APOD: Star Size Comparisons (2011 Feb 22)

[NoelC]

It would take you much longer than 1100 years to circle that star.
It would take you at least half that time to get through the TSA.

Ya, we wouldn't want something like a dirty bomb to be brought on board.

-Noel

[Belmont5]

Surely the "centre of the universe" is totally dependant upon there being a discernible boundary?

[trevor.ockenden]

I believe the most glaring inaccuracy is that it would take approximately 1,100,000 years to fly around this hypergiant at 900 km/hr given that the suns diameter is stated to be 2,800,000,000 km. This figure stood out as too small as soon as I saw it.

[Chris Peterson]

I believe the most glaring inaccuracy is that it would take approximately 1,100,000 years to fly around this hypergiant at 900 km/hr given that the suns diameter is stated to be 2,800,000,000 km. This figure stood out as too small as soon as I saw it.

You might want to try that calculation again.

[Guest]

my favourite video on my least favourite day... February 22nd which was the day i was in an earthquake that destroyed my home and everything

[Terry P-B]

I come to this a bit late (just working my way through the archives of this marvelous site), but i have basic issue with the video: size. In the video, Jupiter's diameter appears 14 times as great as Earth's, where in reality, the ratio is more like 11:1. The ratio of the Sun's diameter to Jupiter's is almost exactly correct--to within 1%, as near as I could measure it.

[e-jit]

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?

Well, at 1.4 Tm in radius (one tera-metre is almost a light hour), it's about ten times as big as Earth's orbit around the Sun - but you did say "distance from it" rather than orbital radius, so you're asking for the orbital period of a body about 1.5 Tm from the centre of VY Canis Majoris. (The Wikipedia entry for it says it's bigger than the orbit of Saturn; or, even accepting the lower estimates of its size that some use, bigger than the orbit of Mars.) It's between 30 and 40 solar masses, so using G.M = w^2.R^3 for our Sun of (turn/year)^2 (AU)^3, its is 30~40 times that and we're using an 11ish AU orbit, so its w^2 is (30~40).(turn/year)^2/(11ish)^3 = ((30~40)/11ish).(turn/(11ish years))^2, so its period is 11ish years divided by the square root of (30 to 40)/11ish, so (very) roughly seven years, if I managed to not goof any of the sums there.

Which, of course, means that an airplane (or, rather, the droplet of molten debris it'd rapidly become) going at 900 km/hr isn't actually a feasible object in that environment; that's below orbital speed, so it'd be losing to gravity. Then again, local gravitational field strength is about 1/500 of that at Earth's surface, so it wouldn't need much force to keep it up. Looking to aerodynamics to model that is fairly hopeless, though: the star's average density is 5 to 10 millionths that of air at STP, so I'm guessing it's tiny even by comparison to that of our stratosphere; and that's an average, so I must guess the surface is even thinner still. It'd be a tiny spec immersed in a magnetohydrodynamic maelstrom, whose matter aspect it'd barely notice; and I don't know enough to guesstimate the strength of the electromagnetic field, much less how that'd affect a droplet of dirty (there used to be a pilot, some furniture and some fuel in there) mostly non-ferrous (iron is too heavy for aircraft) metal.
But I think that's an unfair point on which to criticise the video: they wanted to convey how far the distance is, so used the fastest speed at which audience members are familiar with travelling to show how long it'd take to travel that distance; they could as easily say that a jet plane flying round our equator at 900 km/hr and refueling continuously in flight would need to keep doing so for 1115 years before it'd have clocked up a distance as big as VY Canis Majoris's circumference.

Not that I think that's a particularly good way to communicate the enormity of astronomical distances. I find the travel time of light to be more useful: one foot is a light nanosecond, a mile is several light microseconds, an air journey East or West that involves a one hour change in time-zone covers several light milliseconds (depending rather on latitude and the politics of time-zone boundaries), the moon is a light second and a quarter away, the Sun is eight and a bit light minutes away, each year the Earth circles the Sun along a path whose length is the distance light travels in 50 minutes and 20 seconds; VY Canis Majoris's diameter is a bit longer than the distance light travels in two and a half hours.
The human brain doesn't deal well with big numbers, but that progression leads from familiar distances matching small quantifiers, through intelligible (if big) distances matching intelligible (if small) times to cast astronomical distances in terms I hope can begin to be grasped.

As for the "centre of the universe", as usual, people misunderstand the idea of an expanding universe. If I insert a fine needle into a drop of soapy water and blow in some air, the drop becomes a bubble that expands and expands; distances within the universe are like distance measured along the surface of the bubble, getting bigger, yet no point in the universe is "the" point from which it is all expanding "away"; a small patch of dye in the surface will expand as the surface expands, outwards from the patch's centre, yet this is no more the centre than that of another small patch of dye elsewhere in the surface. The only centre the universe has is the whole of it at its beginning, before its parts started expanding away from one another.

We are, of course, at the centre of the observed universe: the inevitable consequence of observing it from here.