A stellar disappearing act
A STRONOMERS often get excited about a star being observed to disappear for a short time, then return.
In these cases, the disappearance has nothing at all to do with the star — it is because something has momentarily passed in front of it, and such an event is called an occultation.
A team of astronomers in Japan recently announced an important discovery, using this technique, of an object about one or two kilometres across in the far reaches of the solar system. It may have important implications for our ideas about the formation of the solar system billions of years ago.
The most well-known example of the passage of one object in front of another in astronomy is an eclipse of the sun, when the moon partly or completely blocks the sun’s direct rays as it passes in front of the sun.
Another example is when either Venus or Mercury passes in front of the sun as seen from Earth. These events are called transits.
Transits of Venus attracted a good deal of public attention when they occurred in 2004 and 2012.
However, stars other than our sun are so distant that their light seems to be coming from a single point in space. Therefore, when a relatively nearby rocky or icy object with no atmosphere passes in front of the star, the star’s light suddenly blinks out completely, and remains that way while the intervening object is obstructing our view.
Astronomers time the duration of the occultation, and these timings give them important information about the size — and sometimes, the shape — of the object, using knowledge or assumptions of the speed of motion of the object through space, while also allowing for the motion of the Earth.
We have known since the 1990s that there are very many objects in the outer solar system beyond Neptune. This region is called the Kuiper Belt (named after astronomer Gerard Kuiper). It is the region currently being explored by the New Horizons spacecraft, which has recently returned great images of the object temporarily known as Ultima Thule, more than a billion kilometres beyond Pluto.
The excitement is all about the likely size of the newly discovered object. The duration of the star’s disappearance — only about two-tenths of a second — allows the object’s size to be estimated to be between about 1.2km and 2.1km in diameter. It is the first Kuiper Belt object of that size to be detected.
Based on our current knowledge of the Kuiper Belt, the astronomers involved with the project had expected that even during their extensive observing campaign, the likelihood of finding any object of this size was very low. The fact that one was found, therefore, is a particular cause for celebration. It opens up the strong possibility that such objects are very numerous.
This is important because our ideas about the formation of the outer planets from smaller objects suggest that kilometre-sized objects play a key role: the important “building blocks’’ in the formation of the planets.
Amateur astronomers around the world, including in Tasmania, participate in occultation observations, partly to gather information on the sizes of asteroids. Though this recent discovery was not made by amateurs, they did use the same type of equipment that many advanced amateurs use: two telescopes less than 30cm in diameter, with the appropriate measuring equipment and software to detect the momentary disappearances.
The astronomers’ use of two telescopes greatly lessened the probability of another type of event, such as a passing bird, causing the momentary drop in light. Specially designed software and mathematical techniques were used to look for events that were observed using both telescopes at the same moment.
Occultations of stars by small solar system objects are only very rarely visible with the unaided eye. You would need a good star chart and to know when to look!
One of the best-observed such events was the occultation of the star called 1 Vulpeculae by the asteroid called Pallas in May 1983, giving astronomers the best data up to that time about the size and shape of the asteroid.