APOD: Star Size Comparisons (2011 Feb 22)

[APOD Robot]

Star Size Comparisons

Explanation: How big is our Sun compared to other stars? In a dramatic and popular video featured on YouTube, the relative sizes of planets and stars are shown from smallest to largest. The above video starts with Earth's Moon and progresses through increasingly larger planets in our Solar System. Next, the Sun is shown along as compared to many of the brighter stars in our neighborhood of the Milky Way Galaxy. Finally, some of the largest stars known spin into view. Note that the true sizes of most stars outside of the Sun and Betelgeuse are not known by direct observation, but rather inferred by measurements of their perceived brightness, temperature, and distance. Although an inspiring learning tool that is mostly accurate, APOD readers are encouraged to complete the learning experience -- and possibly help make future versions more accurate -- by pointing out slight inaccuracies in the video.

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[Guest]

How much convection happens in supergiants?...I thought they had convective interiors rather than surfaces in which case the video depiction might be a little off...

[_Phile]

The Moon and planets should be depicted as rotating prograde rather than retrograde (save Venus--and Uranus had it appeared). It would be nice if their relative rotational speeds could also be represented.

[eps0mu0]

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.

[Beyond]

Well, one inaccuracy is that you don't go anywhere when you click on--->pointing out slight inaccuracies

[Fictionize]

technically all points in the universe are the center of the universe right? an observer at any point would see the entire cosmos running away in every direction. the end should have something to say about your importance in the universe or your influence on it, not your centeredness in it.

[RBolzen]

We are the center of the observable univers, arent we.

[bystander]

Well, one inaccuracy is that you don't go anywhere when you click on--->pointing out slight inaccuracies

If you clicked on it from the APOD, you would come here.

[bactame]

Seems like size is the theme so sticking to theme should have shown a star or two smaller than the sun since the sun has a respectable size and is relatively large on a count basis along the main sequence.

[Uh Huh]

Rbolzen beat me to the punch. That last bit of the video is 100% incorrect. A point in space even one foot from your head is always in the past. Not the center. Check Einstein. The universal center exists everywhere, it's what's called the "here" and "now" inside our own heads (neither has any basis in reality but refer to a single process of becoming).

[blastoff]

Arcturus is considered a K-type "orange" giant. at 4300 K its much hotter than "red" giants which burn at under 3700 K.
to J.F. , the video states that there are several hundred billion stars forming our galaxy, not forming in our galaxy.

[Chris Peterson]

We are the center of the observable univers, arent we.

Not just the observable universe, but the entire Universe as well.

[Kim Nilsson]

Wait....you mean the EARTH is not the Center of the Universe???

[drewSearing]

WHERE is Uranus? It's mysteriously gone missing!?

[Ann]

Arcturus is considered a K-type "orange" giant. at 4300 K its much hotter than "red" giants which burn at under 3700 K.
to J.F. , the video states that there are several hundred billion stars forming our galaxy, not forming in our galaxy.

I'm not happy with the star colors here. The so-called "red" stars are shown as very unrealistically red, because as anyone knows who observes stars through a telescope, "red" giants are really yellow-orange in color. Our own Sun, which would look white to us in space (since daylight, which is sunlight, is white) is shown as deep dark yellow-orange here! Last time I looked at W Orionis, which belongs to the reddest class of stars apart from the brown dwarfs, it was not as deeply orange as the Sun looks here!

Also, if you want hyper-correctness, there is no way that a cool hypergiant like VY Canis Majoris can be spherical in shape. Not even Betelgeuse, which is not a hypergiant, is spherical in shape! Also VY Canis Majoris wouldn't have a well-defined surface, not in the sense that the Sun has a well-defined visual "edge". So you couldn't make a nice circular flight above the surface of this star, even if you could miraculosuly protect yourself from the star's heat, stellar wind, convectional hiccups, sudden local outbursts and other dangers. (If you flew at 900 kilometers per hour above this star, wouldn't the gravity of the star overwhelm the relatively puny velocity of the airplane and simply suck you in?)

Click to view full size image

This is what the antics of Betelgeuse are supposed to look like, and that star isn't even a hypergiant!

It is really true that several hundred billion stars make up our galaxy, the Milky Way. But our galaxy's glory days of star formation are over, and these days only about ten stars form in the Milky Way every year.

Ann

[HotBlue]

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?

[Alnilam]

I'm french and I hope you will excuse my poor English.

Few comments about this video :

- why all planets except Uranus ?

- why all planets with their atmosphere and Venus without ? (giant planets would be very strange without their huge atmosphere !!!)

- almost all planets rotate in the wrong way ! (see previous comments)

- our sun is not so orange ! (this "color" is popular through "false color" images from SOHO/SDO, in fact it's almost white, lightly yellow, from a human visual point of view)

- all stars will be very very much brighter if seen at a so "close" point of view ! (incoherent with the planets, except if a very selective filter is used, but the video don't suggest that)

- the size of the biggest star seems to be under discussion yet : from 1.400 to 2.100 sun diameter ? The video gives the upper estimation.

