# Tag Info

26

After some playing around with wolfram alpha and google my best comparison has been The sun compared to VY Canis Majoris is like a donut compared to the London Eye. The London Eye is about 120m in diameter, this divided by 1500 is about 8cm which is roughly the diameter of a ring donut.

22

On the banana scale, it's an ore freighter. Every physicist worth their salt knows that the most important scale in the galaxy is the banana. Now, your average banana is between 7-8 inches in length. Approximately the same shape are the ore freighters that go on the Great Lakes. They're called 1000 footers because, go figure, they're roughly 1000 feet long. ...

19

According to current knowledge, yes. If the gas cloud is too massive, the pressure of the radiation prevents the collapse and the star formation. The article Stars Have a Size Limit by Michael Schirber, it's about 150 Solar Masses. However, there's the Pistol Star, which is speculated to be 200 SM. In the article 'Das wechselhafte Leben der Sterne' by ...

18

It is true that a surprisingly large number of stars are smaller (and thus less massive) than the Sun. However, the stars that are bigger than the Sun are often much bigger. Look at this chart: Image courtesy of Wikipedia user Jcpag2012 under the Creative Commons Attribution-Share Alike 3.0 Unported license. Notice how small the Sun is compared to some of ...

16

Without checking the numbers in detail, according to Wikipedia, the volume of the observable universe is about $3.5\cdot 10^{80} \mbox{ m}^3$, and the volume of Earth is about $1.08321\cdot 10^{21} \mbox{ m}^3$. By dividing the two volumes we get a factor of $3.2\cdot 10^{59}$, or written as decimal number: The observable comoving volume of the universe is ...

14

Yes, there is a limit. Anything with a mass larger than about 13 times that of Jupiter would be called a brown dwarf (a failed star), though whether such an object would consist entirely of gas, or had a rocky/icy core as is probable for most giant planets, is not presently observable. Any larger than about 75 Jupiter masses and we would just call it a star. ...

14

The confusion comes from the difference between the nucleus and the coma. The nucleus is a small icy body, only a few km across. The coma is the cloud of gas and dust released from the nucleus as it warms up. With not much gravity, the coma spreads out into space, and it can be hard to say exactly where the edge of the coma lies, however, a coma "the size ...

13

What I mean is, even if it is capable of expanding, if everything originated at the big bang how can that space turn to be infinite in a finite time - the age of the universe. In the standard ΛCDM model of the Big Bang, the universe is infinite and has always been such. The Big Bang singularity happened everywhere, in the sense that far back enough in time, ...

12

A decent part of this answer is based on the introduction to Kroupa & Weidner (2005), though I’ve obviously gone into a lot more depth on all of the references. Our story starts, as do many concerning stellar astrophysics, with Sir Arthur Eddington. In his 1926 book, The Internal Constitution of the Stars, he derived the Eddington luminosity, the ...

9

I think the source of confusion between the two concepts - the Big Bang singularity and an infinite universe - is the misconception that the universe began as a finite expanse originally. This misconception easily arises from analogies using present-day logic and numbers that were not applicable in the early universe. For example, I've heard it said that ...

9

The vast majority of the particles in Saturn's rings are small, on the order of $\sim10^{-1}$ m or lower. The columnar number density, according to data from Voyager 1 and Earth-based observations, can be approximated as a function of particle radius by a power law for all particle radii $a$ in meters such that $0<a<1$, as can be seen on this log-log ...

9

How big is one arcsecond at various distances? An arcsecond is a small angle, 1/3600 of a degree or about 5 millionths of a radian ($4.85\times10^{-6}$). To estimate the size of something that appears 1 arcsecond across you can use the small angle approximation to trigonometry: Multiply the distance to the object by $4.85\times10^{-6}$ Examples: One ...

8

The expansion rate of space is not itself the reason that the radius $R_\mathrm{Uni}$ of the observable Universe is larger than 14 billion lightyears (Gly). Just the fact that space expands is the reason. If space did not expand, then $R_\mathrm{Uni}$ would be the expected 14 Gly, as this is the distance that light can travel in the 14 billion years (Gyr) ...

