Corona Australis Molecular Cloud

• 1. Introduction

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  Corona Australis Molecular Cloud sits just 430 light-years from Earth. That distance makes it one of the closest star-forming regions to our solar system. It occupies the constellation Corona Australis, near the border with Sagittarius, and spans roughly 8 light-years from end to end. Inside, temperatures sink to between 13 and 22 degrees above absolute zero. Within that cold envelope of gas and dust, an entire population of objects in various stages of becoming stars fills the space. How that population is organized, and what its youngest members look like, are questions observers have worked to answer for over a century. A globular cluster appears to sit at the cloud's edge, but its true location is tens of thousands of light-years further on.

• 2. Mass, Scale, and Extinction

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  The total material inside the CrA Cloud adds up to roughly 7,000 times the mass of the Sun. Yet that mass is not spread uniformly. More than half of it concentrates in one compact sub-region, which is also where star formation runs most actively. The cloud sits about 18% below the galactic plane and covers an angular area of 16.0 by 6.5 degrees on the sky. Despite that extent, astronomers have directly confirmed only 55 distinct optical members within it. Extinction explains the small count. At certain wavelengths, the cloud absorbs visible light by as much as 45 magnitudes. That level of blockage makes the cloud effectively opaque to ordinary telescopes. Surveys in the infrared, X-rays, radio waves, and millimeter continuum have proven far more productive for probing its interior. Within that compact core sits the Coronet protostar cluster, the cloud's most concentrated gathering of young stars.

• 3. The Coronet

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  About 30 young stars make up the Coronet protostar cluster. Each is at a different stage of its evolution, spanning a wide range of masses. A few late-stage B stars circle its edges. Surrounding the cluster, several young stellar objects and numerous Herbig-Haro objects fill the neighborhood. Herbig-Haro objects appear where jets of material driven by young stars strike surrounding gas, producing bright knots of shocked material. At the lower end of the mass range, two brown dwarfs have been confirmed within the region. Seven more candidates await classification. The broader cloud complex holds hundreds more young objects beyond the Coronet, catalogued by infrared surveys of the L1688 sub-cloud.

• 4. A Census of Young Stars

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  A total of 425 infrared sources have been detected near the L1688 cloud, the complex's central sub-region. Astronomers classify most of them as young stellar objects. Sixteen have been confirmed as protostars, bodies still actively drawing in gas from surrounding envelopes. A further 123 are T Tauri stars carrying dense circumstellar disks. T Tauri stars are pre-main-sequence objects, still gathering enough mass to sustain stable hydrogen fusion. Another 77 T Tauri stars have thinner, weaker disks. Estimated ages for both T Tauri groups range from 100,000 to a million years. A star a million years old is extraordinarily young on any astronomical timescale. The reflection nebulae embedded in the cloud drew the attention of observers long before these infrared surveys were possible.

• 5. Nebulae and an Impostor

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  Four reflection nebulae stand out among those embedded in the Corona Australis complex: NGC 6729, NGC 6726, NGC 6727, and IC 4812. NGC 6726 and NGC 6727 are genuinely part of the cloud system. NGC 6729 shares the same patch of sky but is not physically associated with the cloud. All four glow because they scatter light from young stars forming in the surrounding material. Three of the four were first recorded by Johann Friedrich Julius Schmidt in 1865.

  The globular cluster NGC 6723 presents a starker case. It appears to sit in the same region of sky as the Corona Australis nebulae and young stars. Its actual distance is 29,000 light-years. Nothing in it connects to the star-forming activity of the Corona Australis complex. It simply occupies the same line of sight, with no physical link to the cloud.

  NGC 6726 and NGC 6727 still anchor wide-field images of the region. The first observers to catalogue them had no way of reaching the cloud's densest zones. Those zones only became accessible to later generations of instruments.