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


  • 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,


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.


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!


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,


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.


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 4

The main focus for this week was ‘The Characteristics of Stars’. For this lass I was back to my usual Tuesday class time, and I had been looking forward to this session for quite some time.

The session started outside at the observatory waiting for the ISS to fly across the Sydney sky at 30,000 km/h. I had never seen this before so I was pretty excited to see a spaceship fly through the sky. It happened at 6.20pm and lasted for a couple of minutes. Incredible to think there are people inside the ISS doing science experiments at 30,000 km/h all day everyday.


Then inside for another amazing 3D presentation by Dr Paul Payne about stars. Starting with the Sun, which produces an enormous amount of light and heat for billions of years. Stars are huge, 100,000 to 400,000 million kilometres across. The Sun is turbulent and the surface, photosphere, can have dark spots and bright patches that can flare up to 1,000x brighter.

Stars vary greatly in temperature, size and brightness. Stars do not burn, as this would not produce enough energy to keep them alive very long, they are nuclear power reactors where hydrogen is fusing to form helium and this processes keeps them alive for billions of years.

The most important property of a star is its mass as it determines everything about it – size, light, length of life. When a star dies it crushes itself to a fraction of its original size leaving behind either a white dwarf, neutron star or a black hole.

The formation of stars

Stars are formed from nebulae – an interstellar cloud of gas (mainly hydrogen and helium) and dust. The contracting ball of gas throws out rings of material to make planets and what was left is the central star. As the central star compressed it gets hotter and hotter and gravity keeps on compressing it even further. With a core temperature of about 10 million degrees atoms of hydrogen in the core move with such immense speeds that they collide and fuse together – called nuclear fusion. This reaction produces an enormous amount of energy and is a million times more efficient at generating energy that during the equivalent amount of coal.

Our Sun

Our Sun is stable and is currently fusing hydrogen at its core, a process that lasts for 90% of its life. Energy from the core radiates through the dense central layers and slowly makes its way to the surface in the form of x-rays and gamma rays. The journey could take a million years to happen. At the surface gas is not as dense so to expel the energy it uses a convection process. Gas below the surface moves in packets carrying heat energy. At the surface each packet radiates energy out into space, and we on Earth see this energy in the form of visible light.

The appearance of the Sun looks cellular, however, each cell is approx. 1000 km across. Small bright patches are where gas is radiating and darker veins are cooler areas where energy has fallen back into the Sun. The temperature of energy varies from 15 million degrees at the core to 5700 degrees at the photosphere. This is why the Sun looks yellow, because more yellow light is produced at this temperature than any other colour.

A sunspot is a region on the surface where the gas is cooler than its surroundings, approx. 4000 degrees cooler. The spot generates less light so appears darker. Galileo correctly recognised them and was able to measure the period of rotation of the Sun. The Sun does not rotate as a solid ball, each latitude rotates at a slightly different rate, for example the equator takes 27 days to rotate once and at a latitude of 40 degrees it takes 29 days.


Solar eruptions are more frequent as the amount of sunspots increases. These eruptions are also called flares, where a small area may increase in intensity and temperatures of a million degrees may be generated so particles are exploded and hurled from the Sun that may even reach Earth and cause auroras visible near the poles.

The Life of the Sun

The Sun will fuse hydrogen for about 9 billion years. The leftover helium ash will build up the core. The Sun is stable during this time. It will eventually start running out of fuel at the core and it will prepare to die. It will take another billion years or so and its energy output will vary, called a variable star, where its brightness may vary.

The Death of the Sun

Eventually the core will collapse causing heat that will drive nuclear reaction faster resulting in the Sun swelling. It will grow to over 100x its present size and be 1000x brighter. The surface will cool and it will become a Red Giant. Finally the ignition of helium in the core, fusing to form carbon, nitrogen and oxygen, generating enormous power. It will glow with a brilliant red, something that happens to all stars prior to dying.

