It is 20 April in the Canberra region. The autumn evening temperature is rapidly dropping toward what will be the lowest temperature of April: 3.6 deg. Celsius. Despite the conditions, five amateur astronomers are setting up across the region to watch the rather nondescript star HIP 59807 in the constellation Corvus for a quarter hour or so. They hope to catch an event a few seconds long. What is this event and why would anyone do this?
We all know that stars twinkle, their brightness varies a little as the atmosphere disturbs our view. There are, however, occasions when a star will completely disappear and reappear a few seconds later. These events are called occultations although a related term might be more familiar; eclipse.
Astronomical occultations occur when an object passes in front of of another completely obscuring it from view. For example, a total solar eclipse is the occultation of the Sun by our Moon. The occulting body might be the Moon, a planet or one of its moons, or an asteroid or other minor body. In the case of our Moon, it is happy coincidence that its size almost perfectly matches the Sun’s size when viewed from their respective distances. This allows the 3,474 kilometre diameter Moon to completely blot out the 1,392,000 kilometre diameter Sun’s glare allowing the faint corona to shine through.
Far less obvious occultations occur from time to time when an asteroid in the solar system’s asteroid belt passes between us and the light from a distant star. Stars are tiny points of light in the night sky, so we cannot expect to see things like the corona and other effects we see during solar eclipses. The best we can hope for is that the star will temporarily disappear from view, or significantly dim, while the asteroid passes in front. On 20 April this occurred when asteroid 324 Bamberga occulted HIP 59807. Prior to the event, predictions had been made using the known, fixed (almost, but that’s another post) position of the star and the known orbit of the asteroid. These calculations allow plotting of the predicted path of the ’shadow’ over the surface of the Earth with known degree of confidence.
Amateur astronomers like these events because it allows them to measure the asteroid’s size and get an idea of its shape. If you measure the time that the star is blotted out you can calculate the diameter of the asteroid. By spreading out across the expected path of the occultation each observer gets a slightly different line of sight and slice through the asteroid’s body. Combining the measurements allows a shape to be derived. For this event five Canberra area observers saw the star disappear, and two in New Zealand did not see the event. Dave Gault’s video of the event can be seen on YouTube. Assuming a roughly elliptical shape, this occultation gave 216.2 by 229.4 km as the best-fit dimensions for Bamberga. Even failing to see the star disappear is useful; if the observer’s point of view passes just outside the body of the asteroid. Such observations can be used to better refine orbital information and put an upper bound on the object dimension.
Occultations are one area where amateurs can add useful data to the scientific pool. Science you can be involved in is always of interest to me.
Other Bits
The HIP 59807 designation comes from the catalogue constructed by the Hipparcos mission. The two plots used here were shamelessly lifted from:
4 responses so far ↓
1 Wes // May 16, 2007 at 11:12 pm
Yet again you’ve made something which is quite complex understandable for even us neanderthals! Great Work.
2 Eris Puts On Weight // Jun 16, 2007 at 5:33 pm
[...] body and some assumptions. Later Hubble Space Telescope observations of Eris as it occulted a star (More on occultations) allowed the size estimate to be refined to 2400±100 km [3]… very similar to Pluto. [...]
3 christine // May 17, 2008 at 2:18 pm
hi just a question last night me and a friend were looking at a star and it was dimming to nothing and then would reappear it was doing this erratically more then once is it a dying star? it may be still in the sky tonight we didn’t see it fall i wish i could give more information on which star my friend said it may be the morning star because it was brighter and lower then the others
4 Chris // May 18, 2008 at 8:00 am
Hi Christine,
Thanks for the question. Unfortunately, probably not.
It is possible that you chanced upon an occultation (like in the article) but these are typically a single disappearance event lasting a few seconds. They’re also fairly rare events on naked-eye visible stars.
Most of the variations you see in a star’s brightness are the result of turbulence in our atmosphere. The path of light from the star to your eye is bent by the air, which as it moves, moves some of the star’s light in and out of your vision. This is the twinkling star of nursery rhyme fame, and also why astronomer’s want to put telescopes (like Hubble) above the air.
Another effect comes about when one stares directly at a point-like light light. Every so often, as your eye naturally twitches about, the star’s light will fall onto the end of your optic nerve… a blind spot in your vision. The star “disappears” but quickly reappears.
If you had seen a star that, over minutes, got massively bright (say as bright as the Moon) before fading gradually then you might have seen a supernova. That would be a truly magnificent event that should be reported to your local observatory with a location in the sky.
I hope this helps,
Chris W
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