Pleiades

• Persons with exceptional eyesight. There are cases of people being able to see stars other people can't; the best known example is that of the binary Mizar in Ursa Major.
• The seventh sister was brighter once, and has since faded. This is the most likely answer. Both Native American and European mythologies tell the same story about one sister conceling herself out of shame.
• Space Aliens told them. Then again, maybe not.

• Alcyone
• Celaeno
• Taygete
• Merope
• Maia
• Sterope
• Electra

This game was a prototype Atari 2600 game by UA Limited that was never released. There are only 2 known copies of this game.

This game is an adaptation of the Arcade Game of the same name.
http://www.klov.com/P/Pleiads.html
Very little is known about the cartridge beside that.

Prototype games like these are the Holy Grail as far as rare Atari Games go. Nothing is harder to find than these. The value of this prototype is unknown. The owner of one of the known copies has refused offers of over $500.

The Rom from this game has not yet been dumped. So you cannot play this game with an Emulator. If you own a copy of this game please get in contact with someone in the Atari Emulation world so that your copy can be dumped. So this rare game can be preserved for the future.

Located in the constellation Taurus, the open star cluster Pleiades (M45) consists of over 500 stars, of which only six are visible to the bare eye, as mentioned earlier in this node. The Pleiades is probably the best visible and most easily recognized star cluster to the naked eye. In the night sky, the whole of Pleiades cover an area approximately four times that of a full moon, and they are at a distance of about 380 light years. Without a telescope, they can appear a bit dim and with a telescope one might see a blue-ish haze surrounding them, which is because they are still young and surrounded by the gas clouds that gave birth to them. 

The stars are really a cluster, and not just an optical illusion of stars that happen to appear in the same area. It was discovered in mid 19th century that the stars have no apparent motion relative to each other,  and therefore is really a real cluster of stars that were born together. Recent calculation show that they are about 100 million years old, and that they will remain a cluster for another 250 million years before they will live on as individual stars. 

The legends in Greek Mythology have it that they were seven sisters, the daughters of Atlas and Pleione. One day whilst walking in the woods, the great hunter Orion lay eye upon them and started chasing them. Only mighty Zeus could rescue them, and he did so by turning them into doves that could fly away into the sky. In the night sky, you can still see them chased by Orion, which is just nearby. Luckily they are protected inside the bull Taurus, which is one of the forms that Zeus take. The faintest of the sisters, Asterope,  is actually a double star. 

In Japan the cluster is called Subaru, after which the car manufacturer with seven stars in its logotype is named. 

As a comment to Gorgonzola's writeup, one can even today see 9 or even 12 stars under perfect conditions and with good eyes. In 1579 astronomer Moestlin accurately drew a map with 11 stars of the Pleiades in it, and this was before the first telescopes were built. 

This is how the seven sisters and their parents are located with respect to each other:

               Merope   o
                                      o   Atlas
    Electra    o               O       o   Pleione
                                   \
    Celaeno    .                  Alcyone
                     O   Maia
      Taygeta    o     
                       .'   Asterope

Several sources used, most notably SEDS, seds.org.

Ple"ia*des [L., fr. Gr. ()]

1. Myth.

The seven daughters of Atlas and the nymph Pleione, fabled to have been made by Jupiter a constellation in the sky.

2. Astron.

A group of small stars in the neck of the constellation Taurus.

Job xxxviii. 31.

⇒ Alcyone, the brightest of these, a star of the third magnitude, was considered by Madler the central point around which our universe is revolving, but there is no sufficient evidence of such motion. Only six pleiads are distinctly visible to the naked eye, whence the ancients supposed that a sister had concealed herself out of shame for having loved a mortal, Sisyphus.

© Webster 1913.