Explanation: It is one of the most important stars in the sky. This is partly because, by coincidence, it is surrounded by a dazzling reflection nebula. Pulsating RS Puppis, the brightest star in the image center, is some ten times more massive than our Sun and on average 15,000 times more luminous. In fact, RS Pup is a Cepheid typevariable star, a class of stars whose brightness is used to estimate distances to nearby galaxies as one of the first steps in establishing the cosmic distance scale. As RS Pup pulsates over a period of about 40 days, its regular changes in brightness are also seen along the nebula delayed in time, effectively a light echo. Using measurements of the time delay and angular size of the nebula, the known speed of light allows astronomers to geometrically determine the distance to RS Pup to be 6,500 light-years, with a remarkably small error of plus or minus 90 light-years. An impressive achievement for stellar astronomy, the echo-measured distance also more accurately establishes the true brightness of RS Pup, and by extension other Cepheid stars, improving the knowledge of distances to galaxies beyond the Milky Way. The above image was taken by the Hubble Space Telescope and digitally processed by a volunteer.
I would be interesting if Hubble was used to make a time lapse of the light echo, though I don't know if the star's brightness varies enough to make a visually clear echo.
The requested URL /apod/image/1309/rspuppis_bryne_3072.jpg was not found on this server.
Sorry about that, seems there was a mishap typing the URL. In the meantime, until it gets fixed, a link to the image is here. There's also one with higher contrast here.
FLPhotoCatcher wrote:I would be interesting if Hubble was used to make a time lapse of the light echo, though I don't know if the star's brightness varies enough to make a visually clear echo.
That would be a great idea! I remember the timelapse images of V838 Monocerotis and it was cool to see the light echo evolution over time.
There are hints of an equatorial ring of stars around the nebula in this image. It is hard to tell if stars in the apparent ring are in the foreground or background. Their possible alignment around this beautiful nebula seems worth mentioning.
Ann wrote:Geckzilla, do you know what filters were used for this image?
ACS/WFC 435W/606W. That is, the Johnson B and broad V filters. Together, these cover the range of about 375 nm to 700 nm, with the channel overlap between 450 nm and 500 nm.
I just checked RS Pup with my software. Its Johnson B-V index is 1.290 ± 0.020, which is relatively red. It is, for example, redder than Dubhe, the yellow K-type star in the Big Dipper. Dubhe's B-V index is 1.061 ± 0.003. It is also a lot redder than Delta Cephei, the prototype star which has given the cepheids their name. The B-V index of Delta Cephei is 0.778 ± 0.046.
The red color of RS Pup could be a consequence of interstellar reddening due to dust. The simple fact that RS Pup is immersed in a reflection nebula definitely suggests that its color has been dust-reddened. Alternatively, it just might mean that RS Pup is big and bright as cepheids go. Evolved stars are typically redder the larger they are.
Variability in stars occurs, (according to what I recall reading) from stars not being able to arrive at a balance between gravity, which works to shrink the star, and the heat released by nuclear fusion reactions deep inside the star, which work to inflate the star. So the regular light curve of a Cepheid works like this, as the star dims from peak luminosity it is contracting under gravity and the core (and shells around the core) heat up until a temp and pressure is reached where some type of fusion can start. The heat from this fusion then halts the contraction and the star puffs up, which lowers the temp and pressure until the fusion reaction shuts down. The star reaches peak brightness and then the process repeats.
My question is, what fusion reaction drives the Cepheid variable phase of a star’s lifetime? In other words, which elements are being fused and what is being produced during this period?
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
Ann wrote:
I just checked RS Pup with my software. Its Johnson B-V index is 1.290 ± 0.020, which is relatively red. It is, for example, redder than Dubhe, the yellow K-type star in the Big Dipper. Dubhe's B-V index is 1.061 ± 0.003. It is also a lot redder than Delta Cephei, the prototype star which has given the cepheids their name. The B-V index of Delta Cephei is 0.778 ± 0.046.
