Rosette Nebula (Caldwell 49)

This beautiful deep-sky object known as Caldwell 49 is a large emission nebula located in the constellation Monoceros approximately 5000 light-years away. The cosmic cloud of gas and dust in this part of the sky resembles the petals of a rose and is a stellar nursery where new stars are being born. This is also where the name originated, as the structure is very similar to a stylized flower pattern used in sculpture since antiquity.

Rosette Nebula in Hubble Palette

Credit: Terry Hancock

The nebula is 130 light-years across and contains many Herbig-Haro objects and Herbig Ae/Be stars, Bok globules, T Tauri stars, and clusters of newly formed stars. The dark dust filaments extending toward the nebula's center, dubbed "elephant trunks," are shaped by stellar winds, radiation from hot young stars, and electromagnetic forces.

NGC 2244

At the center of the Rosette Nebula is NGC 2244, an open star cluster containing several O-Type Stars that generate large amounts of radiation and stellar wind. Only a few million years ago, the stars of NGC 2244 were born from the surrounding gas, and they have since been burning and affecting the surrounding nebula in Caldwell 49. This cluster spans 50 light-years across the night sky, is 5,200 light-years away, and can be seen with binoculars near the Unicorn constellation (Monoceros).

Credit: Don Goldman, NASA

I find it fascinating that such a deep-sky object exists where a nebula and star cluster coincide, as the bright hot stars of NGC 2244 are currently dramatically chaning the formation of the center of the Rosette Nebula. In many thousand years, astronomers will look at this phenomenon and see a completely different nebula than what we currently see in the twenty-first century.

Location & Best Time to See it

The Rosette Nebula is known as "The Rose of the Night Sky" for its sparkling view of a well-structured nebula and star cluster during winter. To observe it for yourself, start by finding one of the brightest stars in the sky, Betelgeuse, which should have an orange tint. Once you've found this star, try to locate the Monoceros Constellation's closet leg, resembling a deer's head (minus the antlers). From there, the corner of this "leg" is in the general area where the Caldwell 49 is in the sky. The photo below from Stellarium is a good representation of where to find the Rosette Nebula in the night sky.

Credit: Stellarium

Image created by Andrew McHaty

The best time to look for the Rosette Nebula from this latitude is from December – March, when it reaches its zenith in the sky. This allows astrophotographers and observers to view this impressive nebula with minimal effects of turbulence in the air and localized light pollution. Unfortunately, by May, Caldwell 49 quickly fades into the horizon as the sun starts to set, out of sight from northern latitudes for six months.

Photographing the Rosette Nebula

During the winter season, many astrophotographers from around the world point their telescopes at this complex structural nebula for hours. Anyone can capture a good image of the Rosette Nebula, you just need to follow the steps. To begin with, you should have a telescope with a focal length of at least 500mm, a dedicated CCD camera or DSLR, and a tracking mount. Using a large camera lens would have the same effect as a telescope, but it won't have the same tremendous amount of detail. Next, with your tracking mount, slew your setup to the coordinates of Caldwell 49 and keep plate-solving until you have the target centered in your frame of view. Now, test different exposures and see which is best for your setup. For me, I can capture the infamous "elephant trunks" with just a short 30-second exposure shot, but I choose to go with a 5-minute exposure shot as it brings out more of the nebula that is hidden with shorter exposures. Once you have everything down, you can start your session plan for the night and capture hours of data of this beautiful rose.

However, as is the case with all deep-sky astrophotography targets, the key to a successful image of the Rosette Nebula is an adequate amount of integration time for a healthy signal-to-noise ratio. We do this by capturing the four main calibration frames (light, dark, flat, and bias) and stacking them together through DeepSkyStacker. A good tutorial on how to use the software is here.

Here is how to take the four main calibration frames:

Light Frames - These are your signal frames; the instructions above are how to take these.

Dark Frames - These frames' purpose is to increase your photo's signal-noise ratio. Keep the same settings you had for your light frames and cover your lens/telescope with the lid.

Bias Frames - The purpose of these frames is to reduce the noise in your photo. Keep the same settings on your camera for your light frames, change the exposure time to the fastest it can possibly be, and cover your lens/telescope with the lid.

Flat Frames - The purpose of these frames are to eliminate vignetting/light falloff and other artifacts in your image due to dust, dirt, or smudges on the sensor or telescope in your photo. These are tricky. In simple terms, cover your lens/telescope with a white shirt and point it at a constant light source, like the morning sky. Then experiment with the exposure times until the histogram looks like the one below. This is shown below for monochrome and color cameras.

Credit: SharpCap

My photo of Caldwell 49 below was captured using a one-shot CCD Camera (ZWO ASI294MC Pro), a dual-narrowband filter (Optolong L-Extreme), and a refractor telescope (100mm Aperture, 900mm focal length) with guiding. To check out my equipment in more detail, click here. Using a duel-narrowband filter, I extracted the Oxygen III and Hydrogen Alpha gasses from the nebula's composition. I'll publish an article soon on how to do this yourself!