Questions about the Cat's Eye Nebula and its halo

[dougettinger]

Please refer to APODs 2011 April 24 and 2010 May 9. I have one very basic question. How does one know whether a nebula is t a supernova remnant or a planetary nebula resulting from the red giant phase of a star ?

What can possibly cause the polar jets indicated in APOD 2011 April 24 ? Polar jets are supposed to occur only in proto-stars and from black holes. Do other Wolf-Rayet stars and/or planetary nebula exhibit polar jets ?

What energy source or mechanism causes the red giant phase of this planetary nebula or any other nebula to expand and disperse into knots of gas and into a halo almost three light years across ?

Why cannot astronomer definitely say that the bubbles near the center of the nebula are the result of expulsion of matter due to the bursts of energy from this evolving Wolf-Rayet star ?

[Chris Peterson]

Please refer to APODs 2011 April 24 and 2010 May 9. I have one very basic question. How does one know whether a nebula is t a supernova remnant or a planetary nebula resulting from the red giant phase of a star ?

There are many differences. SNRs expand much faster, contain much more mass, are excited more or less from the outside as they interact with surrounding gas (as compared with PNs, which are excited by their central stars), SNRs and PNs have different types of central stars, they have different sorts of emission and absorption lines, and they look quite different at UV and radio wavelengths.

What can possibly cause the polar jets indicated in APOD 2011 April 24 ? Polar jets are supposed to occur only in proto-stars and from black holes. Do other Wolf-Rayet stars and/or planetary nebula exhibit polar jets ?

Jets can occur whenever you have material falling into a body with a rotating magnetic field. That said, I don't believe this APOD shows what would be considered polar jets. We're just seeing an asymmetry in the material ejection pattern, which is common with PNs.

What energy source or mechanism causes the red giant phase of this planetary nebula or any other nebula to expand and disperse into knots of gas and into a halo almost three light years across ?

The energy source of all stars is the same: gravity. Stars become red giants when they stop hydrogen fusion and heat up dramatically, blowing material outwards and increasing in radius (although the outer part of these stars is very tenuous). They produce PNs when they start producing energy by unstable fusion reactions, which create energy pulsations strong enough to blow off much of their atmosphere.

Why cannot astronomer definitely say that the bubbles near the center of the nebula are the result of expulsion of matter due to the bursts of energy from this evolving Wolf-Rayet star ?

I don't understand the question. Which Wolf-Rayet star? That isn't the type of star at the center of the Cat's Eye.

[dougettinger]

Thanks for your prompt and precise answers. I have a difficult time not believing that opposing polar jets are indicated in APOD 2011 April 24 - The Cat's Eye Nebula from Hubble. These jets may have their origin in the magnetic fields inside the "bubbles" and not directly from the star.

Wikipedia does list the star of NGC 6543 as being an O7 + [WR] - type star. A Wolf-Rayet has pulsations over short periods of time and may have caused the "bubbles" seen the the APOD. But the "bubbles" are considered a mystery. Why?

[Ann]

The reason why the central star of the Cat's Eye Nebula is not a classic O-type or Wolf Rayet star has to do with size.

Yes, the central star of the Cat's Eye Nebula is undoubtedly hot. But it is not nearly as big as a classic O-type star. Because while a classic O-type star is always bigger than the Sun, the central star of a planetary nebula is, I believe, always smaller than the Sun.

Why is the central star of a planetary nebula so small compared with a classic O-type star? That is because the classic O-type star is very massive, at least ten times more massive than the Sun. A classic O-type star shines hot and blue "on the surface" and is able to ionize any hydrogen that happens to be nearby. The classic O-type star is ferociously hot in its center. I don't know how hot it will be in its center, but the Sun is supposedly 15 million degrees Kelvin in its center. That's a lot, but an O-star will be much hotter.

The central star of a planetary nebula, however, can be described as the exposed and cooling core of a star that was never an O-star when it was on the main sequence, that is, when it shone by fusing hydrogen to helium in its core. The central star of a planetary nebula may well have been more massive than the Sun in its youth, but it was never as hot and as massive as an O star. And now that it has become the central star of a planetary nebula, its mass is never more than 1.4 times that of the Sun, or so I believe anyway.

As this "never-an-original-O-star" ages, it sets off other fusion processes than the hydrogen to helium one in its center. Like Chris said, these new fusion processes will turn the star into a bloated red giant, and eventually this giant will shed all of its atmosphere and reveal its hot but cooling core. The recently shed atmosphere of the former red giant will still be relatively nearby, and the fiercely hot naked core of the star will ionize this previously shed gas and make it glow.

