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STARS
When we look up to the sky on a clear night we can see the stars. You will notice that some appear to be brighter than others and you may also notice that some appear to differ in colour. In fact there are many different types of star and we will try to give you a brief introduction of how they form, what they are made of and how they die.

The closest star to our planet is the Sun, which happens to be a fairly ordinary star. Most of the stars that you see in the sky are far larger and brighter than the Sun but because they lie at vast distances from us, they appear as tiny dots in even the most powerful telescopes.

Magnitude

We measure a star's brightness on a magnitude scale.
  • The dimmest stars visible from a dark site for someone with good eyesight
    are magnitude 6.
  • As they increase in brightness they move up the magnitude scale through
    5, 4, 3, 2, 1, 0, -1
  • The brightest star in the sky is Sirius at magnitude -1.44
  • The Full Moon is magnitude -12 and the Sun is -27
  • The dimmest objects observed with the largest telescopes are at magnitude 27
  • The scale is not linear, each increase in magnitude corresponds to almost
    2.5x reduction in brightness and so a difference of 6 magnitudes equals a
    brightness difference of 100x

    However, the observed brightness is not indicative of a star's true brightness. A dim star close to us can appear brighter than a bright star further away. The stars that make up the Summer Triangle asterism are a good example of this. Altair in Aquila is 10x brighter than the Sun. Vega in Lyra is 50x brighter, and Deneb in Cygnus is 54,000x brighter!




    Colour

    The colour of stars is directly related to their temperature which is measured in Kelvin. A change of temperature of one Kelvin is a change of 1 degree Celsius but 0 Kelvin equals -273.15 degrees C or Absolute Zero.
    The blue stars are hotter and the red stars are cooler.
    They are divided into spectral types which are arranged from the hottest blue to the coolest red stars. There are now further classifications that cater for much dimmer stars, however most of these are out of the reach of small telescopes.



    This can be easily remembered by the mnemonic
    Oh Be A Fine Girl/Guy Kiss Me

    They are also categorised by the elements found in their atmosphere.

    Stellar evolution

    Stars form from gas and dust that is found in dense extremely cold nebula known as molecular clouds or stellar nurseries.



    At these temperatures which are only a few degrees above absolute zero (10-20Kelvin) the gas molecules which are mainly Hydrogen, Helium and Carbon monoxide, tend to clump together when they collide due to their electrostatic charge. As more and more dense clumps stick together, gravity takes over and this draws ever more gas into a swirling highly dense protostar.
    When the temperature in the core of the protostar rises high enough due to the force of gravity generating higher pressures in the core, the star begins to shine and release energy.

    The energy is released from nuclear fusion in the core of the star which is converting Hydrogen atoms into Helium atoms, the resulting Helium atom has less mass than the four Hydrogen atoms that created it. The missing mass is released as energy in the form of charged particles and electromagnetic waves of which light is a part.

    Stars live their life in a balance between the force of gravity that's trying to compress them and the internal nuclear pressure trying to expand them.

    Strangely the more massive stars have a shorter lifespan than the smaller stars, as the higher gravity increases the temperature in the core of the star which therefore burns fuel quicker and therefore the star lives for less time.

    When a star with the mass of our star, the Sun, runs out of Hydrogen in its core, the outward pressure will decrease resulting in the core collapsing and getting hotter. This extra heat then begins to fuse the previously fused Helium into Carbon and in turn ignites the remaining Hydrogen in the outer layers causing the star to expand and cool into a Red Giant. When our Sun goes through this stage in around 5 billion years it will swell and engulf all of the inner rocky planets except Mars. The remaining core will then collapse again and become a White Dwarf with the outer shell expanding outwards to create a Planetary Nebula.


    The above Hubble image shows the planetary nebula M57 (the ring nebula) in the constellation of Lyra. The small White Dwarf can be seen in the centre.

    White Dwarves have no method of generating heat and gradually cool into a Black Dwarf. It has been calculated that the time it would take for this to happen would be longer that the time since the Big Bang and so no Black Dwarf stars actually exist.


