In the movie “Castaway”, Tom Hanks’ character, once he’s discovered he can spear them with a stick and hold them over a fire to cook them opines, “Ya gotta love crab…”
Well I feel the same way, but about the Crab Nebula! I don’t know why I have such a fascination about this object but I remember really being struck by it as far back as when I was a kid reading my Larousse Encyclopedia of Astronomy. It could be because it so graphically represents what it is, an exploded star; or perhaps because it’s got so much sharp detail but whatever the reason, this one is in my ‘top ten’ for sure.
As I’ve acquired bigger and better telescopes I always go back to the Crab during ‘crab season’ to see what I can see now but am always disappointed. Even in my 18-inch scope there’s just the slightest hint of the major two or three filaments and just is never the super interesting thing you know it is.
I’ve been trying to get an astrophoto of the Crab going all the way back to my attempts in the era of film(!) and always came up empty-handed, mostly because I was using equipment that wasn’t capable of succeeding but didn’t know it at the time.
Well fast-forward 30 years and suddenly I do have the right equipment but have still been unsuccessful in all my previous three attempts in the past few years due to a variety of factors regarding scope type, object size, camera quality, and mount capability. Seems like I always got something in that list wrong.
New Recipe
My best effort to date was in 2018 and while it’s not terrible, it isn’t very good either. The stars are round, the sky is black but the color is a bit weird and there’s a bunch of “hot pixels” on the camera masquerading as stars in the background, but most disappointingly there is only a hint of the fantastic detail you see in this thing in Hubble shots. So my goal this time was to conquer all those hurdles and get the shot I really wanted after all these years.
A crappy Crab
So, in one of those rare instances when all the factors that normally vex you move out of the way and align in your favor, I finally have the shot of the Crab Nebula that I’ve been trying to take my whole life!
Supernova remnant the Crab Nebula, Messier 1, in the constellation of Taurus.
Better Ingredients
So what makes this thing so hard to shoot? Well first of all, it’s actually pretty small in the sky. When you look at astrophotos they’re always cropped to show off the object to its best advantage so unless you actually know the object already they all look about the same size in the photos. For instance, my photo of the North American Nebula from a couple months ago covers an area of sky about the size of a ping-pong ball held at arm’s length. The crab nebula is about the size of the head of a pin held at arm’s length so you’re going to need as much magnification as you can get!
Secondly, a long focal length (highly magnified) telescope reveals any errors in sky tracking much more readily so your mount’s tracking speed needs to be really, really accurate and your polar alignment better be pretty close to perfect.
10-inch Ritchey-Chretien Astrograph; Losmandy G11 mount
Thirdly, your camera needs have lots of pixels, more dots in your photos means more detail and since the part of the image you’re actually going to end up keeping is probably only about 20% of the total camera frame you want as many pixels, as tightly packed together on your camera sensor as possible.
Lastly, you need to get lots of really good data. My previous image was 20 exposures stacked together (likely 30 seconds each, maybe a minute tops); the good new photo is about three and a half hours across four different filters which took me from 7pm to 4am to collect including setup.
Murphy’s revenge
In my previous post here I remarked, “Why is it never easy?” Well tonight it actually was! Polar alignment using the star drift method took 25 minutes instead of an hour and my accuracy numbers were a new record. All the hardware was responding to all the software, there was a perfect guide star to lock onto and the guiding software kept my mount trained on it to within an arc-second or so instead of the usual 1.8 so almost twice as accurate. The night was perfectly clear and pretty chilly so the camera’s cooling circuits got the detector temperature down to -25°C; take that you stray noisy electrons!
And finally, the “meridian flip” worked perfectly as well although I still don’t trust things enough to let the software do it on its own. A meridian flip is when you have been shooting an object in the east and it starts to cross the Meridian–the imaginary line in the sky from the North Star to the South dividing the sky into the east half and the west half. Well at some point past the Meridian your telescope is going to get so low on the mount it’s going to crash into the legs of the tripod so you have to stop, rotate the telescope 180° to the other side (now the east side pointing west) and now you’re good for the rest of the night. Well this maneuver has always been fraught with disaster for me with the scope going the wrong way, wires getting tangled, having a hard time reacquiring the target (which is now upside down in your camera frame) but tonight it just all worked and I was shooting again in 10 minutes.
The practical upside of all this is that out of a run of 50 exposures with a certain filter I only had to throw away four instead of the usual fifteen with this scope/mount setup.
What is this thing called Crab?
