To see a World in a Grain of Sand
And a Heaven in a Wild Flower,
Hold Infinity in the palm of your hand
And Eternity in an hour.
- William Blake
Whirlpool galaxy - M51
M51 is Whirlpool galaxy, actually a pair of them, M51a and M51b. The large one is devouring the small one. You can see the dust lane that is diffusing around both of them. Those lanes span across trillions of miles, and constitute mainly hydrogen and helium gas and particles of heavier elements created during explosions involving star deaths.
M51 has around 100 billion stars, and it is around 23million light years away from earth. One light year is about 6 trillion miles. The side to side measurement across the pair is 60000 light years, which is smaller than that of our home galaxy, Milky Way, which is 100000 light years across. In the night sky, the galaxy pair makes around 11 arc-minutes in our eyes. There is one more very tiny longish fuzzy galaxy that can be seen in the bottom left corner of the image. It is IC4263, which is about 130 million light years from us. That is about 5 times farther than the M51 system, and the farthest object I have imaged so far.
I imaged this on three separate nights. There are 50 exposed images stacked, with 4min of sub-exposure each. So, total 200 min exposure. Stacking helped increasing the signal to noise ratio. It was imaged with a 600mm lens (f/6.3) on a crop-sensor mirrorless body with 24million pixels, with moderate sensor amplification set at ISO 800. The galaxy, with quite small apparent size, occupied only around 40,000 pixels out of available 24million.
What makes me feel spiritual is to think that the photons that created this image in the camera sensor left M51 23 million years ago, when human species did not show up on our planet. While those photons were on their way with the speed of light (186000 miles per second), human being evolved, got to know the Cosmos incrementally, and developed engineering tools to capture the photons and to try to make sense of them. All while the photons were on their voyage through almost empty space with roughly 5 protons per cubic meter! Also to think, we are looking into 23 million years in the past! Can be humbling for our prefrontal cortex!
Wizard Nebula - NGC 7380
NGC 7380 (referred to as Wizard Nebula) is an emission nebula situated in constellation Cepheus, which is part of the Perseus arm of our galaxy Milky Way. The nebula is about 8500 light years away from us, and it is around 20 light years across. It is about 10 million years old, which is considered fairly young compared to the age of our galaxy. Energy radiated from two large nearby stars are responsible for ionizing the gas cloud within NGC 7380. The final image is rendered in what is known as the Hubble Palette. Emission from ionized Oxygen is represented in blue, and those from ionized Hydrogen and Sulfur are represented in green and red respectively. The same two stars, performing gas ionization, are also responsible for creating gravitational tidal perturbation within the gas cloud. Such perturbations lead to the seeding of new stars. So far, around 31 newborn stars are identified within NGC 7380.
The final image was constructed by stacking sub-exposures with Hydrogen-Alpha, Sulfur-II, and O-III filters, two hours of exposure (60 images, 2 minute each) with each filter. Total, 180 images taken during six hours over four Fall nights in November 2020. All images were taken from my backyard in Okemos, MI, which is reasonably dark (Class 5 in Bortle scale) for imaging with narrowband filters, as used for this image. My western sky, however, is light polluted by the city of Lansing, which makes it almost unusable for the Oxygen work.
The images were taken with a 400mm 8-inch wide-field Newtonian telescope (f/2 aperture), and a 16 megapixel monochrome camera cooled at -20 degree Celsius.
Galaxy Pair - M81/M82
The galaxy-pair in the image are approximately 12 million light years away from the solar system. The right one, M82, also known as Cigar Galaxy (called so due to its shape) is gravitationally bound to the left M81, which is known as Bode’s Galaxy. M81, like our home galaxy Milky Way, is a twin-arm grand spiral galaxy. The structure of M82 is less obvious optically because its galactic plane is aligned with Milky Way such that only a sideway projection can be seen. Recent infrared observation, however, found M82 to also be a spiral galaxy.
