APOD: Planetary Nebula Abell 7 (2013 Dec 05)

[APOD Robot]

Planetary Nebula Abell 7

Explanation: Very faint planetary nebula Abell 7 is some 1,800 light-years distant, just south of Orion in planet Earth's skies in the constellation Lepus, The Hare. Surrounded by Milky Way stars and near the line-of-sight to distant background galaxies, its generally simple spherical shape, about 8 light-years in diameter, is outlined in this deep telescopic image. Within its confines are beautiful, more complex details enhanced by the use of narrowband filters. Emission from hydrogen and nitrogen is shown in reddish hues with oxygen emission mapped to a bluish-green color, giving Abell 7 a more natural appearance that would otherwise be much too faint to be appreciated by eye. A planetary nebula represents a very brief final phase in stellar evolution that our own Sun will experience 5 billion years hence, as the nebula's central, once sun-like star shrugs off its outer layers. Abell 7 itself is estimated to be 20,000 years old. Its central star is seen here as a fading white dwarf some 10 billion years old.

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[/b]

[Beyond]

It reminds me of the something really bad that was coming to earth in the movie... The Fifth Element. Of course, in the movie, it never made it because the first four elements were combined with the fifth element and stopped it in it's tracks. No worries about Planetary Nebula Abell 7, though. It's different. It has that small white dot in the middle of it.

[Ann]

Planetary nebulae are not my forte, but I appreciate this image and the heroic effort that was necessary to bring out so much detail in the nebula. Don Goldman's own comparison between the faintness of Abell 7, compared with the brightness of the Tarantula nebula, is indeed interesting.

What I find most interesting is the very strikingly blue color of the central star, the white dwarf. No other stellar object in this image is that blue (except for a very faint blue dot at the upper rim of the nebula, at about two o'clock). The very blue color of the central star testifies to its high temperature. How hot is it? I have no idea, but I would be surprised if it not at least, say, 40,000K. It could well be hotter.

Ann

[AllanH]

When the explanation says the central star is a 10 billion year old white dwarf, does it mean that it has been a white dwarf that long, or was that when it entered the main sequence? (The nebula being only 20.000 years old suggest the latter...)

[neufer]

When the explanation says the central star is a 10 billion year old white dwarf, does it mean that it has been a white dwarf that long, or was that when it entered the main sequence? (The nebula being only 20.000 years old suggest the latter...)

• Indeed. A 10 billion year old white dwarf is an invisible black dwarf.

---

Phase.................... Lifetime
-----------------------------------------------------
Protostar................ 5 x 10⁷ years
Main Sequence......... 9 x 10⁹ years
Red Giant............... 5 x 10⁸ years
Horizontal Branch...... 5 x 10⁷ years
AGB...................... 1 x 10⁶ years
White Dwarf............ 9 x 10⁹ years

The temperature and luminosity of the coolest WD gives
the age of the Galactic disk : around 10 Billion years old.

[Ann]

When the explanation says the central star is a 10 billion year old white dwarf, does it mean that it has been a white dwarf that long, or was that when it entered the main sequence? (The nebula being only 20.000 years old suggest the latter...)

The star whose hot core is now a blue white dwarf may well originally have been born ten billion years ago. Maybe it was originally an F-type star like Procyon in the sky. Maybe it was originally a G-type star like Alpha Centauri A. However, whatever spectral class this star was during its main sequence days, it certainly hasn't been a white dwarf for much longer than the planetary nebula has existed.

Ann

[starsurfer]

I'am incredibly overjoyed to see this image on APOD!!! Don Goldman represents the future of planetary nebula imaging, I'm surprised he hasn't discovered anything new yet.

Ann, I also absolutely love the intense blue central star, for some reason it reminds me of the one in the planetary nebula PK 164+31.1, possibly my favourite central star of a planetary nebula.
I'm surprised the description doesn't mention that the central star is a binary, a possible companion was found in HST images.

