Starship Asterisk*

The Spotty Surface of Betelgeuse

Explanation: Betelgeuse really is a big star. If placed at the center of our Solar System it would extend to the orbit of Jupiter. But like all stars except the Sun, Betelgeuse is so distant it usually appears as a single point of light, even in large telescopes. Still, astronomers using interferometry at infrared wavelengths can resolve the surface of Betelgeuse and reconstructed this image of the red supergiant. The intriguing picture shows two, large, bright, star spots. The spots potentially represent enormous convective cells rising from below the supergiant's surface. They are bright because they're hotter than the rest of the surface, but both spots and surface are cooler than the Sun. Also known as Alpha Orionis, Betelgeuse is about 600 light-years away.

Maybe it's just an artifact of the picture but it seems as though the surface may be bumpy also.

I was wondering about that too, orin. I think I read on this forum that supergiants are thought to have diffuse borders rather than relatively sharp ones, like our sun. I wonder if the gravity would still keep it bound in a spherical shape. I wish there was a method of getting a better view of distant objects. I suppose I should be amazed that we are able to resolve Betelgeuse beyond a point source but the blurry dots are still pretty underwhelming.

orin stepanek wrote:Maybe it's just an artifact of the picture but it seems as though the surface may be bumpy also.

It probably is somewhat lumpy, but I don't think we're seeing any of that in the image. When you have diffuse light and dark areas like this, our brains try to interpret them as high and low areas, with highlights and shadows.

This might be a cylinder - and perhaps this gives us some insight into why end-of-life stars expel their shells in cylinders, rather than spheres - most vividly seen in Eta Carinae, I think... In this particular picture, it seems that the white spots could be the top and bottom of a cylinder (with rounded caps), surrounded by a lumpy, banded, rough, churning sphere of cooler material - part of the process that will result in a planetary nebula as this star reaches it's end of life...

So why the hot spots at the top and bottom of the cylinder? What is it about stars that causes this (we've seen it in many planetary nebulas...)? One possibility that comes to my mind is that the star's rotation causes a certain amount of counter-force to gravitational contraction that is maximal at the "equator" and diminishes in latitude, relative to the radius of rotation at any given latitude. At the top and bottom of such a spherical, rotating star, that counter-force would drop to zero, so there would be more gravitational pressure present at the poles of a star (extending all the way down to the core) than at the equatorial region. Fusion happens due to this gravitational pressure, and really only occurs in the "core", so maybe the "fusion region" of stars (the "core") is not actually a sphere, even if the star is generally shaped like a sphere - maybe the "core" is typically shaped more like a rounded cylinder... and when a star gets as big, old, and cool (in it's outer layers) as Betelgeuse, you can actually see the "poles" of that core-cylinder (at least in infrared).

Why don't we see our own sun's "core-cylinder poles"? Besides that our sun is still young and too dense to see into, I think we would all agree that the temperature difference between the fusion core and the external shells would become greater and greater as a star reaches the end of it's life, expands, and becomes less dense - so maybe it's a combination of both - greater temperature difference and decreasing density of the outer layers making the inner hot regions more visible.

Also in this picture, the central dark band to my mind looks like it might be roughly the equatorial region. That ought to be dark and better at obscuring the core simply because the equatorial region would be "deeper" - there's more material to see through here, so the surface literally appears darker here, close to the equator, in an aged star...

Am I totally off my rocker with this hypothesis? I wonder if anyone has ever done computer modeling to see how much solar rotation would affect the fusion core and whether it might explain cylindrical expansion shells?

Did some calculations - the average density of the Sun is about 1400 kg/m^3, or about 1.4 grams per cubic centimeter - 50% more dense than water, but less than a fifth as dense as iron (of course, the core being much more dense than the outer layers). But the average density of Betelgeuse is only about 0.0000328 kg/m3 - a thousands of times less dense than even "air" at 1 atmosphere... So the outer layers of Betelgeuse would be just thin wisps of hot material ... very transparent and quite variable (lumpy)...

Two thoughts:

1) The bright spots on Betelgeuse correspond to regions
where solar sunspots/activity take place on our own Sun.

2) If there was less Betelgeuse activity in the past (due to Betelgeuse sunspot cycling)
could a recent enhancement of such smaller scale brightening be confused
for an apparent shrinkage of Betelgeuse.

What does "10 mas" in the lower right of the picture mean?

hartwigr wrote:What does "10 mas" in the lower right of the picture mean?

10 milli-arc-seconds.

Pluto is approximately 110 mas in diameter.
Charon is approximately 55 mas in diameter (about the same as Betelgeuse].
Pluto's moon Hydra is approximately 7.5 mas in diameter.

neufer wrote:Pluto is approximately 110 mas in diameter.
Charon is approximately 55 mas in diameter (about the same as Betelgeuse].
Pluto's moon Hydra is approximately 7.5 mas in diameter.

