Starship Asterisk*

ta152h0 wrote:on this image is it possible to discern a disc,or would this still be a point of light ?

Anthony Barreiro wrote:

BDanielMayfield wrote:Thanks Stephen. The kappa mechanism was completely new to me. It’s good to have misconceptions corrected.

Well then, next question: When or where in a star’s lifetime does the Cepheid variable phase occur? It would have to be after the main sequence burning of Hydrogen, right?

A cepheid variable is a yellow supergiant, fusing hydrogen in a shell around its core, in an unstable period between the main sequence and the red supergiant phase. The outer layer of the star pulsates, and dims and brightens, because doubly ionized helium in the outer layer absorbs more light than singly ionized helium. Here's what wikipedia has to say:

http://en.wikipedia.org/wiki/Yellow_supergiant wrote:A yellow supergiant (YSG) is a supergiant star of spectral type F or G.[1] These stars have initial masses between about 10 and 40 solar masses, although some yellow supergiants will have lost over half of that. Lower mass stars have lower luminosities and are seen as yellow giants. Higher mass stars do not expand beyond blue supergiants.
Most yellow supergiants are cooling and expanding rapidly towards red supergiants after leaving the main sequence, spending only a few thousand years in that phase, and so are much less common than red supergiants.[2] Yellow supergiants are burning hydrogen in a shell after exhausting the hydrogen in their cores. Core helium ignition occurs smoothly at some point during the development of a red supergiant, but models vary on whether this occurs at the yellow supergiant stage or after the star has become a red supergiant.[3][4]
Yellow supergiants are in a region of the HR diagram known as the instability strip because their temperatures and luminosities cause them to be dynamically unstable. Most stars observed in the instability strip appear as variables, subgiants as RR Lyrae variables, giants as W Virginis variables (type II Cepheids), and brighter giants and supergiants as Classical Cepheids. ...

References
[1] p. 366, The evolution of massive stars with mass loss, Cesare Chiosi and Andre Maeder, Annual review of astronomy and astrophysics 24 (1986), pp. 329–375. Bibcode: 1986ARA&A..24..329C. doi:10.1146/annurev.aa.24.090186.001553.
[2] Neugent; Philip Massey; Brian Skiff; Georges Meynet (2012). "Yellow and Red Supergiants in the Large Magellanic Cloud". arXiv:1202.4225v1 [astro-ph.SR].
[3] Bibcode: 2011BSRSL..80..266M
[4] Georges Meynet; Sylvia Ekström; André Maeder; Patrick Eggenberger; Hideyuki Saio; Vincent Chomienne; Lionel Haemmerlé (2013). "Models of rotating massive stars: Impacts of various prescriptions". arXiv:1301.2487v1 [astro-ph.SR].

http://en.wikipedia.org/wiki/Cepheid_variable#Dynamics_of_the_pulsation wrote:The accepted explanation for the pulsation of Cepheids is called the Eddington valve,[37] or κ-mechanism, where the Greek letter κ (kappa) denotes gas opacity. Helium is the gas thought to be most active in the process. Doubly ionized helium (helium whose atoms are missing two electrons) is more opaque than singly ionized helium. The more helium is heated, the more ionized it becomes. At the dimmest part of a Cepheid's cycle, the ionized gas in the outer layers of the star is opaque, and so is heated by the star's radiation, and due to the increased temperature, begins to expand. As it expands, it cools, and so becomes less ionized and therefore more transparent, allowing the radiation to escape. Then the expansion stops, and reverses due to the star's gravitational attraction. The process then repeats.
The mechanics of the pulsation as a heat-engine was proposed in 1917 by Arthur Stanley Eddington[38] (who wrote at length on the dynamics of Cepheids), but it was not until 1953 that S. A. Zhevakin identified ionized helium[39] as a likely valve for the engine.

