APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

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APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by APOD Robot » Wed Aug 29, 2018 4:05 am

Image Nearby Cepheid Variable RS Pup

Explanation: In the center is one of the most important stars on 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 type variable 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 featured image was taken by the Hubble Space Telescope.

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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by FLPhotoCatcher » Wed Aug 29, 2018 6:44 am

Is there a clear time lapse movie of the light echos?

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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by neufer » Wed Aug 29, 2018 11:09 am

Click to play embedded YouTube video.
FLPhotoCatcher wrote:
Wed Aug 29, 2018 6:44 am

Is there a clear time lapse movie of the light echos?
Art Neuendorffer

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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by Ann » Wed Aug 29, 2018 12:16 pm

APOD Robot wrote:

An impressive achievement for stellar astronomy, the echo-measured distance also more accurately establishes the true brightness of RS Pup
Isn't it an intolerable act of cheating to tell us that the true brightness of RS Pup has been established, without telling us what the true brightness of RS Pup actually is?

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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by neufer » Wed Aug 29, 2018 12:34 pm


Ann wrote:
Wed Aug 29, 2018 12:16 pm

Isn't it an intolerable act of cheating to tell us that the true brightness of RS Pup has been established, without telling us what the true brightness of RS Pup actually is?
RS Puppis (or RS Pup) : Apparent magnitude (V) 6.5-7.6

Absolute magnitude (MV): -5.70 :arrow:

RS Puppis is one of the brightest known Cepheids in the Milky Way galaxy and has one of the longest periods for this class of star, of 41.4 days.
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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by Ann » Wed Aug 29, 2018 12:42 pm

neufer wrote:
Wed Aug 29, 2018 12:34 pm

Ann wrote:
Wed Aug 29, 2018 12:16 pm

Isn't it an intolerable act of cheating to tell us that the true brightness of RS Pup has been established, without telling us what the true brightness of RS Pup actually is?
RS Puppis (or RS Pup) : Apparent magnitude (V) 6.5-7.6

Absolute magnitude (MV): -5.70 :arrow:

RS Puppis is one of the brightest known Cepheids in the Milky Way galaxy and has one of the longest periods for this class of star, of 41.4 days.
Thanks, Art! I knew I could trust you! :D

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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by BDanielMayfield » Wed Aug 29, 2018 5:57 pm

Ann wrote:
Wed Aug 29, 2018 12:16 pm
APOD Robot wrote:

An impressive achievement for stellar astronomy, the echo-measured distance also more accurately establishes the true brightness of RS Pup
Isn't it an intolerable act of cheating to tell us that the true brightness of RS Pup has been established, without telling us what the true brightness of RS Pup actually is?

Ann
Intolerable? Cheating? Isn't that being a little hard on the APOD writers Ann? After all, the true brightness of RS Pup is continuously changing over time, and it's still only known to a closer approximation.

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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by Chris Peterson » Wed Aug 29, 2018 6:08 pm

Ann wrote:
Wed Aug 29, 2018 12:16 pm
APOD Robot wrote:

An impressive achievement for stellar astronomy, the echo-measured distance also more accurately establishes the true brightness of RS Pup
Isn't it an intolerable act of cheating to tell us that the true brightness of RS Pup has been established, without telling us what the true brightness of RS Pup actually is?
Actually, the true brightness itself is completely irrelevant to the story here. Indeed, stating the true brightness might even confuse the actual point being made.
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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by Boomer12k » Wed Aug 29, 2018 10:54 pm

Wonderful image... love the detail... I could not find if Cepheids go supernova... I guess they collapse, but don't blow???

Love the "light echo"...
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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by Chris Peterson » Wed Aug 29, 2018 11:12 pm

Boomer12k wrote:
Wed Aug 29, 2018 10:54 pm
Wonderful image... love the detail... I could not find if Cepheids go supernova... I guess they collapse, but don't blow???
Classical Cepheids (like RS Pup) can easily have enough mass to end up as supernovas. This particular star, at about 9 solar masses, is right on the edge of the slightly uncertain boundary that determines whether it will end as a white dwarf or a supernova and neutron star.
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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by neufer » Wed Aug 29, 2018 11:37 pm

Boomer12k wrote:
Wed Aug 29, 2018 10:54 pm

Wonderful image... love the detail... I could not find if Cepheids go supernova... I guess they collapse, but don't blow???
RS Puppis is borderline massive enough to go supernova... but it appears to be loosing a lot of mass now so probably not.
https://en.wikipedia.org/wiki/Classical_Cepheid_variable#Properties wrote: <<RS Puppis is a 9.2 M supergiant [whose] spectral type varies between F9 and G7 as its temperature changes. It lies on the instability strip and based on the rate of change of its period is thought to be crossing it for the third time. The third crossing occurs as a star is evolving towards cooler temperatures for the second time after performing a blue loop. The third crossing of the instability strip occurs much more slowly than the first crossing just after a star leaves the main sequence. Stars with at least 9 M (possibly as much as 12 M) evolve in a complex fashion, progressively burning heavier elements at hotter temperatures in their cores. The star becomes layered like an onion, with the burning of more easily fused elements occurring in larger shells. Although popularly described as an onion with an iron core, the least massive supernova progenitors only have oxygen-neon(-magnesium) cores. These super AGB stars may form the majority of core collapse supernovae, although less luminous and so less commonly observed than those from more massive progenitors.