At this stage the star will produce heavier elements than helium up to iron. Elements heavier than iron when fused will not produce energy but absorb it, making the star very unstable. A planetary nebula is then produced, shells of gas are released cocooning itself in a cloud of its own material. They appear as faint blue discs surrounding dying stars. When the star tries to fuse iron the reaction absorbs energy from the core and the interior behaves more like a refrigerator than an oven. The core collapses, gravity compresses the star the size of the Sun to the size of Earth. It is extremely dense, a sugar cube sized peeve of its material would weigh 5 tonnes. The star is dead, it does not produce energy and it slowly cools off, it is now a white dwarf.

The power of stars

Stars are rated like light bulbs. Some stars produce 100,000x more power than our Sun. To measure the power we have to measure the brightness and how far away it is. If we know any two of power, brightness and distance we can find the third quantity.

Stars are hot!

The surface temperature of a star can be measured by its colour. Cool stars are red at 3000 degrees. Hot stars are blue at 15,000 degrees on the surface. We can see the colours of stars at night with the naked eye. We notice blue stars more as they generate more power. Examples are Sirius, Rigel. They are blue stars and are 8 and 815 light years away respectively. Examples of red stars, which do not last as long in the night sky, are Betelgeuse, Antares and Gamma Crucis.

Composition of stars

The composition of a star can be measured from the light from it. Telescopes equipped with a spectrometer can break the light into its different component features, called an absorption spectrum. Dark lines throughout the spectrum, like a barcode, reveal the elements, such as hydrogen and helium. The spectrum can also reveal the velocity as the lines will be red-shifted, meaning the star is moving away from us. The greater the velocity the greater the shift. If the star is moving towards us it will be blue-shifted. This shift in spectral lines is called the Doppler Effect.

An emission spectrum is produced by a glowing cloud of gas, like a nebula. Dark lines are now bright and the bright, rainbow continuous spectrum background is gone. Bright lines coincide with light lines and can measure all the properties listed for an absorption spectrum.


A supernova is a very violent event in the universe when a massive star explodes. Elements heavier than iron are produced in the explosion. Gravity is the driving force behind the collapse of the core. The remnant is a compressed star smaller than a white dwarf, even 20 km across. A sugar cube sized piece can weigh 5,000,000 tonnes. This is known as a neutron star. It emits pulses of light, like a light house. Two beams of light are generated from the magnetic poles and as it rotates the beams are swept out into space. An example is the Crab Nebula Pulsar which flashes at 30x per second, meaning it rotates 30x per second.

Black Holes

A star with 3x the mass of our Sun can collapse to form a black hole. This is when the star pulls itself out of existence due to the amount of gravity it has. Nothing can escape a black hole, not even light. The star is reduced to a singularity into which all matter is pulled to. An envelope of space around it is called the event horizon, this is the point of no return. A typical size would be 10km. They are small and black so very hard to detect, we can only hope to detect their gravitational influence on objects around them. A famous black hole is in the binary system called Cygnus X1.

Cyg X-1 generic

Binary Stars

Most stars come in pairs. What appears as one point of light in the sky is actually the accumulation of two stars close together. These stars are in orbit around each other, this is called a binary system.

Next week… Telescopes.

Exploring the Heavens – Week 2

The focus of this week’s class was Celestial Rhythms and the development of the constellations.

We started the class upstairs in the observatory and outside on the balcony with Paul describing our session and then identifying a few of the objects on display in the night sky, including a crescent Moon. Paul described how the Moon is moving around the Earth and how the position of the Sun in relation to the Earth and Moon affects the different phases of the Moon. He explained that next week the moon will have moved across the sky in counter-clockwise direction and showed us approximately where it would be.

We then headed downstairs and outside to a large model of the solar system painted on the ground, as seen in the image below during the daytime. We sat around the model and Paul described the motion of the Earth and other planets around the Sun. This demonstration was a great way to visualise our solar system and how various planets move in relation to the Sun. Paul also described how our view on Earth is affected by the Sun and how the Sun blocks various stars at certain parts of the year.


This session was about the importance of understanding how the sky moves during a night and throughout a whole year.

“The sky is on a continual march that presents us with different constellations at different times of night, and different times of year.”

