Observing the night sky: constellations, planets and parallax

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Science Revision – The

Mysterious Universe

6.1 Observing the night sky

·

Constellations: Groups of stars that were given a

particular name because of the shape the stars seem to form in the sky when

viewed from Earth.

·

Planets: Large object that orbits a star. Planets do not

produce their own light. They are known as the ‘wanderers’ because they wander

around the Sun.

·

Galaxies: Very large groups of stars and dust held together by

gravity.

· Nebulae:

Clouds of dust and gas that may

be pulled together by gravity and heat up to form a star.

·

Parallax: Is the effect of the apparent movement of objects

at different distances is due to the actual movement of the observer.

The development

of the telescope in the sixteenth century allowed Earth-bound astronomers

to see objects in the sky with much greater precision than ever before.

Observations using telescopes showed that many different types of objects in

the sky could be identified. These included single or double stars, groups

of stars called galaxies, clusters of galaxies, and clouds

of gas and dust called nebulae.

Perhaps some stars are closer to Earth

than others. Being closer, they would

appear brighter than other stars and their motion would be detectable

against the backdrop of more distant,

and therefore dimmer, stars.

In 1837, German

astronomer Friedrich Bessel became the first person to provide proof of a parallax effect when observing stars.

As the Earth orbits the sun, the positions of stars change very slightly

relative to each other. If all the stars were the same distance from the Earth,

this would not happen.

Observations of

a stellar parallax effect indicate that

some stars are relatively close to us while others are much further away.

6.2 The Sun

· Nuclear Fusion: Joining

together of the nuclei of lighter elements to form another element, with the

release of energy.

· Corona: Sun’s bright, hazy

atmosphere, obvious only when there is a total solar eclipse.

· Sunspots: Dark

spots on the surface of the sun that are cooler than the rest of the surface;

they are regions of intense magnetic activity.

· Aurora Australis: Spectacular

light show mainly visible from areas close to the South Pole. It is caused by

solar particles interacting with the Earth’s magnetic field. It is sometimes

called the southern lights.

· Aurora Borealis: Spectacular

light show mainly visible from areas close to the North Pole. It is caused by

solar particles interacting with the Earth’s magnetic field. It is sometimes

called the northern lights.

· Ionosphere: Highest

layer of the atmosphere where the air is extremely thin. This layer reflects

radio waves, enabling communication between many parts of the Earth.

Possibly the most important single object we see in the sky is our nearest star, the sun.

Without it, life would not exist on Earth.

We depend directly on the sun for the light and heat it shines onto the surface of the Earth.

Energy from the sun is stored in plants for food and heat, and the

sun's energy controls the world's weather.

The movement of the Earth on its axis and in its orbit around

the sun gives us day, night and the

seasons; these regular patterns determine

our natural biological rhythms.

Despite its importance to life on Earth, the sun is a very

ordinary star in a relatively unremarkable part of the universe. It is only the

sun's closeness to our planet that

makes it so important to us.

Most of the other stars in the universe have a mass somewhere between one-half and four times that of our

sun.

Stars like the sun are thought to have formed from the material found in the space between the stars.

Although this interstellar matter has an extremely

low density, it has been estimated that there is as much matter in the space between the stars as there is in the

stars themselves. Most of this interstellar matter is hydrogen gas and small amounts of helium. Only one per cent is made up of heavier elements found in particles of

dust.

Like other stars, our sun was formed

from a collapsing cloud of gas and dust. Its core temperature and pressure eventually rose high enough for atomic nuclei to become joined together by a

process called nuclear fusion. As a result

of fusion, hydrogen is transformed into

helium and vast amounts of energy are released. It has been estimated that the sun has been turning hydrogen into helium for about five billion years. It is very

stable and will probably stay as we now see it for another five billion years.

A visit to the surface of

the sun would reveal violently churning

gas and massive amounts of particles and energy spreading out into space.

Billowing out from the surface are huge

luminous arcs of gas that are usually seen only through a telescope on

Earth during an eclipse or by sensors on board interplanetary space probes.

During an eclipse, the moon blocks out the main body of the sun as

seen from Earth and leaves the corona visible.

The amount of activity on

the sun's surface is not constant. It varies over a fairly regular cycle,

which takes about 11 years to complete.

Each stage of this cycle can be seen from Earth because the number of visible sunspots provides clues about the level of activity. Sunspots look dark because they are cooler than the surrounding areas. However, the temperature of sunspots is still extremely high.

This activity produces a large number of particles that spread out from the sun. The Earth's

magnetic field captures some of the particles, often producing patterns of

light visible from the surface known as the aurora

borealis and aurora

australis. The captured particles in turn interfere with the Earth's own magnetic field. This has a severe

effect on long-distance radio

transmission.

Long-distance radio transmission relies on the reflection of high frequency radio waves from the ionosphere, high in the

Earth's atmosphere. The particles captured by the Earth's magnetic field during

high sunspot activity cause small changes in the Earth's own magnetic field.

These changes interfere with the

reflection of radio waves by the ionosphere, severely disrupting

long-distance radio transmission.

Low sunspot activity also disrupts

long-distance radio communication. It reduces the ability of the Earth's

ionosphere to reflect high-frequency (short-wave) radio waves.

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