Brilliant A-type star Sirius, the brightest-looking of all stars in the sky, is twice as massive as the Sun, has a radius about 1.75 times that of the Sun, and has a "surface temperature" of 9880 Kelvin, according to Professor emeritus Jim Kaler. See http://stars.astro.illinois.edu/sow/sirius.html. However, Sirius has a white dwarf companion. The tempereature of this companion is 24,800 Kelvin, according to Jim Kaler. That is probably hot enough to classify Sirius B as a B0 star. In spite of that, however, the total luminosity of Sirius B, including its ultraviolet light, is just 2.4 percent that of the Sun, according to Kaler. Why is that? It is because Sirius B is so tiny, with a radius just 0.92 the size of Earth.

Click to view full size image

Sirius A and Sirius B. If you ask me, this picture exaggerates the size of Sirius B.

Because Sirius B was never an O star (although, according to Jim Kaler, it may have been a B3-B5 star with a mass 5 to 7 solar), it never got hot enough in its core to get all those fusion processes going that eventually would have turned its core into iron and set off a supernova explosion.

Ann

[dougettinger]

I understand all that you stated. Planetary nebulas are generated from lower mass stars and supernovas come from higher mass stars such as O and B types. So then we can safely say that Wikipedia is in error when it stated that the central star of the Cat's Eye Nebula is an O7 + [WR] type star ?

Ann, you bring up another question in discussing Sirius A and B. I will start a New Topic about this question so as to keep different subjects apart.

[Chris Peterson]

Thanks for your prompt and precise answers. I have a difficult time not believing that opposing polar jets are indicated in APOD 2011 April 24 - The Cat's Eye Nebula from Hubble.

Again, I don't think there is much to suggest that we are seeing jets. Both true jets as well as structure in planetary nebulas ultimately arise from the same place: the natural asymmetry introduced by an axis of rotation. But that's the only thing they have in common. A jet is an active structure, where material entering the system primarily on the equatorial axis is ejected along the polar axis after undergoing interaction with a rotating magnetic field. What we see in the Cat's Eye does not look like a process at all, but rather like a physical structure created by the earliest or fastest dust and gas to be blown off the progenitor star.

[dougettinger]

I will accept your explanation that I am not seeing true polar jets.

Chris, is Wikipedia correct in stating that the central star is a O7 + [WR] type star ?

[Ann]

So then we can safely say that Wikipedia is in error when it stated that the central star of the Cat's Eye Nebula is an O7 + [WR] type star ?

I don't think so, not necessarily. Because a white dwarf too will be gradually cooling. A newborn white dwarf will be much hotter than an O star will ever be on its surface. But a true, massive O star will be hotter in its core than a white dwarf will ever be.

As for Sirius B, I wouldn't think that it is classified as an O star, since its temperature is "only" 25,000 degrees Kelvin. That's a bit low for an O star. But Sirius B is definitely hot enough to qualify, as far as its temperature is concerned, as an early B-type white dwarf. As Sirius B keeps cooling, it will "drop down the spectral class sequence" from B, to A, to F, to G, to K, to M, and eventually it will stop emitting any energy altogether and become a "black dwarf".

Ann

[neufer]

Planetary nebulas are generated from lower mass stars and supernovas come from higher mass stars such as O and B types. So then we can safely say that Wikipedia is in error when it stated that the central star of the Cat's Eye Nebula is an O7 + [WR] type star ?

<<The ejected gases form a cloud of material around the now-exposed core of the star. As more and more of the atmosphere moves away from the star, deeper and deeper layers at higher and higher temperatures are exposed. When the exposed surface reaches a temperature of about 30,000 K, there are enough ultraviolet photons being emitted to ionize the ejected atmosphere, making it glow. The cloud has then become a planetary nebula.>>

<<Planetary nebulae (PNs) are the descendents of intermediate (1–8 M⊙) mass stars. The central star of a PN will evolve into a white dwarf with a mass between ∼0.55 and ∼1.0 M⊙. While on the upper asymptotic giant branch (AGB), much of the star's initial mass is lost in a more or less spherical outflow at rates of up to 10-4 M⊙ yr-1 and at speeds of ∼10–20 km s-1. As the star leaves the AGB, the UV radiation from the emerging white dwarf (WD) ionizes the red giant ejecta, producing the short-lived (∼3 × 104 yr) PN. The star's surface escape velocity also increases at this stage, and its wind speed may increase to ∼1000 km s-1 as its mass-loss rate drops to ∼10-8 M⊙ yr-1. This fast wind is shocked as it collides with the previously ejected slow wind. This process is expected to play a role in PN shaping.

