Gravitational Fields and Star Formation NCEA Physics Gravitational Fields and Star Formation

The birth of stars Aims: To describe how stars form from very large clouds of hydrogen, helium and dust which collapse under the influence of gravity so that the core becomes hot enough for nuclear reactions to begin.

Protostar formation

Stars often form in the spiral arms of galaxies where there is a lot of gas and dust. Galaxy arms

Nebulae, where stars are born

Nebulae Nebulae are huge clouds of gas and dust that are the birth place of new stars. The gas in a nebula is nearly all hydrogen with a small amount of helium. When light from nearby stars hits these atoms they emit beautiful colours that give nebulae their distinctive look.

Nebula picture 1

Orion nebula (M42)

Dust particles The dust particles are very small. Smoke particles are about the same size. These dust particles mix with the hydrogen in the nebula.

Areas of star formation Parent cloud A 10pc B

Star birth clouds

Elements that form an average star Abundance Hydrogen 91 % Helium 8.3 % Oxygen 0.06 % Carbon 0.02 % Nitrogen 0.01 % The rest …

Close up picture 1

Close up picture 2

Collapse of gas into a protostar A huge cloud of gas has a massive amount of GPE. As it shrinks some of this energy is transferred to kinetic energy as the protostar starts to spin. The rest is transferred as kinetic energy to warm the gas and as radiation that is emitted from the protostar.

Clusters of stars A large dense cloud of gas and dust will break up into smaller parts as it shrinks, scientists are still trying to figure out why! Stars form in clusters

Protostar Collapse The spinning of the protostar collapses clouds into disks through which material flows down to the central object.

More than anything else, mass plays a critical role in determining whether the central object will become a star. Layer upon layer of gas encircling the centre adds its weight to the layers beneath. . Imagine laying on the floor while layers of sand bags are being piled on top of you. The more layers of sand bags there are, the more crushed you feel beneath them and the hotter the temperature rises.

Maturing protostars When temperatures reach about 5 million degrees C at the centre, nuclear reactions begin. This takes about 80 times the mass of Jupiter (0.08 of the sun's mass). Any less mass and the temperature does not reach a high enough value to ignite the fusion reaction. The gas ball does not become a star

Protostar disks

Getting hotter As the protostar collapses it gets hotter. The bigger the initial protostar the quicker it gets hotter. Massive protostars evolve to be normal (main sequence) stars very quickly (10,000 years); less massive stars evolve more slowly, up to 10 million years. Eventually the core of the protostar becomes hot enough for nuclear fusion to take place, this needs a temperature of at least 15 million Kelvin / degrees Celsius.

Nuclear fusion - The simple picture

Nuclear Fusion The star is full of hydrogen moving about very fast. We can imagine the gas as full of protons and electrons. At a million Kelvin the electrical repulsion force between the protons prevents them becoming close when they move towards each other. As the temperature increases further the protons become closer. During nuclear fusion two protons become so close that the ‘Strong nuclear force’ grabs them and holds them together, this releases lots of energy. This force only works over very short distances so the protons have to move very fast to get close, moving fast means being very hot!

Fusion diagrams At low temperatures the two positively charged protons repel each other and cannot come into contact.

Fusion diagrams As the particles become hotter they move faster. When they fly towards each other they become closer but the force of repulsion is still too strong to allow them to come into contact.

Fusion diagrams At extremely high temperatures the protons are moving so fast that the force of repulsion cannot stop the protons from hitting one another. When they impact on one another nuclear fusion takes place and they stick together.

Hydrogen Fusion As soon as two protons fuse together there will be a radioactive decay as two protons are not stable. When a proton decays into a neutron it also emits a positron (beta plus) particle. Within a fraction of a second the positron will meet an electron and both will be annihilated to produce pure EM radiation. p n p p p p positron e+ e-

P-P Chain H1 H1 He3 H1 109 years 1 sec He4 H1 106 years H1 Other nuclear reactions can take place at these temperatures so that hydrogen eventually turns to helium. The fuel of the Sun is being used up to make new elements.

Stability at last At each radius Fgrav=Fpressure Nuclear fusion now creates enough pressure to stop the star from collapsing any further. A star will spend most of its life in this hydrogen to helium phase.

The Sun as a power station Near the centre of the core of the Sun nuclear fusion is proceeding in generating tremendous amounts of energy. Using E = m c2 we know that 4.7 million tons of material is being converted each second. The Sun has been a main sequence star for approximately 4500 million years. Small stars like our Sun are called yellow dwarfs.

The sun converts 4. 3 million tons (4 The sun converts 4.3 million tons (4.3x109 Kg) of mass into energy every second ... equivalent to the mass of 1million elephants. It has been doing this for the past 4.6 billion years.

The sun is constantly consuming its own mass to generate the energy it radiates into space. Over its 4.6 billion year lifetime it has lost some 600 trillion trillion (6x1026) Kg. Relax ... this is only a few hundredths of one percent of the sun's total mass of 2x1030 kg. The sun has barely noticed the weight loss.

The sun is slowly changing - in the solar core, 37% of the available hydrogen has already been fused to form helium. For every 600 million tons of hydrogen fused to helium, 5 million tons are converted into energy and eventually released to space. At this rate, the sun will continue to burn hydrogen for the next 5 billion years.

The Sun: Our nearest star Property Value Surface T 5500 K Central T 15  106 K Luminosity 2  1033 ergs Mass 4  1030 kg Lifetime (ms) 10 billion years

Solar corona The Sun’s temperature reaches a minimum in the photosphere, then increases in the chromosphere and increases again to millions of degrees in the beautiful corona region. The corona can only be seen during an eclipse.

Corona and sun spot

More corona

Sun spots The sun suffers from acne. Spots of magnetic distortion appear on its surface. Solar flares made of hot charged particles (mostly p+ and e-) evaporate from the Sun and stream into the Solar System About 1 million tonnes/second stream away at 1 million mph.

These solar flares can stream upwards by distances bigger than the size of the Earth. Small solar flares

Huge solar flares

Solar Wind The solar wind increases when there are more flares from sun spots. Solar wind `storms’ can have powerful effects on the Earth, even overloading electrical cables. Satellites in space are not protected by the Earth’s magnetic field and can have their electrical circuits destroyed by powerful solar winds.

The Earth’s protection We are protected by the dangerous solar winds by our atmosphere. At the poles a small amount of wind enters the atmosphere causing a beautiful effect.

Northern lights The solar wind creates light in the same way as a neon strip light in our classroom. The magnetic field at the poles of the Earth is straight downwards so that some particles can travel until they hit a particle in the atmosphere.

N lights

N lights

Summary – The birth of stars Huge amounts of gas and dust from nebulae are slowly drawn together over millions of years by gravity. As the gas compresses it changes its gravitational potential energy changes into kinetic energy. The KE changes into heat and the protostar becomes hotter and hotter as it becomes smaller. Eventually the protostar becomes hot enough for nuclear fusion to start (10 million degrees). The star is now in its main sequence, converting hydrogen to helium.