I was also wondering about the signification of mas... Charon about the same as Betelgeuse?? I don't think so

What is our sun's mas?

FrankTKO wrote:Charon about the same as Betelgeuse?? I don't think so

Hubble image of Charon
Hubble image of Betelgeuse

Why does the picture look so geometric? It can't be the resolution, since that's high enough…

FrankTKO wrote:What is our sun's mas?

The question would be better phrased, "what angle does our sun subtend?" The Sun and Moon both have an angular size of about 0.5°, which is 1800 arcsec, or 1,800,000 mas (about 42,000 times larger than the apparent angular diameter of Betelgeuse).

Betelgeuse shrinking!
Betelgeuse Resolved (Aug 5 2009)

Chris, thanks for that quick explanation of comparative sizes. I was just wondering, in terms of magnification, what would the approximate magnification be of that photo of Betelgeuse if one were looking through an eyepiece? Is it 10,000X? or even more..? (I wonder if those cheap e-bay telescopes will start marketing their equipment as being able to see Betelgeuse as a disk? After all, they do advertise that with THEIR scopes, you can see MILLIONS of miles into space....!)
Clear skies,

astroarchitect wrote:I was just wondering, in terms of magnification, what would the approximate magnification be of that photo of Betelgeuse if one were looking through an eyepiece?

Well, very roughly, I'd say the resolution in the image looks something like what you see on Mars at 50X. So given Mars at 15,000 mas and Betelgeuse at 40 mas, that means you'd need a magnification of something like 20,000X. That's feasible with a 10 meter aperture and no atmosphere.

Yes, Thank You Chris Peterson, you are excellently knowledgeable in the observational disciplines of astronomy !
(however much we may differ on the theoretical astrophysics side)

I was wondering if milli (as in m.a.s.) meant to multiply by 1000, it does !

As to the unevenness of the surface brightness of Betelgeuse, I believe it is 'an artifact', a result of the interferometer process that was used to produce the image, akin to digital photos at certain enlargements or angles will get a sideways lineyness to them, like polarization effects in a way.


http://www.space.com/scienceastronomy/a ... y_101.html

"In astronomy, we will be dealing with the interference of two light waves. If both waves are in step or in phase, that is, the crest of both waves coincide, the two will add together to form a single wave. This combined wave will have a higher crest and deeper trough (or larger amplitude). In the case of light waves, two dimmer light beams will add together to form a brighter beam -- this is called constructive interference.

"On the other hand, destructive interference occurs when the two waves are out of step with each other; that is, the crest of one coincides with the trough of the other. Here, although the waves are still adding together, they cancel each other out. So, the amount of interference that occurs depends on both the amplitudes of the two waves and the degree to which their respective crests and troughs are in phase with each other.

"What is looked for are alternating bands of light and dark, called fringes. Fringes are bright where the beams are constructively adding together and dark where they are canceling each other out.

"What makes the interferometer such a precise measuring instrument is that these fringes are only one light-wavelength apart. In visible light, about 590 nanometers --that corresponds to 1/43,000th of an inch! Any movement along the optical axis by either flat mirror will cause the fringes to shift an equal amount in lockstep. The measurement of this movement is made by literally counting the number of fringes - each dimming and brightening of light - one wavelength at a time!

"Such a precise system is also incredibly sensitive -- so much so that any vibration, movement, thermal expansion, etc. is picked up as well. In fact, Michelson's early experiments were affected by street traffic vibrations up to 1,000 feet away! Using shorter wavelengths of light allow greater precision, but are much more difficult to work with (the fringes are closer together).

"In astronomical interferometry, the most important parameter is the "baseline," the distance between the flat mirrors. Another key parameter is called "visibility,'' which is the difference in brightness between a fringe and the relative darkness between it and the next fringe.

"If one plots visibility versus baseline, the maximum visibility occurs at a baseline of zero, and decreases as baseline is increased. At some point, the visibility drops to zero (and the fringes disappear). This is called the "resolving point.'' At greater baselines, the fringes reappear and visibility increases, but only to about 10% of the peak visibility before dropping again. The heights of subsequent peaks taper off.

"The significance of the resolving point is that if you are observing a star, it gives a direct measurement of the apparent diameter of the object against the sky. If the distance to the star is known, then the actual diameter can be calculated. (By analogy, the apparent size of coin held up varies by the distance you hold the coin up from your eye.) Although stars are large, they are at very great distances, and so the apparent diameters are very small, typically a few thousandths of an arcsecond (1 milliarcsecond is about 275 billionths of a degree)."

- - -

And thusly I suspect that some of the variations in Betelgeuse's surface brightness are partly a result of the process used to obtain the image.

