1 of 63

Labs

Virtual Labs

Images / Videos

Data Sets

Readings

Home learning

Exit ticket

Take notes

Understanding check

Discuss online

On your own

With a partner

With your group

Develop consensus

Extend

Anchor

Guiding question

Investigation

Putting it together

Assessments

2 of 63

How are elements created?

What physical processes lead to the evolution of stars?

Can observations of the present state of the universe predict the future outcome of the universe?

3 of 63

Understandings

  • that the stability of stars relies on an equilibrium between outward radiation pressure and inward gravitational forces
  • that fusion is a source of energy in stars
  • the conditions leading to fusion in stars in terms of density and temperature
  • the effect of stellar mass on the evolution of a star - the main regions of the Hertzsprung-Russell (HR) diagram and how to describe the main properties of stars in these regions
  • the use of stellar parallax as a method to determine the distance d to celestial bodies
  • how to determine stellar radii.

4 of 63

Guidance:

  • Energy release calculations are required.
  • The conversion between astronomical units (AU), light years (ly) and parsecs (pc) is required.
  • The sketching and interpretation of HR diagrams, including the location of main sequence stars, red giants, super giants, white dwarfs, the instability strip and lines of constant radius, is required.
  • HR diagrams will be labelled with luminosity on the vertical axis and temperature on the horizontal axis.
  • The surface temperature and composition of a star can be determined from the stellar spectrum.
  • The determination of stellar radii using luminosity and surface temperature is required.
  • Cepheid variables are not required.

5 of 63

How can we know how far away stars are?

6 of 63

How can we know how far away stars are?

Luminosity - total power radiated (W)

Apparent brightness - power received per unit area of the detector (B2)

7 of 63

8 of 63

The solar constant (i.e. amount of energy falling on the Earth) is 1.37x103 Wm-2 and the sun’s luminosity is 3.9x1026W.

Calculate the distance between the sun and the Earth.

1 AU 1.5x1011 m

9 of 63

We need to know the Luminosity.

10 of 63

Which star is hotter?

11 of 63

Shape of the curve

If we can measure the wavelength at which the emitted radiation has its maximum intensity, we can calculate the surface temperature of a star.

Blackbody Curves (NAAP)

See what the effect of the atmosphere is

Filters Simulator (NAAP)

12 of 63

Finding the surface temperature of a star

If we can measure the wavelength at which the emitted radiation has its maximum intensity, we can calculate the surface temperature of a star.

Area under the graph = total Energy radiated

  • A blackbody is a perfect emitter and absorber of radiation. It emits radiation dependent on its temperature only. Stars can be modelled as blackbodies.

13 of 63

Absorption spectra: light absorbed by a cool gas

Emission spectra: given out by a hot gas

Here is a reminder of the difference between absorption and emission spectra

Three Views Spectrum Demonstrator

14 of 63

15 of 63

Wien’s law tells us the relationship between the surface area of the star, the temperature, and its luminosity

16 of 63

17 of 63

Parallax

18 of 63

19 of 63

20 of 63

Astronomical distances

An arc second is 1/3600 of a degree

1 degree = 3600 arc-seconds > not in data booklet

21 of 63

Limitations

How many metres are there in a pc?

What happens to the parallax angle when a star is far away?

Parallax - limited to stars that are 100pc away (within our galaxy)

22 of 63

23 of 63

24 of 63

Deneb

Vega

25 of 63

26 of 63

27 of 63

Doppler effect for light

Start at 19:25!

source

Speed of light is the same in all frames of reference!

RECAP!

28 of 63

Redshift > moving away

Blueshift > moving towards

RECAP!

29 of 63

30 of 63

How are elements created?

What physical processes lead to the evolution of stars?

Can observations of the present state of the universe predict the future outcome of the universe?

31 of 63

32 of 63

Look at the size tab: At which stage in its life cycle will the Sun be at its largest?

Look at the light bulb tab: At which stage in its life cycle will the Sun be at its brightest?

Look at the thermometer tab: At which stage in its lifecycle will the Sun be at its hottest?

Look at the pie chart tab: In which stage of its life will the Sun spend most of its time?

Look at the mass tab: What happens to the mass of the Sun as it gets older?

Red Giant�

Red Giant�

Beginning of white dwarf�

Main sequence, 9000 Myr�

Stars lose mass gradually by converting hydrogen into helium and heavier elements (mass of parts > mass of product)

33 of 63

If we know the luminosity and temperature, we can also find the radius!

34 of 63

35 of 63

36 of 63

1.4 solar masses

1.4 -3 solar masses

3 solar masses

Oppenheimer–Volkoff limit

Chandrasekhar limit

37 of 63

“It can take thousands of years for a photon to reach the surface of the sun from its core”

For more detail see https://www.askamathematician.com/2013/08/q-why-does-it-take-thousands-of-years-for-light-to-escape-the-sun/

38 of 63

The p-p cycle

Neutrinos are important because they allow scientists to peek into the interior of the sun and learn about the processes there.

