# How do we know the size of these suns?

so ive been reading through these threads and have realized most of you are really ****in smart. REALLY SMART. im just an average joe so this might be a dumb question but i was watching this video on youtube:

and kept wondering how do they know? what way did they figure a sun can get THAT BIG?

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anybody? there must be some equation to find out the size of them dosnt anybody know?

marcus
Gold Member
Dearly Missed
not everybody here feels very smart, some just love astronomy and enjoy learning and talking about it

many here could give you an introduction to how astronomers figure out the masses and sizes of stars. I dont have time to watch that Youtube, but I will tell you part of the story.

you can tell the TEMPERATURE of a glowing surface by its COLOR (how red versus how blue) so they measure the surface temp of stars essentially by putting thru a prism and separating out the colors and studying the spectrum (rainbow)

a certain LAW (which can be tested in lab) tells the WATTS OF POWER PER SQUARE METER output of a glowing surface just from the temperature. If you have a square meter of hot iron or ceramic and you tell me the temp, then I can predict the watts of light coming off fairly accurately. It is a beautiful law going back to 1870-1880 if I remember right, one of the most beautiful in all physics. A theorist name Boltzmann and an experimentalist Stefan discovered it. The surface does not have to be solid. It can be the surface of a star, that works too.

with a telescope and light-meter you can TELL THE WATTAGE OF A STAR

If you know the total watts output of a star and you also know the watts per square meter of surface area then you can easily calculate the total surface area.

If you know the surface area you can quickly find the radius because the surface area of a sphere of radius R is 4 pi R2

So finding the radius of a star from what you can tell about it with a telescope comes down mostly to High School Algebra (and a couple of simple laws that can be verified in the laboratory)

What I told you works fairly generally but there are probably some special cases where other methods to find the size are appropriate. I didn' watch your Youtube so I don't know what stars in particular they may be talking about. hope this helps some.

I dont have time to watch that Youtube, but I will tell you part of the story.

hope this helps some.
hahaha writing that probably took longer than the video itself. it was just a video of the planets and some stars in scale and some douche was on there saying how scientists dont know anything and that theyre just random guesses on the size so thank you for answering my question so thourghly so i can stop his ignorance by informing him.

Chronos
Gold Member
Astronomy is an imprecise science, by the standards of earthbound sciences. Mass calculations of binary systems are fairly precise. A variety of disciplines, including spectral imaging and nucleosynthesis models, are used to approximate masses of solitary stars. The SB model, as noted by marcus, also figures prominently. Here is one example using interferometry:
http://arxiv.org/abs/astro-ph/0411054

marcus
Gold Member
Dearly Missed
... so i can stop his ignorance by informing him.
if the guy is determined to be a jerk and scoff at over a century of careful painstaking work by dedicated men and women, then what he should target is not the estimates of SIZE of stars but the estimates of their DISTANCE

the reason is that the process of building up the astronomical distance scale is complicated and can't be summed up in a few sentences---it involves measuring different ways and checking one against the other, and generations of detective work, analyzing clusters of stars to learn the relation of temperature to luminosity
and discovering special types of stars which pulsate regularly according to their brightness
constructing the distance scale has been a great human achievement and it is still going on.

everything else in astronomy depends on the distance scale, so if an antiscience person wanted to attack the creds of astronomers I think that would be the natural place to attack. It is vulnerable in the sense that the building up of ability to tell distance is a long fascinating story. If someone starts shouting that it is all wrong it would be difficult to counter because you'd have to lead him---and whoever else had their doubts raised---patiently thru many steps to give an honest account of it.

I should have pointed out that in the short account I gave you of how one can tell the size of a star, there is a point where you need to know the distance (see, it's basic ).
That is where you use a telescope and light-meter to tell the WATTAGE of the star.

the amount of light you get falls off as the square of distance, so if you know the distance you can adjust for it and calculate how much light you would be getting if you were closer. The telescope is only sampling a tiny sector of the whole output---but knowing the distance and the wattage of that tiny sector lets you figure the total.

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