If the lower estimation is good, other stars could be bigger : VV Cephei A, V354 Cephei, KW Sagitarii, KY Cygni, Mu Cephei. And what about "objects" like IRS5 ?

[Alnilam]

Ooouuupppsss ! One more :

If planets rotations could be corrected, please use their proper own speed (ex.: Moon about 27 days, Mars and Earth about 1 day, Jupiter and Saturn about 10 hours, and Sun about 25 days).

[Mongeeses]

The video is wrong, I am fairly positive that indeed I AM the center of the universe.

[Ann]

Alnilam, what a great username! The real Alnilam is a magnificent blue supergiant!



That's Alnilam in the center!

Now to some of your questions:

- why all planets except Uranus ?

If you ask me, I think they thought Uranus was unnecessary and boring. Uranus is very similar in size to Neptune, and it is paler in color and visually featureless, so it looks more boring than Neptune.

- why all planets with their atmosphere and Venus without ? (giant planets would be very strange without their huge atmosphere !!!)

I think the reason for the denuding of Venus is the same as the reason for the exclusion of Uranus - they thought Venus with its atmosphere looks boring! Like Uranus, Venus has featureless cloudtops. Also, Venus is visually white, and I think these guys prefer red and orange over white. By divesting Venus of its atmosphere, they could show the planet's rocky surface features and color them orange, too!

- almost all planets rotate in the wrong way ! (see previous comments)

I didn't notice that, and I wouldn't pay attention to that, but I believe you.

- our sun is not so orange ! (this "color" is popular through "false color" images from SOHO/SDO, in fact it's almost white, lightly yellow, from a human visual point of view)

I couldn't agree more!!!

- all stars will be very very much brighter if seen at a so "close" point of view ! (incoherent with the planets, except if a very selective filter is used, but the video don't suggest that)

Artistic licence, I suppose. It would be hard to show off star colors if you attempted to even approach the true brightness of stars here - and of course, you would blind anyone who stared at your video and saw all those blindingly bright stars pass by!

- the size of the biggest star seems to be under discussion yet : from 1.400 to 2.100 sun diameter ? The video gives the upper estimation.

If the lower estimation is good, other stars could be bigger : VV Cephei A, V354 Cephei, KW Sagitarii, KY Cygni, Mu Cephei. And what about "objects" like IRS5 ?

Interesting. I didn't know that. I have a confession to make - since I am so strongly into things that can be at least potentially blue, the biggest stars don't interest me much, I'm afraid, since they can never be blue.

Well, good points, Alnilam!

Ann

[Rare White Ape]

If you flew at 900 kilometers per hour above this star, wouldn't the gravity of the star overwhelm the relatively puny velocity of the airplane and simply suck you in?

It probably would, but then again, doing that on Earth would do that, too, if not for its atmosphere and the ability of the aircraft's wings to generate lift.

This is off topic, but I think it would be fun to calculate the density of a star's atmosphere at certain altitudes and see what the diameter would be at the average pressure of Earth's atmosphere. And further to that, a 747 is designed to work most efficiently at the air pressures found at cruising altitude, so find that altitude on a massive star, see how large the diameter is at that altitude, then find out how long it would take to fly around it.

[Sandgirl]

I just wondered - it would take 1100 years for jet plane to orbit that giant star - but how long would it take the Earth to orbit that star if it was at the same distance from it as it is from our Sun?

[stem]

As a matter of fact you are the center of the Universe to the extent that the Universe has a center.

[DJ Joe Sixpack]

The "center of the universe" discussion revolves (if you'll pardon the expression) around articles of faith... That is, Einstein's semi-paradoxical axiom that "the center of the universe is wherever you happen to be" has a nicely perverse ring to it, and has some basis in quantum physics as well. However, I think it is also possible that at some point in time we many be able to map out the perceivable physical limits of our universe and -- setting aside the question of what is "outside" of "everything" -- determine what seems to be an objective center. (I think it is also likely that the patterns of cosmic expansion will prove to be multi-linear and chaotic, producing swirling patterns of interaction that are more like terrestrial weather patterns than like the traditional concept of one great outward push of mass and energy from a single, central "big bang" point. I think it is also highly likely that we will discover that there was more than one "big bang" and that the stuff of our universe emerged out of the ether from multiple locations, more or less simultaneously, when whatever conditions made this emergence possible. We'll see.

[Alnilam]

Hello Ann ! The pseudo "Alnilam" is realy inspired by the Orion belt : it's the first asterism that I noticed when observing the sky during winter evenings when I was walking from the school bus stop to my home (long, long, long ago...). And then I decided to learn more about stars and constellations... I use it on my favorite astronomy forum (one of the french most popular : astrosurf.com). So here too.

About the possibility of a plane flying near the surface of a big star : don't you think that it will simply fuse ?
(the surface temperature of VY CMa is estimated to be about 3.000 K)