[NGC3314]

The very blue color of the central star testifies to its high temperature. How hot is it? I have no idea, but I would be surprised if it not at least, say, 40,000K. It could well be hotter.

Above about 25,000 K, optical colors no longer give much information on stellar temperature - blackbody spectra saturate so all the continuum changes go into the UV. There is a schematic illustration of this here. So in optical images 25,000 K, 100,000 K, all give the same colors.

Mildly relevant news note - the Chang'E 3 lunar lander now en route carries a 15-cm UV telescope, designed, among other things, to do color and variability studies on stars too bright for the GALEX mission.

[rstevenson]

... The very blue color of the central star testifies to its high temperature. How hot is it? I have no idea, but I would be surprised if it not at least, say, 40,000K. It could well be hotter.

At this white dwarf's young age, it should be radiating at almost 10⁷ K. It's effective surface temperature though is likely to be in the 40,000 K to 150,000 K range, depending on the size of the star it evolved from.

Rob

[neufer]

I'm surprised the description doesn't mention that the central star is a binary,
a possible companion was found in HST images.

• A possible binary.

<<The central white dwarf star is designated as WD 0500-156.
This star may be part of a binary pair (De Marco, et al., 2012) based
upon a Hubble image showing a red dwarf companion 0.91" (500+ AU) away.>>

[sgwmunro]

If the white dwarf of Abell 7 is 10 billion years old, is that the age of it's dwarfness (ie the age of the planetary nebula) or the age of it's first stellar evolution?

[Ann]

The very blue color of the central star testifies to its high temperature. How hot is it? I have no idea, but I would be surprised if it not at least, say, 40,000K. It could well be hotter.

Above about 25,000 K, optical colors no longer give much information on stellar temperature - blackbody spectra saturate so all the continuum changes go into the UV. There is a schematic illustration of this here. So in optical images 25,000 K, 100,000 K, all give the same colors.

You may be right. However, a photographic film or filters may respond differently to color than the human eye.

It seems obvious to me that the white dwarf in this picture is hotter than any other stars in the picture.

Ann

[Ann]

If the white dwarf of Abell 7 is 10 billion years old, is that the age of it's dwarfness (ie the age of the planetary nebula) or the age of it's first stellar evolution?

It is not the age of its dwarfness. If it had been a white dwarf for that long, it would have cooled to a reddish color by now, or, alternatively, it would be so cool as to be optically invisible.

Ann

[Chris Peterson]

You may be right. However, a photographic film or filters may respond differently to color than the human eye.

It seems obvious to me that the white dwarf in this picture is hotter than any other stars in the picture.

You are probably correct. But you are correct for the wrong reasons. You can't infer anything about the temperature of the central star from its color given the complex mixing of broadband and narrowband data in this image. Although effort has been taken to produce a somewhat natural color balance, the reality is that this is not an image that has accurate enough color to reliably assess star colors. Once you mix in narrowband data, you lose the accuracy of the continuum information (which is already somewhat degraded by the different color space created by the filters in comparison with our eyes), and therefore you lose much of the information about the blackbody temperatures.

I'll bet that if you measured this star using photometric filters to get a simple color index, it would be considered white; i.e. the standard indexes would be very near zero.

[geckzilla]

Planetary nebulas look uniform in color in broadband filters. Or, rather, I should say that the ones I found in the Hubble archive look like this. I suspect the knots in these two would shine brightly in red if an H-alpha filter were used. Smaller, fainter stars completely disappear in some narrowband data. Anyway, I guess the point is that planetary nebulas are almost always imaged in narrowband because it's a lot more informative and interesting. Then again, H-alpha seems to make everything in space much more interesting.


NGC 6309


NGC 2452

[Chris Peterson]

Then again, H-alpha seems to make everything in space much more interesting.

Except stars. Stars shine so faintly in Ha that we often use Ha filters to produce images of nebulas that aren't all cluttered up with bright stars.

[Ann]

I'll bet that if you measured this star using photometric filters to get a simple color index, it would be considered white; i.e. the standard indexes would be very near zero.

