Australian Sky & Telescope - 04.2019

18 AUSTRALIAN SKY & TELESCOPE April 2019

HOW FAR AWAY ARE THE STARS?You might t hink

that astronomers should know, but distances to the stars

are something very difficult to figure out. In daily life, we

estimatenearbydistancesusingatrigonometrictrickbuilt

into our bodies: Our eyes see the world from two slightly

different perspectives, and our brain processes this difference

tobuildathree-dimensionalimageofourenvironment.This

shift in an object’s apparent position, calledparallax,enables

ustocompleteamyriadoftasks,fromthreadinganeedleto

catchingaballinmid-air.

Since classical antiquity astronomers have laboured to

use the same method on the stars, by observing the apparent

shift of the position of a star while the Earth moves along its

orbit. But the stars are so far away that it was only in the 19th

centurythatastronomersfinallysucceededinmeasuringa

handfulofstellarparallaxes.Measurementsonagrandscale

had to wait for modern technology.

Near the end of the 20th century, the European Space

Agency(ESA)designedaspacetelescopetomeasurestellar

parallaxes. The High Precision Parallax Collecting Satellite

(Hipparcos, named in honor of the Greek astronomer

Hipparchus of Nicaea from the 2nd century BC), observed

a predefined set of stars over four years. The result was the

Hipparcos Catalogue, published in 1997 and containing

preciseparallaxesforalittlemorethan100,000stars,all

within 300 light-years of Earth. The precision achieved was

Placing the Pleiades

about 1 milliarcsecond (1 mas), which is^1 /1,000of an arcsecond

or^1 /3,600,000 of 1°. That’s like being able to see an astronaut

standing on the Moon.

Along with its expected successes, the Hipparcos mission

delivered several surprises. The most notable was the distance

to a famous star cluster, the Pleiades (Messier 45, in Taurus).

This group of bluish stars is easily visible by eye in dark spring

and summer skies — although there are many more members

than your naked eye can discern. Hipparcos found a distance

of roughly 380 light-years, rather less than the 440 light-

years of previous calculations, which were based on the stars’

brightnesses and considerations of stellar physics.

This was an embarrassing problem. On the one hand,

if astronomers had been using the wrong distance for the

Pleiades, then it could have implications on a much larger

scale: The Pleiades are a nearby open cluster and for this

reason are frequently used to test our models of stellar

evolution and to calculate the distances to farther stars. On

the other hand, if Hipparcos were wrong, then the dubious

result might challenge the entire Hipparcos catalogue. Was

there some instrumental or systematic error astronomers

had overlooked? Was there a problem with just the Pleiades,

or also with other measurements? Or were the Pleiades really

closer and thus didn’t fit into our models of stellar formation

and evolution?

Astronomers now have an answer.

STAR SLEUTHING by Guillermo Abramson