APOD: Star Size Comparisons (2011 Feb 22)

[Chris Peterson]

I'm starting to be bugged by these "biggest star" announcements - and the resulting pictures and Youtube videos - because if you calculate the (overall) DENSITY of this star, it works out to between 0.000005 and 0.000010 kg/m^3. To get an idea how UN-star-like that is, consider that it is about the density of the Earth's atmosphere at 90 km altitude!!!

So what? By this measure, what size would you consider the Sun to be? After all, at its surface (the top of the photosphere, where it becomes opaque) the density is similarly low- a decent vacuum by most standards.

Despite the low densities at the outside of these very large stars, the dimensions given define the size of their opaque, radiative surfaces. That's a very reasonable way to define the size of a star.

[pabloften]

The identifying tent cards appear to get proportionately larger. However, there is no mention which label is at the center of their universe.

[Thorne]

First time here, but I've always wondered this:

So, if there were a solar system around one of these larger stars, how many planets could it (theoretically) hold? How many of those would fall into a Goldilocks zone?

[Regulus]

The question of quantitative size is not meaningful, since there are fundamental principles operating within stars that are universal to them all. What is important then is an appreciation of those principles. The geometry of stars and galaxies for instance, also makes itself known on a much smaller scale physically, i.e. a model of an atom looks similar to the model of a solar system. Therefore one can see that it's the geometry that is higher and therefore more important than the quantitative size of its expression.

[Chris Peterson]

...a model of an atom looks similar to the model of a solar system.

Actually, they look nothing alike at all. Not even remotely. So the comparison of scale falls apart in this case.

[starswarm magellan]

I didn't care for the techno music. I much prefer the music from the "Powers of Ten" APOD from Feb 1. Weirdly cool and freaky in a Gumby sort of way. Today's soundtrack is too busy to reflect the awe one feels or the vast emptyness that is most of observable space. They probably omitted Uranus because of it's bad reputation for harboring Klingons.

[Beyond]

Harboring Klingons The Federation wouldn't put up with Klingons being so close

[k_atl_pablo]

The sequence on the comparison between planets and between stars introduces visual errors in the respective sizes of these round objects since they all appear in perspestive. The viewer never sees any two objects whose centers are at the same distance from the viewer. Wide angle perspestives of objects at close distances also create visual "deformation".

[Wonder]

Nice video. I was only bugged by the center of universe comment at the end. The rest was great.

The video is about size, not rotation rate, color, atmosphere or actual rank.

Colors could be thought of as seen at the coolest wavelength (venus by S-band, earth by visible).
Rotation rates were all the same. I know what they were doing here: it makes the images pop out in 3-D. I used to do the same on a 80286 (oops, there goes my age!!)
Who cares if the biggest is really the biggest? It's the biggest SIZE we are considering likely for any star.

Oooo, how bout this: I think the universe is so HUGE, mainly just so that gravity on Earth will be as gentle as it is, relative to chemical bond strengths (look up VSL gravity). You are loved.

[CallMeSempronio]

Surely the centre of the Universe is the point at which the "Big Bang" went ... BANG! Has anyone attempted to estimate where in the observable universe this point might be?

With the prevailing "expanding universe" theory model, all possible points are the center of the universe and that means, yes, YOU ARE the center of the universe from your point of view.

Also, I think the "BANG" point is impossible to define because of the dark matter problem.

[jjgravity]

The last statement ofthe video is completely inaccurate. It states, "No, You are not the center of the universe!" Actually, everything is at the center of the universe. The universe began everywhere when it comes down to it. We can make the point that our star is at the center of the universe, as can a civilization 50,000,000 ly away from us about their star. Both are accurate.

[Laurelin]

I think the video used the words "form" (make-up) our galaxy, not "are forming," although since several have mentioned this, that syntax issue is definitely worth examining.

Yes the color a star appears depends on if you are observing from Earth, or from Space. Even the color of our own Sun is distorted due to our atmosphere and it should be more towards a whitish color (tinge of yellow). That being said, a LOT of factors go into how we interpret the color of a star--human vision being a big one (rods are not very sensitive to red), atmospheric interactions, atmospheric glow, the instrumentation involved (especially if filters or ccds are involved), the fact stars are point sources (again a factor of how our eyes work, along with the brain's interpretation--similar in concept to combining red, green, and blue light to get what is interpreted as white light). Looking at peak wavelengths and blackbody spectra, the star colors should be more extreme than they appear to us. What we see in a telescope is highly influenced by many factors, including as I stated our own eyes--they can't always be trusted 100%

Yes we are at the center of the observable Universe. And a hypothetical being on the other side of the universe would likewise see itself as the center of its observable universe. I think the key point is that we are all so small, on our own little "pale blue dot," and there's so much more than ourselves, our struggles, etc., although it's easy to forget sometimes.

How spherical an object is depends on several factors although given the dynamics, I agree that VY Canis Majoris would likely not be that spherical, and yes it would for multiple reasons be a bad idea to fly around it. I don't think the point of the video is that one should or could take a trip around that star or that it would be a very "round" trip (sorry for the pun). I think the point of the video is to give students and the general public a means/comparison to gain a perspective of size and scale on the Universe (hypothetical). I have a background in formal as well as informal education, from ages pre-school through adult, and especially when it comes to dealing with sizes that are hard to comprehend (either the really large or the really small), these types of metaphors and visuals can go SO far. One example people get a kick out of is the idea that space is closer to us than the distance from San Francisco to Las Angeles. Yes we can't drive to space, but IF we could, it would take less time than the drive between SF and LA (at the same constant speed of course)! I think what's important to consider is in what context will this video be used? If it's designed as an educational tool for say middle/high school students, yes you want them to learn accurate information; at the same time, I'm happy if they can get the basics down, I can guarantee that only in certain situations and with select student groups would the star's shape be a point worth focusing on. It could make for an interesting mathematics word-problem, or physics discussion. But I think if we get too caught up in details (however fascinating and important they might be), the bigger picture could get lost. That's just my take though--this is the timeless debate in science education--what to focus on, not focus on, to go knee deep on one topic or inch deep on many...it's a challenge and a balance.

