APOD: Stars and Dust across Corona Australis (2023 Jun 22)

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APOD: Stars and Dust across Corona Australis (2023 Jun 22)

Post by APOD Robot » Thu Jun 22, 2023 4:06 am

Image Stars and Dust across Corona Australis

Explanation: Cosmic dust clouds cross a rich field of stars in this telescopic vista near the northern boundary of Corona Australis, the Southern Crown. Part of a sprawling molecular cloud complex this star forming region is a mere 500 light-years away. That's about one third the distance of the more famous stellar nursery known as the Orion Nebula. The 2 degree wide frame would span 15 light-years at the clouds' estimated distance. Mixed with bright nebulosities the dust clouds effectively block light from more distant background stars in the Milky Way and obscure from view embedded stars still in the process of formation. Large dark nebula Bernes 157 is on the left. To its right are a group of pretty reflection nebulae cataloged as NGC 6726, 6727, 6729, and IC 4812. Their characteristic blue color is produced as light from hot stars is reflected by the cosmic dust. The more compact NGC 6729 surrounds young variable star R Coronae Australis. Just below it, filamentary arcs and loops are identified as Herbig Haro objects associated with energetic newborn stars. In fact, at the heart of this area lies the Coronet Cluster, one of the nearest and most active star forming regions.

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Ann
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Re: APOD: Stars and Dust across Corona Australis (2023 Jun 22)

Post by Ann » Thu Jun 22, 2023 5:37 am

corona_aus1024[1].jpg
Stars and Dust across Corona Australis
Image Credit & Copyright: Alessandro Cipolat Bares

It should come as no great surprise that I like this APOD, and that I like this part of the night sky! It's so blue!
APOD Robot wrote:

To its right are a group of pretty reflection nebulae cataloged as NGC 6726, 6727, 6729, and IC 4812. Their characteristic blue color is produced as light from hot stars is reflected by the cosmic dust.
That's only partly true. The stars lighting up the blue reflection nebulas are hot enough to create a blue reflection nebula, but not too hot, because that would turn the blue reflection nebulas into red emission nebulas. On the other hand, even stars no hotter than the Sun could probably create a bluish reflection nebula!



The stars lighting up the blue reflection nebulas of Corona Australis, HD 176269, HD 176386 and TY CrA, are all B9 type objects. They are around 10,000 K and just a little bit hotter than Vega and Sirius.

I recommend this annotated picture of the Corona Australis nebulosity area:


The picture is very small, but you can see a larger version here. Just scroll down a little bit.


R Corona Australis (R CrA) is an actively starforming region, one of the closest such regions to the Earth.

R Corona Australis Don Goldman.png

The whitish-pink sort of airplane-shaped nebula is NGC 6729, and inside it is the variable star R Corona Australis. I'm not sure which of the bright spots in it is R CrA itself! But R CrA is the central star of a newborn cluster, the Coronet Cluster:


The funny "inverted comma-like" object next to R Corona Australis is a Herbig-Haro object, created by outbursts from not yet fully formed stars. I do recommend the picture by Johannes Schedler that the APOD caption linked to as an example of Herbig-Haro objects. Just look at all those little red arcs! :shock: :D

Ann
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Re: APOD: Stars and Dust across Corona Australis (2023 Jun 22)

Post by Chris Peterson » Thu Jun 22, 2023 1:41 pm

Ann wrote: Thu Jun 22, 2023 5:37 am
APOD Robot wrote:

To its right are a group of pretty reflection nebulae cataloged as NGC 6726, 6727, 6729, and IC 4812. Their characteristic blue color is produced as light from hot stars is reflected by the cosmic dust.
That's only partly true. The stars lighting up the blue reflection nebulas are hot enough to create a blue reflection nebula, but not too hot, because that would turn the blue reflection nebulas into red emission nebulas. On the other hand, even stars no hotter than the Sun could probably create a bluish reflection nebula!
Perhaps more to the point, dust reflects brown light. It scatters blue light. Blue reflection nebulas would be more accurately called scatter nebulas!
Chris

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Re: APOD: Stars and Dust across Corona Australis (2023 Jun 22)

Post by orin stepanek » Thu Jun 22, 2023 4:16 pm

corona_aus1024.jpg
Thanks Chris; I really like the scattered blue light; it's so colorful! :D
ngc6726_wide_tanlbl1.jpg
Nice; I like annotations! 8-)
Orin

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