APOD: Star Size Comparisons (2013 Jun 06)

[geckzilla]

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.

Interesting. So it's too hard to even guess at, then. Maybe the JWST can help. What's Betelgeuse going to resolve to with it, 16 pixels instead of 4? Heh.

[bill@wwheaton.com]

I guess we are agreed that VY Canis Major is spectacularly poorly represented, with its sharp, definite edge and perfectly round shape? Also the other red giants, and probably hypergiants too. How about having the smaller (huge) stars shown through the outer atmospheres of the largest ones? Then we could suggest the variable opacity with radius, like the Sun at sunset. This is obviously a difficult thing to get across, but we mustn't show very misleading stuff. Nice try, KEEP WORKING ON IT!!

[Ann]

Click to view full size image

Copyright © Michael Richmond

It's fun to consider how astronomers realized that some stars are gigantic in size. Astronomers have long known that blue stars have higher surface brightness than red stars, because blue stars emit a lot more visual light per surface unit than red stars. The spectra on the left show the energy distribution of stars with temperatures corresponding to spectral classes A0 (10,000K), late class G (5,000K) and late class M (3,000K). Note that the blue and the red curves cross at about 7,000 Angstroms, which is at the red end of the visible spectrum for humans.

Click to view full size image

Source: http://www.csupomona.edu/~pbsiegel/phy303/ch13.html

The higher surface brightness for blue than for red stars is an important reason why the main sequence looks the way it does, where stars are brighter the bluer (and therefore hotter) they are.

Astronomers soon learnt of small orange or red stars. 61 Cygni, a pair of main sequence orange K-type stars, were the first to have their parallax measured. The parallax of a star tells you the distance to it, and from the distance and apparent luminosity of the star you can calculate its absolute luminosity. The two stars of 61 Cygni have a combined luminosity of about 12% that of the Sun (according to my software), so they are dim bulbs. They are smaller than the Sun, but not that much smaller, so their puny luminosity is caused by their low temperatures and low surface brightness.

Well-known red dwarf Barnard's star turned out to be even redder, even cooler, even smaller and even dimmer than the two components of 61 Cygni. The visual luminosity of Barnard's star is only 0.0004 that of the Sun.

So how could it be, then, that some stars are as red as Barnard's star, yet shine brightly in the night sky, and some of them are so far away that yesterday's astronomers were absolutely unable to measure their parallaxes? How could these stars be so bright?

There was really only one answer - they had to be big, humongously big, even. Their temperatures are low and their surface brightness low, so in order to shine as brightly as they do even though they are very far away, they simply have to have a tremendously large "surface" from which they can emit all the light that they do produce.

And that's why astronomers know that some stars are just unbelievably big.

Ann

[Boomer12k]

I do not believe we are at the center of the Universe...We are probably in a Super Arm Structure in a side Vein of the that structure, and probably a side vein of a side vein, and so it is likely that we are not exactly at the center of the Universe....we are not the center of our own Local Group probably...when the Universe expanded we moved OUT, AWAY, from that central point....everything, as the Big Bang, or big expansion, or Big Whoosh, implies...would be moving outward from that central location....leaving the actual center a VOID....Like a Doughnut Hole....a Sphere with a void in the actual center....either that or a very large black hole...as a large amount of Mass would have been compressed in that explosion....

However....from my observations....those observation tell me I am at the center of the Universe...but that is an illusion of Ego, which is what all that "centrist" stuff was....I am not the only person in the Universe....and fortunately....I HAVE LOTS OF FRIENDS TOO!!!!!!!

As for the existence of God...as proposed by some other posts...I have my own examples of the Spirit...and helpful, friendly spirits...and consciousness...so, I do accept the concept of Spirituality, as for the Closed Religious Views of "God" a single Spiritual Creator, or a Pantheon of them, those are all Cultural if you look at them, and as they spread they evolved and influenced other cultures that they took over....they are all too limiting, and "Egocentric".....but that is not to say that the Spirit or Spirituality does not exist...maybe we ALL create the Universe...as we all have the power to create.

:---[===] *

[erhoads]

It would be nice to have this video include black holes.

Except most black holes would be smaller than the smallest sized stars, at least in size.

[Chris Peterson]

It would be nice to have this video include black holes.

Except most black holes would be smaller than the smallest sized stars, at least in size.

Indeed, you'd need to include several. The vast majority of black holes are a few tens of kilometers across, so much smaller than the starting point of the Moon. There are probably some intermediate ones around the size of the Earth, but they're rare. At the other end, we have the supermassive black holes found in the center of galaxies, which at their largest are the size of solar systems, 20 or 30 times larger than the largest star.

[elopez]

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.

