Images

P7.4 The Sun, the stars and their surroundings

The Pleiades is a group of bright stars about 500 light-years away

With a spectrometer, the light from each star is broken up into

a spectrum, revealing slight differences in their colour.

All objects (including stars) emit some electromagnetic radiation.

As the temperature rises, the amount of radiation at high frequencies increases

A graph of the spectrum tells us about intensity as well as

showing which frequencies are

present.

What TWO things do you think will happen to this graph as the object gets hotter ?

Robert Bunsen used his burner to heat different elements and investigate the light given off.

Not all colours of the spectrum are seen. In fact different elements gave off (emitted) different colours

We call these line spectra or emission spectra

What causes an element to produce its own unique emission spectrum?

An electron gives out a single photon of light as it drops from one energy level to another.

Only certain energy levels are allowed so only certain colours

are emitted

The spectrum of the Sun – its like the opposite of an emission spectrum – see the dark lines?

Light photons from the surface of the Sun must pass through its atmosphere before it reaches us.

Gases in the Sun’s (or star’s) atmosphere absorb some of these light photons

An electron jumps from one energy level to another when the atom absorbs a single photon of light.

All elements have different energy levels

Only those photons with particular energy values will be absorbed

5

6

3

4

The missing frequencies in the absorption spectrum of hydrogen correspond exactly to the frequencies seen in its emission

spectrum.

Lines in the emission spectrum of hydrogen (bottom) correspond to some of the dark absorption bands in the Sun’s spectrum (top). Other lines in the absorption spectrum are due to other elements

in the solar atmosphere, including helium.

1. recall that the removal of electrons from atoms is called ionisation and explain how electron energy levels within atoms give rise to line spectra

2. recall that specific spectral lines in the spectrum of a star provide evidence of the chemical elements present in it

3. use data on the spectrum of a star, together with data on the line spectra of elements, to identify elements present in it

You should be able to...

The Gas Laws and Absolute Zero

Mass of gas Pressure of gas

Volume of gas Temperature of gas

Particles of a gas collide with the walls of its container. This

causes pressure.

The Gas Laws and Absolute Zero

Mass of gas Pressure of gas

Volume of gas Temperature of gas

Keep this constant

Keep this constant

Pressure (atmos)

1 1.2 1.4 1.6 1.8 2.0

Volume (cm3)

12 10 8.6 7.5 6.7 6.0

Understand that the volume of a gas is inversely proportional to its pressure at a constant temperature and explain this using a

molecular model

Flash animation

Here we keep the mass of gas constant again and the VOLUME of gas

Flash

Here we keep the mass of gas constant again and the PRESSURE of gas

Flash

understand that both the pressure and the volume of a gas are proportional to the absolute temperature

explain why the pressure and volume of a gas vary with temperature using a molecular model

Can you interpret absolute zero using a molecular model

and kinetic theory ?

You should be able to recall that –273°C is the absolute zero of temperature, and

convert temperatures in K totemperatures in °C (and vice versa)

0°C = 273K

27°C = 300K

350K = 77°C

Pressure Volume

0.2 16

0.4

0.8

1.6

Pressure Volume

0.2 16

0.4 8

0.8 4

1.6 2

Pressure Volume P x V

0.2 16 3.2

0.4 8 3.2

0.8 4 3.2

1.6 2 3.2

As long as the temperature does not change....,

pressure × volume = constant

P1V1 = P2V2 = P3V3 = ....

Temp (K) Pressure P / T

100 2000

200

300

400

Temp (K) Pressure

100 2000

200 4000

300 6000

400 8000

P / T

200

200

200

200

As long as the volume does not change....

pressure / temp (K) = constant

P1 = P2 = P3

T1 T2 T3

Temp (K) Volume

80 160

160

240

320

Temp (K) Volume

80 160

160 320

240 480

320 640

V / T

2

2

2

2

As long as the pressure does not change....

Volume / temp (K) = constant

V1 = V2 = V3

T1 T2 T3

A star is born - Protostars

A protostar forms from a cloud of gas (mostly hydrogen and helium) which is pulled together by gravity. Gravity ‘compresses’ the gases (increased pressure) so increasing the temperature.

Every particle in the dust cloud attracts every other particle. As they ‘fall inwards’ they move faster and particles collide with each

other. The fastest particles are in the centre of the cloud and faster moving particles means a higher temperature.

The young protostar attracts even more dust and gas. Eventually its core becomes so dense and hot that nuclear fusion starts and

a star is born.

A protostar at the centre of a new planetary system.

Cross-section of a star

Cross-section of a star

The core of a star is where the

temperature and density are highest

and where most nuclear fusion takes

place

...and by convection currents through the convective zone....

Energy is transported by photons of radiation through

the radiative zone....

....and radiated into space as electromagnetic radiation from the star’s

surface (photosphere)

The surface of the Sun, showing the granularity caused by the presence of convective cells.

If brought close enough together, hydrogen nuclei can fuse into helium

nuclei releasing energy. This is called nuclear fusion

Would have burned out in only 50000 years!!

Nuclear processes discovered in the early 20th Century provided a possible explanation of the Sun’s energy source

The fusion process is a little more complicated than this

During the fusion process, one of the protons becomes a neutron. This releases two tinier particles — a positron and a neutrino

Gamma rays

2 protons (hydrogen nuclei) fusing to make deuterium

NB energy is liberated when light nuclei fuse to make heavier nuclei with masses up to that of the iron nucleus

Can you complete & interpret nuclear equations for fusion in stars to include the emission of positrons to conserve charge

Use Einstein’s equation E = mc2 is to calculate the energy released during nuclear fusion and fission

(where E is the energy produced, m is the mass lost and c is the speed of light in a vacuum)

Mass numbers balance. Emission of positron (+vely charged electron) keeps atomic number the same before and after.

Surprisingly mass on the right is slightly less than on the left. At each stage in the fusion, mass changes into energy

Data for stars – the H–R diagram.

HR Diagram – Interactive Lab

Watch the two animationsHR_an

Star Life Cycle _an

recall that .....

...the Hertzsprung-Russell diagram is a plot of temperature and luminosity and identify regions on the graph where supergiants, giants, main sequence and white dwarf stars are located

...in a main sequence star, hydrogen fusion to helium takes place in the core

...a star leaves the main sequence when its core hydrogen runs out; it swells to become a red giant or supergiant and its photosphere cools

...the more massive the star, the hotter its core and the heavier the nuclei it can create by fusion

... in a red giant or supergiant star, helium nuclei fuse to make carbon, followed by further reactions that produce heavier nuclei such as nitrogen and oxygen

...a low-mass star (similar to the Sun) becomes a red giant, which lacks the mass to compress the core further at the end of helium fusion; it then shrinks to form a white dwarf

...in a white dwarf star there is no nuclear fusion; the star gradually cools and fades

...in a high-mass star (several times the mass of the Sun) nuclear fusion can produce heavier nuclei up to and including iron; when the core is mostly iron, it explodes as a supernova creating nuclei with masses greater than iron and leaving a dense neutron star or a black hole.

...and also....

...but no evidence of extraterrestrial life (at present or

in the past) has so far been detected.

Astronomers have found convincing evidence of planets

around hundreds of nearby stars

but even if only a small proportion of stars have planets, many

scientists think that it is likely that life exists elsewhere in the Universe