After our outside demonstration and further observing of the night sky we moved back inside for a much needed tea break and to escape the chilly Sydney evening weather, should have taken a jacket!

After our break we headed downstairs to the 3D theatre and enjoyed another presentation by Paul. The focus of the inside session was to demonstrate the motion of the Earth travelling around the Sun and its rotation on its own axis, what causes the seasons as well as examine closely what makes a day and a year, which is not quite what we thought.

I learnt so much over our 2 and a half hour class. As we spin around our axis once per day we actually move about 1 degree around the Sun. We rotate and orbit in a counterclockwise direction and in a year there are 365.25 days. So, the Earth, rotating at 1360 km/h, must spin on its own axis 360 + 1 degrees to have the Sun reappear in the same position. A day as we know it is 24 hours long, this is actually a solar day as our clocks have been tuned to the combined motion of our rotation and orbit around the Sun.

A different kind of day is called a sidereal day, which is 23 hours 56 minutes and 4 seconds long. This is the time astronomers use to predict the location of stars on the night sky. All of this means that we will see a slightly different sky each night.

We all know we have four seasons on Earth and the seasons are caused by our axis of rotation being tilted by 23.5 degrees to the plane of our orbit, called the ecliptic. The axis points in the same direction as we orbit the Sun, and this is what causes the seasons and the variation in the length of a day throughout the year. Paul gave us examples of the equinoxes and the solstices. The equinoxes occur approx. half way between the solstices on March 21 and 21 September, the solstices on 21 June and 21 December. In June the axis is pointed towards the Sun so the northern hemisphere will see the Sun high in the sky and will receive many hours of sunbathed daylight. In the southern hemisphere people will see the Sun much lower in the sky and will experience less daylight compared to the northern hemisphere. The reverse is true for December. At equinox the day and night are the same length for both hemispheres – equal.


“We spend our days and nights humbly on planet Earth. We suffer from the illusion that we feel stationary on the surface of the Earth and that the Sun and stars seem to revolve around us.”

Infant, our axis is slowly precessing, like a spinning top over a period of 26,000 years. This means that the celestial poles on the celestial sphere sweep out a circle every 26,000 years carrying our coordinate system of the stars with it. This small drift was actually measured by Hipparchus around 140 BC. Paul has recommended we purchase a planisphere, seen below, a dynamic map that can portray the night sky for a given location at any time of night throughout the year. These are only designed for one latitude and do not contain much information.

There are 88 constellations in the night sky. Each constellation is an area of the sky and are recognised and classified by the International Astronomy Union (IAU). We learnt heaps about the constellations:

  • Each culture developed its own constellations
  • Constellations often symbolised mythological creatures
  • Constellations are a reminder of a lesson or a seasonal event
  • Many originated in Babylonia
  • The Greeks adopted many and developed their own mythologies to explain them
  • The constellations the Sun passes through along the ecliptic are called the zodiacs
  • There are actually 13 zodiacs, including Ophiuchus, although the Sun does not spend too much time in this sign
  • The Babylonians set up the zodiacs and their counting system was based on base 12, so they stuck with just the 12 constellations and that is why we have 12 hours of day, 12 hours of night and 12 months a year
  • The Sun now spends more time in Ophiuchus than in some neighbouring constellations

I am a Pisces and here is my constellation from the IAU (


A few facts about Pisces:

  • One of the most ancient constellations
  • Depicts 2 fishes swimming in opposite directions with their tails joined
  • In mythology the two fishes represent Aphrodite (Venus) and Eros (Cupid)
  • One day they hide to hide in rushes along the Euphrates to escape a monster called Typhon. The two fishes swam away to safety
  • Pisces is watery and faint, it lies in an undistinguished part of the sky
  • Its brightest stars are only magnitude 4
  • One way to locate Pisces is by reference to the square of Pegasus
  • The Sun passes through Pisces between Feb 19 and March 20
  • The 12th sign
  • Famous Pisces include Steph Curry (Mar 14), Einstein (Mar 14), Steve Jobs (Feb 24), Daniel Craig (Mar 2), Jon Hamm (Mar 10)

Pisces stars


Eta Piscium is the brightest star at 316x that of the Sun. It is 294 light years from Earth and is a G class bright giant star.

