by Ann » Sat Oct 22, 2022 10:15 am
AVAO wrote: ↑Sat Oct 22, 2022 8:37 am
Ann wrote: ↑Sat Oct 22, 2022 5:48 am
But, okay! I should have clicked on all the links in today's APOD caption to find the best portrait of the California Nebula, the Pleiades and IC 348. Here it is, by Rogelio Bernal Andreo!
Ann
Why isn't there a red glow at the Pleiades?
Like Xi Persei, these are very bright and also radiate strongly in the X-ray.
The Pleiades are way too cool to ionize a red emission nebula! The brightest and hottest of them are spectral classes B6-B8. That is to say that their temperatures are likely below 16,000 K. The temperature of Menkib, Xi Per, which ionizes the California Nebula, is 35,000 K.
To ionize a nebula, a star has to be at least spectral class B1. A star that could have ionized a nebula if there had been any significant amounts of gas in its vicinity is Beta Cephei, Alfirk. Its temperature is 27,000 K.
A star that is a little cooler than Beta Cephei, so that it just borderline barely ionizes parts of its nebula, is HD 200775, a young star of spectral class B2. It is surrounded by a lot of blue reflection nebulosity and a hint of pink Hα emission nebulosity in its own nebula, the Iris Nebula:
Beta Cephei in constellation Cepheus by Will Gater.
The Iris Nebula and star HD 200775. The arrows point at weakly ionized regions.
I don't remember why I put the question mark there. Photo: Adam Block.
A star of spectral class B1 that does ionize a red emission nebula is BD+46 3474, ionizing star of the Cocoon Nebula:
Also it is not X-rays that ionize hydrogen and make it glow red. It is ultraviolet light, which is emitted in copious amounts by stars whose temperature is around 30,000 K and higher.
Ann
[quote=AVAO post_id=326710 time=1666427879 user_id=144694]
[quote=Ann post_id=326707 time=1666417711 user_id=129702]
But, okay! I should have clicked on all the links in today's APOD caption to find the best portrait of the California Nebula, the Pleiades and IC 348. Here it is, by Rogelio Bernal Andreo! :D
[img3="From California to the Pleiades
Image Credit & Copyright: Rogelio Bernal Andreo (Deep Sky Colors)"]https://apod.nasa.gov/apod/image/1311/2013-11_C2P_950px.jpg[/img3]
Ann
[/quote]
Why isn't there a red glow at the Pleiades?
Like Xi Persei, these are very bright and also radiate strongly in the X-ray.
[/quote]
The Pleiades are way too cool to ionize a red emission nebula! The brightest and hottest of them are spectral classes B6-B8. That is to say that their temperatures are likely below 16,000 K. The temperature of Menkib, Xi Per, which ionizes the California Nebula, is 35,000 K.
To ionize a nebula, a star has to be at least spectral class B1. A star that could have ionized a nebula if there had been any significant amounts of gas in its vicinity is Beta Cephei, Alfirk. Its temperature is 27,000 K.
A star that is a little cooler than Beta Cephei, so that it just borderline barely ionizes parts of its nebula, is HD 200775, a young star of spectral class B2. It is surrounded by a lot of blue reflection nebulosity and a hint of pink Hα emission nebulosity in its own nebula, the Iris Nebula:
[float=left][attachment=1]Beta Cep and constellation Cepheus Will Gater.png[/attachment][c][size=85][color=#0040FF]Beta Cephei in constellation Cepheus by Will Gater.[/color][/size][/c][/float][float=right][attachment=0]Iris Nebula Adam Block annotated.png[/attachment][c][size=85][color=#0040FF]The Iris Nebula and star HD 200775. The arrows point at weakly ionized regions.
I don't remember why I put the question mark there. Photo: Adam Block.[/color][/size][/c][/float]
[clear][/clear]
A star of spectral class B1 that does ionize a red emission nebula is BD+46 3474, ionizing star of the Cocoon Nebula:
[img3="The Cocoon Nebula and its ionizing star of spectral class B1, BD+46 3474. Photo: Adam Block."]https://upload.wikimedia.org/wikipedia/commons/9/9d/Ic5146s.jpg[/img3]
Also it is not X-rays that ionize hydrogen and make it glow red. It is ultraviolet light, which is emitted in copious amounts by stars whose temperature is around 30,000 K and higher.
Ann