APOD: Star Size Comparisons (2013 Jun 06)

[mwhidden]

Hugo,
By definition, a Black Hole has no size. It is a point in space with no diameter at all.
Perhaps a BH should be the focus of a similar size comparison video going smaller rather than larger.
This video ends at the Plank length - a BH should be smaller than that!
https://www.youtube.com/watch?v=MKtfBd8QnYE

John

I think you have confused a black hole with a singularity. A black hole is the diameter of the event horizon, which can be 10s of km up to hundreds of AU.

[anushkafromindia]

the shape of the universe has not been defined with proof till now and it is most probably an infinite universe that means that every point in the universe should be the center of it . Thus each person is the center of the Universe

[JohnOOOO]

Why did you skip Uranus?

[Kwells]

Philosophically, size doesn't matter. Suppose you contracted Naegleria fowleri ("Brain-eating amoeba" in the news of late). This is a single organism - vastly simpler and enormously smaller than your own body, and yet it is now infinitely significant to you if treatment is not successful. one can assume (if the little amoeba could think) the reverse would be true for the bug if treatment were successful. Asking a scientist what makes something more or less significant, philosophically, is like asking the same of a trainspotter: he is going to say "The more wheels it has, of course!".

Consider that if the human race had never heard of Betelgeuse or the andromeda galaxy -- as large as they are -- nothing whatsoever would be significantly different in anyone's everyday life. They huge objects are enourmously insignifcant.

[Jordan]

Planets and Sun are rotating in reverse.

Than watch the video upside down!

[Irishman]

Planets and Sun are rotating in reverse.

Either that or they're upside down

(well, in the apod video today, they also rotate correctly, if viewed "upside down" with the south pole at the "top", where top/bottom make no sense in space anyway)

I thought about that, but no, that doesn't work, because the planets show the surface features. Look at the Earth, it shows the continents. You can't just say you're looking at the South Pole at the top.

[AndyInSpain]

The larger stars are basically spheres of hot gas with a central core of dense matter.

That's a pretty good description of ALL stars really!

Relative relationship of multi star formations deserves attention: in the more exotic circumstances a sun-like star may be expected to orbit a field of blue tens of light years distant. Another viable probability is a sun-like star orbiting a blue supergiant at apprx 0.11 light years distance. What would be the mass-orbital identity of such a stellar association? Only a sophisticated computer programs capable of modeling existing solar systems can answer that question.

I'm sorry, but none of that makes any sense at all.

The solar system is an example of a G type star with a dim brown dwarf companion.

Really? The Sun has a companion? Where did you find that out? Have you told astronomers about this?

[Psnarf]

9.77 million hours = 407,000 days = 1115 years

...does that include leap years and round-off errors? The jet in the image would not be able to fly on the surface of the star, the distance of one circumnavigation might need to be adjusted to the altitude of the edge of the corona. Of course, the distance is imaginary. A jet engine needs a source of oxygen; given the combined power of all of its engines, the plane would fall toward the center. Ignoring the temperature, what kind of rocket engine would you need to reach orbital velocity at the surface? Have to take into account the mass of the star to calculate the gravitational force the rocket must counteract. Besides, it is a mind game that ignores those actual forces. It's not about the nail.
http://vimeo.com/66753575

I, myself, am rotating in the opposite direction.

[Chris Peterson]

the shape of the universe has not been defined with proof till now and it is most probably an infinite universe that means that every point in the universe should be the center of it . Thus each person is the center of the Universe

The fact that any 3D point in the Universe can be treated as its 3D center does not depend on the shape of the Universe, nor on whether or not it is infinite in size. It depends upon the fact that the Universe is four-dimensional.

[mwhidden]

Planets and Sun are rotating in reverse.

Either that or they're upside down

(well, in the apod video today, they also rotate correctly, if viewed "upside down" with the south pole at the "top", where top/bottom make no sense in space anyway)

I thought about that, but no, that doesn't work, because the planets show the surface features. Look at the Earth, it shows the continents. You can't just say you're looking at the South Pole at the top.

Perhaps it is time that is moving in reverse in the video.

[Sandy]

We can visualize that the two dimensional surface of a three dimensional sphere has no center. The universe can be thought of as a four dimensional sphere with a three dimensional surface. Similarly there is no center to the three dimensional surface of a four dimensional sphere.