8

You assumption is wrong. Universe can( and is ) expand faster than the speed of light. The photon emitted towards our planet in the early universe, had to traverse a universe that is expanding. That photon experienced redshift, that means his wavelenght increased and frequency decreased. We measure that photon's wavelength, compare it to a photon that was ...

8

An airplane flying along the surface of the sun would take about 6.6 months to circle it once. The same airplane would take 787 years to complete one trip around VY Canis Majoris. Aircraft's speed: 900 km / h Sun's radius: 696,000 km One circle around the sun: 4,373,096.97 km Time for aircraft to complete this circle: 4858 hours ≈ 6.6 months VY Canis ...

7

Far away galaxies are receding from us faster than $c_{0}$ = 299 792 458 m/s, but it does not mean that they are breaking the speed of light limit. These galaxies would still measure the speed of light locally to be $c_{0}$ (assuming the speed of light and the laws of physics are the same throughout the universe), and they would never catch up and move ...

7

More massive stars have a more massive core and produce more massive white dwarfs. The relationship between the initial mass of the main sequence star and the final mass of the white dwarf is monotonic, but not linear. The Sun is expected to produce a white dwarf with a mass of around $0.5 M_\odot$, whilst a $8M_\odot$ star is expected to produce a carbon/...

6

It really depends what you mean by "rock". At the temperatures and pressures at the cores of stars (and at which nuclear fusion reactions are possible), "rocks" as I suspect you are thinking of, do not exist. Thermonuclear reactions do not occur because the gas is "flammable", they occur because the kinetic energies of the nuclei in the gas (at these ...

6

Well, they are different shapes, but we can get a rough answer by doing a ratio calculation based on their radii. Let $g$ be the radius of the galaxy, about $5\times 10^{20}\ \mathrm m$. Let $e$ be the radius of the Earth, about $6371\ \mathrm{km}$. Let $s$ be the radius of the scaled-down Earth. Then, $$\frac e g = \frac s e$$ So s = \frac{e^2} g = \...

6

An asteroid resting on Earth would be a mountain. Or, for smaller asteroids, a pile of gravel. Mountains are limited in altitude by the strength of stone to resist compression: a too tall mountain would sink down as the base crumbled and spread out. The limit on Earth is about 10 km. Besides the strength issue mountains are also floating ("isostasy"...

5

To expand on self.'s answer - Derek from Veritasium on Youtube explains the "(and is)" portion - ...This doesn't violate Einstein's theory of relativity since nothing is moving through space faster than light, it's just that space itself is expanding such that far away objects are receding rapidly from each other. ... Link to video on Youtube

5

It is known that the universe that we can see in our telescopes is less than the total universe. Since we cannot see what is beyond the visual edge, we cannot determine if the universe is infinite or finite.

5

Discovery of Titan, 1655: Unknown diameter. Dollfus, 1970: 4,850$\pm$300km (1). Measured by Filar micrometer (2) and diskmeter / double-image micrometer (3). (Apparently a summary of earlier measurements, currently trying to find print copy) NASA SP-340, 1974: Summary of above techniques, propose settling on 5,000km diameter until it can be measured by ...

5

Short Answer: In part Two of the long answer below, it is stated that a planetary mass object needs to have a mass of at least 0.1, 0.12, 0.23, or 0.25 Earth mass to be habitable. Worlds with those masses and with a radius of 0.58 Earth radius should have densities of 2.827, 3.392, 6.502, or 7.067 grams per cubic centimeter. Such densities are possible, ...

4

The first order theoretical limit on stellar size is from the Eddington Limit. As the star collapses it is balances by radiation pressure from fusion. However, the fusion rate scales strongly with density (which is why the most massive stars have extremely short lifetime) so if the star was massive enough, the radiation pressure would probably blow it apart. ...

4

There is an explanation for why rings flatten out here. The general mechanism is that particles collide, and gets a very uniform momentum. Thus, any set-up giving unusually thick rings is in essence "cheating". Here are some ways: Moons can cause spiral waves in the rings, giving them more of a structure in the z direction. The ones known in Saturn's rings ...

4

James K's answer is right on, and probably what you're after. I just wanted to mention an effect which comes into play if you look really, really far away: Because light moves at a finite speed, we see galaxies (and other things) as they were in the past. And because the Universe expands, everything was closer together in the past, so we see distant ...

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