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Star Sequence (Life Cycle). Born in Stellar Nebula Stars are large balls of hot gas. They look small because they are a long way away, but in fact many are bigger and brighter than the Sun. Stars are made (or “born”) in giant clouds of dust and gas.
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Star Sequence (Life Cycle) • Born in Stellar Nebula • Stars are large balls of hot gas. • They look small because they are a long way away, but in fact many are bigger and brighter than the Sun. • Stars are made (or “born”) in giant clouds of dust and gas.
Sometimes part of the cloud shrinks because of gravity. • As it shrinks it becomes hotter and when it is hot enough, nuclear reactions can start in the centre….. • … and A Star is Born!
Once nuclear fusion is producing heat in the centre of the new star, this heats stops the rest of the star collapsing. • The star then stays almost exactly the same for a long time (about 10 billion years for a star like the Sun). • The balance between gravity trying to make the star shrink and heat holding it up is called Thermodynamic Equilibrium.
Depending on the star’s mass it becomes one of three general types of stars • Brown Dwarf – never fully develops into a star – not enough mass (hydrogen for fusion to continue) • Average Star (Our Sun) • MASSIVE Star
Average Stars • Also known as Main Sequencestars – make up 90% of stars in space • Become Red Giantsas hydrogen resources are depleted • Then can break up into a Planetary Nebula • Remaining part of the star becomes a White Dwarfas it cools • Once all heat is gone it then becomes a Black Dwarfand dies.
Massive Stars • BecomeSuper Red Giantsas hydrogen resources are depleted • Then explodes into a Super Nova • Remaining part of the star becomes either • aNeutron Star, or • aBlack Holeand dies.
The life of a star • During its “life” a star will not change very much. • However, different stars are different colour, size and brightness. • The bigger a star, the hotter and brighter it is. Hot stars are Blue. Smaller stars are less bright, cooler and Red. • Because they are so hot, the bigger stars actually have shorter lives than the small, cool ones.
How does a star “die”? • Eventually, the hydrogen (the “fuel” for the nuclear fusion) in the centre of the star will run out • No new heat is made and gravity will take over and the centerof the star will shrink. • This makes the very outside of the star “float up” and cool down, making the star look much bigger and redder - a Red Giantstar.
The second Red Giant stage • As the centre collapses, it becomes very hot again, eventually getting hot enough to start a new kind of nuclear fusion with Helium as the fuel. • Then the Red Giant shrinks and the star looks “normal” again. • This does not last very long, though, as the Helium runs out very quickly and again the star forms a Red Giant.
The end of a Sun-like star • For a star like the Sun, no more nuclear fusion can take place, so the centre of the star will then keep collapsing. • Eventually it can become almost as small as the Earth, but with the same mass as a whole star! This very dense object is called a White Dwarf. • A piece of White Dwarf the size of a mobile phone would weigh as much as an elephant on the Earth!
Hetrzsprung- Russell (H-R) Diagram • Illustrates important things about stars • Brightness • Absolute Magnitude • usually shown on right-hand Y-axis • Luminosity • usually shown on left-hand Y-axis • Temperature/Color • Spectral Class • shown on X-axis
Nebulae Emission Nebulae Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states (in much the same way as a neon light). These nebulae are usually red because the predominant emission line of hydrogen happens to be red (other colors are produced by other atoms, but hydrogen is by far the most abundant). Emission nebulae are usually the sites of recent and ongoing star formation. (M 42 shown)
Reflection Nebulae Reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. Reflection nebulae are also usually sites of star formation. They are usually blue because the scattering is more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae. (NGC 7023 shown)
Dark Nebulae Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across. (NGC 2264 shown; see also the Horsehead Nebula)
Planetary Nebulae Planetary nebulae are shells of gas thrown out by some stars near the end of their lives. Our Sun will probably produce a planetary nebula in about 5 billion years. They have nothing at all to do with planets; the terminology was invented because they often look a little like planets in small telescopes. A typical planetary nebula is less than one light-year across. (M 57 shown)
Supernovae Supernovae occur when a massive star ends its life in an amazing blaze of glory. For a few days a supernova emits as much energy as a whole galaxy. When it's all over, a large fraction of the star is blown into space as a supernova remnant. A typical supernova remnant is at most few light-years across. (M 1 shown)
M 1 Supernova was discovered in 1054 and was visible for 23 days during the daytime, and easily seen for 2 years at night. It apparently was depicted in “cave paintings”