Classical Cepheid variables are 4–20 times more massive than the Sun, and around 1,000 to 50,000 times more luminous. Spectroscopically they are bright giants or low luminosity supergiants of spectral class F6 – K2. The temperature and spectral type vary as they pulsate. Their radii are a few tens to a few hundred times that of the sun. More luminous Cepheids are cooler and larger and have longer periods. Along with the temperature changes their radii also change during each pulsation (e.g. by ~25% for the longer-period l Car), resulting in brightness variations up to two magnitudes. The brightness changes are more pronounced at shorter wavelengths.

The majority of classical Cepheids are thought to be fundamental mode pulsators, although it is not easy to distinguish the mode from the shape of the light curve. Stars pulsating in an overtone are more luminous and larger than a fundamental mode pulsator with the same period.

When an intermediate mass star (IMS) first evolves away from the main sequence, it crosses the instability strip very rapidly while hydrogen shell burning. When the helium core ignites in an IMS, it may execute a blue loop and crosses the instability strip again, once while evolving to high temperatures and again evolving back towards the asymptotic giant branch. Stars more massive than about 8-12 M start core helium burning before reaching the red giant branch and become red supergiants, but may still execute a blue loop through the instability strip. The duration and even existence of blue loops is very sensitive to the mass, metallicity, and helium abundance of the star. The rate of change of the period of a Cepheid variable, along with chemical abundances detectable in the spectrum, can be used to deduce which crossing a particular star is making.

Classical Cepheid variables were B type main sequence stars earlier than about B7, possibly late O stars, before they ran out of hydrogen in their cores. More massive and hotter stars develop into more luminous Cepheids with longer periods, although it is expected that young stars within our own galaxy, at near solar metallicity, will generally lose sufficient mass by the time they first reach the instability strip that they will have periods of 50 days or less. Above a certain mass, 20-50 M depending on metallicity, red supergiants will evolve back to blue supergiants rather than execute a blue loop, but they will do so as unstable yellow hypergiants rather than regularly pulsating Cepheid variables. Very massive stars never cool sufficiently to reach the instability strip and do not ever become Cepheids. At low metallicity, for example in the Magellanic Clouds, stars can retain more mass and become more luminous Cepheids with longer periods.>>
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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by FLPhotoCatcher » Thu Aug 30, 2018 1:11 am

https://en.wikipedia.org/wiki/Classical_Cepheid_variable#Properties wrote:
RS Puppis is a 9.2 M supergiant [whose] spectral type varies between F9 and G7 as its temperature changes.
...Stars with at least 9 M (possibly as much as 12 M) evolve in a complex fashion, progressively burning heavier elements at hotter temperatures in their cores. The star becomes layered like an onion, with the burning of more easily fused elements occurring in larger shells. Although popularly described as an onion with an iron core, the least massive supernova progenitors only have oxygen-neon(-magnesium) cores...
How do scientists know that "The star becomes layered like an onion"? I'm sure they have done calculations, but I would be interested to know how they came up with the idea. Wouldn't the high temps in and near the core create convection currents that would thoroughly mix the gasses?

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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by BDanielMayfield » Thu Aug 30, 2018 3:20 am

FLPhotoCatcher wrote:
Thu Aug 30, 2018 1:11 am
https://en.wikipedia.org/wiki/Classical_Cepheid_variable#Properties wrote:
RS Puppis is a 9.2 M supergiant [whose] spectral type varies between F9 and G7 as its temperature changes.
...Stars with at least 9 M (possibly as much as 12 M) evolve in a complex fashion, progressively burning heavier elements at hotter temperatures in their cores. The star becomes layered like an onion, with the burning of more easily fused elements occurring in larger shells. Although popularly described as an onion with an iron core, the least massive supernova progenitors only have oxygen-neon(-magnesium) cores...
How do scientists know that "The star becomes layered like an onion"? I'm sure they have done calculations, but I would be interested to know how they came up with the idea. Wouldn't the high temps in and near the core create convection currents that would thoroughly mix the gasses?
They know with very high confidence because both they've been able to experimentally find the energies required to fuse verious elements in accelerators and because the theoretical math works out. Simulations of what must be happening inside stars produce predictions that agree with observations.

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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by Ann » Thu Aug 30, 2018 3:41 am

BDanielMayfield wrote:
Wed Aug 29, 2018 5:57 pm
Ann wrote:
Wed Aug 29, 2018 12:16 pm
APOD Robot wrote:

An impressive achievement for stellar astronomy, the echo-measured distance also more accurately establishes the true brightness of RS Pup
Isn't it an intolerable act of cheating to tell us that the true brightness of RS Pup has been established, without telling us what the true brightness of RS Pup actually is?

Ann
Intolerable? Cheating? Isn't that being a little hard on the APOD writers Ann? After all, the true brightness of RS Pup is continuously changing over time, and it's still only known to a closer approximation.

