Stars and the Sun

BC Science Probe 9Section 13.3-13.4

Pages 425-436

Stars

• Stars have a life cycle.– Beginning (birth)– Midlife– End (death)

• The Sun, which is the star closest to us, has been around for 5 billion years and is expected to last another 5 billion years.

Stellar Birth

• Stars are born when nebulas collapse on themselves.– Small amounts

of matter collide and stick together making bigger masses that have more gravity.

Stellar Birth

• When the nebula begins to collapse, the region with the greatest density will pull more of the hydrogen gas toward it than the rest and will begin to grow.

• This is called a protostar.

Stellar Birth

• Once the protostar gains enough mass and gets dense enough, that material in the centre will become hot enough to start nuclear fusion!

Nuclear Fusion

• This happens in the core of the star.• Hydrogen nuclei fuse together to form helium

nuclei.• 6 hydrogen protons fuse to produce 1 helium

nucleus with 2 protons and 2 neutrons as well as 2 free protons and a ridiculous amount of energy!

• The energy can be in the form of heat, light, x-rays, gamma rays and particles.

Nuclear Fusion

Nuclear Fusion

• The energy of the nuclear fusion causes thermal expansion.– The force of the expansion counters and balances

the gravity to stop the collapse.• Our Sun went through this process. It probably

took 30 million years to condense and ignite.

Stellar Midlife

• The fusing of hydrogen to helium continues just like what is happening in the Sun right now.

• The energy that is released can take thousands of years to reach the outer layers of the Sun where it is released as light.

Solar Anatomy

• The part of the Sun that produces the energy is the core.– Temp. - 1.5 x 107 ⁰C– Radius – 1.75 x 105 km

Solar Anatomy

• The next layer of the Sun is the radiative zone.– 3.5 x 105 km thick

Solar Anatomy

• The outer layers of the Sun are the convective zone and the photosphere.

• Convective zone – 2 x 105 km thick– 2 x 106 ⁰C

• Photosphere – the visible part of the Sun– 300 km thick– 5500 ⁰C

Solar Anatomy

• The layers all rotate at different speeds and once the energy reaches the convective zone, it moves by convection currents to the surface.

Solar Anatomy

• Outside of the photosphere are the chromosphere and the corona.

• Chromosphere– Layer of gases at 60 000 ⁰C

• Corona– Layer of gases at 2 x 106 ⁰C

The Sun’s Surface

• The surface of the Sun is not smooth and featureless.

• It looks more like the surface of a boiling liquid.

The Sun’s Surface

• Sunspots– Dark spots on the surface of the Sun.– The vary in size and regularity.– Caused by disturbances in the Sun’s magnetic

field.– They appear in pairs with opposite magnetic poles

which disturb the magnetic field more.– The magnetic disturbances caused by sunspots

can lead to solar flares and solar prominences.

The Sun’s Surface

• Solar prominences– 1 x 1011 tonnes of glowing hydrogen released from

the photosphere.– Last 4-5 minutes

The Sun’s Surface

• Solar flares– Ejection of large quantities of gas and charged

particles.– Last a very short time

The Sun’s Surface

• Solar wind– This is happening all the time.– Ionized gas emitted from the Sun.– Stronger when there are flares and prominences.– Creates aurora borealis.– Disrupts communications equipment and other

electronic systems.