VY Canis Majoris definitely does not really have a photosphere like our sun, and it's a pretty messy system having blown off much of its outer layers. In fact while it's the largest (volume/size wise) it's not the most massive having expelled much of its mass through stellar winds as a result of the increase in internal pressure. The problem I do see with comparing main-sequence and giant stars is where the size factor comes from--due to "bloating" from fusion processes beyond H-fusion (increasing volume), or due to mass. Many blue giants and supergiants would outweight red giants, but may take up less space depending on their mass and where they are in their life cycles. That said, again--if the point is to give students/the general public a perspective of size and our place in the universe, than that might not matter too much.

Overall I think the video could make a great educational tool and really puts size into perspective, and when teaching about size, distance and scale in relation to the universe, I would certainly utilize it.

[Laurelin]

Also, I agree that the music is not the best choice. Music more along the lines of those by composers such as Hans Zimmer or Steve Jablonsky perhaps?

[TimeLord]

First time here, but I've always wondered this:

So, if there were a solar system around one of these larger stars, how many planets could it (theoretically) hold? How many of those would fall into a Goldilocks zone?

Only problem with this would be that a high mass star would not last long enough for a full planetary system to form. That would take about 100 MYears. A star of just 10M(sun) would only last 10MYears. These massive stars shown in the video would last even a much shorter time. But as to a theoretical "goldilocks zone", that would depend on the effective temp and actual size, as well as how stable the star is.

[xcjorr]

Actually, everything is at the center of the universe. The universe began everywhere when it comes down to it.

Wouldn't that then mean there is a center to the center of the universe - being that the universe was once the crumpled up 'center' itself? Or, is this where the fourth dimension of time comes into play?

(Dare I raise the spectre of additional spatial or time dimensions curled up in the universe?...)

[Chris Peterson]

Wouldn't that then mean there is a center to the center of the universe - being that the universe was once the crumpled up 'center' itself? Or, is this where the fourth dimension of time comes into play?

Exactly. The center of the Universe is at t=0.

[Duane Williams]

The sequence appears to move away from the viewer, rather than slide right to left along a line perpendicular to the viewer. It gets worse as the video progress and objects get much larger. As a result, one doesn't get a good sense of the relative sizes of adjacent bodies, since the larger one on the right can simply be seen as being closer to the viewer and the ones on the left farther away. The apparent size difference may be interpreted as being a result of objects of similar size being displayed in perspective, rather than as a result of real size differences.

[morn1415]

My comment:
Towards the end, the text states that the last star (the hypergiant) is only one of many hundreds of millions of stars forming in our galaxy. My interpretation of "to form" would interpret this statement as many hundreds of millions of stars are in the process of being created. The author undoubtedly meant to say that the star is one of a many hundreds of millions of stars in our galaxy.
Perhaps there was a slight problem with translation. For example, if the creator of the video was a German speaker, then the German word "formen" might have been mis-translated with "to form".
Regards,
J.F.

Indeed I am. I was not aware of this. Probably I meant ...stars "shaping" our galaxy. Regards, C.U.

[Uh Huh]

Wouldn't that then mean there is a center to the center of the universe - being that the universe was once the crumpled up 'center' itself? Or, is this where the fourth dimension of time comes into play?

Exactly. The center of the Universe is at t=0.

Yep. Space-time is a process, but say the word expanding to most people and they immediately get balloon imagery in their heads. The "closed universe" theory is another symptom of this stinkin 3d thinking. Our universe is becoming not expanding, and as close as we can get to the center of this becoming is inside each of our heads.

[mollwollfumble]

Don't you just hate this image of Venus!

It's the false colour image derived from the Magellan spacecraft radar reflection results.
It in no way would resemble what Venus would look like if its atmosphere were stripped off.
Please stick to what Venus looks like in some wavelength of the electromagnetic spectrum, such as U, V or I bands, and not this computer generated travesty.

[BMAONE23]

They might have chosen the color for the surface of the planet by the only true (falsly colored) surface images we have

Click to view full size image

source
a few other false color images from the Venera 13 Probe
http://rst.gsfc.nasa.gov/Sect19/Sect19_7.html
http://www.strykfoto.org/venera.htm

[Lorenzo]

Where's Eta Carina?

[edlb]

The scale effect gets lost as you move up to the larger stars. I never could have guessed the size of the earth from reference to the imaginary center line shown in the video. Maybe there's a better way to provide a frame of reference so it's easier tell the change in scale from beginning to end rather than from one object to the next.

[Dmpalmer]

In order to expand the FOV enough to go from The Moon to the largest stars, you really have to pull back the camera (since zoom lenses with that much range are beyond the budget of an educational film.)

In fact, you have to pull back faster than the speed of light, so at any given time you are actually looking at light that is from earlier than the previous frame. (You actually use a Matrix bullet-time style multicam instead of actually loading a single camera on an FTL starship; that way you don't have to unduly break the laws of physics.) Therefore, the Earth only appears to be rotating the wrong direction.

[Javier]

2.800.000.000 km of diameter are 61 trillion km of circunference, flying at 900 km/h, ¿and only in 1100 years?