• Planetary nebulae harboring relatively cool, WC-type Wolf-Rayet central stars (typically designated [WCL] objects, where L differentiates the cooler, later objects from the earlier [WCE] objects; e.g., Leuenhagen et al. 1996) represent important test cases for understanding the origin and nature of X-ray emission from PNs. The WC central stars of PNs are characterized by their overall spectral resemblance to "bona fide," massive WR stars, but as a group they extend to both hotter and cooler effective temperatures. The luminosities determined for the WC central stars of PNs, given reasonable distance estimates, confirm that these stars are much less luminous than the WC remnants of massive stars and establish beyond doubt their post-AGB nature.>>

<<On February 11, 1960, Jack Paar walked off his [NBC Tonight] show after NBC censors edited out a segment, taped the night before, about a joke involving a "W.C." (water closet, a polite term for a flush toilet) being confused for a "wayside chapel." As he left his desk, he said, "I am leaving The Tonight Show. There must be a better way of making a living than this." Paar's abrupt departure left his startled announcer, Hugh Downs, to finish the broadcast himself.

• An English lady is visiting Switzerland. She asks about the location of the "W.C." The Swiss, thinking she is referring to the "Wayside Chapel", leaves her a note that said (in part) "the W.C. is situated nine miles from the room that you will occupy... It is capable of holding about 229 people and it is only open on Sunday and Thursday... It may interest you to know that my daughter was married in the W.C. and it was there that she met her husband... I shall be delighted to reserve the best seat for you, if you wish, where you will be seen by everyone."

Paar returned to the show on March 7, 1960, strolled on stage, struck a pose, and said, "As I was saying before I was interrupted..." After the audience erupted in applause, Paar continued, "When I walked off, I said there must be a better way of making a living. Well, I've looked... and there isn't.">>

[Ann]

In Sweden, we don't use polite euphemisms to describe the place where you go when you have to go. The appopriate term is "toalett", toilet. Back in the 1970s a Swedish girl visited the United States and was amazed at the sheer number of "restrooms" that were found everywhere. She told her American hosts that she couldn't believe all the restrooms everywhere, even in restaurants! "We don't have them in Sweden!" the proud girl informed her American hosts. "We wait until we get home!"

Ann

[owlice]

Back in the 1970s, an American woman visited Europe, and was amazed at the sheer variety of flush mechanisms on toilets. Each WC brought a search for the appropriate knob/chain/lever/etc. to pull up/pull down/push/etc. She commented frequently to her continental-traveling daughter about the variety of flush mechanisms, so her daughter, when she thought of it, took photos of toilets while on various trips. After some years, she gathered together the photos intending to give them to her mother, as she thought her mother exaggerated greatly; as the daughter looked through them, however, she noticed that none of the toilets -- not a one! -- flushed in the same way any other pictured toilet flushed. Familiar with the Doors of Ireland poster, she had the pictures made into a poster (Toilets of Europe) instead.

[dougettinger]

Planetary nebulas are generated from lower mass stars and supernovas come from higher mass stars such as O and B types. So then we can safely say that Wikipedia is in error when it stated that the central star of the Cat's Eye Nebula is an O7 + [WR] type star ?

<<The ejected gases form a cloud of material around the now-exposed core of the star. As more and more of the atmosphere moves away from the star, deeper and deeper layers at higher and higher temperatures are exposed. When the exposed surface reaches a temperature of about 30,000 K, there are enough ultraviolet photons being emitted to ionize the ejected atmosphere, making it glow. The cloud has then become a planetary nebula.>>

<<Planetary nebulae (PNs) are the descendents of intermediate (1–8 M⊙) mass stars. The central star of a PN will evolve into a white dwarf with a mass between ∼0.55 and ∼1.0 M⊙. While on the upper asymptotic giant branch (AGB), much of the star's initial mass is lost in a more or less spherical outflow at rates of up to 10-4 M⊙ yr-1 and at speeds of ∼10–20 km s-1. As the star leaves the AGB, the UV radiation from the emerging white dwarf (WD) ionizes the red giant ejecta, producing the short-lived (∼3 × 104 yr) PN. The star's surface escape velocity also increases at this stage, and its wind speed may increase to ∼1000 km s-1 as its mass-loss rate drops to ∼10-8 M⊙ yr-1. This fast wind is shocked as it collides with the previously ejected slow wind. This process is expected to play a role in PN shaping.

