Dying Stars

Now let's move our attention all the way across the sky to this area. This is the Big Dipper or Ursa Major. Unfortunately, at this time of the year, it is low in the northeastern sky shortly after sunset. However, it does get higher in the sky later in the night. At this point in the Dipper's bowl, lies another example of a planetary nebula, M97 (NGC 3587) called the "Owl Nebula."

Estimates of the distance to the Owl Nebula vary greatly, but it is agreed that it is several thousand light years away. You'll notice that right in the center of the nebulosity, there is a faint star. This star is an example of a white dwarf star and is the key to the nature of the planetary nebulae. In fact, the Owl Nebula has about as much to do with owls as it does with planets! The Owl Nebula is so-called due to its shallow resemblance to the face of an owl. The class of objects in the sky called "planetary nebulae" are similarly named for their resemblance to planets as viewed through a telescope. The origins of these objects is still not understood in detail, but they are most likely the result of a star in the red giant phase having shed its outer atmosphere through a strong stellar wind. Contrary to popular belief, most stars do not explode at the end of their lives (although it is true that very massive stars do) but instead die a quiet, slow death. In fact, our own Sun will likely form a planetary nebula in about 5 billion years. Only the core of the Sun will then be left in the center of the nebula as a white dwarf star.

Another beautiful example of a planetary nebula is the Ring Nebula (M57/ NGC 6720) in Lyra.

It is easy to visualize how this structure could have resulted from a star swelling up and loosing its outer atmosphere. [Point out central star.] This object is quite an impressive sight in a small telescope such as those at the ETSU Observatory. Unfortunately, it is not visible this time of year, but come to an open house at the observatory in the Summer or Fall and I promise that you will see it!

As I mentioned above, very massive stars do die in violent explosions called supernovae. After these explosions, the bulk of the star is thrown out into space and only a small neutron star is left in the center. The most famous example of a so-called "supernova remnant" is the Crab Nebula (M1/NGC 1952) in Taurus. It is located near this star which is called Zeta Tauri. It to is visible through a moderate size telescope. Here's a photograph.

By measuring the rate of expansion of the cloud, it is estimated that the star exploded about 950 years ago. In fact, on the night of July 4, 1054 it is recorded in the Chinese Annals of the Sung Dynasty that a "guest star" appeared near the star Zeta Tauri. There are also records of the "new star" in native American pictographs which date from the same time. It is estimated that the Crab Nebula is about 6300 light years away. It is a source of both radio and X-ray radiation. The remnant of the original star is now a very dense neutron star only a few miles wide. It was discovered in 1968 that this star pulses off and on at a rate of 30 times per second and was the first example of a "pulsar." The star flashes this way due to the fact that it has a very strong magnetic field which produces a beam of radiation. The star is rotating at a rate of 30 times per second and this beam of radiation sweeps past the Earth, similar to a lighthouse.

[Display meteor shower!]

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