Bruce
I was most definitely exaggerating.

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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by Ann » Thu Aug 30, 2018 4:06 am

Chris Peterson wrote:
Wed Aug 29, 2018 6:08 pm
Ann wrote:
Wed Aug 29, 2018 12:16 pm
APOD Robot wrote:

An impressive achievement for stellar astronomy, the echo-measured distance also more accurately establishes the true brightness of RS Pup
Isn't it an intolerable act of cheating to tell us that the true brightness of RS Pup has been established, without telling us what the true brightness of RS Pup actually is?
Actually, the true brightness itself is completely irrelevant to the story here. Indeed, stating the true brightness might even confuse the actual point being made.
I was obviously grumbling and very much exaggerating. Nevertheless, I'm not backing down all the way. RS Pup is interesting not because it is a bright star, but because it is one of the brightest known examples of a pulsating star whose true brightness is intimately linked to the length of its pulsation periods. Therefore stars as RS Pup are used as standard candles for distance estimation.

Certainly it is interesting to know exactly how bright RS Pup is, given the fact that it belongs to a class of stars that are used as standard candles?

According to a source quoted by Art, the true (V?) magnitude of RS Pup is about -5. Interesting! If the true V magnitude of RS Pup had been -4 instead, then the pulsating Cepheids would have been different as standard candles. And if the true V magnitude of RS Pup had been -6, then again the pulsating Cepheids would have been another kind of standard candles. And maybe, just maybe, astronomy may have had to reassess its "cosmic distance ladder" a little bit, if it had turned out that RS Pup was either brighter or fainter than expected.

You might still argue that it doesn't matter exactly how bright RS Pup is, as long as its brightness agrees with what astronomers had expected, so that the cosmic distance ladder doesn't have to be adjusted. Okay, fine.

I still don't see why the APOD caption couldn't have said, for example, that the true V magnitude of RS Pup when it is at its brightest is -5, which makes it some 10 magnitudes brighter than the Sun, which, I guess, makes it some 10,000 or so times brighter than the Sun in visual light. And then the caption could have added that this is how bright classic Cepheids can get, and to be that bright they have to have a period of over 40 days.

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Re: APOD: Nearby Cepheid Variable RS Pup (2018 Aug 29)

Post by neufer » Thu Aug 30, 2018 12:35 pm

FLPhotoCatcher wrote:
Thu Aug 30, 2018 1:11 am
https://en.wikipedia.org/wiki/Classical_Cepheid_variable#Properties wrote:
RS Puppis is a 9.2 M supergiant [whose] spectral type varies between F9 and G7 as its temperature changes.
...Stars with at least 9 M (possibly as much as 12 M) evolve in a complex fashion, progressively burning heavier elements at hotter temperatures in their cores. The star becomes layered like an onion, with the burning of more easily fused elements occurring in larger shells. Although popularly described as an onion with an iron core, the least massive supernova progenitors only have oxygen-neon(-magnesium) cores...
How do scientists know that "The star becomes layered like an onion"? I'm sure they have done calculations, but I would be interested to know how they came up with the idea. Wouldn't the high temps in and near the core create convection currents that would thoroughly mix the gasses?
Cepheid variability can only be explained by assuming that the helium burning occurs in a thin helium shell around a carbon core:
https://en.wikipedia.org/wiki/Triple-alpha_process#Reaction_rate_and_stellar_evolution wrote:
<<The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon. The triple-alpha steps are strongly dependent on the temperature and density of the stellar material. The power released by the reaction is approximately proportional to the temperature to the 40th power, and the density squared. In contrast, the proton–proton chain reaction produces energy at a rate proportional to the fourth power of temperature, the CNO cycle at about the 17th power of the temperature, and both are linearly proportional to the density. This strong temperature dependence has consequences for the late stage of stellar evolution, the red giant stage.

For lower mass stars, the helium accumulating in the core is prevented from further collapse only by electron degeneracy pressure. As the temperature rises, increased pressure in the core would normally result in an expansion, reduction of density, and thus reduction in reaction rate. However, due to the high pressure at the center of the star this does not occur and energy production continues unmoderated. As a consequence, the temperature increases, causing the reaction rate further increase in a positive feedback cycle that becomes a runaway reaction. This process, known as the helium flash, lasts a matter of seconds but burns 60–80% of the helium in the core. During the core flash, the star's energy production can reach approximately 1011 solar luminosities which is comparable to the luminosity of a whole galaxy, although no effects will be immediately observed at the surface, as it is hidden by the star's overlying layers.

For higher mass stars, carbon collects in the core, displacing the helium to a surrounding shell where helium burning occurs. In this helium shell, the pressures are lower and the mass is not supported by electron degeneracy. Thus, as opposed to the center of the star, the shell is able to expand in response to increased thermal pressure in the helium shell. Expansion cools this layer and slows the reaction, causing the star to contract again. This process continues cyclically, and stars undergoing this process will have periodically variable radius and power production. These stars will also lose material from their outer layers as they expand and contract.>>
Art Neuendorffer