[Guest]

Center of the universe or not: We, you and I, are the ones who conceive of the universe (several different ones so far); it doesnt conceive of us.
So while the universe may have conceived us-- U wanna b a mindless *? Or the Imagining Mind?

[geckzilla]

I have an interest in the way we visualize the larger stars. I know we've never resolved any star other than our sun into much more than a blurry blip 4 pixels across, so we may never have a direct image of one, but do we know enough about them to guess a little better at their appearance? For instance, would the edge of the star be so clearly defined or would it be kind of tattered like a cloud? Or maybe blurred like a fog settled close to the ground? Or perhaps there is a visible layering effect? I also wonder if we could look at the star a bit longer with our own eyes without some sort of filter.

Jim Kaler wrote about Betelgeuse:
The star is so big that its angular diameter is easily measured. Indeed it was the first to have such a measure, of 0.047 seconds of arc, from which we find a true radius of between 4.1 (compromise distance) and 4.6 (larger distance) AU, considerably greater.

However, the star is surrounded by a huge complex pattern of nested dust and gas shells, the result of aeons of mass loss, that extends nearly 20,000 AU away (Betelgeuse so far having lost over a solar mass). That, an extended atmosphere, and the pulsations make it difficult to locate an actual "surface" to tell just how large the star actually is.

Moreover, because of changes in gaseous transparency, the "size" of the star depends on the color of observation. Long-wave infrared measures give a vastly larger radius of up to 5 AU and greater, as big as the orbit of Jupiter, while shorter-wave infrared light gives as small as 3 AU. Moreover still, infrared measures reveal Betelgeuse to be shrinking (by some 15 percent over about 20 years), and other measures show that the star is not even round, but somewhat oval.

Sure, that's the same as any empirical evidence I've ever read about for Betelgeuse. I'm more interested in simulations of stars using the physics we understand about them to create a kind of visual equivalent if we were to actually get up close and personal with such a star and snap a photo. There's a lot of artistic renditions of giant stars similar to the ones in today's APOD video which show them as being nothing more than enlarged Suns with similar surface textures and a ridiculously emphasized change in color. I haven't studied astrophysics in much depth (I am on Ch. 5 of a physics review book we have laying around the house--circular motion and gravitation. Newb alert.) but this common perception of stars can't be very close to correct, especially when compared to the relatively small amount of empirical evidence we do have which certainly shows with one example that a very large star is visually nothing like our Sun.

Further searching brings me back to this video which is probably what first got me thinking about this.

Click to play embedded YouTube video.

[owlice]

Why did you skip Uranus?

See here:

Click to play embedded YouTube video.

[Chris Peterson]

Click to play embedded YouTube video.

I like the responses, and I like the way the author thinks. But there are a couple of important errors.

(1) "Agnostic" doesn't mean "I don't know". That's what "skeptic" means. Essentially all atheists are skeptics, as are many theists. Agnosticism means believing a question cannot be answered, that it asks something fundamentally unknowable. It does not lie on a line between atheism and theism. Everybody is either an atheist or a theist (with varying degrees of conviction, of course). Within those two categories, one may be agnostic or not.

(2) The author is confused about what it means to be at the center of the Universe, confounding this with the Cosmological Principle. While it is quite certain that every point is at the center of expansion (the Cosmological Principle), this is rather trivial compared with the realization that every point is also at the geometric center- something that follows from a more profound understanding of the dimensional structure of the Universe.

[Chris Peterson]

I have an interest in the way we visualize the larger stars. I know we've never resolved any star other than our sun into much more than a blurry blip 4 pixels across, so we may never have a direct image of one, but do we know enough about them to guess a little better at their appearance? For instance, would the edge of the star be so clearly defined or would it be kind of tattered like a cloud? Or maybe blurred like a fog settled close to the ground? Or perhaps there is a visible layering effect?

The surface details of stars quite different from the Sun are very speculative, and I'd suggest they are just guessed at in every simulation we've encountered. But the general appearance probably is not. While the interior of very large stars could be called a mild vacuum in any lab, these stars are very large, and the nature of gravitational binding and optical depth is such that all will have a photosphere that is shallow compared with the stellar diameter. In other words, all stars, regardless of size, should appear visually to be sharply defined, as if they had a solid surface. This is also why we see nebulas with sharply defined edges, despite being even more tenuous than the largest of stars.

