APOD: In the Glare of Alpha Centauri (2012 Jun 28)

[APOD Robot]

In the Glare of Alpha Centauri

Explanation: The glare of Alpha Centauri, one of the brightest stars in planet Earth's night sky, floods the left side of this southern skyscape. A mere 4.3 light-years distant, Alpha Centauri actually consists of two component stars similar in size to the Sun, locked in a mutual orbit. Much smaller and cooler, a third member of the same star system, Proxima Centauri, lies outside this field of view. Still, the telescopic scene does reveal often overlooked denizens of the Milky Way's crowded galactic plane that lie beyond the glare of Alpha Centauri, including a planetary nebula cataloged as Hen 2-111, an estimated 7,800 light-years away. The gaseous shroud of a dying star, the nebula's brighter core and fainter halo of reddish ionized gas span over twenty light-years, seen just right of picture center. Farther right are two notable open clusters of stars, the compact Pismis 19 also nearly 8,000 light-years away whose light is reddened by intervening dust, and the looser, closer NGC 5617. Just visible in the glare of Alpha Centauri is the dim glow of a shell-like supernova remnant, above and right of the closest star system's bright core.

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[Flase]

Is it certain that Proxima Centauri is part of the same star system or is it a wanderer?

In fact what is the latest estimation of the number of smaller bodies, brown dwarfs, that might also be wandering by?

[Ann]

I think this is an excellent picture!

The description given by the APOD Robot is very satisfying, but a few things can be added. First, the fact that Alpha Centauri looks so overwhelmingly bright, even though it is the intrinsically faintest of all stars apart from the Sun that look bright and obvious to the naked eye. Fascinatingly, 99% of all naked-eye stars are brighter than the Sun, but 95% of all stars in the Milky Way are fainter than the Sun!

In this Sun-centered view of the universe - sorry about that - all the white dots making up the constellations represent stars that are intrinsically brighter than the Sun. The rare bright stars in the sky shine so brightly that they are seen over vast distances and dominate the sky, whereas all stars similar to the Sun look faint and insignificant - all of them except the Sun and Alpha Centauri. The Sun is only eight light minutes away, and Alpha Centauri, at four light-years, is the second nearest star to the Earth in the cosmos. What a coincidence that both the Sun and Alpha Centauri are G-type hydrogen-fusing dwarfs of similar brightness!

Alpha, Beta and Proxima Centauri. Photo: ESO

The fact that Alpha Centauri is a multiple star is a fact that we don't have say much about here, except that it's a pity that today's excellent APOD isn't wide enough to show Proxima Centauri, the probable third member of the Alpha Centauri system. But this ESO photo shows the glare of the Alpha A+B Centauri system, with the tiny, ultrafaint red dwarf in a very wide orbit around the two main components.

I guess that Proxima Centauri would have showed up in today's APOD if the picture had been wide enough to include it, but the tiny star would have looked insignificant indeed!

In the ESO picture, you can see the clusters NGC 5617 and, faintly, Pismis 19. You can also see cluster Trumpler 22, which is visible in today's APOD below Pismis 19. But in the ESO picture you can't see planetary nebula Hen 2-111 very well, certainly not its outer halo. The supernova remnant which is seen in today's APOD is also invisible in the ESO image. Photographer Marco Lorenzi has used an Ha filter to tease out these fascinating details in the background of Alpha Centauri.

The wonderful deep-sky jewels seen in Marco Lorenzi's image have nothing to do with Alpha Centauri itself. We happen to see the clusters and the remnants of dead stars in the same direction as we see Alpha Centauri. But Alpha Centauri is our next-door neighbour, while the deep-sky wonders belong to the rich treasure trove of the distant Milky Way.

The Milky Way is like a city in the distance, while Alpha Centauri is like a small cottage a few steps away.

Ann

[Ann]

Is it certain that Proxima Centauri is part of the same star system or is it a wanderer?

In fact what is the latest estimation of the number of smaller bodies, brown dwarfs, that might also be wandering by?

I don't think it has been established that Proxima Centauri is definitely a part of the Alpha Centauri system.

http://stars.astro.illinois.edu/sow/rigil-kent.html wrote:
Alpha Centauri has yet another member, a faint eleventh magnitude (11.05) companion called "Proxima" that is a huge 1.85 degrees (at least 8500 AU) away from Alpha proper and that orbits with a period of at least a three-quarters of a million years. If indeed it does orbit (and that is not certain), it is now on the near side of its path and some 6,000 astronomical units closer than the bright pair, making it actually the closest known star (but since it is part of Alpha, surely it is still fair to call Alpha the closest star).