Pisces contains a Messier object called Messier 74, a spiral galaxy located between the stars alpha Arietis and eta Piscium. It is also known as the Phantom Galaxy, shown below.



The grand-design spiral galaxy Messier 74 as photographed by the Hubble Space Telescope in 2007. Image: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration. Acknowledgment: R. Chandar (University of Toledo) and J. Miller (University of Michigan)

The red areas indicate pockets of hydrogen gas. They glow due to the radiation from hot, young stars. Astronomers call these areas H2 regions. The brighter stars are not part of the galaxy and are actually located a lot nearer to us. The galaxy appears face-on and is approx. 30 million light years from Earth. It is roughly the same size as the Milky Way with a diameter of 95,000 light years. Two supernovae have been seen exploding in recent years in this galaxy. It contains about 100 billion stars. It is not easy to observe due to low surface brightness and requires clear skies. The only other object with a lower surface brightness is M101, the Pinwheel Galaxy, shown below.



The Phantom is an example of a grand design spiral galaxy with 2 clearly defined arms which extend for about 1,000 light years. The arms contain clusters of young blue stars and star forming nebulae. It is receding at a speed of 793 km/s.

Zoom into M74:

M74 was first observed in 1780 by French astronomer Pierre Méchain who told his good friend Charles Messier about it, Charles added it to his famous catalogue.

More about M74 here.

Thank you Hubble!

Top 10 Solar System facts so far…

10. The total number of stars in all the galaxies is comparable to the number of dry sand grains on all the beaches on Earth. There are about 100 billion galaxies and the total number of stars in all those galaxies is about 1000 billion per galaxy.

9. We are always in motion:

  • We spin around Earth’s axis at 1000 km/hr
  • We orbit the Sun at 100,000 km/hr
  • Our solar system moves randomly at 100,000 km/hr
  • Our solar neighbourhood orbits the centre of the Milky Way at 1,000,000 km/hr
  • Our galaxy moves randomly among our local group of galaxies at 300,000 km/hr

8. The Earth wobbles like a spinning top. The path the wobble carves out is called precession. It takes approx. 26,000 years for the Earth to complete one top wobble.


7. Solar eclipses happen because of coincidence. The Sun is 400 times larger than the Moon, and the Sun is also 400 times farther away. These 2 factors are such that the Moon can completely overlap the Sun. In the distant past the Moon used to be a lot closer to the Earth so eclipses were not possible. Also, the Moon is slowly drifting away from Earth at 1 cm per year, so in the distant future it will be too far away to block the Sun.

6. The names for the 7 days of the week come from the 7 known objects known to change their position: Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. The names come from the Norse-Celtic barbarians that overthrew Rome.

  • Sun-day
  • Moon-day
  • Tiu (Mars) – day
  • Woden (Mercury) – day
  • Thor (Jupiter) – day
  • Freya (Venus) – day
  • Saturn – day


5. Astronauts orbiting the Earth are weightless because they are continuously falling toward the Earth, not because there is no gravity. There is plenty of gravity in space. Astronauts and the ISS are continually falling, but they never reach it, that is why they are weightless.

4. The human eye was the first astronomical detector, however, it can only detect 1 in every 1000 photons. Astronomy’s newest detector, the Charge-Coupled Device (CCD), captures 1 in every 1.1 photons.

3. The Sun contains about 99.9% of the total mass in the solar system.


2. Our solar system formed from the gravitational collapse of a large cloud of gas and dust. This theory is called Solar Nebular theory. As the cloud collapsed is flattened out into a disk. Our young protosun is at the middle of the disk, where the temperature is hotter than the outside. This determined the difference in the types of planets that formed, rocky terrestrial planets on the inside and gas giants on the outside. When the Sun finally turns on strong solar winds burn away left over material from the disk.

1. The Apollo 11 mission boot prints on the surface of the Moon will remain for around 200 million years due to the lack of erosion, due to very little atmosphere on the Moon.