I imaged this on June 5th, 6th, and 14th nights in between cloud covers. There are 62 exposed images stacked with average 3 min exposure each. So, total about 3 hours worth of photon integration happened. Stacking helped increasing the signal to noise ratio. It was imaged with a 600mm lens (f/6.3) on a crop-sensor mirrorless body with 24million pixels, with moderate sensor amplification set at ISO 1600. The galaxy, with quite small apparent size, occupied only around 3% (i.e., 720,000) of the pixels out of available 24million.
M81 has around 250 billion stars, and it is about 45000 light years across, which is quite a bit smaller than that of our home galaxy, Milky Way, which is 100000 light years across. In the night sky, M81 makes around 27 arc-minutes in our eyes, where an arc-minute is 1/60th of one degree. This is referred to as the apparent size of the object as seen from our planet. All the stars in the image are in foreground, and they are in our own galaxy. On a closer look, one may see bright spots in M81 that are aligned with its spiral arms. Those are not stars since they are too far to be resolved at the level of stars with my camera. They are the star-forming hot areas, knows as nebulae, teeming with hot gas, dust, and mostly newborn stars, each with the potential to have a solar system like the one we reside in.
These two galaxies are outside our local group of less than 100 galaxies, which are gravitationally bound. Meaning, these are receding from Milky Way, with a rate of approximately 150 miles per second, and that rate is increasing due to the expansion of space. At some point in future, these galaxies will be receding faster than the speed of light, since unlike the movement of mass in existing space, the rate of expansion of space itself is not bounded by the speed of light.
The grim reality is that we will not be able to see these galaxies beyond that point. As a matter of fact, other than our neighboring gravitationally bound 100 or less galaxies, the entire Universe will set to oblivion for us... for ever! Parts of it are already gone. Gravity will eventually combine all the galaxies in our local group, within 10 million light years, into a single clump of particles. That will be it! There is no bright future that awaits us .. so, let us get the living done well for ourselves, and ensure that for others!
Andromeda Galaxy - M31
Andromeda (i.e., M31), which is about 2.5 million light years from Milky Way, is the largest of around 100 or so galaxies in our local group. The galaxies in the local group, including our Milky Way, are gravitationally bound. They are mutually converging towards an eventual mega merger. Andromeda and Milky Way are hurtling towards each other with a relative speed of about 190 miles per second. They are expected to collide and merge into a single galaxy in 4.5 billion years from now. Like Milk Way, Andromeda is a spiral galaxy with a diameter of nearly 220,000 light years, and it is home of nearly a trillion stars.
This image was created from 4 hours of Red, Green, Blue, and Hydrogen-Alpha data at 400mm focal length. The red dotted areas around the periphery of the galaxy are hot star-forming nebula with heavy Hydrogen-Alpha emission. Imaging Andromeda requires looking through near Milky Way’s galactic plane where the star density is very high. That explains the dense foreground star distribution in the image. Two of Andromeda’s satellite galaxies M32 and M110 are visible in the image.
Great Globular Cluster - M13
M13, the Great Globular Cluster, is a collection of around half a million stars situated in the halo of our galaxy Milky Way. It is around 25,000 light years from us, and spreads across a sphere with radius approximately 72 light years.
This image was created by combining ten 4 minute exposure images. I am continuing to take more images, which will be combined with this one for bringing out more details.
M13 occupies around 20 arc minutes in the night sky with a dazzling array of very old stars. Some of the stars are thought to be as old as the Universe, which is around 13.7 billion years old. The yellow/orange stars are the oldest ones. The bluish ones are younger. Unlike most of the stars in a galaxy, globular clusters are not on the galactic plane. They are situated “above” the plane, referred to as the galactic halo. Milky Way has around 150 known globular clusters.
Unlike the usual stars on our galactic plane, most of the old stars in M13 did not not go through many generations of stellar death-birth cycles. Multiple generations involve supernova explosions during star deaths, which is when most of the elements heavier than iron are created in the Universe. Since the stars in M13 did not go through many such cycles, they are low on heavy elements compared to the stars on the galactic plane.
