond equal to?

Answer 5

1/3600 degrees

Question 6

What is luminosity and what is it also known as?

Answer 6

Luminosity is the total amount of energy emitted in the form of electromagnetic radiation each second. It’s also known as the power output of a star.

Question 7

What is meant by intensity?

Answer 7

The power received from a star per unit area at Earth.

Question 8

What is apparent magnitude?

Answer 8

A measure of brightness of an object in the sky.

Question 9

Who came up with the apparent magnitude scale?

Answer 9

An Ancient Greek called Hipparchus.

Question 10

Outline the logarithmic scale of apparent magnitude.

Answer 10

• Difference in 5 magnitudes is 100x difference in brightness
• Difference of 1 on the scale corresponds to a difference in intensity of 100 ^ 1/5 = 2.51x

Question 11

State an equation which can be used to calculate the brightness ratio between two stars and what each letter means.

Answer 11

I2 / I1 = 2.51 ^ (m1 - m2)

I = intensity of star
m = apparent magnitude of star

Question 12

What is the apparent magnitude of the Sun?

Answer 12

-26.7

Question 13

What is the limit of the naked eye on the apparent magnitude scale?

Answer 13

About 6.

Question 14

What is absolute magnitude?

Answer 14

The apparent magnitude of a star if it were located 10 parsecs away from Earth.

Question 15

How can you use standard candles to calculate distances to objects?

Answer 15

• Standard candle are objects that you can calculate the absolute magnitude of directly e.g. type 1a supernovae
• All type 1a supernovae have the same peak absolute magnitude, therefore you can work out how far it is by comparing its apparent magnitude to its absolute magnitude