Astrophotography is more than capturing light; it's about telling the story of the Universe. Few targets tell it better than the Heart Nebula (IC 1805) and the Soul Nebula (IC 1848), two vast star-forming regions side by side in the constellation Cassiopeia, close to its border with Perseus. Together, they are known as the Heart and Soul Nebulae, one of the favorite pairings of the autumn sky for astronomers and astrophotographers alike.
Both lie about 7,500 light-years away, in the Perseus Arm of the Milky Way (published estimates range from about 6,500 to 7,500 light-years). Each spans roughly 2–2.5° of sky, about four or five full Moons side by side, which at that distance means each nebula is around 300 light-years across. The pair together covers about 6° of sky.
This is our second time on this target. Last autumn, we captured almost 8 hours of Hα and OIII. This October, we went back with the same telescope and the same framing, added another 16 hours, including sulfur (SII) for the first time, and combined both seasons into a single image.
The Heart and Soul Nebulae lie in eastern Cassiopeia, near the border with Perseus. From Hong Kong, they are northern targets, reaching about 51° altitude as they cross the meridian, in the direction of the Shenzhen light dome. Sky and Telescope's Pocket Sky Atlas – Roger W. Sinnott.
History
The first part of the complex to be discovered was not the Heart itself but its brightest knot. On 3 November 1787, William Herschel recorded a small, bright patch of nebulosity at the western edge of what we now call the Heart, later cataloged as NGC 896. The full extent of the two nebulae, far too large and faint for the eyepiece, only came to light with photography, and they entered the Index Catalogue (IC) in the early 20th century as IC 1805 and IC 1848, named after the star clusters at their centers.
In 1959, Stewart Sharpless listed them in his catalog of emission nebulae as Sh2-190 (Heart) and Sh2-199 (Soul). The Soul is also known to radio astronomers as Westerhout 5 (W5), from Gart Westerhout's 1958 survey of radio sources along the Milky Way.
The Heart (upper left) and Soul (lower right) Nebulae — 23h 51m total exposure over two autumns (Askar FRA 300 Pro, ZWO ASI 2600 MC Pro), HSO palette.
Astrophysics
Both nebulae are H II regions: clouds of hydrogen ionized by the ultraviolet light of hot, young, massive stars. They belong to the same giant star-forming complex, sometimes called W3/W4/W5, after its three main radio sources.
The Heart and Melotte 15: at the center of the Heart sits the open cluster Melotte 15, also cataloged as IC 1805 and Collinder 26. A few of its stars are close to 50 times the mass of the Sun, surrounded by many smaller ones. Their radiation and stellar winds light up the gas and carve the nebula from the inside, leaving dark pillars and dust lanes pointing back towards the cluster, like the ones near the bright core in our image.
The Soul and its cavities: the Soul has its own cluster, also cataloged as IC 1848 (Collinder 32). Its massive stars have hollowed out large cavities in the surrounding gas, the teal-blue hollows inside the Soul in our image. As the gas is pushed outward, it piles up and compresses along the cavity walls, and that compression can trigger new generations of stars. Astronomers see this in the ages of the young stars around the nebula: the farther from the cavity centers, the younger the stars tend to be.
Reading the colors: in our HSO rendition, Hα (hydrogen) is mapped to red, SII (sulfur) to green, and OIII (oxygen) to blue, then blended into the warm and cool tones you see.
The orange and red clouds are dominated by hydrogen and sulfur, typical of the cooler outer edges of the nebulae.
The silver-blue areas in the cores of the Heart and the Soul are where OIII is strongest: doubly ionized oxygen needs the most energetic ultraviolet light, so it glows closest to the hottest stars.
The dark lanes are dust in front of the glowing gas, blocking its light.
What is an H II region? "H II" is the astronomers' name for ionized hydrogen: hydrogen atoms that have lost their electron. When ultraviolet light from a hot star strips the electrons away, they later recombine with the protons and release light at very specific wavelengths, the strongest being the deep red Hα line at 656.3 nm. That's why emission nebulae like the Heart and Soul glow red in photographs, and why narrowband filters, which let through only those wavelengths, cut through light pollution so well.
NASA's Wide-field Infrared Survey Explorer (WISE) mission captured this mosaic of the Heart and Soul nebulae. WISE's infrared vision allows it to see into the cooler and dustier crevices of clouds like these, where gas and dust are just beginning to collect into new stars. - NASA/JPL/Caltech/UCLA
A Closer Look: The Heart Nebula
The Heart Nebula (IC 1805) — 3h 20m total exposure in 5-minute subframes (Askar 103 APO + 0.8x flattener/reducer, ZWO ASI 2600 MC, Askar D1), November 2025. HSO-style palette synthesized from the D1's Hα and OIII.
This close-up of the Heart was one of our first images with the Askar 103 APO and its 0.8x flattener/reducer. At a longer focal length than the FRA 300, it trades the wide view of both nebulae for a closer look at the Heart's core: the cluster Melotte 15 and the dark pillars and dust lanes its stars have carved, with the bright knot of NGC 896 at the nebula's edge.
The Heart Nebula, plate solved and annotated in PixInsight.
Cosmic clouds form fantastic shapes in the central regions of emission nebula IC 1805. The clouds are sculpted by stellar winds and radiation from massive hot stars in the nebula's newborn star cluster, Melotte 15. Credit: NASA Astrophotography of the Day (APOD) - Ivan Eder.
Observation
The Heart and Soul are best placed in the late autumn and winter evening sky. They cross the meridian around midnight in early November and remain well placed into January. At a declination of about +61°, they are circumpolar from much of Europe and North America, but from Hong Kong, they climb to only about 51° above the northern horizon.
What you can see visually:
Both nebulae are large and faint, with very low surface brightness, so they are difficult visual targets.
The clusters are the easy part: Melotte 15, IC 1848, and the foreground NGC 1027 show well in binoculars or a small telescope.
Under dark skies, a wide-field telescope with a nebula filter (UHC or OIII) can show NGC 896 and hints of the brightest parts of both nebulae.
From light-polluted skies like Hong Kong's, they are photographic targets. The full shapes, the pillars, and the cavities only appear after many hours of narrowband exposure.
Astrophotography Tips
Choose your focal length. The pair spans about 6° of sky. To fit both comfortably on an APS-C sensor, you need about 135–200 mm. At 300 mm, our field of view is 4.3° × 2.9°, so we used a diagonal composition, the Heart in one corner and the Soul in the other, accepting that part of the Fishhead region and the bottom of the Soul fall outside the frame.
Put the extra time into SII. Sulfur is the faintest of the three signals in this region. We spent all three nights of the new narrowband time on the Askar D2 to give the sulfur enough signal, since a noisy SII channel turns into blotchy color once stretched.
Use broadband data for star color only. With narrowband outweighing broadband by more than 20 to 1, we used the L-Pro data only for the stars, through a star mask. Narrowband stars come out magenta or teal; the L-Pro gives them natural color without bringing Shenzhen's sky glow back into the nebulae.
