The famous Ring Nebula, also known as M57, is one of the strangest looking sights in the night time sky. Under low to medium magnification (35-100 power) with at least a 6 inch reflector telescope, it appears as a small pale-blue donut or smoke ring, surrounding what appears to be an empty hole. When you see it for the first time it looks unreal, as if it isn't really out there in space but something somebody painted on the lens or mirror of your telescope.(See University of Arizona's Lunar and Planetary Laboratory website for beautiful time exposure photograph of the Ring Nebula - don't expect to see such color, size or detail in your telescope).To find the Ring Nebula, first find the star Vega (in the constellation Lyra). Vega is the brightest star (not including planets) in the summer and early autumn evening sky. It is the northwest corner of the "Summer Triangle", which also includes the bright stars Deneb and altair. Along the imaginary line between Vega and Altair (Altair is the south-most corner of the Summer Triangle), about one half fist width from Vega, are two dim stars in a diagonal line. The Ring Nebula is almost exactly in the middle of these two dim stars. You need a telescope to see it.
The Ring Nebula is the best known member of a class of stars called "planetary nebula". Despite the name they have nothing to do with planets. They are dying stars. It is believed our own sun will one day end its existence as a planetary nebula.
Stars go through a series of fascinating stages as they "die". A star's fate as it begins to die is totally dependent on how much matter it is made of - its mass. It is believed that any star with about as much mass as our sun, and up to about two times larger, goes through the following stages of death:
- As it uses up the hydrogen that fuels it, it can no longer fight off the gravity that pulls it inward, and its interior begins to contract.
- Further spurts of energy from the contraction pushes out its cooler outer layers and causes a pulsating swelling in size. The star becomes a "red giant".
- Eventually the outer layers of some red giants, containing as much as 20% - 50% of the original star, become disconnected - an expanding shell of gas thrown into space, no longer part of the star.
- The central core continues to become denser, cooling from a "white dwarf" to eventually a cold, dead "black dwarf" about the size of the Earth.
The donut-like ring we see when looking at the Ring Nebula is the disconnected shell of gas from a sun that once was - a sun with approximately the mass of our own sun. Somewhere inside the shell, too small or dark to be seen with most amateur telescopes, is the remnant of the remaining star.
In his book Cosmos, astronomer Carl Sagan describes a similar fate for the death of our own sun in about 5 billion years. He states:
"In its death throes, the Sun will slowly pulsate, expanding and contracting once every few millennia, eventually spewing its atmosphere into space in one or more concentric shells of gas. The hot exposed solar interior will flood the shell with ultraviolet light, inducing a lovely red and blue fluorescence extending beyond the orbit of Pluto. Perhaps half the mass of the Sun will be lost in this way. The solar system will then be filled with an eerie radiance, the ghost of the Sun, outward bound."In viewing the Ring Nebula we see an image of the future death of our sun, and with it all the life giving forces and energy on Earth.
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