Home > Stargazing > Watch a Star Disappear Behind an Asteroid: Wednesday 10 September 2014

Watch a Star Disappear Behind an Asteroid: Wednesday 10 September 2014

Many thanks to the always-excellent Astronomy Now magazine for this story. Their full article is here, and is well worth a read.

In the early hours of the morning of Wednesday 10 September (at around 0306BST) stargazers in the northern part of the British Isles have the chance to witness a star disappearing, if only for a few seconds.

The star in question – HIP 22792 in the constellation of Taurus – is faint, though, and so you won’t see it with your naked eye. The good news is that you can see it through even a modest pair of binoculars mounted on a tripod, and it’s easily seen through a telescope.

So why is it blinking off and on again? In fact it isn’t, but for those few seconds a much smaller but much closer object, an asteroid called (569) Misa will pass in front of it, perfectly obscuring it for observers in a 90km wide band running from Galway in Ireland all the way up to Peterhead near Aberdeen. Observers within this band will see the occultation, and those near the centre line (including me in Glasgow and stargazers in Sligo, Londonderry, and Dundee) will get the best view, with the occultation lasting longest (3.6 seconds!).

The path of occultation. Image from Astronomy Now.

The path of occultation. Image from Astronomy Now.

How to find HIP 22792

The faint star in question is located in the constellation of Taurus, and will be around 38° above the eastern horizon at the time of occultation. Luckily there are plenty of bright stars nearby to signpost you there.

The location of HIP 22792 in Taurus at 0306 Wed 10 September 2014

The location (crosshairs) of HIP 22792 in Taurus at 0306 Wed 10 September 2014

Here’s a close up of the area in question:

The stars around HIP 22792 at 0306 Wednesday 10 September 2014

The stars around HIP 22792 (crosshairs) at 0306 Wednesday 10 September 2014

The stars labelled in red (by me) are the signposts to HIP 22792. Stars a and b (ι Tau and τ Tau) are both naked eye in anything other than bright city light pollution, shining at magnitudes +4.6 and +4.3 respectively. This makes them very easy to spot in binoculars. Draw an imaginary line between a and b and cut it with a perpendicular line moving in the opposite direction from the bright star of Aldebaran, and the next bright-ish star you come to is labelled c, HIP 22743, which shines at +6.5. Continue along this line past a faint star (unlabelled) shining at +7.4, then double that distance again to find the target, HIP 22792, which is the faintest so far, at +7.6. The stars labelled d and e are there for reference, and are at +5.8 and +6.3 respectively.

Practical tips for finding HIP 22792

If you have a telescope with in-built goto and tracking you’re good to go but for the rest of us we need to do a bit of prep.

  • At the very least you’ll need to mount your binoculars on a steady tripod, or have your scope aligned, so that you can track the target for several minutes.
  • Finding it may take some time so don’t just fall out of bed expecting to locate it easily. Give yourself at least 20 minutes of set-up (or more, if you’re new to this!)
  • Make sure you’re observing from a site that has a good eastern view, that isn’t obscured by buildings or trees
  • As always, the further you can get from the glare of street lights the better.
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