Hubble Telescope: False Stars

Photo Courtesy: NASA
One of the Fine Guidance Sensors being replaced by astronauts during Servicing Mission 2 in 1997.

How Hubble Accidentally Finds Celestial Objects All the Time

Jaclyn Wiley/Former Editor-in-Chief

The Hubble Space Telescope is famous for discovering galaxies and stars with its large powerful optical system. Usually, scientists tell Hubble where to look to find new celestial objects, but sometimes, Hubble makes unexpected discoveries all on its own.

Scientists use guide stars to help Hubble locate an object in the sky. The guide stars must be single, steady light sources called point sources, so that Hubble’s targeting system can lock onto them. A normal star, like the sun, is a point source of energy because it emits a steady amount of light without wavering or moving.

The instruments that find and lock onto the guide stars are called the Fine Guidance Sensors.  These sensors only lock onto a point of light if it is a point source. If the guide star is not a point source of light, then the fine guidance sensors reject it.  If a guide star is not a point source, then the star itself is not a normal star. Scientists were puzzled – if these celestial objects were not normal stars, what were they?

Sometimes, anomalous guide stars turn out to be two stars instead of one.  From the Earth’s perspective, these bright stars are very close together, to the point where they cannot be discerned from one another with the naked eye. Some even challenge the strongest telescopes.

Double stars can be binary systems, where two stars orbit one another, or optical doubles, where the two stars appear to be close to one another but are actually very distant.  In both cases, there are two sources of light instead of one, so the fine guidance sensor rejects the false point source.

In a paper released in 1998, scientists went through the fine guidance sensor records for these rejected guide stars and were able to identify over 250 double star systems.

Other times, the false guide star is actually a galaxy. Even though galaxies are made up of millions of stars and planets, the sheer distance between the galaxy and the Earth makes the galaxy appear as a single point of light.

The fine guidance sensors find objects that were previously and wrongly assumed to be normal stars.  Since the false stars appeared to be normal, they did not receive the scrutiny that would have revealed their true nature.  The use of the fine guidance sensors challenges scientists to question everything, especially things that might appear normal or insignificant at first glance.