From KenCroswell.com:

Doesn't a star turn into a white dwarf after it creates a planetary nebula? where's the planetary nebula if this happened recently?

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"I'm as accurate as any psychic. And I'm a cartoon!" -- Squidward

"Arrrgh, the laws of physics be a harsh mistress!" -- Bender

Title: On the origin of the ultramassive white dwarf GD50
Authors: P. D. Dobbie (1), R. Napiwotzki (2), N. Lodieu (1), M. R. Burleigh (1), M. A. Barstow (1), R. F. Jameson (1) ((1) University of Leicester, UK, (2) University of Hertfordshire, UK)

We argue on the basis of astrometric and spectroscopic data that the ultramassive white dwarf GD50 is associated with the star formation event that created the Pleiades and is possibly a former member of this cluster. Its cooling age (~60Myrs) is consistent with it having evolved essentially as a single star from a progenitor with a mass M>6Msun so we find no need to invoke a white dwarf-white dwarf binary merger scenario to account for its existence. This result may represent the first direct observational evidence that single star evolution can produce white dwarfs with M>1.1Msun, as predicted by some stellar evolutionary theories. On the basis of its tangential velocity we also provisionally identify the ultramassive (M~1.2Msun) white dwarf PG0136+251 as being related to the Pleiades. These findings may help to alleviate the difficulties in reconciling the observed number of hot nearby ultramassive white dwarfs with the smaller number predicted by binary evolution models under the assumption that they are the products of white dwarf mergers.

Read more (22kb, PDF)

http://www.naic.edu/~gibson/pleiades/pleiades_myth.html

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`Irony` actually does mean `metal like`...

Planetary nebulae last only a few tens of thousands of years. According to the article, the white dwarf is 60 million years old--so the planetary nebula that arose when it was born is long gone.

Then this star could not be the "lost sister" of legend.

I was confused by the subject line.

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"I'm as accurate as any psychic. And I'm a cartoon!" -- Squidward

"Arrrgh, the laws of physics be a harsh mistress!" -- Bender

It is a joke. According to the mythology, there were seven Pleiads, one of which became mortal and wasn't placed into the sky. Now they're find a star that may have been a part of the cluster, so it is the lost "seventh sister".

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The diameter of a white dwarf or a neutron star depends on how massive it is. More massive the object is, the smaller is its diameter. Sirius B, which is a relatively massive white dwarf, has a diameter about that of the Earth. According to Ken Croswell's webpage, the diameter of GD 50 is only 54% of Earth's diameter!

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Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.
-- Richard Feynman

Side question:

1) If the Pleaides are the seven sisters.
and
2) Only six stars are easily visible to the naked eye.
and
3) Two of those stars are their father (Atlas) and mother (Pleione).

Where are the other three sisters?

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Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

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Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.
-- Richard Feynman

Most people count 6 or 9 Pleides without optical aid. If you can see 9, then the 2 parents and 7 sisters add up. Otherwise...who knows?

Does "seven-sisters" form an illiteration in Greek?

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PW -- Plant Whisperer