The red color of RS Pup could be a consequence of interstellar reddening due to dust. The simple fact that RS Pup is immersed in a reflection nebula definitely suggests that its color has been dust-reddened. Alternatively, it just might mean that RS Pup is big and bright as cepheids go. Evolved stars are typically redder the larger they are.
http://en.wikipedia.org/wiki/Classical_Cepheid_variables wrote:
<<Classical Cepheids (also known as Population I Cepheids, Type I Cepheids, or Delta Cephei variables) undergo pulsations with very regular periods on the order of days to months. Classical Cepheids are population I variable stars which are 4–20 times more massive than the Sun, and up to 100,000 times more luminous. Cepheids are yellow supergiants of spectral class F6 – K2 and their radii change by (~25% for the longer-period I Carinae) millions of kilometers during a pulsation cycle.
There exists a well-defined relationship between a Classical Cepheid variable's luminosity and pulsation period, securing Cepheids as viable standard candles for establishing the Galactic and extragalactic distance scales. HST observations of Classical Cepheid variables have enabled firmer constraints on Hubble's law. Classical Cepheids have also been used to clarify many characteristics of our galaxy, such as the Sun's height above the galactic plane and the Galaxy's local spiral structure. Over 700 classical Cepheids are known in the Milky Way Galaxy, and several thousand extragalactic Cepheids have been discovered. The Hubble Space Telescope has identified classical Cepheids in NGC 4603, which is 100 million light years distant.>>
BDanielMayfield wrote:Variability in stars occurs, (according to what I recall reading) from stars not being able to arrive at a balance between gravity, which works to shrink the star, and the heat released by nuclear fusion reactions deep inside the star, which work to inflate the star. So the regular light curve of a Cepheid works like this, as the star dims from peak luminosity it is contracting under gravity and the core (and shells around the core) heat up until a temp and pressure is reached where some type of fusion can start. The heat from this fusion then halts the contraction and the star puffs up, which lowers the temp and pressure until the fusion reaction shuts down. The star reaches peak brightness and then the process repeats.
My question is, what fusion reaction drives the Cepheid variable phase of a star’s lifetime? In other words, which elements are being fused and what is being produced during this period?
Actually, it seems to be totally dependent on the opacity of the ionized atmosphere of the star, AKA, the k mechanism.
"The κ–mechanism is the driving mechanism behind the changes in luminosity of many types of pulsating variable stars. Here, the Greek letter kappa (κ) is used to indicate the radiative opacity at any particular depth of the stellar atmosphere. In a normal star, an increase in compression of the atmosphere causes an increase in temperature and density; this produces a decrease in the opacity of the atmosphere, allowing heat energy to escape more rapidly. The result is an equilibrium condition where temperature and pressure are maintained in a balance. However, in cases where the opacity increases with temperature, the atmosphere becomes unstable against pulsations. If a layer of a stellar atmosphere moves inward, it becomes denser and more opaque, causing heat flow to be checked. In return, this heat increase causes a build-up of pressure that pushes the layer back out again. The result is a cyclic process as the layer repeatedly moves inward and then is forced back out again."-Wiki
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?
What is the speed of light in the dust/gas cloud of the refelcting nebula. Surely one cannot assume that is is the same as in a vaccum? Is anyone aware of tests to measure the speed of light through various types of smoke and dust?
Roland wrote:What is the speed of light in the dust/gas cloud of the refelcting nebula. Surely one cannot assume that is is the same as in a vaccum? Is anyone aware of tests to measure the speed of light through various types of smoke and dust?
The density of dust and gas in a nebula is equivalent to about the hardest vacuum we can make in a lab. So the speed of light in a nebula is very, very close to c - unmeasurably different, in fact.
Roland wrote:
What is the speed of light in the dust/gas cloud of the refelcting nebula. Surely one cannot assume that is is the same as in a vaccum? Is anyone aware of tests to measure the speed of light through various types of smoke and dust?
The density of dust and gas in a nebula is equivalent to about the hardest vacuum we can make in a lab. So the speed of light in a nebula is very, very close to c - unmeasurably different, in fact.
The problem is not to be distracted by the superluminal_motion due to features in the foreground.
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)
Roland wrote:
What is the speed of light in the dust/gas cloud of the refelcting nebula. Surely one cannot assume that is is the same as in a vaccum? Is anyone aware of tests to measure the speed of light through various types of smoke and dust?
The density of dust and gas in a nebula is equivalent to about the hardest vacuum we can make in a lab. So the speed of light in a nebula is very, very close to c - unmeasurably different, in fact.
The problem is not to be distracted by the superluminal_motion due to features in the foreground.
Nearby Cepheid Variable RS Pup
