GCSE Astronomy: The Sun

Detailed notes covering the 'Sun' section of the GCSE Astronomy specification.

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The Sun
Candidates should be able to:
2.8 Describe the dangers of direct observation of the Sun and simple techniques for safe observation.
If you look directly at the Sun with a telescope, you will focus all the light and heat on to your eye, and you will blind
yourself- probably permanently. This is true even when the Sun is low down and misty, and looks deceptively
harmless. Dark caps and filters over the eyepiece are not to be recommended because they can never give proper
protection and they are always liable to shatter without warning.
To observe sunspots, point the telescope towards the Sun by squinting along the top of the tube, or by casting the
shadow of the telescope itself on to a screen held or fixed behind the eyepiece. The Sun's image will then fall on to the
screen, and you will have an excellent view if any spots. This is safe, but always keep your eye well away from the
2.9 State that the Sun is a star.
2.10 State that the Sun's energy is produced by nuclear reactions in its interior, converting hydrogen into helium.
2.11 State the approximate diameter of the Sun (1.4 million km), its approximate distance from the Earth (150 million
km) and its surface temperature (5700 K).
2.12 Describe the solar corona, photosphere and chromosphere.
The solar corona is the hot, tenuous, outer atmosphere of the Sun and is much fainter than the photosphere so can
only be observed when the Sun is eclipsed by the Moon, or by using a special instrument called coronagraph which
occults the Sun's disk. The corona changes shape with the phase of the sunspot cycle and is made up of a thin gas at a
very high temperature.
The photosphere is the visible surface of the Sun, from which almost all the radiation energy is emitted into space. It
is only a few hundred kilometres thick and is composed of gas at relatively high pressure and so it produces a
continuous spectrum. (500km thick)
The chromosphere is a layer of much thinner gas. The gases are capable of absorbing wavelengths which they would
normally emit. The result is a dark-line or absorption spectrum as the lines are reversed and show up as dark
Fraunhofer lines crossing the background rainbow. (10,000km thick)
2.13 Describe the temperature distribution through the layers of the Sun and the high temperature of the corona.
Corona- temperatures up to 2 million degrees
Chromosphere- temperatures up to 400,000 degrees
Photosphere- temperature about 5,500 degrees
Core- temperature about 14 million degrees

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Describe the influence of the Sun's magnetic field on the appearance of the corona.
(Refer to image on page 5 in lesson 3- The Sun)
The size and shape of the Sun's corona is affected by its magnetic activity.
At solar maximum (highest magnetic activity), the density and temperature of the corona both increase, and its
shape shows its most symmetrical form as the corona is evenly distributed.
At solar minimum (lowest magnetic activity), the corona has long streamers which extend from the equatorial
regions.…read more

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Describe the solar wind and explain the influence of the solar wind on the Van Allen belts and aurorae.
Solar Wind- Flow of charged particles streaming out from the Sun; it is made up of protons, alpha particles and
electrons. The charged particles originate from the Corona.
On reaching Earth, the particles can have speeds of several hundred km/s. The particles ar e deflected by the Earth's
magnetic field however some of these charged particles get trapped in the Earth's magnetic field.…read more


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