Questions tagged [size]

Questions regarding the physical dimensions of a celestial object.

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Does "Angular Diameter Turnaround Point" solve the Great Wall Problem?

According to Wikipedia, The Hercules–Corona Borealis Great Wall is the largest known structure in the observable universe, measuring approximately 10 billion light-years in length. But since it's ...
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1 vote
1 answer
170 views

How much energy to shorten a synodic month by about 1.56%?

Suppose the moon underwent a single, massive, large-object bombardment event. About what number (or range) of about what mean mass of objects could shorten the synodic month by about 1.5633%? (Assume ...
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9 votes
1 answer
218 views

How to determine the scale on astronomical images?

I've always been amazed at the many beautiful images of objects in the night sky. The Andromeda galaxy is a superb example. But it was only a few years ago that I discovered that Andromeda in those ...
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24 votes
4 answers
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Is there any planet bigger than a star?

Or a star smaller than a planet? Which star and planet would be an example of this?
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4 votes
6 answers
811 views

Logically, how can the universe be infinite in size?

Many people have told me that the “universe doesn’t care what you think” in my regards to it being infinite in size, and I know something that seems logical doesn’t mean anything when measured by ...
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2 votes
0 answers
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How big could a regular star in supergiant/hypergiant phase theoretically get, not counting quasi-stars?

For clarity, I'm not asking about the super-behemoths that are quasi-stars. I'm interested in knowing how large a regular star could theoretically get once it reached supergiant/hypergiant phase, ...
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2 votes
1 answer
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Largest projected images (such as shadows) observed

Serpens Nebula around the star HBC 672 (source: heic1819b) Which are the largest observed images projected onto interstellar bodies (such as nebulae) by means of electromagnetic radiation? By “largest”...
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Volume of the observable universe [duplicate]

What about the volume of the observable universe? Can we find it? And what is the result in cubic light years?
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3 answers
105 views

How big can a planet get, with respect to its star for it to sustain life? [closed]

Earth is 108 times smaller than the sun in terms of its diameter while Jupiter is 10 times. Assuming Jupiter was located such that it could sustain life, theoretically how big a planet can get with ...
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4 votes
1 answer
57 views

Long-lived supernova remnants?

We cannot see the supernova remnant of the star that triggered the formation of the Sun and the Solar System, as the remnant dispersed and became mixed into the interstellar medium. So I am wondering, ...
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3 votes
1 answer
139 views

How big and how far away would the moon be if earth was the size of Jupiter?

If the earth were the size of Jupiter, what would its 'moon' be in both relative size and distance? I did some quick figuring and think I roughly figured out that if placed at the position of the sun, ...
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1 vote
0 answers
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Is a star with 2/3 solar masses and 9/10 solar radii still in the main sequence?

As far as I know, K-type and G-type stars (among others) grow in radius as they age, eventually leading to red giant phases. My question is, would a star that is about 0.66 solar masses and 0.94 solar ...
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Is is possible for a main sequence star to have 0.77 solar masses yet only be 0.54 solar radii?

I know ordinarily a star's mass and radius tend to be proportional, so a 0.5 solar masses star tends to be about 0.5 solar radii and have a density of 4.82 g/cm³, 3.42 or about three and a half times ...
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2 votes
1 answer
229 views

What increases or decreases the mass and density of a fixed radius star?

Assuming a relatively even proportion of mass and radius, a 0.25 solar masses and radii star would have a density of 22.5003 g/cm³, or about 16 times our Sun's density. Keeping the radius of 0.25 ...
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2 answers
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What factors influence a star's temperature and density?

Assuming a relatively even proportion of mass and radius, a 0.25 solar mass and radius star would have a density of 22.5003 g/cm³, or about 16 times our Sun's density. However, there is obviously a ...
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6 votes
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Can a habitable planet be smaller than 0.58 Earth radii?

According to the below image, the lowest escape velocity a planet can have in order to still be able to retain water on its surface and have a temperature above freezing is 6.5 km/s minimum. With ...
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4 votes
1 answer
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How do pressure, temperature, and gravity affect a smaller body than Earth with the same bulk composition?

