Astronomical Concepts – Week 5

The main topic this week was the Sun.

Here are a few facts about the sun:

  • It is a hot ball of glowing gas
  • It is a yellow dwarf, main sequence star
  • Approx. 150 million km from Earth (this distance is known as 1 astronomical unit (AU))
  • Formed 4.5 billion years ago
  • Formed from a giant cloud of spinning and collapsing gas
  • Light from the sun takes 8 minutes and 20 seconds to reach our eyes
  • Sunlight takes 170,000 years to get from the core to the sun’s surface

Composition:

  • 91% Hydrogen
  • 8.9% Helium
  • 0.1% other elements such as oxygen, carbon, nitrogen, silicon, magnesium, neon, iron

The theory of how our sun and planets formed is called the Solar Nebula theory. Our solar system formed from the gravitational collapse of a large cloud of gas, 98% hydrogen and helium. As it collapses it spins and the centre becomes hot, where the protosun is located. It is colder on the outside of the spinning disc. As the cloud continues to collapse conservation of energy, momentum and angular momentum flatten it out. Further in towards the centre the resulting planets are warmer and further away the planets are colder.

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The image below shows the structure of the sun,

sun_structure

The temperature at the core is around 15 million celsius. The surface of the sun is around 5,500 celsius. In its core the sun is burning hydrogen and helium via nuclear fusion, this is what stars do and it is why they shine. The sun contains about 99.9% mass of the entire solar system and utterly dominates the gravity of the orbiting planets.

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The sun also emits a solar wind, charged particles flowing outwards from the sun that causes space weather and on Earth causes the northern lights.

Our sun has been around for over 4.5 billion years but will not live forever. The graphic below indicates the life cycle of our sun and in about 5 billion years time something rather dramatic will happen!

lifecycle-of-the-sun

So, in around 5 billion years time the sun will effectively run out of gas. The sun will begin to puff up in size and quite a lot bigger, around 30 times great in size, the Earth will literally be inside the sun. It will become a red giant. A red giant is red because its exterior has cooled from 9,000 to 3,000 Fahrenheit. This red giant stage will last for another 2 billion years. Eventually the sun will start to contract and become a bit larger than its original radius but give off 10 times as much energy than at present. This phase will last only 500 million years. Our sun will become a white dwarf and then a black dwarf.

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The left hand side is for stars like the sun, the right hand side is for stars that are much bigger than our sun.

We were lucky enough to be able to observe the sun from one of the observatory’s telescopes and through a h-alpha filter. It looked something like this,

sunha

This means to block out most types of light and view just a very narrow bandwidth focused on the hydrogen alpha spectral line. It means it is safe to observe the sun. The light occurs when a hydrogen electron falls from the third to the second lowest energy level. It is useful for observing prominences.

Did you know… NASA has a spacecraft orbiting the sun called the Solar Heliospheric Observatory (SOHO). The objective of the mission is to observe all aspects of the sun. It was launched in 1995 and is still going strong now. You can view the latest images of the sun on its webpage here. The image below is the sun taken with extreme ultraviolet imaging telescope (EIT 195) – 1.5 million kelvin.

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Anyway, there is so much information online about the sun and it is so interesting. Keep reading and learning!

Next week …. more about stars!

Exploring the Heavens – Week 3

The topic for this week was ‘The Characteristics of the Solar System’. The main items covered were the formation of the solar system, the planets and other celestial bodies such as comets and asteroids.

Due to being ill this week I attended the Thursday class instead of my usual Tuesday class, but the format was exactly the same. Paul’s 3D presentation was fantastic as it put us right in the middle of the solar system and we could experience it from many different angles and points of view, which really helped in developing our understanding of how it all works.

One of the main themes throughout this course has been the ‘ecliptic’ – a plane on which all the planets sit and orbit the Sun. Our solar system consists of our star – the Sun – an object which dominates our neighbourhood, consisting of 99% of the mass and also gravitationally. The 8 planets and countless comets and asteroids all belong to the Sun. The unit by which we measure the distance of the planets from the Sun is called an astronomical unit, and the Earth is 1 AU from the Sun (approx. 150m km). It is possible that the distances of all the planets from the Sun can be explained with a mathematical formula that proves they are not just random distances.

A sidereal period is the time it takes a planet to orbit the sun. The further the planet from the Sun the greater the period. Kepler’s third law – the law of periods – found a simple relationship between the distance and the period. The ratio of the average distance from the Sun cubed to the period squared is the same constant value for all planets.

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His equation:

kep3

Our best guess at how our solar system was formed is called the Solar Nebula theory. Any theory has to explain the characteristics of our system:

  1. The order of the orbits of planets
  2. The categories of planets – terrestrial and jovian
  3. The amount of comets and asteroids
  4. Anomalies

formation

  1. Our solar system formed from the gravitational collapse of a large cloud of gas and dust
  2. As the cloud collapses, conservation of energy, momentum and angular momentum flatten it out into a disk
  3. The diffuse clouds end up as a spinning disk of gas and dust with the young protosun at the middle
  4. The spinning disk is hotter at the centre and colder on the outside, so closer in are more material of rock and iron, further out more hydrogen compounds such as methane, explaining the makeup of our planets
  5. Finally the Sun ignites and releases a strong solar wind to clear away the remaining dust other material. We are left with the planets we have now

We talked about all of the planets and their characteristics. When we were outside looking up we could clearly see Jupiter and Mars on the ecliptic. The signs of the Zodiac pass through the ecliptic. This is a good way to find a planet!

The synodic period is the time it takes for a planet to return to the same angle with respect to the Sun. The synodic period for Mars is 780 days for it to move from opposition to opposition. Prior to opposition is when the planets move in retrograde motion.

We had an amazing view of the Moon tonight through a telescope on the balcony at the observatory. This was my first time viewing the Moon through a telescope and it looked amazing, we could see so much detail. I can only imagine the view from the Moon looking back at Earth.

Moon

The Moon has a sidereal period (orbit) of 27.3 days and appears to go through its phases every 29.5 days (synodic period). The basis for our month. The phases of the Moon are due to the changing appearance relative to the Sun, however, we only ever see one side of the Moon as it has been locked in its orbit, due to the tidal effect from Earth.

The Moon is the major force behind tides on Earth. The gravity of the moon pulls the water  up towards it, creating an uneven distribution. The Earth and moon orbit a common centre of mass, located close to the surface of the Earth. As the Earth rotates on its axis each point on the surface is subjected to a sequence of high and low tides every 6 hours. These tides are also causing the moon to slowly drift further away from Earth at 3.7 cm per century.

Finally, we talked about eclipses, both solar and lunar. Paul said that we should all try to make it to a total solar eclipse, when the moon obscures the Sun. This happens when the 3 bodies are in exact alignment, which happens every 6 months or so. This phenomenon is possible because the Sun and Moon look the same size in the sky. The Sun is approx. 400x larger than the moon but is is 400x times further away, hence a total eclipse of the Sun is possible. Very few places on Earth will see a total eclipse due to the precision needed to make this happen.

Next week… The Stars!