Acquisition Details:

Observed on: 10/13/2022 10/14/2022 10/15/2022 10/19/2022 10/25/2022 10/24/2022
Filter: H-Alpha : Exposure time: 6h 52m 00s (24720.0 seconds)
Filter: L: Exposure time: 1h 20m 00s (4800.0 seconds) Stars only
Filter: OIII : Exposure time: 7h 16m 00s (26160.0 seconds)

Commentary:

IC 1805, also known as the Heart Nebula, is very frequently imaged object. An emission nebula in Cassiopeia, at its center is a cluster, Melotte 15, which its namesake discoverer, P. J. Melotte identified in 1915. Melotte was cataloguing star clusters found during his examination of the first high-quality photographic atlas of the sky, the Franklin-Adams charts. Melotte’s notes described “A very loose cluster, ... similar to NGC 1027 but stars not so numerous and generally brighter.”

At Lick Observatory in 1930, Robert Trumpler was attempting to understand the distances, dimensions, and distribution of clusters including Melotte 15. Using magnitude estimates and spectroscopy, he found the many of the stars were anomalously redder than their spectral identifications suggested. Intervening gas and dust was selectively scattering blue light from the stars — Trumpler had discovered interstellar reddening.

Today we know that Melotte 15 is an OB Association, consisting of hot, blue-giant stars blasting their environment with UV radiation. Because these stars are young and burn out quickly, OB associations are relatively homogenous in age. The most familiar OB association may well be the Great Nebula in Orion, with its white-hot Trapezium stars lighting up the gases and clearing out the gas and dust in the nebula thorough photoevaporation and radiation pressure.

Determining cluster membership can be difficult — which stars are from the foreground, and which belong to the cluster? In 1995, Phil Massey and others were looking to understand the evolution of clusters including Melotte 15. They needed to know which stars were members, and they undertook a comprehensive photometric and spectroscopic survey from Kitt Peak using the 36-in and 4-meter telescopes there. They identified 1024 proposed members, and carefully characterized their reddening to make color corrections to the photometry. This allowed them to conduct a more comprehensive analysis of stellar ages, masses, and cluster evolution. 

The data for this image were taken in October of 2022. It is an HOO narrow-band image with just H-Alpha and OIII, since the SII signal at the core of the nebula was relatively weak. This also afforded me the opportunity to render the colors in a truer way, with H-Alpha and OIII appearing their actual pinkish and green, respectively.

Curious about the distribution of the hotter stars in the cluster, I downloaded Massey’s list of cluster members, applied his reddening correction to the B-V colors and converted the B-V to approximate temperatures using Ballesteros' formula. I created a custom catalog of stars with temperatures of 10,000K and over, and they are marked in Revision B.

Data from Gaia, with its high-precision parallaxes able to pinpoint cluster members, will revolutionize our understanding of these objects.

Subsequent work on this cluster by Wolf et al. have attempted to look for circumstellar disks around the stars in the cluster. Unsurprisingly, disks are rare around O stars, with their fierce stellar winds rapidly eroding away any orbiting  dust and gas. 

Any life that might evolve around an O-star might be almost as exotic as those described in Robert Forward’s science fiction novel, Dragon’s Egg!