Though all of the rings probably are inclined to our view so that they appear to intersect , they probably are in three different planes. The small bright ring lies in a plane containing the supernova; the two rings lie in front and behind it.
To create the beams illuminating the outer rings, the remnant would need to be a compact object such as a black hole or neutron star with a nearby companion.
Material falling from the companion onto the compact object would be heated and blasted back into space along two narrow jets, along with a beam of radiation. As the compact object spins it might wobble or precess about its axis, like a child's top winding down.
The twin beam would then trace out great circles like jets of water from a spinning lawn sprinkler. If the rings are caused by a jet, however, the beams are extremely narrow collimated to within one degree. This leads Burrows to conclude: "This is an unprecedented and bizarre object.
We have never seen anything behave like this before. The jet model explains why the rings appear to be mirror imaged, and why they appear to be symmetrical about a point offset from the center of the explosion. Burrows got the idea for the beam explanation when he noticed that where a ring appears brighter, an equally bright region appears on opposite ring.
By connecting lines between the similar clumps on opposite rings Burrows found a common center. This type of supernova will be discussed later in this chapter. At a distance of approximately In the image, you can see reddish plumes of hydrogen coming from the central region of the galaxy, where a considerable number of young stars are being born. Our most detailed information about what happens when a type II supernova occurs comes from an event that was observed in Before dawn on February 24, Ian Shelton, a Canadian astronomer working at an observatory in Chile, pulled a photographic plate from the developer.
Where he expected to see only faint stars, he saw a large bright spot. Concerned that his photograph was flawed, Shelton went outside to look at the Large Magellanic Cloud. He soon realized that he had discovered a supernova, one that could be seen with the unaided eye even though it was about , light-years away. The supernova remnant with its inner and outer red rings of material is located in the Large Magellanic Cloud.
This image is a composite of several images taken in , , and —about a decade after supernova A was first observed. Now known as SN A, since it was the first supernova discovered in , this brilliant newcomer to the southern sky gave astronomers their first opportunity to study the death of a relatively nearby star with modern instruments. It was also the first time astronomers had observed a star before it became a supernova. The star that blew up had been included in earlier surveys of the Large Magellanic Cloud, and as a result, we know the star was a blue supergiant just before the explosion.
By combining theory and observations at many different wavelengths, astronomers have reconstructed the life story of the star that became SN A. Formed about 10 million years ago, it originally had a mass of about 20 M Sun. At this time, its luminosity was about 60, times that of the Sun L Sun , and its spectral type was O. When the hydrogen in the center of the star was exhausted, the core contracted and ultimately became hot enough to fuse helium.
By this time, the star was a red supergiant, emitting about , times more energy than the Sun. While in this stage, the star lost some of its mass. This lost material has actually been detected by observations with the Hubble Space Telescope Figure 4.
The gas driven out into space by the subsequent supernova explosion is currently colliding with the material the star left behind when it was a red giant. As the two collide, we see a glowing ring.
Figure 4: Ring around Supernova A. These two images show a ring of gas expelled about 30, years ago when the star that exploded in was a red giant. The supernova, which has been artificially dimmed, is located at the center of the ring. The left-hand image was taken in and the right-hand image in Note that the number of bright spots has increased from 1 to more than 15 over this time interval. These spots occur where high-speed gas ejected by the supernova and moving at millions of miles per hour has reached the ring and blasted into it.
The collision has heated the gas in the ring and caused it to glow more brightly. The fact that we see individual spots suggests that material ejected by the supernova is first hitting narrow, inward-projecting columns of gas in the clumpy ring. The hot spots are the first signs of a dramatic and violent collision between the new and old material that will continue over the next few years.
By studying these bright spots, astronomers can determine the composition of the ring and hence learn about the nuclear processes that build heavy elements inside massive stars.
Challis, R. Sugerman STScI. Helium fusion lasted only about 1 million years. When the helium was exhausted at the center of the star, the core contracted again, the radius of the surface also decreased, and the star became a blue supergiant with a luminosity still about equal to , L Sun.
This is what it still looked like on the outside when, after brief periods of further fusion, it reached the iron crisis we discussed earlier and exploded.
Some key stages of evolution of the star that became SN A, including the ones following helium exhaustion, are listed in Table 1. Once iron was created, the collapse began. It was a catastrophic collapse, lasting only a few tenths of a second; the speed of infall in the outer portion of the iron core reached 70, kilometers per second, about one-fourth the speed of light.
In the meantime, as the core was experiencing its last catastrophe, the outer shells of neon, oxygen, carbon, helium, and hydrogen in the star did not yet know about the collapse. Information about the physical movement of different layers travels through a star at the speed of sound and cannot reach the surface in the few tenths of a second required for the core collapse to occur.
Thus, the surface layers of our star hung briefly suspended, much like a cartoon character who dashes off the edge of a cliff and hangs momentarily in space before realizing that he is no longer held up by anything. The collapse of the core continued until the densities rose to several times that of an atomic nucleus.
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