What is it about this object that gives it its unique look? It’s a supernova remnant, and a pretty recent one at that. It’s also fairly close at 6200 light-years on the next spiral arm out in the Milky Way. I’ve got another image of a supernova remnant, now known as the Veil Nebula which is at the top of my Top Ten list.
The Veil Nebula, supernova remnant in the constellation of Cygnus The Swan.
Even though this is a supernova remnant as well it’s a super old one and sort of indirect. What happened here was that a supernova explosion (now long gone) and the shock wave from it ripped through this part of the galaxy causing all the gasses that were hanging out there to compress and glow.
Supernovas are really big, bright explosions. We can see them literally all the way across the entire universe and indeed, having the Hubble Space Telescope hunt for supernovas in distant galaxies is one of the best ways we have to understand how far away things are and therefore how big the universe actually is!
Harder than chinese astronomy
So imagine you’re a Chinese Astronomer on July 4th, 1054 AD when all of a sudden there’s a brand new star in the sky, a ‘guest star’ as they referred to it. Remember since this is 450 years before the invention of the telescope an astronomer was a guy who just looked at the sky and wrote things down that seemed interesting. Well, this brand new star was four times brighter than Venus, was visible during daytime for 23 days and was seen in the night sky for a year and nine months after. So everything you see in the Crab Nebula Is the actual star itself after a thousand years of explosive expansion. It was also observed by Native Americans in Arizona and New Mexico as shown by petroglyphs we’ve discovered. Pretty much everybody who was alive then saw it because how can you miss a star so bright you can see it during the day?
When a star goes supernova there are basically three possible ultimate outcomes:
The star is completely destroyed
The star leaves behind an earth-sized or smaller ball of pure neutrons. This stuff is so dense a teaspoon of it would weigh millions of tons on the Earth. We call this a Neutron Star. If it’s spinning and giving off radio waves from all the nearby stuff that’s falling onto it, we call it a Pulsar.
If the amount of left-over neutrons in the neutron star exceeds roughly three times the mass of our Sun then it collapses into a Black Hole.
The Crab chose what was behind curtain #2 and has a pulsar with 1.4 times the mass of the Sun compressed into a ball only 20 miles across, rapidly spinning in the center or the Nebula, sending a beam of X-rays our way every 33 milliseconds, marking the original location of the supernova explosion. The nebula itself is expanding fast enough that professional astronomers can actually measure that expansion year by year. The Hubble goes and has a look every now and then each year since it’s been up there which they’ve stitched into a movie! This isn’t the HST version but it does show the expansion over time:
Crabby History
The Crab’s nebulous remnant was first telescopically discovered by John Bevis in 1731. Charles Messier independently found it in 1758, when he was looking for Halley's comet on its first predicted return. Although it turned out not to be a comet Messier included this in his catalog of, ‘things that look like comets but aren’t but I’m writing them down anyway so I’m not fooled again later” which we now call the Messier Catalog and the Crab is the first entry so it’s referred to as “M1”. I’ve heard descriptions about the type of equipment these guys were using back in the 1700s and I’m amazed they could see some of the stuff they saw!
The Crab pretty much existed as a ‘cloudy oval smudge’ kinda thing until around 1844 when a drawing of it by Lord Rosse in Ireland (astrophotography didn’t exist at that time) showed some of the filamentary structure and the way he drew it, it looked like a crab and the name stuck. Other famous astronomers of the day like William Herschel correctly noted that the Crab can’t be resolved into stars but thought as scopes got bigger it would be, apparently mistaking those filaments as not quite resolved stars.
The first real photo was taken in 1892 with a 20-inch scope and that really revealed the gaseous nature of the Crab for the first time. In 1921 they compared the 1892 photo with a much more recent one and discovered the expansion of the nebula. By measuring the expansion over the period of time that allowed them to ‘run the clock backwards’ mathematically and that’s when the connection between the 1054 supernova and the Crab was discovered. In 1949 we were sending radio source detectors up on captured German V2 rockets for some quick peeks above the atmosphere and the Crab was identified as a strong radio source. In 1963 X-rays were found to be coming from the Crab as well. In fact, the X-Ray signal is 100 times brighter than the optical. Five years later the Pulsar was discovered and one year later was also found to be pulsing in sync in the visible light spectrum, a first.
Location Location Location
Here’s a diagram of where the Crab is in our galaxy relative to the Sun.
The distance to the Crab is about 6200 light-years and the object itself has expanded out from a single star to 13 x 11 light-years over 970 years since the explosion.
Crab Out
Well that’s probably more than you wanted to know but suffice it to say I’m crossing this one off of the Astrophoto Bucket List.
Bill
a.k.a. Bill the Sky Guy
Hilton Head, South Carolina