39 of 63

Nuclear fusion in the main sequence

What is a main sequence star?

What nuclear equations describe the fusion processes in a main sequence star?

40 of 63

Fusion in main sequence stars

Main sequence stars like our Sun

41 of 63

42 of 63

Fusion in stars

Fill in the missing bits in the nuclear equations

43 of 63

Fusion in stars

Fill in the missing bits in the nuclear equations

44 of 63

Fusion in stars

Fill in the missing bits in the nuclear equations

This is another way to produce Helium.

All elements in the intermediate stages are used up

This happens in more massive stars.

It is called the CNO cycle.

45 of 63

More massive main sequence stars

CNO cycle

The carbon nucleus has a charge of +6, so the barrier that must be overcome for carbon to fuse is much higher. This requires higher temperatures

This is another way to produce Helium.

46 of 63

Time that stars will spend on main sequence

After about 12 % of the total mass of Hydrogen in the star has been fused the star will no longer be in equilibrium

The ‘lifetime’ of the star as a main sequence star depends on

  • original mass of Hydrogen
  • rate of nuclear fusion.

More massive stars have more concentrated cores at higher temperatures so they have a higher fusion rate.

47 of 63

48 of 63

49 of 63

The HR diagram

50 of 63

Red giant phase

  • The core runs out of hydrogen
  • the core begins to contract under its own gravity
  • The core temperature increases, allowing hydrogen fusion to continue in a shell around the core.
  • The outer layers continue expanding, cooling, and reddening
  • The helium core continues contracting and heating up.
  • Eventually the core is hot enough to start fusing Helium
  • For stars > 8 M the core temperature is high enough to fuse other elements up to iron

51 of 63

Helium flash

The first element to be produced as it enters the red giant stage is carbon

For stars up to 8 M

Triple alpha process:

52 of 63

Helium fusion in core.

Since the hydrogen shell was the primary contributor to the star's high luminosity, the reduction in its energy output causes the total luminosity to drop

53 of 63

What is the subsequent evolution for these stars?

Stars with an initial mass less than 8 solar masses

Outer layers are ejected (planetary nebula)

Dense core remains (white dwarf)

a white dwarf shines because of the residual thermal energy trapped in its dense core

54 of 63

White dwarf

Thus, a white dwarf shines because of the residual thermal energy trapped in its dense core, which it slowly radiates away over billions of years.

Planetary nebula

55 of 63

Stars more massive than 8M

Formation of Oxygen

Neon

Sodium

Magnesium

Silicon

56 of 63

The instability strip

A star more massive than the sun may go back and forth on the HR diagram several times as shown in the diagram. The star is now in the instability region of the HR diagram.

The delicate balance between radiation and

gravitational pressures has been disturbed; the star

tries to find a new equilibrium state but instead the star

pulsates, i.e. it grows and then shrinks in size.

57 of 63

Iron cannot undergo fusion to release energy, so no outward radiation pressure exists to counteract gravity.

The core collapses under its own gravity, compressing protons and electrons into neutrons.

This creates a dense core. The collapsing gas suddenly rebounds, generating a powerful shockwave.

The outer layers of the star are blasted into space, releasing enormous energy

58 of 63

Fusion reactions

The Sun is powered by the fusion of Hydrogen into Helium. This happens in a series of reactions called the p-p cycle. Write an equation for each step and hence one overall equation for the whole pp cycle

RECAP

59 of 63

Energy released in fusion

Calculate the energy released in these reactions

Δm=5.3 x 10-3 u

E=5 MeV

Mass of proton 1.007276 u

Mass of deuterium 2.014102 u

Mass of tritium 3.016049 u

Mass of He-4 4.002602 u

Mass of He-3 3.016029 u

Mass of neutron 1.008665 u

40 million Kelvin temperature to overcome the “Coulomb barrier”.

RECAP

60 of 63

Calculate the time for which the sun would take to burn all of its hydrogen.

61 of 63

Summary

Nebula→ Main sequence star

A star with an initial mass < 8M

Red giant → Planetary nebula→ white dwarf

A star with an initial mass > 8M

Red supergiant→ Supernova→ neutron star or black hole

< 1.4 M

White Dwarfs (gradually cool and fade)

1.4 - 3 M

Neutron Stars (some become pulsars)

> 3M

Black Hole from which no light escapes

Core Mass

62 of 63

All of the hydrogen and most of the helium in the universe were produced at the very earliest moments in the life of the universe.

Elements up to iron are produced in stellar cores.

Other elements are produced in supernovae explosions

63 of 63

  1. Try first

  • Then ask a buddy

  • Then look at the Answer (not the video solution) to see if you can work it out

  • Then look at the video solution

  • Then ask your teacher

Higher Level