Tuesday, August 18, 2009

Sizing the Universe

The night of July 21 was the first time I ever photographed the moons of Uranus with a DSLR camera. As I mentioned in a
previous post, while I was processing the image I noticed a number of fuzzy objects to the South-West of the planet that looked very much like galaxies. I did a quick search and found out that Uranus was just few arcminutes away from the Compact Group of Galaxies (CGC) named Hickson 97. The presence of a planet right beside a very distant group of galaxies convinced me to take a longer exposure. The night of July 24 looked promising so I went for a 1h 42min 30sec total imaging time (my camera battery died before the end of the 1h 45s planned session...). I kept the exposure of individual subframes pretty short (30 sec) to minimize the adverse effect of light pollution. Speaking of which, I was really impressed with the darkness of my backyard. That night I could see all the stars of the Little Dipper with the exception of the dimmest, η UMi (mag +4.95) so the limiting magnitude of my skies must have been about +4.8: not bad for an urban location!

Regardless of how "dark" my location is, finding Uranus these days is not that easy. Currently Uranus is about 30 degrees high (at culmination) in Pisces, close to the border with Aquarius and Cetus:

Its apparent magnitude (+5.8), the low altitude and the fact that it sits in a region of the sky deprived of bright stars contribute to the challenge of finding this interesting planet. So how do I find Uranus? I use the only method that cannot fail: star-hopping. I have to admit that star-hopping with a 10" f/4.7 reflector is quite easy. The limiting magnitude I can reach with direct vision is probably around +11 or +12. I still have to find a single low power field of view with my scope that does not contain any star that dim. I usually begin the hop from a naked eye star that is not too far from the target. After pointing the scope towards such a star, I look into the finder hunting for dimmer stars that are closer to the target. Then I launch Cartes du Ciel on my laptop and superimpose the field of view provided by a 25mm Plossl eyepiece (about a degree). I place a series of circles (field-of-views) that go from the beginning of the hop to the target (typically a star visible in the finder), making sure I follow the directions provided by Declination and R.A. Here is the hop that led to Uranus:

The reason why I lay the field-of-view circles in the directions of the celestial coordinates is simply because my telescope is attached to an equatorial mount which moves along declination and R.A. If I had an alt-azimuth mount, I would follow the directions altitude-azimuth instead. Star λ Psc (the start of the hop) is magnitude +4.5 and I could not see it naked eye from my backyard (the atmospheric extinction is significant at an altitude below 30 degrees). I could see α Peg (Markab, mag.+2.5), instead. After centering Markab in the 6x30 finder I was able to find λ Psc.

The arrangement of the moons on the night of the 24th was quite favourable:

Ariel was too close and Miranda was just too faint, but the other three were within reach. As I mentioned at the beginning of this post, I took a 1h 42min 30sec total exposure of the region around Uranus. I used a Baader coma corrector and an Orion Starshooter autoguider to minimize aberrations and star trailing. The camera gain was set to ISO1600. Here's the processed image with a close-up of the Uranian system. As you can see I was unable to get all four main moons, but between this image and the one I took on July 21 I did manage to image all of them.

It is also interesting to see how much Uranus moved against the background stars from July 21 to July 24.

Although I was interested in capturing the moons again, my primary focus was on Hickson 97 and other fuzzies:

that were barely visible in my previous (shorter exposure) image:

So here's the processed image without the inset.
If you look carefully you will notice a number of faint deep sky objects in this image.

Here's an annotated, enhanced version of the previous image.

Inset 1 shows Uranus and its moons as it's been discussed before.