Heart Nebula - IC1805
Our galaxy has a big heart, a heart-shaped nebula that is 165 light years or 1000 trillion miles across. The nebula is about 7500 light years away from Earth and is located in the Perseus Arm of Milkway, in the constellation Cassiopeia. It is an emission nebula showing glowing ionized hydrogen and other gas, and darker dust lanes. The nebula's intense energy output is driven by the radiation emanating from a small group of stars near its center. This open cluster of stars, known as Collinder 26 or Melotte 15, contains a few bright stars nearly 50 times the mass of our Sun, and many more dim stars that are only a fraction of our Sun's mass.
The top-right corner of the huge gas cloud is designated as a separate nebula called the Fish Head nebula or IC 1795. The brighter region of IC 1795 is designated NGC 896 and is the home to many massive young stars.
The above false color image is rendered in Hubble Palette, which is common with many spectacular Hubble Space Telescope images.
IC1848 - Soul Nebula (in Hubble Palette)
Rosette Nebula in Constellation Monoceros; True Color Image with Hydrogen Alpha filter
Distance to Earth: 5,219 light years, and Radius: 65 light years
Rosette Nebula Rendered in Hubble Palette with Sulfur-II as Red, Hydrogen Alpha as Green, and Oxygen-III as Blue
Veil Nebula West - NGC6960
This is a part of a supernova remnant in Cygnus Loop, which is in the constellation Cygnus. A supernova is a massive explosion that happens to a star when it runs out of hydrogen, which is its primary nuclear fuel during the energy production stage. These are one of the most violent types of explosions in the Universe, and these happen during the deaths of stars that are born larger than a critical size. NGC6960 is part of a large remnant of a supernova, or stellar dead body, that can be seen around the constellation Cygnus (the swan). Four hours worth of data for the above image was taken with Hydrogen-Alpha and Sulfur-ii filters.
Veil Nebula East - NGC6992
This is the eastern part of Cygnus Loop supernova remnant, taken with Hydrogen-Alpha, Sulfar-ii, and Oxygen-iii filters to produce the Hubble Palette image. Cygnus loop is around 1760 light years from earth, and around 130 light years in diameter. The supernova explosion happened relatively recently, estimated around 21,000 years ago.
Pinwheel Galaxy - M101
Horse Head and Flame nebulae
Both the nebulae and their surrounding extensive gas clouds are part of what is known as the Orion complex. The left most star of the constellation Orion’s belt Alnitak appears as the brightest star with its halo just above Flame nebula, which is an emission nebula. Horse Head is a dark nebula, which constitutes mainly neutral (i.e., non-ionized) Hydrogen without any electromagnetic emission in the visible spectrum. That is why the horse head appears as dark, silhouetted in the background of hundreds of trillions of miles of ionized Hydrogen radiating in the Hydogen-alpha part of the visible red spectrum. The horse head itself emits microwave radiation at 1.42 GHz (i.e., 21cm), which is used by radio telescopes for mapping out the neutral hydrogen distribution in different parts of the Universe. Current estimates of the size of horse head is 4 light years tall and 3 light years wide. In miles, its around 24 trillion miles tall by 18 trillion miles wide. It is around 1500 light years from the solar system. The entire Orion complex is within the Orion spur of the Milky Way in which our solar system is located. In that sense, the Orion constellation and all its nebulae and stars are right in our cosmic backyard.
NGC281 - Pac-Man Nebula (in Hubble Palette)
Markarian's Chain is a stretch of galaxies that forms part of the Virgo Cluster. When viewed from Earth, the galaxies lie along a smoothly curved line. Out of the 8 galaxies, M84 and M86 are two prominent ones. The chain is visible in the constellation Virgo (the Virgin).
Pelican Nebula - IC507
This is a bit dramatic color rendition of Pelican Nebula in the constellation Cygnus (the great swan), processed in Hubble Palette. The hue of orange and blue were slightly nudged towards blood-orange and turquoise to bring out an apocalyptic appearance of the nebula, which is about 1800 light years away. It is mainly hydrogen gas and dust particles of heavier elements. Light was collected for four hours with a 400mm f/2 scope and a monochrome CCD camera cooled at -20 degree centigrade for noise reduction. Pelican is an emission nebula, with ionized hydrogen emitting mainly in the red part of the spectrum (represented as green in this false color image). Ionization happens by two extra-bright nearby stars that can be seen in the mid-bottom and top-left parts of the image.
