Unit 5 - Astrophysics - Classification of Stars

Question 1

Define ‘Apparant Magnitude’.

Answer 1

The brightness of a star as seen from Earth.

Question 2

Define ‘Absolute Magnitude’.

Answer 2

The brightness of star if it were at a distance of 10 parsecs from the observer

Question 3

How do you determine the ratio of brightnesses between adjacent star classes.

Answer 3

Question 4

What is the relationship between apparent and absolute magnitude?

Answer 4

Question 5

Describe for Class O star:

Temperature.

Colour.

Prominent absorbtion lines.

Answer 5

25,000-50,000K

Blue

ionised and neutral Helium and weak hydrogen

Question 6

Describe for Class B star:

Temperature.

Colour.

Prominent absorbtion lines.

Answer 6

11,000-25,000K

Blue/White

Neutral helium and moderate hydrogen.

Question 7

Describe for Class A star:

Temperature.

Colour.

Prominent absorbtion lines.

Answer 7

7,500 - 11,000K

White with blue tinge

Strong hydrogen and ionised metals

Question 8

Describe for Class F star:

Temperature.

Colour.

Prominent absorbtion lines.

Answer 8

6,000-7,500K

White

Ionised metal and weak hydrogen

Question 9

Describe for Class G star:

Temperature.

Colour.

Prominent absorbtion lines.

Answer 9

5,000K-6,000K

Yellow/White

Neutral and ionsied metal, weak hydrogen

Question 10

Describe for Class K star:

Temperature.

Colour.

Prominent absorbtion lines.

Answer 10

3,500-5,000K

Orange

Neutral metals

Question 11

Describe for Class M star:

Temperature.

Colour.

Prominent absorbtion lines.

Answer 11

2,000-3,500

Red

molecules/neutral metals/Titanium oxide

Question 12

What class of star is the sun in?

Answer 12

Class G

Question 13

Define a black body.

Answer 13

A black body is a body that completely absorbs all wavelengths of electromagnetic radiation that fall upon it and reflects and transmits none.

Question 14

Draw a black body curve for three different temperatures: P,Q,R where P>Q>R

Answer 14

Question 15

What is Wien’s Displacement Law

Answer 15

That the product of the maximum wavelength and the temperature is constant.

Question 16

What is Stefan’s Law?

Answer 16

The energy emitted per second by a black body is given by:

Question 17

Draw a Hertzsprung - Russel diagram.

Answer 17

Question 18

What is the lifecycle of a star with similar mass to the Sun

Answer 18

When the hydrogen runs out, the star collapses causing rapid temperature rise and helium fusion begins which causes the star to expand to a huge size whilst cooling - a red giant.

When the star is mainly carbon it collapses again to form a white dwarf.

Question 19

What is the Chandrasekhar limit?

Answer 19

The maxiumum possible mass that a white dwarf can have without collapsing under its own weight to become a neutron star or black hole.

Question 20

Define a neutron star.

Answer 20

The left over remains from a supernova

Question 21

What is a black hole?

Answer 21

A concentration of matter which has a gravitational field strong enough to curve space-time around itself so not even light can escape.

A place where the escape velocity is greater than the speed of light.

Question 22

What is the Schwarzchild radius?

Answer 22

The distance from the centre of a black hole where the escape velocity is equal to the speed of light.

Question 23

Define the event horizon.

Answer 23

An imaginary surface around a black hole where the escape velocity is equal to the speed of light.

Question 24

How will the Sun change when it evolves into a red giant?

(Three marks)

Answer 24

When hydrogen in the core runs out gravitational forces overwhelm radiation pressure and the star initially collapses.

This collapse causes a rapid rise in core temperature and helium fusion begins.

The gas/radiation pressure now overwhelm gravity and the star expands to a huge size cooling as it does so.

Question 25

Explain how a white dwarf is formed and describe what a white dwarf is.

Answer 25

Once fusion stops gravity overwhelms gas pressure and the star collapses to a very small, very dense and very hot white dwarf made mainly of carbon.

No more fusion takes place and the star gradually cools.

Question 26

In what conditions is a black hole formed?

Answer 26

If a neutron star left after a supernova is greater than three times the mass of the Sun then it will continue to collapse under gravity to a singularity and form a black hole.