Inset 2 shows Hickson 97 as imaged from my telescope (for comparison an image of the same region from the STScI Digitized Sky Survey is shown in the bottom right corner). Hickson 97 is an example of Compact Group (CG) of galaxies. CGs are defined as "small systems of several galaxies in a compact configuration on the sky" (Hickson, 1997, Introduction). CGs challenge Astronomers in many ways. For example, when the red shift of the first CGs were measured at the beginning of the 60ies, it was found that in some cases one of the galaxies had a redshift quite different from that of the other members. What seemed peculiar was the fact that the probability of finding a completely unrelated galaxy (different redshifts imply different distances from Earth) overlapping with a CG was supposed to be very low and yet at least three examples of CGs having a galaxy with different redshift within them had been found . To complicated matters even further was the fact that Halton Arp, one of the most prominent experts in Galactic Astronomy and author of the famous Catalogue of Peculiar Galaxies, proposed that the enormous redshift of quasars was not due to the expansion of the universe. Instead, according to Arp, quasars were physically related to much closer galaxies which, in turn, showed red shift anomalies. Arp published an interesting albeit controversial book on the subject entitled Quasars, redshifts, and controversies. The work of Hickson devoted to generate a much more homogenous list of CGs helped clarify that the galaxies with discordant redshifts were in all likelyhood the result of projection alignments and were not physically related to the CGs they were superimposed to. For more information please refer to the very readable review by Hickson himself available online. Hickson 97 is about 300 million light-years away and consists of five galaxies identified with the letter a, b, c, d and e (data from the Principal Galaxy Catalogue):

ComponentDimensions (arcminxarcmin)Integrated MagnitudeGalaxy Type
HCG 97a1.6'x0.8'14.04S0 - Lenticular
HCG 97b1.2'x0.3'15.72Sc - Spiral
HCG 97c1.0'x0.5'14.98Sa - Spiral
HCG 97d1.7'x0.8'14.64E1 - Elliptical
HCG 97e0.5'x0.3'16.65S0a - Lenticular

My image shows all the components of Hickson 97, plus a couple of smaller galaxies, one of which I could not identify.

Inset 3 shows two fairly bright galaxies ("bright" here is a relative term...) as compared to Hickson 97. PGC 72461 in particular is a mag.+14.54, 1.1'x0.6' elongated lenticular galaxy (S0 class) with a bright core. PGC 72457 is a smaller 0.9'x0.7' spiral (Sb) of mag.+15.06. The image seems to indicate a hint of a spiral arm, but it is difficult to distinguish actual details when the levels are stretched to the limit.

Inset 4 shows a region with three faint galaxies of magnitudes +15.8 (PGC 196678), +16 (PGC 3080162), +17.22 (PGC 1086503) and a pair of galaxies, PGC 72432 and PGC 72433 of magnitude +16.56 and +16.12, respectively.

Scattered througout the entire field of view I was able to identify at least 19 more faint galaxies. Here's the complete list in order of apparent magnitude including the component of Hickson 97 and the galaxies in inset 3 and 4. In total the camera was able to detect 28 galaxies! The distance of each galaxy was calculated from the radial velocity as provided by the Principal Galaxy Catalogue (PGC) using the well-known formula d = H0 x v, where d is the distance in millions of light-years (Mly), H0 = 74(km/s)/Mpc is the Hubble constant and v is the radial velocity in Km/sec as deducted from red shift measurements. The PGC provides also the apparent major and minor axis of each galaxy in arcminutes. Knowing the distance and the apparent size, it is possible to estimate the actual size of a galaxy. In the following table, the size (intended as the average radius of the galaxy) is given in light years.

PGC#Integrated Magnitude Radial VelocityDistance (Mly)Size (ly)
72408 HCG 97a14.046922305142000
72404 HCG 97c14.646239275135000
72409 HCG 97d14.98600126477000
72430 HCG 97b15.726924305106000
72405 HCG 97e16.651189452443000

It is interesting to see that the galaxies of Hickson 97 are among the brightest in the list. The reason is that at an average distance of about 300 million light-years they are relatively close (as compared to the other galaxies). They are also large galaxies: component a, for example, is about 140,000 light years across, about 40% larger than the Milky Way. Components c and b at 135,000 and 106,000 light-years across are also respectable. To find the actual size of these galaxies is simply a matter of geometry: the PGC Catalogue provides their angular sizes. The distance is known from their radial velocity and the Hubble's Law, as explained above. So the actual size in light-years is the product of angular size (in radians) and distance (in light-years).