The splotches follow another pattern. One that might be created by looking at opposing wave patterns as they cross, or looknig through two screens that aren't precisely ligned up. Perhaps there was a slight misalignment in the interferometry calculations?
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kovil wrote:As to the unevenness of the surface brightness of Betelgeuse, I believe it is 'an artifact', a result of the interferometer process that was used to produce the image...

There is good evidence that the image is showing real structure on Betelgeuse. Time series images show the patterns moving, as you'd expect with a rotating star. Also, the HST has sufficient resolution to resolve the surface of Betelgeuse, and these patterns are seen in single images that undergo conventional processing, no interferometric reconstructions involved. Finally, in addition to the spot structure there is clear evidence of limb darkening, which is difficult to explain in terms of reconstruction artifacts or interference effects.

As pointed out by Stormculture, the average density of Betelgeuse is 3e-8 g/cc. That means the density at the "surface" of Betelgeuse is much less, probably even far less than the density of our sun at it's surface (1e-9 g/cc).

So my question is: What force holds this very tenuous matter in place, since the gravity on the surface of Betelgeuse (6e-4 g's) is much lower than the gravity on the surface of our sun (28 g's)? Wouldn't the solar wind on Betelgeuse rapidly blow the "surface" away? And since the density of the surface of Betelgeuse is presumably far less than the density of our sun's surface, how can we "see" Betelgeuse's more tenuous and cooler surface?

Similar questions arise for blue giants and super giants, which presumably have far greater solar winds than Betelgeuse.

What holds Betelgeuse's thin outer layers "in orbit" is the same as how Earth's thin outer atmosphere stays in place despite solar wind - gravity. Yes, Betelgeuse is a star with a solar wind that is pushing against the outer layers - but that's why Betelgeuse is so large in diameter. Assuming it is at equilibrium, the "surface" of Betelgeuse is the point at which those thin wisps of hot plasma experience equal forces from gravity pulling them back to Betelgeuse's core and the solar (particle) wind, pushing them out.

That said, Betelgeuse could very well NOT be at equilibrium - it's at the end of it's life and would be experiencing the "bounces" that occur as a star runs out of the easier fuels (Hydrogen, then Helium, and so on) and has to start fusing larger atoms. Each time there is a shift to a larger atom, there is a "bounce" that causes a sudden increase in the pressure against the outer shell of material, but only after a period of contraction. This is why you see multiple, distinct "shells" in planetary nebulae from dying or dead stars.

The actual process, put more clearly, is this - lets say we're talking about our Sun, just for a concrete example:

- Sun starts running out of Hydrogen available to fuse
- total real-time fusion decreases, so the force pushing out on the outer layers (and total radiation) decreases
- Sun starts to collapse - ie its apparent radius diminishes much faster than the normal loss due to radiation/solar wind, and it's density increases
- density in the core increases, which increases the temperature
- at a particular point, the temperature and pressure become great enough to start fusing Helium into Beryllium. This suddenly increases the amount of fusion (and thereby also increases the "radiation" the core is producing.
- the outer shells that had been collapsing on the core are suddenly met with new expanding radiation and particles from the new type of fusion taking place, and so are "blown" back. This is violent - and gets more violent each time it happens - each time the star moves up to heavier fusion. The outer shell now starts expanding again and "bounces" out to an equilibrium point that is actually further away from the core than it had been. Some material is also completely ejected into space at very high speed - this is what we see in a planetary nebula.

This cycle repeats until all that's left in the star is Iron atoms, or the density becomes so great that atomic nuclei can no longer stand the gravitation pressure on them, and they, too, collapse - which is the process that creates neutron stars, black holes, and supernovas.

So the core of dying stars, by necessity, increases in density and temperature continually (until the star "dies" - becomes merely a white dwarf, neutron star, or black hole), but the outer shell and the "apparent diameter" of the star increases with each "bounce", until death.

"I was also wondering about the signification of mas... Charon about the same as Betelgeuse?? I don't think so"

[My apologies for not knowing better how to handle quotes.]

A milli-arc-second [one millionth of one arc-second, I assume] is an angular measurement.
Think of looking through a surveyor's theodolite and measuring the angle from one point to another. The breadth of the object viewed then depends on it's distance from the viewer.
Pluro and Charon are distant from Earth in multiples of A.U. Betergeuse is about 600 light-years away, so its breadth is vastly larger.
Peace and health to all,
Phil G

Phil G wrote:A milli-arc-second [one millionth of one arc-second, I assume] is an angular measurement.

A milliarcsecond is a thousandth of an arcsecond.

"A milliarcsecond is a thousandth of an arcsecond."

Thanks, Chris.
I was going too much by sound and too little by brain.
But maybe, at 75, it's forgivable?
Peace and health,
Phil G

I'm willing to bet the visual patterns look like that just because that's how big the pixels are in our interferometry calculation. It looks to be 3 pixels wide and 7 pixels tall. The pixel grid is angled about 15 degrees from vertical. Probably because that's the direction the telescopes were tipped.