That's where you and I strongly disagree, Chris. Of course, to have a meaningful discussion, we have to define what we mean by "white". I consider the Sun to be white, since humanity has evolved in response to the light of the Sun, and it has been immensely practical for us to see the total light from the Sun (which is direct sunlight + sky) as white. But others consider the Sun to be yellow, even though that means that the blue light from the sky becomes an utter mystery.

Those people who consider the Sun to be yellow also consider A0-type star Vega to be white, since its B-V index is 0.0. To me Vega is quite blue, certainly when seen through a telescope, and therefore a color index of 0.0 indicates a blue star, in my opinion. By comparison, the B-V index of the Sun is 0.656 ± 0.005, which to me is a good definition of what the human eye sees as a white color.

What is the color index of the central star of Abell 7? I have no idea, but I may hazard a guess. The apparent color index of any hot star is strongly dependent on the dust reddening of this object, which can be determined by measuring the photon intensity of the star at several different wavelengths. But let's toy with the (unlikely) idea that this star is completely unreddened by dust. What would its B-V index be?

We still can't begin to guess at its B-V index before we consider whether or not this star is "equally blue" from any direction, or if it is bluer when seen pole on and redder when when it presents its equator to us. Several hot stars that are "surprisingly blue" - i.e. Tau Scorpii, the star which is surrounded by a large faint emission nebula southeast of Antares - are likely seen pole on. (The B-V index of B0-type star Tau Scorpii is -0.20.) Other hot stars that are "surprisingly non-blue" - i.e. Sigma Scorpii, the binary star with an O-type component and a B-V index +0.30, are likely particularly dust-reddened due to interaction between the two components. Still other hot stars that are surprisingly non-blue - i.e. Achernar, whose Johnson B-V index is -0.158 but whose Tycho B-V index is only -0.064 - are likely strongly oblate and seen from a "thick" equatorial point of view.

Let's assume, nevertheless, that a circa 40,000K or hotter star that is not producing dust by interacting with another component, and which is seen in a part of the sky where there is little dust in our line of sight, and which is neither pole-on nor equator-on but something in between, might have a color index of about -0.25. Is that a white color?

No, it is quite strikingly blue, in my opinion. I most certainly don't mean to imply that we are talking about a saturated blue hue. Such a claim would be nonsensical. Nevertheless, to me this color is quite strikingly blue.

Others may disagree. You may also claim that Vega is white and the Sun is yellow. Anyone is perfectly entitled to that opinion.

I just don't see how those who believe in an intrinsically yellow Sun can explain the blue color of the sky or the overall neutral or "white" color of daylight.

Ann

[c]Abell 39. Photo: Adam Block.[/c]

This LRGB image of planetary nebula Abell 39 shows a strikingly blue central star. (There are much larger versions of this picture, but you will have to google them yourself.)

[geckzilla]

Then again, H-alpha seems to make everything in space much more interesting.

Except stars. Stars shine so faintly in Ha that we often use Ha filters to produce images of nebulas that aren't all cluttered up with bright stars.

...that's why it makes space more interesting. By getting rid of the boring stars.
I tried to get interested in stars by taking a closer look at globular clusters but they still bore me. If you've seen one, you've seen them all... and I say that in the entirely superficial sense. Clusters and stars themselves much more interesting when verbally described for their individual characteristics and what can be learned from them. I guess you could say imagery fails them all except for our Sun.

[Chris Peterson]

That's where you and I strongly disagree, Chris. Of course, to have a meaningful discussion, we have to define what we mean by "white". I consider the Sun to be white, since humanity has evolved in response to the light of the Sun, and it has been immensely practical for us to see the total light from the Sun (which is direct sunlight + sky) as white. But others consider the Sun to be yellow, even though that means that the blue light from the sky becomes an utter mystery.

Nobody who is informed considers the Sun to be yellow. It is classified as yellow based on its temperature, but that does not mean it is visually yellow. As you know.