About 16 of the 28 galaxies shown in the image are at about 300 light years away from us. I was not able to find any information as to whether these galaxies belong to some larger structure, but, given their distance, density and location, I am guessing they belong to the Perseus-Pisces supercluster. The rest of the galaxy in the list is farther away. The three farthest from us seem to be 977 million (PGC 196644), 1.3 billion (PGC 1084361) and 1.8 billion (PGC 1096772) light years away!
I am sure these are the three most distant objects I ever imaged. I find quite exciting that a few photons which had travelled across the Universe for a couple of billion years ended up being converted in electic current inside my camera!

Finally the last question I wanted to answer was what the sky would look like from a planet located in one of the Hickson 97 galaxies. How big would the other components of the Group appear in the night sky of a planet located in one of those galaxies? Geometry comes to the rescue. Knowing the distance from Earth and the celestial coordinates (RA and Dec) of each of the Hickson 97 galaxies, with the help of some trigonometry it is easy to calculate the relative distance between each pair. The following table provides the distance (in Mly) of each galaxy from the others:

Where distances are given in million of light years. The content of each cell provides the distance between the component on the row and the component on the column. For example, the distance between component a and component b seems to be only 100,000 light years which is less than their respective sizes! If that's the case the two galaxies must exert a significant gravitational pull on each other and orbit around their common centre of mass like a gargantuan dumbbell. The other components seems to be located between 10 and 40 million light years away from each other, which means that Hickson 97 is about 4 times bigger than our own group of galaxies, the Local Group. It is interesting to note that the Local Group would look like a bit smaller than Hickson 97 if seen from one of the component of Hickson 97 itself, with the Milky Way and the Andromeda galaxy replacing the roles of component a and b (although not that close to each other). I guess that helps put things in perspective! But let's go back to the original question which was: what the other galaxies of Hickson 97 would look like in the night sky of a hypotetical planet located around a star of component b, say? Knowing the apparent size of each galaxy and knowing their mutual distance it is easy to calculate how big they would look like from each other. It turns out that component a would span about 60 degrees across as seen from component b and it would look brighter than the Milky Way. Imagine the fun astronomers of component b must have when the sky is clear! The other galaxies would be between a tenth and half a degree across, probably very similar to our familiar Messier galaxies. The night sky from a planet in component a would be very interesting as well, since component b would span a little less than 50 degrees.

One last observation. In my image there is one planet, stars and galaxies. Light takes about two and a half hours to get to Uranus, many years to get to the stars (20 Psc, the brightest star in the image, is about 290 light years away), 300 million years to get to Hickson 97 and more than a billion year to get to the farthest galaxies captured in the image: that's one handy way of sizing the Universe!

The next table summarizes the

Wednesday, August 5, 2009

Uranus and its moons

The moons of the Outer Planets have always fascinated me. I was fourteen years old when the images of the Jovian system started pouring down on Earth as Voyager 1 and 2 passed Jupiter on their way to the rest of the Solar System. What captured my imagination was the variety of sizes, colours and surface features of the moons of the gas giant. It seemed to me that each one of them was a completely different world with a long story to tell. Think of Io and Europa, for example: one the most volcanic body in the Solar System, the other encased in a crust of ice probably hiding an ocean! As the years passed, the probes flew by Saturn and then started following diverging trajectories: probe #1 towards the interstellar space which is about to enter 30 years later, probe #2 towards Uranus and Neptune, never explored before. Like Jupiter, each of the Outer Planets came with a variety of moons, many of which revealed features that are still keeping astronomers busy these days.