The above references greatly aid my understanding about the connection between red giants and planetary nebulas. Hence, the most recent conclusions are that red giants, planetary nebulas, WR type stars, and white dwarfs will evolve from intermediate (1-8 Msolar) mass stars. Do any of the previously mentioned evolutions result for solar masses greater than 8 solar masses ? I suppose WR-type stars result from the gaspings of any star above 2 solar masses.

Oh, by the way, I enjoyed your thread of thought about WC's on the Jack Paar Show. Thanks, Art.

[Ann]

I suppose WR-type stars result from the gaspings of any star above 2 solar masses.

"Classic" WR stars represent an evolutionary phase of massive stars that started out as original O stars. I don't believe that a "classic" WR star can result from a star weighing less than 10 solar masses. In fact the star may have to weigh a lot more.

In this picture you can see two massive O stars, Gamma Velorumon in the lower right and Zeta Puppis in the upper right. But Gamma Velorum, the brighter of the two, is paired with a "classic" WR star.



This is what Professor Emeritus Jim Kaler says about the two components of Gamma Velorum, an O-type dwarf (meaning that it is still fusing hydrogen to helium in its core) and a WR star (see http://stars.astro.illinois.edu/sow/regor.html):

Estimating from other stars, the O7 dwarf should have a luminosity of around 180,000 times that of the Sun, a temperature of 32,500 Kelvin, a radius of 13 solar, and a mass around 30 solar. The WR star is much hotter (57,000 to 70,000 Kelvin, most of the light coming out in the ultraviolet part of the spectrum) with a luminosity of 100,000 solar. Measures of the angular separation between the stars coupled with the known orbit then give a distance of 1200 light years, over 40 percent greater than previously adopted. More massive stars always evolve the faster, so while the Wolf-Rayet component is now less massive than the O star, it is much farther along in its evolution, and hence had once to be the more massive of the two. Both stars blow powerful mass-losing winds, the WR star at a rate of a hundred- thousandth of a solar mass per year (more than 100 million times that of the solar wind), while the mass-loss rate of the O star is some 25 times less than that of the WR star. The collision between the winds produces X-ray emission. The windy WR star probably started with somewhere around 40 solar masses and has now stripped itself down by an unknown amount, perhaps to under 10. Only a few million years old, the visually fainter Wolf-Rayet component is almost certainly in the last stages of preparing to blow up as a supernova. The O star will follow much the same path and go supernova as well.

Ann

[dougettinger]

I am still confused. I thought O7 dwarfs came from intermediate mass stars (1 to 8 Msolar). O7 dwarfs are very recently evolved white dwarfs that are still very bright. But yet previously mentioned O7 dwarf's progenitor is a star of about 30 solar masses. How is this possible ?

[neufer]

Luminous O7-9 stars Main Sequence 'dwarfs' & 'supergiants' merge in the upper left corner of the HR diagram

---

I am still confused. I thought O7 dwarfs came from intermediate mass stars (1 to 8 Msolar). O7 dwarfs are very recently evolved white dwarfs that are still very bright. But yet previously mentioned O7 dwarf's progenitor is a star of about 30 solar masses. How is this possible ?

Ann is quoting Jim Kaler:

<<Estimating from other stars, the O7 [Main Sequence] dwarf should have a luminosity of around 180,000 times that of the Sun, a temperature of 32,500 Kelvin, a radius of 13 solar, and a mass around 30 solar.>>

Whereas, Wikipedia states:

<<The Gamma Velorum system is composed of at least six stars. The brightest member, γ² Velorum or γ Velorum A, is actually a spectroscopic binary composed of a blue supergiant of spectral class O9 (30 M☉), and a massive Wolf-Rayet star.>>

[Ann]

I am still confused. I thought O7 dwarfs came from intermediate mass stars (1 to 8 Msolar). O7 dwarfs are very recently evolved white dwarfs that are still very bright. But yet previously mentioned O7 dwarf's progenitor is a star of about 30 solar masses. How is this possible ?

It is the terminology that is the problem, not the stars themselves.

A dwarf star is a star that is fusing hydrogen to helium in its center. The Sun is a dwarf star. Sirius, which is much bigger than the Sun, is nevertheless a dwarf star, because it is fusing hydrogen to helium in its center.