Earth has a density of about ${\rm 5.513 ~g/cm^3}$ with one Earth radius and one $g$. Venus, a very similar planet to Earth and likely with about the same bulk composition, has a density of about ${\...
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1 vote
1 answer
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What if you 'placed' the Chicxulub asteroid onto the Earth? [closed]

We all know what sort of devastating impact an asteroid could have if it collided with the Earth. But I'm curious to know what the effects would be if you removed the speed element from it, and were ...
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2 votes
1 answer
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Why doesn't star size distribution conform to space rock size distribution?

The number of space rocks is exponentially related to the size of the rocks. There are more small space rocks than big ones. Stars are most commonly the size of the sun, big and smaller stars are rare,...
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2 votes
1 answer
148 views

How do we know what the biggest star is?

Kurzgesagt claims that the largest (observable) star in the universe would be Stephenson 2-18 which is in line with Wikipedia: It is among the largest known stars, if not the largest, and one of the ...
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3 votes
1 answer
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Do more massive stars become larger or smaller white dwarfs?

I wonder whether e.g. stars more massive than the Sun (but below the mass where they'd go supernova) would become larger or smaller white dwarfs than a white dwarf Sun. Since Procyon B is larger than ...
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2 answers
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Why are event horizons described in Schwarzschild radii rather than diameters, and either way can they change equator to pole?

I never heard of Schwarzschild diameters, only of Schwarzschild radii. Why is it like this? Wouldn't it be better to portray the (possible) size of a black hole in a diameter? Either way, can we use ...
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2 votes
1 answer
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When was the diameter of Titan first measured?

Titan is enshrouded in a thick opaque cloud of methane : with a telescope, you can't see the moon's surface. Because of this, from a distance, you can only see the diameter of the moon + its ...
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Is a literal instance of Russell's teapot possible?

Much has been said about Russell's teapot, and I accept as obvious that a teapot would be too small to be detectable in an orbit around the Sun by any reasonable method today, or by any method that ...
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-4 votes
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Why is the Moon's volume so small compared to the Earth's?

The Moon has 27.3% of the Earth's radius/diameter but only 2% of the Earth's volume. I don't quite get it why the Moon's volume is that small despite having more than a quarter the 2-dimensional size ...
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3 votes
2 answers
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Best way to simulate star sizes to scale in celestial sphere

Using the Hipparcos catalogue, I am trying to create a celestial sphere. As all the stars are a fixed distance from the centre of this sphere, the only way to differentiate the distances and magnitude ...
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1 vote
1 answer
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can we calculate the velocity of galaxies at the edge of the observable universe?

I read this post Does the edge of the Universe travel faster than the speed of light? but there was no calculation, so I tried it thusly: The observable universe is 28.5 Gpc (gigaparsecs) = 28500 ...
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-2 votes
1 answer
116 views

What are the necessary steps required for calculating the size of a star? [closed]

Title pretty much says it all. Basically, what exact variables are used, and what steps need to be taken, in the calculation of the size of a star? Preferably, some example data for how a the size of ...
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3 votes
0 answers
55 views

Is there a minimum size for a galaxy to have spiral arms?

Is there a minimum size or mass for a galaxy to present spiral arms? I don't remember many small galaxies with spiral arms. Does the mechanism responsible for producing spiral arms no longer hold if ...
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8 votes
1 answer
3k views

Size and mass of comet ATLAS (C/2019 Y4)

I read some news about this new comet and some unreliable sources said it is twice as big as Jupiter. I tried to find a reliable estimation of its size or mass, but I couldn't find anything. Would you ...
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18 votes
1 answer
646 views

How small can a spiral galaxy be?

The smallest observed spiral galaxies I can find are NGC 2976 and NGC 4605, both with a diameter of 20 kly, but I don't have anything like an exhaustive source to search. I've also found a mention ...
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4 votes
1 answer
322 views

Volume of Galaxies

Pretty much awkward question though. I would like to ask how is the volume of a Galaxy X is measured? For example consider milky way, what are the techniques to measure the same? How accurate and ...
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1 vote
1 answer
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Different sizes of bow shocks

In this image of LL Ori bow shock, there is a smaller bow shock at the upper right. Is that second bow shock substantially smaller than the LL Ori bow shock, or is it just much further away? If ...
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6 votes
1 answer
457 views

What is the smallest diameter black hole that can be formed naturally?