Three decades after the Voyagers flew by Jupiter my interest in the moons of the Outer Planets is still alive. I guess there's a reason why three of the seven (so far) posts on this blog are about the satellites of Saturn, Jupiter and Neptune...As I described in a previous post, imaging the moons of the outer planets is well within reach of amateur astronomers. A mid-size telescope like mine (10" reflector f/4.7) and a digital SLR camera at prime focus are more than adequate for the task. As a point in case, I was able to image Triton (14 degrees above the horizon) with only a 15 minute total exposure from my urban location (a slower focal ratio will need a longer imaging session).

After the success with Triton, I decided to go for the moons of Uranus. The Uranian system presents a different set of challenges when compared to the Neptunian system. First of all, Uranus is brighter than Neptune by 2 magnitudes, +5.8 against +7.8, which implies that the glare from the planet affects a larger area around it assuming the same optics, camera sensor, camera settings (exposure and gain in particular) and image processing steps. Because of that a faint moon too close to the planet might not be detected by the camera sensor simply because the signal from the moon is below the threshold of the signal caused by the planet's glare. So if there were a moon exactly like Triton orbiting Uranus, it would probably be a little more difficult to image than the actual Triton in orbit around Neptune. Second, the satellites of Uranus are dimmer than Triton (+13.46). The brightest is Titania at +13.97, followed by Oberon (+14.18), Ariel (+14.4), Umbriel (+15.05) and Miranda (+16.55). The others are too dim to be of interest for the amateur astronomer. Third, at the moment (2009), while the inclination of Triton's orbit as seen from Earth is very high, the inclination of the Uranian moons is almost zero, which means that, as seen from Earth, Triton moves on a low eccentricity ellipse around Neptune of radius about 15 arc seconds while the moons of Uranus moves on an almost straight line from one side of the planet to the other:

The implication is that while any time is good for observing Triton (provided it is night!), the moons of Uranus required a little bit of planning to make sure that they are not too close to the planet during an imaging session. On the evening of July 20th, the weather looked promising, so I decided to check the arrangements of the Uranian moons in Cartes du Ciel. On July 21th at 2:45am they looked like this:

The arrangement didn't look very favourable, with Umbriel and Titania only 4.5" and 4.8" away from Uranus. The reason why I am saying that is because the image scale, IS, measured in arc-second/pixel of my setup is:
IS = 206265 x (pixel size) / (focal length) =
206265 x 5.2 microns / 1200 mm = 0.89 arcsecond/pixel
Given that Uranus is 3.8" in diameter, the distance of Umbriel (say) from the limb of the planet was 2.6". The seeing on that night was around 3", so it would have been extremely difficult for my scope and camera to resolve Umbriel and Titania. The other two moons, Ariel and Oberon were more accessible, being 13.4" and 24.4" away from the planet. Miranda, being so faint and so close, wouldn't have a chance, but that would be OK for my first attempt. So I waited for Uranus to clear the roofs of my neighbourhood and climb to about 25 degrees above the SE horizon. Based on the experience acquired with Neptune and Triton, I decided to go for 20sec individual exposures instead of 15 and bump up the total exposure to 30min instead of 15. The fact that the moons of Uranus are fainter than Triton and that Triton was barely visible in my Jupiter-Neptune image, convinced me to go for longer individual and total exposures. The gain was set to 1600ISO. The camera (Canon XSi) was connected to the scope in the prime focus configuration. A Baader Coma Corrector was used to contain coma. The reason for the coma corrector was because the entire field of view around Uranus is quite pretty, with star 20 Psc of magnitude +5.5 (very similar to Uranus) less than 35 arc-minutes away. 20 Psc is a giant star of spectroscopic class G8, so yellow-orange in colour. That would make a nice contrast with the light blue cast of Uranus. To eliminate trailing (even with short subframes) I used an autoguider (Orion Starshoot). After 30 minutes I packed up and left the processing for the following day, but not before launching DeepSkyStacker to take care of the lengthy aligning and stacking of the 100 individual frames.