But Sirius B is a "white dwarf star". The term is unfortunate. The term was coined, or so I believe, before the word "dwarfs" became the normal word to describe stars of any mass that are fusing hydrogen to helium in their centers.

Back when Sirius B was first detected, astronomers knew that "red dwarfs" existed. The red dwarfs were so called because they were redder than the Sun and much fainter. These dwarfs had to be cooler as well as smaller than the Sun in order to be so faint. At this time, astronomers also realized that a number of well-known reddish stars such as Betelgeuse, Antares, Aldebaran and Arcturus had to be very large, since they were so cool and yet so luminous.

When astronomers detected Sirius B, they didn't know what kind of star it was. They didn't realize that Sirius B is the still hot but cooling core of a dead star, a star which is no longer producing energy, but merely radiating away heat that was produced before. Sirius B produced heat both when it was a main sequence star of class B, fusing hydrogen to helium in its core, and then when it was a red giant, fusing helium to carbon and oxygen in its core and fusing hydrogen to helium in a shell outside its core. The heat that was built up during these energy-producing phases is now slowly radiating away into space as the dead core cools.

As I said, the astronomers that discovered Sirius B didn't know what kind of star it was, but they could see that it was very hot and very faint. Back then, no stars were described as blue, even though stars that are hotter than 20,000 degrees Kelvin are certainly bluish in color. To the astronomers who discovered Sirius B, the color representing the hottest stars was white. And so, because Sirius was very small and hot, it was described as a "white dwarf".

This has unfortunate consequenses. Personally I'm always going to argue that the Sun is white, because if it wasn't, then daylight, which is sunlight, wouldn't be neutral in color. If the Sun is really yellow, then we ought to see daylight as yellow. Since we don't, I'm always going to say that we should describe the Sun as white.

So if we accept that the Sun is white, and if we accept that a "dwarf" is a star that fuses hydrogen to helium in its core, then the Sun really ought to be called a "white dwarf". But the Sun is extremely different from what astronomers mean when they say "a white dwarf".



This image shows IK Pegasi, an A class dwarf, smaller and a bit cooler than Sirius, on the left, IK Pegasi B, a white dwarf, in the middle, and the Sun on the right.

As I said, I regard the Sun as white. But if we accept the established wisdom, which says that the Sun is yellow and A-class stars like Sirius and IK Pegasi are white, then, since both Sirius and IK Pegasi fuse hydrogen to helium in their cores, these two stars are both "white" "dwarfs" orbited by "white dwarfs"!

And if the hottest stars are described as white, then the Sun, which is clearly yellower than the hottest stars, will be described as yellow. Yellowish stars like Arcturus will be described as red, since they are redder than the Sun, which is "defined" as yellow. Really red stars, which are so small and faint that they only emit red light, will be described as "brown", since the term "red dwarf" is already taken.

To describe the Sun as a "yellow dwarf" is doubly unfortunate, in my opinion. It creates the idea that the Sun is yellow, which it isn't, in my opinion. And it creates the idea that the Sun is a very small star, when in fact it is bigger than most other stars, since most other stars are small class M stars, so called red (but really yellow-orange) dwarfs.

This picture shows you stars of various colors:

Click to view full size image

On the far left is Alpha Centauri, a main sequence star of spectral class G2. Alpha Centauri resembles the Sun, and in this picture it is appropriately white in color.

In the middle of the picture is the Southern Cross. The topmost star of the Southern Cross is Gacrux, a red giant star of spectral class M, which is yellow-orange in color. There are also many blue stars in the picture, and almost all of them are of spectral class B.

As for the term "white dwarf", it should be replaced with the term "degenerate star" or something, to point out that this is a star which has shrunk to an incredibly small size after all fusion processes have ceased in it. In a white dwarf, the atoms are packed so closely that only the so called "electron pressure" prevents the star from shrinking further.

Ann

[dougettinger]

Thanks to both Ann and Art. Now I understand my confusion. I thought "dwarf" was reserved for stars of the same type that are runts and "white dwarf" was reserved for degenerate stars. As Ann clearly pointed out, I was wrong.

The problem with any science that is rapidly changing is that the terminology becomes inappropriate. Astronomers should clarify the terminology by making it more understandable by producing tables that cross-classify old and new terminologies; this would be the same people that are trying to classify celestial bodies in our solar system.

I will now try to write an outline of star evolution as I now think I know it. Wikipedia has not been that helpful for me because the writers are wrangling with this terrible terminology.