References
[37] Webb, Stephen, Measuring the Universe: The Cosmological Distance Ladder, Springer, (1999)
[38] Eddington, A. S. (1917). "The pulsation theory of Cepheid variables". The Observatory 40: 290. Bibcode:1917Obs....40..290E.
[39] Zhevakin, S. A., "К Теории Цефеид. I", Астрономический журнал, 30 161–179 (1953)

neufer wrote:

Code: Select all

Designation  	        App. Mag.(Max/Min)   Period 	 Spectral class 	
---------------------------------------------------------------------------------------------- 
Polaris	Ursa Minor   	1m.86 / 2m.13 	 3.9696  d 	F8Ib or F8II
δ Cep 	 Cepheus	      3m.48 / 4m.37 	 5.36634 d 	F5Ib-G2Ib
l Car     Carina 	  	 3m.28 / 4m.18 	35.53584 d 	G5 Iab/Ib
RS Pup 	Puppis 	  	 6m.52 / 7m.67 	41.3876  d 	F8Iab

Dust-reddened[/url] for sure [/list]

geckzilla wrote:
I might be confused about light echoes. Near the center of the lower left quadrant of the image, there is a steady set of repeating linear structures. The way I understand it, those structures don't actually look like that but are actually cross sections of the nebula being brightly illuminated and dimly illuminated over time. Is that correct?

geckzilla wrote:
I think what confuses me the most is my brain keeps interpreting the nebula as moving when I watch the animation of V838 Monocerotis.

geckzilla wrote:I came back to this thread to make sure my understanding was correct because as I was messing around processing the Hubble 5 nebula I saw some repeating structures that seemed like they could be light echoes. Tried searching for any mention of it with no luck.

Chris Peterson wrote: Maybe, but their high symmetry makes me think these are diffraction artifacts.

geckzilla wrote:

Chris Peterson wrote: Maybe, but their high symmetry makes me think these are diffraction artifacts.

Hmm, it might actually be slightly easier to see them in my version.

geckzilla wrote:
Hmm, it might actually be slightly easier to see them in my version.
I circled some of the others that aren't so close to the star and which take on a circular shape.
http://www.geckzilla.com/astro/Hubble5_ ... choes1.jpg
http://www.geckzilla.com/astro/Hubble5_ ... choes2.jpg

geckzilla wrote:
Yes, but there are sometimes strange cases, are there not? I definitely wouldn't say that any of the other planetary nebulas I've looked at are similar to this one. I have to look at more bipolar ones, though.

ta152h0 wrote:
Maybe some B&W taken by , let's say, Mr Hubble himself ?

http://www.astro.washington.edu/users/balick/WFPC2/hb5.caption.html wrote:
Nickname: Hubble's Double Bubble
observed by HST: Sep 9 1997
distance 0.7 kpc (2200 l.y.)
constellation: Sagittarius

<<Hubble 5 is a strikingly lovely "butterfly" or bipolar (two-lobed) nebula which has received relatively little attention. Internal motions in the nebula have been measured spectroscopically to be in excess of 200 miles per second. The heat generated by the winds causes the each of the lobes to expand, much like a pair of balloons with internal heaters. The expanding lobes encounter older material ejected previously. Supersonic shocks form where the ambient gas is compressed and heated ahead of the rapidly expanding lobes. Atoms caught in the shocks radiate the visible light seen in this image.>>

geckzilla wrote:
I had read that before.

Thinking about it again, though, shocks might be comparable to light echoes.

http://en.wikipedia.org/wiki/Schlock wrote:
<<Schlock is an English word of Yiddish origin meaning "something cheap, shoddy, or inferior (perhaps from German Schlag, Yiddish shlak, meaning 'a stroke'; also possibly from German "Schlag"/"Schlagsahne", whipped cream)" In the field of science, "schlock" refers to shoddy methods or unreliable results.

In Art, "schlock" is sometimes used as a synonym for kitsch.>>

neufer wrote:In Art, "schlock" is sometimes used as a synonym for kitsch.>>

ta152h0 wrote:
scientist humor is sometimes overwhelming

neufer wrote:

http://en.wikipedia.org/wiki/Boston_Molasses_Disaster wrote:
<< ... Anthony di Stasio, walking homeward with his sisters from the Michelangelo School, was picked up by the wave and carried, tumbling on its crest, almost as though he were surfing. Then he grounded and the molasses rolled him like a pebble as the wave diminished. He heard his mother call his name and couldn't answer, his throat was so clogged with the smothering goo. He passed out, then opened his eyes to find three of his four sisters staring at him.>>