STELLAR+EVOLUTION+-+kate+&+elizabeth+group+two

=Stellar Evolution= By: Kate St. Clair and Lizzie Johnson Yellow Class

The definition of stellar evolution is the sequence of radical changes that a star undergoes during its lifetime (the time in which it emits light and heat).

This is the life cycle of our sun: Compared to other stars in our galaxy, our sun is an average sized star. Although it may seem like a large star to us, it is just the usual size as stars go, but because it is the closest star to Earth, it seems huge.

Stars are massive balls of bright plasma that group together to form galaxies. The closest star to the earth is our sun, which is part part of the reason it appears so large and luminous, even though it is just an average sized star. This is also why we can see it during the day. Middle size stars last for billions of years. The Sun is the nearest star to Earth

Creation:
Stars are created when other stars die. When a star dies, it leaves behind a large cloud of dust made of hydrogen, helium, and other elements. These elements are compressed through many complex processes, including nuclear fusion, and convection. Some of these elements support the star enough so that it won't collapse on itself. Eventually the star will become a Red Giant, and from there possibly morphing into a degenerate form.

Stars are formed in Stellar Nurseries. Stellar Nurseries are huge gaseous dust clouds where tiny litte particles congregate to form protostars, and possibly planetary systems, with more than one star.

Gravity is constantly pushing a star in, fighting the star so that it will collapse on itself, but the stars' pressure is pushing out, fighting to keep it un-collapsed.



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The following two images are photos of stellar nurseries:
[|Hubble telescope] image known as //pillars of creation,// where stars are forming in the [|Eagle Nebula] [|LH 95] stellar nursery in Large Magellanic Cloud

The following photo is of a star field, or, planetary system:
A dense starfield in [|Sagittarius]

Low-mass stars:
When a low-mass star such as a red or white dwarf stops producing enough energy, it will slowly begin to collapse into itself. It a low-mass star does not collapse onto itself, it may become a red giant. If a star's core becomes inactive it will be surrounded by hydrogen layers which the star may draw from, but if it is still able to transfer heat through convection, it will not be surrounded by these layers of hydrogen. Low-mass stars are very luminous due to their low mass. Low mass stars last for trillions of years.

The evolution of a star with the mass of the Sun. The star begins as a bok globule (1) then undergoes a contraction period as a protostar (2) before joining the main sequence (3). Once the Hydrogen at the core is consumed it expands into a red giant (4), then sheds its envelope into a planetary nebula and degenerates into a white dwarf (5).

Mid-sized stars:
Some examples of mid-size stars, also known as red giants are Aldebaran which is in the constellation Taurus, and Arcturus in the constellation Bootes. These mid-size stars develope a helium core. Gravity compresses the hydrogen, so it fuses faster than it would normally. This makes the luminosity of the star greater, so the effective temperature decreases.



Massive stars:
Massive stars, also known as red super giants, are not very luminous. They are formed from lower mass stars that were very bright and luminous. This occurs because the core is so large that helium ignition will occur before pressure can become effective. Massive stars lose mass rapidly due to pressure, so they lose their shells before they can become supergiants, and because of this they mantain extremely high surface temperatures. Massive stars only last for millions of years.

This photo displays the layers within a massive star, just before it collapses.



Variable stars:
This photo is one example of a variable star. This is the reflection nebula GMC 1999. Variable stars are stars that may have random or intermittent changes in luminosity. Some stars also go through phases where they may become pulsating variables. These stars change in size, shape, and luminosity. The time it takes to complete one 'pulse' varies from star to star, and may range generally speaking between minutes and years, depending on the size of the star. Eruptive variables are stars that experience sudden and unpredictable pulses. This category includes stars called protostars, Wolf-Rayet stars, Flare stars, giant stars, and supergiant stars. Many variable stars have one last, exciting pulse before errupting into a cloud of dust to create a new baby star.



The above is a picture of Mira, one example of a variable star.

Reference Section:

 * Flip book


 * star notes


 * Text book pages: chapter 21, sections 2 and 3. page numbers 382-388.

=__GOOD LUCK ON ALL YOUR EXAMS!!__=