Topic 3.3 - Physical Properties Of Stars

Question 1

Explain the difference between Optical Double stars and True Binary Stars.

Answer 1

True Binary stars are linked together gravitationally and orbit a common centre of mass. e.g. Castor in Gemini. e.g. Algol in Perseus. Optical Double stars merely appear nearby but may be light years apart.

Question 2

What is magnitude?

Answer 2

The brightness of a star. A difference of of 5 magnitudes between two stars is equivalent to one star appearing 100 times brighter than the other

Question 3

What is Pogson’s ratio?

Answer 3

It follows that a magnitude difference of exactly 1 corresponds to brightness ratio of the fifth root of 100 (which is 2.512) but in GCSE all you need is 2.5

Question 4

Magnitude difference to brightness ratio chart.

Answer 4

Magnitude Brightness
Difference Ratio

1                     2.5
2                    6.25
3                    16
4                    40
5                    100
6                    250

Question 5

What are the 4 main factors that the apparent magnitude of a star depend on?

Answer 5

1. The total energy radiated by the star - this depends on the star’s size and temperature
2. The distance to the star
3. The amount of interstellar gas and dust
4. The amount of light absorbed by the Earth’s atmosphere

Question 6

Why might a star appear bright?

Answer 6

Because it’s near to us or highly luminous due to its large size, and/or high temperature (or a combination of both of these factors)

Question 7

What is Sirius’ magnitude and why is it so bright?

Answer 7

Sirius’ magnitude is -1.5 but is os bright because it is one of our closest neighbours.

Question 8

What is inverse square law?

Answer 8

The intensity of light, or any other form of radiation from a star, obeys a mathematical relationship know as the inverse square law

Question 9

What does ‘inverse square’ mean?

Answer 9

It means that if the distance to the star (hypothetically) doubles, the star will appear 4 times dimmer (2²) dimmer: if the distance increases by a factor of 5, the star appears dimmer by a factor of 25 (5²) and so on

Question 10

What is absolute magnitude?

Answer 10

A measure of the true brightness and is defined as the apparent the magnitude the star would have if it were observed from a standard distance of 10 parsecs

Question 11

What is the formula to work out the apparent or absolute magnitudes?

Answer 11

M = absolute magnitude
m = apparent magnitude
d = distance to the star in pc

M = m + 5 - 5 log d

Question 12

What are the two main classes of variable star?

Answer 12

Binary stars and cepheid

Question 13

What is a binary star?

Answer 13

Two stars orbiting around their common centre of mass

Question 14

What are Cepheid variables?

Answer 14

Giant yellow stars that regularly expand and contract in size

Question 15

How can you determine the period of variability?

Answer 15

Graphs showing how the intensity of light from a variable star changes with time are called light curves. These can be used to determine the period of variability by simply reading off the time axis

Question 16

In a Binary system which star is which?

Answer 16

The brighter star is called the primary star and the dimmer star is called the secondary star

Question 17

What is parallax?

Answer 17

The apparent motion of near and more distant objects in respect to each other, caused by the motion of an observer.

Question 18

Talk about heliocentric parallax.

Answer 18

In 6 months the Earth lives halfway around the sun. The position of a star against very distant stars on the sky wobbles.
The angle on its position changes by in 3 months and is the parallax angle. You can draw a right angle with this angle and the opposite side of length 1 AU.
So you can work out the adjacent side which is the distance to the star, using trigonometry, the opposite side (1 AU) and the parallax angle.
One limitation is that if a star is too far away so it never appears to wobble but the solution to this is standard candles which are stars that you know the actual absolute magnitude of. CEPHEID VARIABLE STARS … vary in brightness with a period proportional to their luminosity

Question 19

What is one limitation to heliocentric parallax and its solution?

Answer 19

One limitation is that if a star is too far away so it never appears to wobble but the solution to this is standard candles which are stars that you know the actual absolute magnitude of. CEPHEID VARIABLE STARS … vary in brightness with a period proportional to their luminosity

Question 20

Describe how Cepheid Variables can be used to determine stellar distances.

Answer 20

Using the period - luminosity law we can work out the absolute magnitude of the star. use the distance modulus formula to calculate distance

Question 21

In what way is the variability of light intensity of Cepheid Variable different to that of eclipsing binaries?

Answer 21

One is due to the star pulsating (Cepheid) and the bother is due to stars eclipsing each other (eclipsing binaries)

Question 22

What is an eclipsing binary?

Answer 22

If two binary stars orbit each other in a plane along our line of sight, two stars will eclipse each other