In my clear Seattle sky tonight, I can see the entire Big Dipper, end to end to the northwest. I am interested in the possibility of seeing something I thought I never would. On 23 August, Palomar Transient Factory (PTF) survey picked up a point of light 10,000 times fainter than can be seen to the unaided eye. This point of light was yet brighter than it had been the day before. By last night it had increased in brightness again by more than a factor of 6. Tonight, while still not visible, 20 times again. It continues to brighten.
The Pinwheel galaxy (M101) is an object of about magnitude 8, pretty much invisible to the naked eye but visible through most binoculars. It sits in a position above the last two stars of the handle in the Big Dipper, “AlKaid” ( القائد ) and “Mizar” ( مئزر ), the two brighter stars on the left most side of the picture above. You’ll recognize Mizar by its nearby, slightly dimmer neighbor, Alcor, (which together form part of a triple binary system). The positions of the three points — AlKaid, Mizar/Alcor, and M101– forming a near equilateral triangle.
Mark Sullivan, Oxford University astronomer and PTF collaborator, predicts the maximum visual magnitude of this explosion will be from 9 to 10. This, he says, “places it within reach of observers using small telescopes or binoculars under clear, dark skies.” A bit dim for the city, I suppose, but it won’t keep me from looking every night.
This newly discovered object, previously an invisible white dwarf stuffed in between a billion other stars, may become bright enough over the next week or two to be seen with binoculars. PTF 11kly is expected to become as bright as magnitude 6 [Edit: It did not become this bright] over the next few days which means that while you won’t necessarily be able to see the Pinwheel Galaxy with the naked eye, you may just see this type 1a supernova, 21 million light years away and brighter than the galaxy it resides in, before it goes black forever.
Of course, having seen the event from this far away, we can conclude that it has been a dead star for nearly 21 million years! This event took place roughly 3 million years before the split of the human lineage from that of apes. Twenty-one million years have passed since this explosion and that burst of light began to propagate far enough for us to see the event as though now just taking place.
The light curve will soon peak, then fade away steadily as a function of the expansion and dispersion of nearly all of the material the former star contained. A glowing, hot shock-wave of material will expand into the surrounding space, bombarding nearby stars with matter traveling at a not insignificant velocity. It will dim within days; thousands of years later, it will fade completely.
In the intervening time between “a few days” and “a thousand years”, someone living on a nearby planet a mere few thousand light years away might see a display not unlike what we see in comparatively nearby Tycho Nebula. That supernova was so bright it could be seen during the day with the naked eye! Not so surprising, though. It is in the same galaxy as our star, after all. First reported by Tycho Brahe in 1572, the appearance, rather, the destruction, but sudden visibility of a new star 13,000 light years away in the constellation Cassiopeia changed the perception of a static sky and the direction of astronomy toward further discovery.
A Note About Finding the Pinwheel Galaxy
Incidentally, I found it not a little challenging to navigate quickly back to this featureless part of the sky (Seattle has a bright sky). But there is a simple way to find M101 without too much trouble.
The trick I finally set on was to follow a chain of easily identifiable start. Locate the pair, Alcor and Mizar (see above). Follow the line this close pair describes up to the next brightest star. This faint star will be one of four describing a jagged line upwards and to the left in a roughly slanted, paired stair-step.
As an aside (if you’re interested) this line of four stars, none of which is any dimmer than magnitude 6 (though all typically unlabeled on most non-technical charts) are designated HIP 66198, HIP 66738, HIP 67231, and HIP 67848. Another naming system designates these four, in the same order, uMa: 81, 83, 84, 86.
Follow that line. If you’ve counted five stars (HIP 68304a, to the left of uMa 86 in the picture below), you’ve gone too far! See Nauyuki Kurita’s excellent descriptions of celestial objects (Stellar Scenes) and his handy graphic (below, linked) describing in a glance how to quickly navigate to M101 from the Mizar/Alcor pair.
When you reach the fourth star, you are near the center of the triangle I described above and just below the Pinwheel Galaxy. Pause and note that above this fourth star you should see a horizontal row of very faint stars; one of these, directly above the fourth star, will have a faint, second star directly beneath it. Above this pair, at a slightly greater distance above the horizontal row as from the row to the last of the fourth stars, will be a blank region in which M101 sits.
If you have a bright, hazy sky lit by city lights or the coming full moon you probably won’t see the galaxy — then again, you might: I found that when I stared at this region until my eyes adjusted, I could almost make out a faint smudge if I didn’t stare directly at it. Maybe wishful thinking. If you have a dark sky, far from the city, you shouldn’t have any difficulty seeing M101.
David Bishop writes for the Rochester Astronomy club and has been keeping track of many of the details of this event and has a link to the latest images. I reckon he or someone else in that club will know before anyone if a new image is available.
Phil Plait of Bad Astronomy did a write up of the various observations which have been made to date. I would recommend checking that out for some great comparison of Type 1a Supernovae, early imagery of what was to become PTF 11kly, and plenty of linked papers and articles describing the data and some of the theoretical repercussions. See also his article from 2006 about M101.