Starship Asterisk*

[attachment=0]corona_aus1024.jpg[/attachment]
Thanks Chris; I really like the scattered blue light; it's so colorful! :D
[attachment=1]ngc6726_wide_tanlbl1.jpg[/attachment]
Nice; I like annotations! 8-)

[quote=Ann post_id=331838 time=1687412256 user_id=129702]
[quote]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.[/quote]

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!
[/quote]

Perhaps more to the point, dust [i]reflects [/i]brown light. It [i]scatters [/i]blue light. Blue reflection nebulas would be more accurately called scatter nebulas!

[float=left][attachment=1]corona_aus1024[1].jpg[/attachment][c][size=85][color=#0040FF]Stars and Dust across Corona Australis
Image Credit & Copyright: Alessandro Cipolat Bares[/color][/size][/c][/float]
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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 [b][size=120][color=#0040FF]b[/color][color=#00BFFF]l[/color][color=#0080FF]u[/color][color=#0000FF]e[/color][color=#4040FF][/color][color=#0040FF]![/color][/size][/b]

[quote]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.[/quote]

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!


[float=left][img3="The Cocoon nebula. The ionizing star, BD +46 3474, is spectral class B1 and therefore only just hot enough to create a red emission nebula. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona"]https://upload.wikimedia.org/wikipedia/commons/9/9d/Ic5146s.jpg[/img3][/float][float=right][img3="A bluish reflection nebula surrounding G2V-type star DI Cha. Image credit: ESA/Hubble & NASA, Acknowledgement: Judy Schmidt"]https://www.nasa.gov/sites/default/files/styles/full_width_feature/public/thumbnails/image/hubble_friday_10302015.jpg[/img3][/float]
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The stars lighting up the blue reflection nebulas of Corona Australis, [url=http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=HD+176269&submit=SIMBAD+search]HD 176269[/url], [url=http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=HD+176386&submit=SIMBAD+search]HD 176386[/url] and [url=http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=TY+CrA&submit=SIMBAD+search]TY CrA[/url], 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:


[img3="Corona Australis nebulas region. Credit: Don Goldman."]https://4.bp.blogspot.com/-BJ1doi_VgOw/V8TD49SSlsI/AAAAAAAAK-E/cdNF2IXrtiouXzqSvj_VKdahvKZo0zADACLcB/s320/8.jpg[/img3]


The picture is very small, but you can see a larger version [url=https://astrodonimaging.com/gallery/ngc-6726/]here[/url]. 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.


[attachment=0]R Corona Australis Don Goldman.png[/attachment]


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:


[img3="The Corona Australis region (containing, at its heart, the Coronet cluster) is one of the nearest and most active regions of ongoing star formation. At only about 420 light-years away, the Coronet is over three times closer than the Orion nebula is to Earth. The Coronet contains a loose cluster of a few dozen young stars with a wide range of masses and at various stages of evolution, giving astronomers an opportunity to observe embryonic stars simultaneously in several wavelengths. This composite image shows the Coronet in X-rays from Chandra (purple) and infrared from Spitzer (orange, green, and cyan).
Credit: NASA/CXC/J. Forbrich, NASA/JPL-Caltech L.Allen (Harvard-Smithsonian CfA), IRAC GTO - http://www.spitzer.caltech.edu/images/2113-sig07-017-The-Coronet-Cluster-in-X-Ray-and-Infrared-"]https://upload.wikimedia.org/wikipedia/commons/thumb/7/7a/Coronet_Cluster_in_X-Ray_and_Infrared.jpg/1024px-Coronet_Cluster_in_X-Ray_and_Infrared.jpg[/img3]


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


[img3="Young suns and arc-shaped red Herbig-Haro objects of NGC 7129. Image Credit & Copyright: Johannes Schedler (Panther Observatory)"]https://apod.nasa.gov/apod/image/1110/NGC7129_schedler_c70_900c.jpg[/img3]

Ann

[url=https://apod.nasa.gov/apod/ap230622.html] [img]https://apod.nasa.gov/apod/calendar/S_230622.jpg[/img] [size=150]Stars and Dust across Corona Australis[/size][/url]

[b] Explanation: [/b] Cosmic dust clouds cross a rich field of stars in this telescopic vista near the northern boundary of [url=http://www.botproductions.com/stellar/corona_australis.html]Corona Australis[/url], 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. [url=https://ui.adsabs.harvard.edu/abs/1977A%26AS...29...65B/abstract]Mixed with bright nebulosities[/url] the dust clouds effectively [url=https://apod.nasa.gov/apod/ap090425.html]block light[/url] from more distant background stars in the [url=http://members.nova.org/~sol/chview/chv5.htm]Milky Way[/url] and obscure from view embedded stars [url=http://arxiv.org/abs/1211.6945]still in the process[/url] of formation. Large dark nebula Bernes 157 is on the left. To its right are [url=https://apod.nasa.gov/apod/image/0407/ngc6726_wide_tanlbl1.jpg]a group of[/url] 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 [url=http://www.solstation.com/stars/r-coraus.htm]R Coronae Australis[/url]. Just below it, filamentary arcs and loops are identified as [url=https://apod.nasa.gov/apod/ap111027.html]Herbig Haro objects[/url] associated with energetic newborn stars. In fact, at the heart of this area lies the [url=https://chandra.harvard.edu/photo/2007/coronet/]Coronet Cluster[/url], one of the nearest and most active [url=https://ui.adsabs.harvard.edu/abs/2008hsf2.book..735N/abstract]star forming regions[/url].


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