I was watching the video I Put a 1mm Size Black Hole Next to Earth and This Happened - Universe Sandbox 2 discussing what would happen if a 1 millimeter black hole is orbiting the earth, and I began ...
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1 vote
0 answers
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Expansion of space - how do we end up with an unobservable region?

I've just been answering a question on this issue,in physics SE,and realised that my answer conceals a point I don't myself understand. I understand that The expansion of metric isn't affected by ...
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10 votes
3 answers
587 views

Existence of planets larger than their host star?

The mass region of objects between ~ 0.5 Jupiter masses and 80 Jupiter masses (gas giants through to brown dwarfs and red dwarfs) is typified by an almost flat relationship with object diameter. There ...
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1 vote
1 answer
292 views

If the whole observable Universe is as big as planet Earth, how big would Earth be?

If the whole observable Universe is as big as planet Earth, how big would Earth be?
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3 votes
1 answer
155 views

If the Milky Way galaxy is as big as planet earth, how big is Earth?

If the Milky Way galaxy is as big as planet earth, how big is Earth?
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11 votes
1 answer
211 views

Why haven't more captured small moons been found?

Shouldn't captured moons have the same size distribution as asteroids? And asteroids are more common the smaller size they are. Moons are likely captured if they are in highly inclined orbits, and ...
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1 vote
1 answer
65 views

Do celestial bodies actually appear larger along the horizon? [duplicate]

Whether it be the moon (especially when full) or tonight, as Mars is closer than it has been in decades, it appears that these bodies are larger when close to the horizon than overhead. Is this an ...
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2 votes
1 answer
166 views

Yet another Question about the expansion of universe

A galaxy 13 billion light-years distant is 94% of the way to the big bang. The universe was 800 million years old when the light was emitted. That galaxy today is traveling away from us at 94% of the ...
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3 votes
2 answers
1k views

Finding the radius of a star in arcsec

If i know the radius r of a star in cm, how can I convert it to arcseconds? For example, if I have a star with r = 3.18e13 cm, and distance to the star d = 220 parsecs, what is the relation to ...
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1 vote
1 answer
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Inconsistent online radius value for Proxima Centauri

I was looking up the radius of Proxima Centauri and got a funny result. Google says it's about 100,000 km, while WolframAlpha says its about 200,000 km. Are the error bars that big, or does someone ...
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1 vote
0 answers
310 views

How big is the Andromeda galaxy in the night sky? [duplicate]

If divide the distance to the sun by its diameter, I get the number 107.517 Likewise, if divide the distance to the moon by its diameter, I get the number 110.650 Both numbers are quite similar which ...
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2 votes
2 answers
785 views

How to Find the Viewing Size of a Star

The sizes of the stars in the picture below have more to do with their brigtness than the actual size of the star. I'm looking for a formula that relates the viewing size of a star with its ...
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0 votes
1 answer
376 views

Why could a gas planet not get much larger than Jupiter? [duplicate]

What is the maximum size for a gas giant?
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1 vote
3 answers
5k views

How big is one arcsecond at various distances?

I've been reading this article https://en.wikipedia.org/wiki/Celestial_coordinate_system and a question came to my mind. The minimum units are seconds ( ″ ). I picture an angle of 1 second as being ...
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1 vote
2 answers
531 views

Retrieving input effective radius from Sérsic profile

I'm playing around with Sérsic profiles, and would like to retrieve the effective (half-light) radius that I put in by integration of the profile. I'm not managing to, so perhaps someone can help me ...
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13 votes
7 answers
3k views

Relatable comparison of VY Canis Majoris to the Sun?

I was trying to describe how vast the largest known star is to someone and felt I wasn't quite able to relate the scale difference. I know it's roughly 1500 times larger than the sun. Anyone know of a ...
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  • 499
1 vote
1 answer
692 views

How do we get the angular size of a crater without lens?

Suppose I want to calculate the diameter of the crater. I apply the small angle formula $$diameter=\frac{\theta_{diameter}}{206265''}\times D$$ where $\theta_{diameter}$ is the angular size of the ...
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