6degrees AstroBlog

Comet Undergoes Massive Outburst and is Visible All Night Long

By Irwin Horowitz, 10-28-07

	(c) Eric Allen

Four days ago, on 2007 October 24, the faint periodic comet 17P/Holmes underwent a superoutburst during which it increased in brightness by a factor of nearly 1 million!  It has gone from a barely noticeable speck in the cosmos to a naked eye object in just a matter of a few hours.

Comets will occasionally undergo an outburst in brightness, though never to such a degree.  The usual cause of such outbursts is the sudden venting of gases and dust from a subsurface pocket.  This pocket gets exposed after the surface material has been ablated by solar heating.  What is so unusual about this event is that this particular comet never approaches close enough to the Sun for this process to strongly affect its surface.

This is not the first time this comet has undergone a dramatic increase in brightness.  When it was first discovered in 1892 November, it was experiencing an outburst which faded away by the middle of December, only to reignite the following month.  Its orbit, located between Mars and Jupiter, was determined to have a period of just less than 7 years.  After its passage in 1906, it was not observed again until 1964.  In each of the subsequent passages, the comet was unremarkable, never gaining much notoriety or garnering much attention from any but the most devoted comet aficionados.  With the events of the past few days, that has most definitely changed.

The favorable weather conditions here in Boise last evening allowed me to set up my telescope on the edge of my driveway to view this mysterious visitor.  Despite the presence of a street lamp across the street and the nearly full moon rising in the east, I was able to easily see the comet.  The photo above by amateur astrophotographer Eric Allen looks just like what I saw while looking through the eyepiece of my scope.  It appeared as a spherical ball of light with a small bright fan-shaped condensation towards the center.  This appearance is likely due to our viewing down the long axis of the dust tail.

If the previous experience is a reliable guide, the comet should remain fairly bright for a few weeks.  This will be long enough that the bright moonlight currently interfering with the view will be gone and the opportunity to view it under truly dark skies later this week and next week will be available.  The Boise Astronomical Society will be holding a star party next Saturday, weather permitting, at either our Black’s Creek Rifle Range location or at Dedication Point located south of Kuna.  I am certain other astronomy clubs throughout the mountain west will be holding similar events next weekend.  If interested in attending, you should contact those clubs for more detailed information.

If you would like to observe the comet for yourself, you can find a good star chart here at the astronomy.com website.  It is located in the northeastern sky after sunset, in the constellation of Perseus near the star Mirfak (alpha Persei).  Starting from the “W” of Cassiopeia located high up in the sky around 9 p.m., go about 1/2 of the way down towards the horizon to find Perseus.  The brightest star in that region is Mirfak (note: do not confuse it with the very bright star Capella located near the horizon).  The comet is the slightly fainter object located about 3 moon diameters below and to the left of Mirfak.  If you have access to either binoculars or a telescope, I strongly encourage you to use them to get a truly spectacular view!

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