Here's the final image.

Ariel and Oberon are clearly visible. I checked against the arrangement provided by Cartes du Ciel and then double-checked using the JPL Ephemeris Generator (HORIZONS), just to be sure of which one is which. No trace of Umbriel and Titania, as expected. I am quite pleased with the result: with some planning it should be possible to see all four main moons. I doubt that Miranda, at magnitude +16.6 and less than 9 arc-seconds maximum elongation will be detected by my setup, but who knows? Perhaps under excellent conditions it might be possible. It certainly won't be easy!

Here's the final image without the inset.

The colour contrast between Uranus and 20 Psc is quite nice.

As I was studying the image in detail, I noticed the pretty asterism to the right of Uranus. What also caught my attention was that some of the "stars" of the asterism looked fuzzy. Upon closer inspection it turned out that those faint fuzzies were indeed galaxies! A quick search confirmed that on that night Uranus was just 12 arc-minutes away from the Compact Group of galaxies Hickson 97. I also noticed other fuzzies disseminated across the image.

The fact that there were a bunch of galaxies in my image convinced me to take a longer exposure of the same field of view. That opportunity presented itself few days later, on July 24. That will be the subject of the next post.


Saturday, August 1, 2009

Planets and moons

One of the interesting astronomical events of the year 2009 is the Triple Conjunction between Jupiter and Neptune in Capricornus. A triple conjunction between two planets occurs when they meet each other three times in the sky. In this context "meet each other" simply indicates that the two planets are in close proximity on the celestial sphere, usually less than a degree apart. In the case of the Jupiter-Neptune 2009 conjunction, the first two encounters (in Right Ascension) occurred on May 25 and July 13; the third and last rendez-vous will occur on December 20.

Around the July 13 conjunction, I took a closer look at the pair using my favourite planetarium software (Cartes du Ciel). While I was fiddling around with dates and times I noticed that the separation between the two planets was about 30 arc-minutes, which is well within the Field Of View of my Canon XSi at prime focus of my 10" f/4.7 Newtonian (about a degree). μ Capricornii (+5.08) made a nice addition right in the middle of the two planets . A couple of days of miserable weather forced me to wait, but on the night of the 17th the sky was perfect.

I took 60 frames 15sec each at ISO1600. With the two planets so low in the sky I figured that 15sec would be a good compromise between having the final image overwhelmed by light pollution and bagging Neptune that at +7.8mag is not exactly a bright object. Not to mention that the extinction coefficient at 14 degrees high is probably significant, so +7.8 was an optimistic estimate of the actual brightness of the Icy Giant.

When I looked at the final image and zoomed in to Neptune to check the colour, I noticed the presence of an object at 2 o'clock, very close to the planet. A possible explanation was that the camera had been able to capture Triton, the main moon of Neptune. I run a couple of checks in Cartes du Ciel and Starry Night, but eventually I used the Ephemeris Generator offered by JPL . The response was that yes, the little orb beside Neptune was indeed Triton! I was actually quite surprised as you can imagine. I was working at prime focus (62.4'x42.8') and the separation between Neptune and Triton is only about 15"! Not to mention that Triton's magnitude is +13.5, like Pluto. I did another check and realized that the limit magnitude my rig was able to achieve in just 15 minutes was +15.2/+15.4 (again, these are optimistic estimates given how low Neptune was).

Here is the image.

This is the second time that I am able to image Triton, but the first time was using a MallinCam HYPER Plus through a 16" SCT.().

All in all I am very happy with this image: Jupiter blazingly bright with the four Galileian moons (from left to right: Callisto, Io, Europa and Ganymede) and then blue Neptune with Triton. Not bad for a 15' image!

P.S. If anybody is aware of an image showing two planets with moons in the same Field Of View please let me know...I could not find any!