ThePiper wrote:Missing / Wanted: Supernovae and Remnants in Star Clusters.
There are a lot of old Red Giants. Will they never die?
Supernova remnants are not missing in globular clusters.
http://en.wikipedia.org/wiki/47_Tucanae wrote:
47 Tuc's dense core contains a number of exotic stars of scientific interest.
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47 Tuc has 23 known millisecond pulsars, the second largest population of pulsars in any globular cluster.
A pulsar is almost certainly a supernova remnant. If there are 23 millisecond pulsars in globular cluster 47 Tuc, then this globular may well have seen at least 23 supernovae in the past.
However, the red giants that exist today in the globular clusters of the Milky Way are not going to explode as supernovae. The reason is that they are not even remotely massive enough to explode. Most of the brightest red giants in today's Milky Way clusters are probably not that much more massive than the Sun. The Sun, too, will become a bright red giant when it runs out of hydrogen in its core, and it will be even brighter when it runs out of helium in its core. But the Sun will never go supernova.
Of course there were massive stars in globular clusters in the past. The massive stars were there long long ago, when the globular clusters were young, and the universe was young. But the massive stars exploded as supernovae long ago, leaving, for example, 23 millisecond pulsars behind in 47 Tucanae.
The bright red giants in the globular clusters of today are actually dying and disappearing "all the time", although the process still takes too long for us to have "caught one in the act". But as a matter of fact, the brightest red giants in globular clusters are on their way out. Very soon, in cosmic terms, they will shed their outer envelopes and become white dwarfs. And as white dwarfs they may actually still go supernova, provided they have just the right sort of companion to interact with. But since the last white dwarf supernova actually witnessed in the Milky Way was seen in the sixteenth (or possibly seventeenth) century, don't hold your breath waiting for one to explode in a globular cluster.
As red giants die in globular clusters, others take their place. Not only are red giants dying all the time in globular clusters, but low-mass stars are leaving the main sequence to become red giants and other stars are leaving the blue horizontal branch to become (red) asymtotic giant branch stars.
Ann
M72: A Globular Cluster of Stars
