The Night Sky - Episode 6

HEART NEBULA

This is episode 6 in my series on our night sky. View episode 5 here.

Here’s a video showing the magnificent Heart nebula. After watching the video, scroll on to read more about one of the most beautiful regions in deep space.

The Heart nebula is an emission nebula located in the Perseus arm of our Milky Way galaxy. In the night sky, it appears in the constellation of Cassiopeia. It is also designated as IC1805 (in the Index Catalogue of Nebulae and Clusters of Stars) and as Sh2-190 (in the Sharpless Catalog).

The Heart nebula is comprised of ionized hydrogen, oxygen, and sulfur gases and is named for the unmistakable heart shape that it takes, which is caused by stellar winds generated by the hot stars at its core. It is at a distance of ~7,500 light years from Earth and spans a rather sizeable portion of the night sky - covering an area of almost 4 times the size of the full moon. It is not naked-eye visible but can be captured using a camera and lens.

To find the Heart nebula, we must first look for the constellation of Cassiopeia.

Several prominent constellations can be seen in the image above, and by star-hopping from one constellation to the next, we arrive at the constellation of Cassiopeia. This constellation is made up of five stars forming the shape of a “W”. The Heart nebula can be found adjacent to the tip of the arm of the “W”.

As the Earth rotates, the stars in the night sky rise and set as well. Depending on where you are located on the Earth, some parts of the night sky are visible only during certain times of the year. From my location (Southern Ontario in Canada), the constellation of Cassiopeia is circumpolar - which means that it is always visible in the night sky year-round.

The blue-gold image of the Heart nebula shown above is what is known as a false-color image. The image below shows a more realistic representation of how our eyes would see the Heart nebula if it were sensitive enough to pick up color at night.

A note about false-color imaging in astrophotography:

The red hue, as seen in the image of the Heart nebula above, is common to most regions of space. This is because there is an abundance of hydrogen in space. When hydrogen atoms are excited by radiation from nearby stars, they are ionized and emit light in the deep red spectrum - at a wavelength of 656.46 nm, to be exact. This red light is known as the h-alpha (Hydrogen alpha or Hα) emission line.

Other elements are present in deep-space objects and especially in nebulae - ionized sulfur (deeper red color than Hα) and ionized oxygen (blue-green color). The h-alpha emissions from the abundant hydrogen overpower light from other elements making most deep-space images appear red to our eyes. However, our eyes can only distinguish between about 16 shades of any given color. So, astrophotographers create false-color images of deep-space objects by mapping the emission lines of ionized sulfur, hydrogen, and oxygen to red, green, and blue respectively on the color wheel. This false-color mapping makes it much easier for our eyes to see intricate structures, dust lanes, and stellar activity.

In fact, there are many possible ways to map the emission lines to colors on the color wheel and different mappings will result in different false-color images. The sulfur→red, hydrogen→green, oxygen→blue mapping is one of the most commonly used mappings because it was popularized by NASA in their rendering of images of space beamed back to Earth by the Hubble space telescope. This mapping is therefore commonly known as the Hubble palette.

The false-color version of the Heart nebula makes it easier for our eyes to pick out distinct regions in the nebula. Blue regions are rich in ionized oxygen, yellow-gold regions are rich in ionized hydrogen, and orange-red regions are rich in ionized sulfur.

There are a number of interesting structures in the vicinity of the Heart nebula. Below is an annotated image of this region in space. Blue markers show objects named in the Sharpless catalog (a directory of hydrogen regions in space). Yellow markers show object from the IC and NGC catalogs. At the bottom of the frame is the very bright Fishhead nebula (IC1795) which is an interesting nebula to photograph by itself. IC1824 is an open star cluster. A beautiful structure can be found at the center of the Heart nebula. Known as Melotte 15, this cluster of young stars (1.5 million years old) drives fierce stellar winds that gives the Heart nebula its shape. Green markers identify distant galaxies and grey markers point to prominent stars visible in the field.

The heart nebula is a relatively easy deep-space object to photograph and can be captured using a simple camera and lens. The key ingredient to getting a good image of the nebula (and any other deep-space target) is time. The more time one sinks into capturing the faint light reaching us from these distant objects, the better the final image.

Capture details

Camera: Astro-modified Canon 80D
Lens: Sigma 100-400mm f/5-6.3 DG OS HSM
Focal length: 250mm (equivalent to 400mm full-frame)
f-Ratio: 6.3
ISO: 800
Sub-exposure length: 180 seconds

Number of exposures: Total of 305 over 3 sessions. Of which 205 exposures were using the Astronomik Hα filter, and 100 exposures were using the Optolong L-Enhance filter.
Total integration time: ~15 hours
Calibration frames: 30 darks, 30 flats, 30 bias for each filter and session.

Mount: SkyWatcher Star Adventurer
Guide camera: ZWO 120mm mini
Guide scope: ZWO Mini Guide scope
Capture control: ZWO ASIAir Pro
Dew control: CooWoo dew heater

Sky quality: Bortle 5
Moon phase: 60-90% illuminated moon

Processing: PixInsight

View episode 5 in this series here.

All episodes in this series