The latest estimate of how many brown dwarfs might be wandering by says that there probably aren't that many. The latest result suggests that the ratio between brown dwarfs and stars might be 1:6, so that there are six stars for every brown dwarf. See http://asterisk.apod.com/viewtopic.php?f=31&t=28852.

WISE Finds Few Brown Dwarfs Close to Home wrote:

Kirkpatrick emphasized that the results are still preliminary: it is highly likely that WISE will discover additional Y dwarfs, but not in vast numbers, and probably not closer than the closest known star, Proxima Centauri. Those discoveries could bring the ratio of brown dwarfs to stars up a bit, to about 1:5 or 1:4, but not to the 1:1 level previously anticipated.

Ann

[starsurfer]

Absolutely stunning image! I love it when overlooked unknown things near really well known things are photographed!

Hen 2-111 is one of my favourite southern planetary nebulae, I love its strange shape and variety of colours! The halo was discovered quite a while ago in 1978 and is apparently the result of a bipolar outflow as opposed to being a conventional AGB-type halo.

I never expected this to be on APOD and I'm so happy I can't stop smiling!

I guess that Proxima Centauri would have showed up in today's APOD if the picture had been wide enough to include it, but the tiny star would have looked insignificant indeed!

I think that even if the image had been wider, I don't think Proxima Centauri would be visible at such a wide scale as the image was taken with a small 6-inch telescope. A mosaic with a larger telescope might have shown it but then it might have been lost in the intense glare of Alpha Centauri.

Also, Ann I love it when you talk science!

[Eon]

What causes this glare? Is it caused by space dust, earth's atmosphere (in case of a ground based telescope) or the telescope itself?

[Chris Peterson]

What causes this glare? Is it caused by space dust, earth's atmosphere (in case of a ground based telescope) or the telescope itself?

Almost all of this glare and stellar bloat is caused by scatter and internal reflections in the optics of the telescope and the camera itself. There may be a small contribution from the atmosphere, as well.

[neufer]

[b][color=#0000FF]Computer-generated image of an Airy disk. The gray scale intensities have been adjusted to enhance the brightness of the outer rings of the Airy pattern.[/color][/b]

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What causes this glare? Is it caused by space dust, earth's atmosphere (in case of a ground based telescope) or the telescope itself?

Almost all of this glare and stellar bloat is caused by scatter and internal reflections in the optics of the telescope and the camera itself. There may be a small contribution from the atmosphere, as well.

It's not just the combined Airy disks
of the primary & secondary mirrors?

[Say "stellar bloat" three times fast.]

[Chris Peterson]

It's not just the combined Airy disks :arrow:
of the primary & secondary mirrors?

No. The image was made with an f/7 refractor (no mirrors), so the Airy disc itself is 10 um in diameter- about one pixel with the optics and camera used. If we were seeing the light of smeared-together Airy rings, the light falloff would be some sort of convolution of a Moffat function and Bessel function- very different from what we see here, with distinct regions of different brightness spreading over more than 1000 pixels. Any visible Airy structure is lost in the central scatter.

In these optics, light can be internally reflected off the lens surfaces, filter surfaces, camera window, and CCD cover glass. Scatter can occur off any optical surfaces that are not perfectly clean (the front of the objective almost certainly has dust on it), as well as internally in filters, the CCD microlens array, and even the CCD material itself.

The dynamic range of objects in this image is huge- I'd estimate at least 18 magnitudes, or more than 10 million to one. Making an image like this creates the same sort of problems as in imaging the solar corona from the ground- the corona being about a million times dimmer than the photosphere. Coronagraphs seek to do this by blocking the light of the Sun, but they require extreme engineering to overcome internal scatter, as well as high altitude, pristine atmospheric conditions.

[saturno2]

Alpha Centauri 4.3 light-years distant.
Proxima Centauri 4.22 light- years distant.
This System of 3 stars has 2 stars very bright and one much smaller and cooler, Proxima Centauri, the nearest star from planet Earth ( Out Sun )

[neufer]

It's not just the combined Airy disks of the primary & secondary mirrors?

No. The image was made with an f/7 refractor (no mirrors), so the Airy disc itself is 10 um in diameter- about one pixel with the optics and camera used. If we were seeing the light of smeared-together Airy rings, the light falloff would be some sort of convolution of a Moffat function and Bessel function- very different from what we see here, with distinct regions of different brightness spreading over more than 1000 pixels. Any visible Airy structure is lost in the central scatter.

In these optics, light can be internally reflected off the lens surfaces, filter surfaces, camera window, and CCD cover glass. Scatter can occur off any optical surfaces that are not perfectly clean (the front of the objective almost certainly has dust on it), as well as internally in filters, the CCD microlens array, and even the CCD material itself.