We can certainly define white in different ways. Color indexes are, of course, very simplistic, giving a best a crude approximation of visual color.

What is important in this case is that we know a star with a temperature as high as this one- given that it's an evolved white dwarf- will have a blackbody spectrum that makes it appear visually white. There is likely to be very little dust reddening in this system.

Certainly, the star is very blue in this image. But that is an imaging artifact. The color we see here is nowhere near what we'd expect for the object itself.

[geckzilla]

Certainly, the star is very blue in this image. But that is an imaging artifact. The color we see here is nowhere near what we'd expect for the object itself.

To further illuminate why this is an imaging artifact (specifically, a result of processing), I have attached an image of the red channel (cropped to just the nebula) so that anyone can see the dark black ring around the central star as well as some other small ones due to sharpening and saturation adjustments which adversely affect the integrity of the objects in favor of aesthetics. Note that the adjustments were applied locally to just the area of the nebula.

[BMAONE23]

It reminds me of the something really bad that was coming to earth in the movie... The Fifth Element. Of course, in the movie, it never made it because the first four elements were combined with the fifth element and stopped it in it's tracks. No worries about Planetary Nebula Abell 7, though. It's different. It has that small white dot in the middle of it.

It is unfortunate though that, in the end of the movie, the remnant of the really bad thing decided to remain and Moon us.

[Ron-Astro Pharmacist]

Beyond wrote:
It reminds me of the something really bad that was coming to earth

To me it looks like it is a ghost image of the Earth following either decades of global warning or millions of years of continental drift or both. A true “planetary” nebula if there ever was one. But of course - it would look that way to me!!

[Anthony Barreiro]

This is a lovely picture of a very planetary nebula, as Ron said, and the central star is a lovely blue color. I understand that narrow-band filters and image processing create artificial colors that are different than our eye would see -- and with an Abell catalogue planetary nebula, a faint blur is about the best you're going to see visually through a telescope. But I'm curious -- in general, would the colors of the outer layers of a planetary nebula influence the apparent color of the central star, or is the light of the central star so bright that it punches through the nebula without any tinting?

[Chris Peterson]

This is a lovely picture of a very planetary nebula, as Ron said, and the central star is a lovely blue color. I understand that narrow-band filters and image processing create artificial colors that are different than our eye would see -- and with an Abell catalogue planetary nebula, a faint blur is about the best you're going to see visually through a telescope. But I'm curious -- in general, would the colors of the outer layers of a planetary nebula influence the apparent color of the central star, or is the light of the central star so bright that it punches through the nebula without any tinting?

I'd say the latter. The star is orders of magnitude brighter than any narrow band emissions, as well as being a continuum source. About the only intervening medium that can affect star color is dust, and that by scattering shorter wavelengths. But a planetary nebula is too small, and has too little dust to have much effect. The only place that leaves for dust is immediately around the central star, but that probably isn't the case here- either an isolated white dwarf or a white dwarf with a red giant companion. Either way, not a dust producing environment.

[Anthony Barreiro]

This is a lovely picture of a very planetary nebula, as Ron said, and the central star is a lovely blue color. I understand that narrow-band filters and image processing create artificial colors that are different than our eye would see -- and with an Abell catalogue planetary nebula, a faint blur is about the best you're going to see visually through a telescope. But I'm curious -- in general, would the colors of the outer layers of a planetary nebula influence the apparent color of the central star, or is the light of the central star so bright that it punches through the nebula without any tinting?

I'd say the latter. The star is orders of magnitude brighter than any narrow band emissions, as well as being a continuum source. About the only intervening medium that can affect star color is dust, and that by scattering shorter wavelengths. But a planetary nebula is too small, and has too little dust to have much effect. The only place that leaves for dust is immediately around the central star, but that probably isn't the case here- either an isolated white dwarf or a white dwarf with a red giant companion. Either way, not a dust producing environment.

Thanks Chris. As I thought about this further, since the ultraviolet light from the very hot remnant central star is making all that gas glow with those pretty colors, the star must be pretty darn bright.