A closeup of Pelican Nebula with its star-forming columns. Notice the small tentacle like structures. They are each roughly 9 to 15 trillion miles long, and were formed and being stretched by moving mass of gas. While moving, they are in the process of creating hundreds of new stars nucleated by the shockwaves of the movement.
Bubble Nebula - NGC7635
Elephant Trunk Nebula (IC 1396) in the constellation Cepheus. The Nebula is a concentration of interstellar gas and dust, and around 2400 light years from our solar system.
The Orion Nebula (M42) is a diffuse nebula situated in the Milky Way, being south of Orion's Belt in the constellation of Orion. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. It is about 1,344 light-years away and is the closest region of massive star formation to Earth. The nebula has a radius of about 12 light year, and it is roughly 3 million years old.
Long Exposure Imaging
Astro-imaging is a special type of photography with which a very dim object, most of the time not visible with naked eyes, can be imaged with a lot of details. The object is not visible with naked eyes because the number of photons that are received by the retina from the object during the persistence of vision (i.e., of the order of 10s of milliseconds) is too small to fully excite the photo-sensors in the retina.
In Astro-imaging, the problem is alleviated by: 1) capturing more photons per unit time, and 2) integrating photons over longer durations. The first one is achieved by using a camera lens or telescope that has bigger aperture than our pupil. The second one is done by keeping the camera shutter open for much longer durations. For example, a reasonably good image of the Whirlpool galaxy (M51) using a standard camera lens would require at least an hour worth of photon integration by keeping the camera shutter open for that long. The end effect can be a dazzling image of a beautiful galaxy that our naked eyes cannot see. All astronomical images taken from our planet or from space use these basic principles. For example, One of the ultra-deep-field images taken by Hubble Space Telescope was created by integrating photons from very distant galaxies over a period of 11 days. And that was through an aperture of around 2.4 meters. Compare that with the diameter of the opening of human eyes of approximately 4 mm.
In practice, a major challenge in astro-imaging comes from our planet’s own spinning motion. The earth spins around its own axis with an angular speed of around 15 arc-seconds per second. One arc-second is 1/3600th of a degree. As a result of this spin of a ground-based camera with respect to a celestial target, any exposure longer than a few fractions of a second creates star trails and blurring, thus spoiling the image. The way to circumvent this is to spin the camera itself in synch with earth’s sidereal motion (i.e., the spin) so that the target celestial object does not drift with respect to the camera sensor during a long exposure. Software-controlled motors are used to drive a camera mount on a tripod to achieve this. Depending on the accuracy of such control hardware and software, this enables an imager to take single exposures up to tens of minutes long.
Multiples such images are then aligned and stacked in order to improve the signal to noise ratio of an image. While signal (i.e., the RGB luminosity of a pixel capturing part of a galaxy or a nebula) is deterministic, most of the noise (i.e., read noise, thermal noise, etc.) is random and fortunately Gaussian. This allows very simple additive processes to enhance the signal while suppressing the noise. Therefore, by combining many long exposure shots, the signal to noise ratio in an image can be enhanced significantly to bring out details in the astronomical objects. Such details are important since gas and dust are the main ingredients of most of such objects, and their subtle variations create intricate texture, color, and the overall beauty. Any additional details in the gas clouds and dust lanes make the image more revealing.
It is not only possible but almost unavoidable to image an object over multiple days, weeks, months, and even seasons. This allows an Astro-imager to continue on taking new shots and improving the quality of an image incrementally. This makes production of a “good” image not just an event, but a long-term project. Unfathomable distances to the celestial objects allow that to happen since the relative perspective of an object from the earth remains fixed over one’s lifetime or even generations.
A surreally beautiful generational project would be to continue imaging a specific part of the heaven by the members of a family and/or friends over many many generations so that the image produced in a generation is built on the exposures taken by the previous generations, potentially hundreds of years in past. This will be another way to celebrate the continuity of life and its work!