To answer your question, we actually do have a very good idea what stars should like from stellar models. Around the edges the dominant effect is Limb Darkening http://en.wikipedia.org/wiki/Limb_darkening. This is due the fact that near the edge of a star we are looking at angle and therefore less deeply. As result, we see cooler and therefore dimmer parts of the star. You can see this effect yourself if you look at the Sun with eclipse glasses. This effect depends only weakly on stellar type with hotter stars having somewhat more darkening http://www.aanda.org/articles/aa/full_h ... 75-09.html.

The other main effect is spots. There can be a lot variation star to star and over time, but in general spots are bigger and more common around younger and lower mass stars due to their faster rotation and stronger magnetic fields.

[ol1bit]

They should add estimated Gravity as well. Yikes!

[copperjet]

Total Perspective Vortex. Infinitesimally small dot on an infinitesimally small dot.

[geckzilla]

. . . At the other end, we have the supermassive black holes found in the center of galaxies, which at their largest are the size of solar systems, 20 or 30 times larger than the largest star.

That is very surprising to me. I've only heard them described in terms of solar masses so I've only ever had a vague idea about the diameter of such black holes since I don't know how many solar masses can be crammed into each square meter.

[geckzilla]

To answer your question, we actually do have a very good idea what stars should like from stellar models. Around the edges the dominant effect is Limb Darkening http://en.wikipedia.org/wiki/Limb_darkening. This is due the fact that near the edge of a star we are looking at angle and therefore less deeply. As result, we see cooler and therefore dimmer parts of the star. You can see this effect yourself if you look at the Sun with eclipse glasses. This effect depends only weakly on stellar type with hotter stars having somewhat more darkening http://www.aanda.org/articles/aa/full_h ... 75-09.html.

The other main effect is spots. There can be a lot variation star to star and over time, but in general spots are bigger and more common around younger and lower mass stars due to their faster rotation and stronger magnetic fields.

Ok, so there's still limb darkening. That's an important even if rather mundane detail. Still, I'm interested more in what I can only define as texture and "lumpiness" (I'm not sure what a more appropriate word would be). I'm ever disappointed that it's unlikely we'll ever image something so far away directly. You can only guess about something so much, right? Not that I would be completely surprised if other stars looked similar to the Sun but I would be sad if there wasn't at least a little visual variation in them or a few exotic examples. Then again, Chris reminded me about the fast spinners which are visually oblate. Those must be quite a sight. Probably need better eclipse glasses.

[geoartist]

Who is to say this is not the center, just because it is a BIG univerese? How does anybody where the center of the universe is, in order to say it is not here? Not that I believe we are, but I don't think I have heard any theories, and there should be some, maybe. There are maps now in existence of galaxy groups and their locations relative to each other and in larger groups, some where on that map should be at least a directional arrow: "Center of Universe that way". If we know how galaxies move in this map, it should give us some indication of where they are moving from, which would be where the big bang was, right?

[geoartist]

Sorry, I needed to correct some spelling errors:

Who is to say this is not the center, just because it is a BIG universe? How does anybody know where the center of the universe is, in order to say it is not here? Not that I believe we are, but I don't think I have heard any theories, and there should be some, maybe. There are maps now in existence of galaxy groups and their locations relative to each other and in larger groups, some where on that map should be at least a directional arrow: "Center of Universe that way". If we know how galaxies move in this map, it should give us some indication of where they are moving from, which would be where the big bang was, right?

[rstevenson]

Still, I'm interested more in what I can only define as texture and "lumpiness" (I'm not sure what a more appropriate word would be).

One way to try to visualize the "surface" of various stars is to think in terms of gravity. The massive gravity of a star tends to smooth out the top of its atmosphere -- which is what the apparent surface of a star really is, kind of like cloud tops here on Earth. But this effect varies with the size and age of the star.

Small stars will have fairly smooth "surfaces," in part because their surfaces are near the core, where the mass is concentrated. Large old stars, having swollen up from radiation pressure on their atmospheres, will have somewhat more turbulent surfaces. The very largest stars regularly burp off some of their atmosphere, which can actually reach escape velocity, forming a nebula at the age where the star is turning into a white dwarf (which is just the core minus the atmosphere.) It's thought that the irregular nature of Betelgeuse is a result of such lumpiness happening close to the end of its giant stage.

So I imagine you can find all gradations of smoothness in stellar appearance somewhere out there, from small smooth cool ones to bloated and lumpy old giants. But it is sad that we won't likely see a picture of another star in any detail any time soon. Here's the best we've got so far, a Hubble pic of Betelgeuse, showing its uneven nature.

Betelgeuse (Hubble).jpg (3.06 KiB) Viewed 2858 times

Rob

[bayareajohn]

also, to answer the question in the video, how could one possibly ever imagine a diameter of 2.8 billion km, that is too easy to imagine;
given Saturn's orbit has a radius of 1.46 billion km, your 2.8 billion km diameter star would fit inside Saturn's orbit, how much easier than that could it get?