    The above diagram shows the relative size of a red supergiant compared to the Sun.

    Larger stars have a different demise! Due to the greater mass, the core continues to fuse the Carbon into Neon, Neon into Oxygen, Oxygen into Silicon and finally Silicon into Iron.

    At this point it takes more energy to fuse Iron than it can possibly release and so the outward pressure stops abruptly, causing the entire star to collapse once more. The core implodes and rebounds into the inrushing outer layers and a cataclysmic explosion occurs. This is called a Type 2 Supernova.

    During this explosion all of the naturally occurring elements heavier than Iron are produced including the stuff that we are made of!

    A remaining stellar core with above 1.4 solar masses will collapse into an incredibly dense star in which the electrons and protons in each atom will combine to create a Neutron Star. It is only the neutron degeneracy pressure that prevents total collapse. These rapidly spinning dense stars are also known as Pulsars as they send jets of radio waves out into space. A single spoonful of the matter in a star of this type would weigh several tons!

    A core with over 10 solar masses will overcome the neutron degeneracy pressure and a Stellar Mass Black Hole is formed. Nothing including light can escape the gravitational field of a black hole.

    Binary and multiple stars

    Most stars in the sky are Binary Stars in which two stars which have formed within the same molecular cloud will live their lives in tandem, rotating around a common centre of gravity.

    Some binary or double star systems consist of stars that are of equal mass and they go through all of the evolution of single stars. However it's when one of the stars is larger than the other that strange things are seen to happen.

    The larger star will reach the Red Giant phase before the other smaller star, and it eventually turns into a white dwarf. When the smaller star reaches the Red Giant stage, some of its outer Hydrogen layer can spiral towards the white dwarf and this will ignite the gas on the white dwarf to create a Nova.

    Novae can recur over time as the red giant expands and contracts. If the mass of the white dwarf exceeds the 1.4 solar mass limit a type 1A Supernova will take place.

    These are extremely violent events during which the star will outshine the combined brightness of the billions of stars in the rest of its galaxy.

    Because these always explode at the same 1.4 solar mass limit they all have the same brightness, we can use them to determine the distance to remote galaxies. Objects like these are known as standard candles.


    The above diagram shows the evolution of both a small and a large star.


    The above image was taken by one of our members in January 2014. It shows a type 1A supernova in the galaxy M82 some 12 million light years distant.

    Astronomers can determine whether a supernova is a type 1A or Type2 by plotting a lightcurve as a graph showing time vs luminosity, and by studying the light through spectography

    Variable stars

    Some stars vary in brightness and these are known as variable stars. Some of these are simply where one star in a binary pair is passing between its partner star and us on Earth and eclipsing the other star which causes a dip in the intensity of the combined light that is measured from the pair when side by side.

    Other variable stars actually vary in brightness and can be very useful for astronomers in determining the distance to other galaxies.
    Cepheid variables have a set brightness to period ratio which means that if you measure the time taken for the star to go through one cycle from dimmest to brightest and back, it will indicate the true luminosity of the star. We can then measure the star's brightness here on Earth (Apparent Brightness) and compare that to its true brightness. By using some maths we can then determine how far away the Cepheid variable star is.

    Therefore when astronomers spot a Cepheid variable in a distant galaxy they simply measure the time taken for one cycle to complete, measure the observed brightness against the known true brightness of the star and calculate the distance using the inverse square law. This law tells us that a star twice as far away will appear - the intensity and one 5 times further away will appear 1/25th as bright.

    All red giant stars are variable due to the instability of the fusion process within the star.


    The above image shows the constellation of Perseus. The star indicated is Algol, a variable star which varies in brightness between magnitude 2.1 and 3.4 every 2.867 days. The entire cycle from bright-dim-bright takes around 10 hours with the main dimming/brightening happening over a 4 hour period.


    'Stars are suns far away, the Sun is a star close up' - Carl Sagan



    Article by Arthur Fentaman

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