The dynamic range of objects in this image is huge- I'd estimate at least 18 magnitudes, or more than 10 million to one.

I'll grant all that... but still the apparent size of Alpha Centauri is basically determined by the ~R^-3 wings of the Airy disc making Alpha Centauri seem to be about 215 [~cuberoot(10 million)] times the basic 10 um Airy disc diameter. A telescope with twice the aperture (and a quarter the exposure time) would have made Alpha Centauri only half as bloated.

[Chris Peterson]

I'll grant all that... but still the apparent size of Alpha Centauri is basically determined by the ~R^-3 wings of the Airy disc making Alpha Centauri seem to be about 215 [~cuberoot(10 million)] times the basic 10 um Airy disc diameter. A telescope with twice the aperture (and a quarter the exposure time) would have made Alpha Centauri only half as bloated.

The physical size of the Airy disc is determined by the focal ratio, not the aperture. So I assume you're keeping the focal length fixed as you double the aperture... resulting in a f/3.5 system and a consequently smaller Airy disc. But in practice, the visual appearance won't change, since the entire Airy structure will still be buried in the scatter from the (imperfect) optics.

[neufer]

I'll grant all that... but still the apparent size of Alpha Centauri is basically determined by the ~R^-3 wings of the Airy disc making Alpha Centauri seem to be about 215 [~cuberoot(10 million)] times the basic 10 um Airy disc diameter. A telescope with twice the aperture (and a quarter the exposure time) would have made Alpha Centauri only half as bloated.

The physical size of the Airy disc is determined by the focal ratio, not the aperture. So I assume you're keeping the focal length fixed as you double the aperture... resulting in a f/3.5 system and a consequently smaller Airy disc. But in practice, the visual appearance won't change, since the entire Airy structure will still be buried in the scatter from the (imperfect) optics.

All I am saying is that Alpha Centauri is bloated (in angular size) simply due to the overexposed R^-3 wings of the Airy disk.

If one wishes to see all those pretty background stars without the distraction of so bloated an Alpha Centauri the solution is a larger aperture (and not cleaning the dust off the optics which can only darken the entire field of view).

Alternatively: one can reduce the R^-3 wings of the Airy disk by apodization:

<<Apodization literally means "removing the foot". It is the technical term for changing the shape of a mathematical function, an electrical signal, an optical transmission or a mechanical structure.

Apodization is used in telescope optics in order to improve the dynamic range of the image. For example, stars with low intensity in the close vicinity of very bright stars can be made visible using this technique, and even images of planets can be obtained when otherwise obscured by the bright atmosphere of the star they orbit. Generally, apodization reduces the resolution of an optical image; however, because it reduces diffraction edge effects, it can actually enhance certain small details. In fact the notion of resolution, as it is commonly defined with the Rayleigh criterion, is in this case partially irrelevant. One has to understand that the image formed in the focal plane of a lens (or a mirror) is modelled through the Fresnel diffraction formalism. The classical diffraction pattern, the Airy disk, is connected to a circular pupil, without any obstruction and with a uniform transmission. Any change in the shape of the pupil (for example a square instead of a circle) or in its transmission results in a change of the diffraction pattern.

The diaphragm of a photo camera is not strictly an example of apodization, since the stop doesn't produce a smooth transition to zero intensity, nor does it provide shaping of the intensity profile (beyond the obvious all-or-nothing, "top hat" transmission of its aperture). The Minolta/Sony Smooth Trans Focus 135mm f/2.8 [T4.5] lens, however, is a special lens design, which accomplishes this by utilizing a concave neutral-gray tinted lens element as apodization filter, thereby producing a pleasant Bokeh. The same optical effect can be achieved combining depth-of-field bracketing with multi exposure, as implemented in the Minolta Maxxum 7's STF function.>>

[Chris Peterson]

All I am saying is that Alpha Centauri is bloated (in angular size) simply due to the overexposed R^-3 wings of the Airy disk.

I understand what you're saying. But the question was about this image, and in this image we're not seeing any bloat of the star from diffraction effects, because all of the diffracted components are lost in the glare of scattered light. Even with a much larger aperture, and consequently smaller (angular) sized Airy structure, the image would look the same.

[neufer]

All I am saying is that Alpha Centauri is bloated (in angular size) simply due to the overexposed R^-3 wings of the Airy disk.

I understand what you're saying. But the question was about this image, and in this image we're not seeing any bloat of the star from diffraction effects, because all of the diffracted components are lost in the glare of scattered light. Even with a much larger aperture, and consequently smaller (angular) sized Airy structure, the image would look the same.

We will agree to disagree then.