I was going to suggest showing the limit of the solar system. This is the furthest distance that humans have travelled away from their home.

Which humans have done that? I'd have expected them to be interviewed on the news...

[Guest]

I don't mean to be a too picky but the last statement in the video regarding the center of the Universe might be wrong. I say might because I am simply going by what I saw on my favourite youtube channel VSAUCE. The link posted will direct you to the specific clip of the video to which I refer. I hopeI (and as a result, Michael) am not wrong

http://www.youtube.com/watch?v=DsXsJtOQnTY

[odysseus]

Venus is rotating the same as all the other planets. However, it is the only one rotating the correct way!

On a more minor note, the planets do not rotate at the same speeds, as the animation suggests.

[odysseus]

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

Well, it doesn't rotate on a vertical axis. If the maker of the animation included it, it may have been a very glaring rotational error, as opposed to the fact that all planets shown (save Venus) are rotating the wrong way and all at similar speeds.

Another reason may be that for size comparison, Uranus and Neptune are not very different in size, relatively speaking.
http://en.allexperts.com/q/Astronomy-13 ... arison.htm

[ro-star]

. . . At the other end, we have the supermassive black holes found in the center of galaxies, which at their largest are the size of solar systems, 20 or 30 times larger than the largest star.

That is very surprising to me. I've only heard them described in terms of solar masses so I've only ever had a vague idea about the diameter of such black holes since I don't know how many solar masses can be crammed into each square meter.

this is correct, there is in fact an equation which relates the radius of a black hole to its minimum mass; so if you know the mass of a black hole you can calculate its radius, and vice versa; and to calculate the mass of an object in space, all you have to do is find another object orbiting the object you're interested in, and once you have the orbital period and distance from primary you can calculate the mass of the primary; and then, if the primary is a black hole, you calculate the radius of the primary given its mass; that is how we know that Sagittarius A* (SGR A-star), the main black hole (there are several) at the center of our galaxy is only a mere 4.3 million solar masses, because there are several stars orbiting around it, such as S2, which has an orbital speed exceeding 5000 km/sec or 1.6% lightspeed, compare that to 8 km/sec to stay in orbit around the Earth; so that gives the black hole a size of Uranus' orbit, and a density of 6.6 grams/cubic meter; the bigger it is, the less dense; and if it is too small, less than 3 solar masses, it even evaporates into radiation;

I agree I was wrong saying that the planets rotate in the right direction viewed upside down, that would be true only if they are featureless, but with continents shown correctly it is not; not to mention their axes are a bit inclined in various directions, especially Uranus which is inclined a lot pointing almost toward the sun;

[JohnD]

It would be nice to have this video include black holes.

Except most black holes would be smaller than the smallest sized stars, at least in size.

Indeed, you'd need to include several. The vast majority of black holes are a few tens of kilometers across, so much smaller than the starting point of the Moon. There are probably some intermediate ones around the size of the Earth, but they're rare. At the other end, we have the supermassive black holes found in the center of galaxies, which at their largest are the size of solar systems, 20 or 30 times larger than the largest star.

Is there an echo in here?
See page 1, fifth post from the bottom.

I said there, Chris, that a BH has no dimension at all, it is a point. Is that wrong?
The event horizon will have a diameter, but that would be hard to illustrate in the sequence that the video used, as it's black!

John

[Chris Peterson]

I said there, Chris, that a BH has no dimension at all, it is a point. Is that wrong?
The event horizon will have a diameter, but that would be hard to illustrate in the sequence that the video used, as it's black!

Yes, that is wrong. A black hole's size is defined by its event horizon. A black hole may have a singularity in its center (but that is by no means certain), and a singularity has no size. But a black hole and a singularity are different things.

I'm sure that a black hole can be visualized by placing it against a lighter background, or against a rich star field. We see black holes visualized this way all the time.

[stephen63]

I'm sure that a black hole can be visualized by placing it against a lighter background, or against a rich star field. We see black holes visualized this way all the time.

I would imagine that if you're close enough to see this, you're doomed?

[TheMiddaySun]

Am I the only one to recognize that the moon was rotating?? The moon does not rotate on its axis. We are presented with the same face of the moon at all times. Hence the saying..."Dark Side of the Moon"

[Chris Peterson]

I would imagine that if you're close enough to see this, you're doomed?

Not necessarily. That should be far enough away that tidal forces won't be extreme. So assuming you're either in a moderately circular orbit, or in a hyperbolic orbit (just passing by), there's every reason to think you'd be fine.

[neufer]

Am I the only one to recognize that the moon was rotating?? The moon does not rotate on its axis. We are presented with the same face of the moon at all times. Hence the saying..."Dark Side of the Moon"

There is no indication that any of these objects was being viewed from earth.

Rather they should be thought of being viewed from an orbiting spacecraft (or jet plane ).