Largest Known Star: Introducing Myself and Cosmology

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The discussion introduces a new member interested in science and cosmology, highlighting a website that emphasizes the vastness of the universe. It explains that the largest known star that can be resolved is Betelgeuse, but emphasizes that most images of stars, aside from the Sun, are not actual telescope images. The conversation details how star sizes are estimated through temperature measurements based on color, with theories calibrated using direct measurements from eclipsing binaries and optical interferometry. It clarifies that while these methods provide estimates, they do not offer the detailed resolution implied by some images. Overall, the exchange enhances understanding of star measurement techniques in astrophysics.
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This is the first time I am posting here. I thought I whould show this website as a way to introduce myself. I'm interested in anything dealing with science, especially cosmology. I have always liked showing this website to everyone because it really shows how small we are.

http://www.techdo.com/images/largest-know-star.htm
 
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Do you know how you measure a stars size which you can't resolve in telescopes? The largets know star that can be resolved is Betelgeuse (as far as I remember)
 
It's important to note that all of the 'images' of the stars that are not the Sun in that slide show are not actual telescope images (apart from the Eta Carina nebula that is actually much bigger than that slide show suggests?). They look like either drawings or re-coloured images of the Sun to give an idea of what that star might look like. We cannot resolve any star other than the Sun with a single telescope (even the biggest and best we have) although we can get some information about sizes using interferometry with multiple optical telescopes. These techniques still don't give you the kind of detail implied in that slide show. Not being critical of it, it's quite cool, but just bear that in mind since it isn't clear from the images alone.

What is normally done to estimate the size of a star is to measure its temperature by measuring its colour (blue is hotter than yellow which is hotter than red). We then have theories that tell us based on the colour and type of star (which we can determine by other aspects of the spectrum) how big the Star must be. These theories have been checked and calibrated on stars where we can measure the size directly, either through eclipsing binaries or optical interferometry (google or ask for more info on any confusing or new terms!) so we have some confidence that they work for stars were we can't get such direct information.
 
I have done quite much astrophysics, but that was a time ago..

I know that the pictures in that perticular slide was'nt real ones, and I know of the temperatur-luminosity-size relations.

Maybe my question was not specific enough:-)

But you shed some light on how we can measure stars size directly, thanx!
 
https://en.wikipedia.org/wiki/Recombination_(cosmology) Was a matter density right after the decoupling low enough to consider the vacuum as the actual vacuum, and not the medium through which the light propagates with the speed lower than ##({\epsilon_0\mu_0})^{-1/2}##? I'm asking this in context of the calculation of the observable universe radius, where the time integral of the inverse of the scale factor is multiplied by the constant speed of light ##c##.
Why was the Hubble constant assumed to be decreasing and slowing down (decelerating) the expansion rate of the Universe, while at the same time Dark Energy is presumably accelerating the expansion? And to thicken the plot. recent news from NASA indicates that the Hubble constant is now increasing. Can you clarify this enigma? Also., if the Hubble constant eventually decreases, why is there a lower limit to its value?
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