I also wonder if we could look at the star a bit longer with our own eyes without some sort of filter.

How long we could look depends on the intensity of the star, and how large it is on our retina. For instance, we can safely look at the Sun for a few seconds with the naked eye, but through a telescope the retina is destroyed in a fraction of a second. The telescope doesn't make the Sun any brighter, it simply results in something of the same brightness covering more of the retina. It is quite certain that we could look at cooler stars for longer. And some large stars have low surface brightnesses, which affects how long we could look.

[rjeff]

I'm definitely naive about astrophotography but have noticed that many images do show various different sizes for stars (for example http://apod.nasa.gov/apod/ap130110.html). More knowledgeable members say that stellar sizes should not be directly resolvable. Why does it look like they can be? Is it an effect of the atmosphere or the optics or what?

[geckzilla]

The surface details of stars quite different from the Sun are very speculative, and I'd suggest they are just guessed at in every simulation we've encountered. But the general appearance probably is not. While the interior of very large stars could be called a mild vacuum in any lab, these stars are very large, and the nature of gravitational binding and optical depth is such that all will have a photosphere that is shallow compared with the stellar diameter. In other words, all stars, regardless of size, should appear visually to be sharply defined, as if they had a solid surface. This is also why we see nebulas with sharply defined edges, despite being even more tenuous than the largest of stars.

Ok, so visually sharp edges due to the ratio of the photosphere to the whole diameter. That makes sense. What about the overall shape appearing smooth and spherical? Does the same concept apply?

[WCF]

Excellent video however the planet Uranus appears to have been overlooked. Was this intentional for some reason?

[Freefall74]

Is my maths wrong, or should it take about 1500 years to fly around the supergiant, not 1100?
d=2.8 bnkm
circumference = pi(d) = 8.8bnkm
vel = 900km/h
therefore flight duration = 9.77 million hours = 1535 years.

Or maybe they're not going around the equator...

You math'd wrong. 1535 = 265 days.

[orin stepanek]

I always liked these starsize comparisons! Space is such a large place!

[Chris Peterson]

I'm definitely naive about astrophotography but have noticed that many images do show various different sizes for stars (for example http://apod.nasa.gov/apod/ap130110.html). More knowledgeable members say that stellar sizes should not be directly resolvable. Why does it look like they can be? Is it an effect of the atmosphere or the optics or what?

Geometrically, stars are effectively point sources, given that their subtended angle is much smaller than the resolution of our detectors. Optically, however, stars are not point sources- their light is spread out into a finite area by diffraction (basically, a kind of spreading out of the light rays by the aperture itself). This results in (usually) circular stars, that decrease in brightness radially. This makes bright stars appear larger than dim stars.

The actual size of stars in images is purely a function of their apparent brightness, and has nothing at all to do with their actual size or distance. Outside of a few special cases, no stars have been resolved, and any image that shows more than one star will not have any of them resolved.

[Case]

The lower lights on this vehicle are probably 1/3 or less in size compared to the main front lights, but they look bigger because of brightness.

---

I have noticed that many images do show various different sizes for stars (for example http://apod.nasa.gov/apod/ap130110.html).

You are seeing differences in brightness, not differences in angular size. When a small area of intense light in a photo gets saturated, it will ‘bleed’ onto the neighboring area, making the light source seem bigger than it really is.

[Chris Peterson]

Ok, so visually sharp edges due to the ratio of the photosphere to the whole diameter. That makes sense. What about the overall shape appearing smooth and spherical? Does the same concept apply?

I think that there are cases where rapidly rotating stars could be sufficiently oblate as to appear visually non-spherical. How "lumpy" the surface appears is probably speculative, given that we don't know much about how internal magnetic fields work in other stars (our knowledge isn't that great even for our own star!) My guess is that in visible light the surface of most stars would look pretty smooth, just as it does with the Sun. Only in narrow wavelength bands like Ha do we typically reduce the continuum enough to see what's really going on.

[owlice]

Excellent video however the planet Uranus appears to have been overlooked. Was this intentional for some reason?

Yes, it was intentional; the creator of the video didn't want Uranus. Please see the video provided in two other answers to this question. Thanks!