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Author Topic: Do stars appear larger than their mass suggests?  (Read 1608 times)

Offline jeffreyH

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Do stars appear larger than their mass suggests?
« on: 09/10/2013 15:42:35 »
when viewing stars do we see a larger radius than we would see for a cooler object and how does this relate to temperature if it occurs?
« Last Edit: 09/10/2013 23:53:16 by chris »


 

Offline Phractality

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Re: Do stars appear larger than their mass suggests
« Reply #1 on: 09/10/2013 17:24:35 »
Denser stars tend to be hotter and brighter. When they run out of fuel, they cool and expand to become red giants. Eventually, the red giant collapses under its own weight and goes nova.
 

Offline jeffreyH

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Re: Do stars appear larger than their mass suggests
« Reply #2 on: 10/10/2013 01:08:27 »
Denser stars tend to be hotter and brighter. When they run out of fuel, they cool and expand to become red giants. Eventually, the red giant collapses under its own weight and goes nova.

Thanks for the reply. My main point was about the apparent size of what would be considered a normal star as opposed to a large planet. This would be before any collapse.
 

Offline Phractality

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Re: Do stars appear larger than their mass suggests
« Reply #3 on: 10/10/2013 03:04:00 »
Denser stars tend to be hotter and brighter. When they run out of fuel, they cool and expand to become red giants. Eventually, the red giant collapses under its own weight and goes nova.

Thanks for the reply. My main point was about the apparent size of what would be considered a normal star as opposed to a large planet. This would be before any collapse.
For a star to have an apparent size, you have to be able to resolve it to more than one pixel in a telescope. Only a handful of stars, notably Betelgeuse, are big enough and close enough for that. How else would you determine the apparent size of a star?
 

If you are talking about gravitational bending of light as it escapes from the star's gravity well, I think the lensing effect would be negligible, except in the case of a black hole or perhaps a neutron star. Any way, that is accounted for in general relativity.
 

Offline jeffreyH

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Re: Do stars appear larger than their mass suggests
« Reply #4 on: 10/10/2013 03:34:38 »
Denser stars tend to be hotter and brighter. When they run out of fuel, they cool and expand to become red giants. Eventually, the red giant collapses under its own weight and goes nova.

Thanks for the reply. My main point was about the apparent size of what would be considered a normal star as opposed to a large planet. This would be before any collapse.
For a star to have an apparent size, you have to be able to resolve it to more than one pixel in a telescope. Only a handful of stars, notably Betelgeuse, are big enough and close enough for that. How else would you determine the apparent size of a star?
 

If you are talking about gravitational bending of light as it escapes from the star's gravity well, I think the lensing effect would be negligible, except in the case of a black hole or perhaps a neutron star. Any way, that is accounted for in general relativity.

Point taken. Thanks for answering. You have given me information I had not considered.
 

Offline CliffordK

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Re: Do stars appear larger than their mass suggests?
« Reply #5 on: 10/10/2013 07:53:31 »
I have always found it interesting the lack of overlap between the lists of the largest known stars, and the most massive known stars

UY Scuti has an estimated radius of 1,708 suns, or 7.9 AU, but merely 32 x the mass of our sun.

It is so large that if it was in our solar system, it would swallow up the Sun, Mercury, Venus, Earth, Mars, Jupiter, and reach more than 80% of the distance to Saturn.

On the other hand, the most massive star listed, R136a1, is nearly 10x as massive, listed at 265 x the mass of our sun, but only 35.4 x the radius of our sun, not even big enough to reach Mercury.
 

Offline evan_au

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Re: Do stars appear larger than their mass suggests?
« Reply #6 on: 10/10/2013 10:59:14 »
When looking at stars with the naked eye, or with a telescope, the disk of the star at the distance of the Earth would cover only 1 rod cell in your eye.
However, the Earth's atmosphere distorts the position of the star so that it covers a wider area of your retina.
Perhaps a brighter star will create a detectable glow on a greater area of your retina, making it seem larger and closer, compared to the fainter and more-distant-seeming background stars.
Not all stars have the same intrinsic brightness - the apparent brightness (apparent magnitude) is a combination of the star's actual brightness (absolute magnitude) and its distance.
 

Offline jeffreyH

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Re: Do stars appear larger than their mass suggests?
« Reply #7 on: 10/10/2013 12:16:16 »
I have always found it interesting the lack of overlap between the lists of the largest known stars, and the most massive known stars

UY Scuti has an estimated radius of 1,708 suns, or 7.9 AU, but merely 32 x the mass of our sun.

It is so large that if it was in our solar system, it would swallow up the Sun, Mercury, Venus, Earth, Mars, Jupiter, and reach more than 80% of the distance to Saturn.

On the other hand, the most massive star listed, R136a1, is nearly 10x as massive, listed at 265 x the mass of our sun, but only 35.4 x the radius of our sun, not even big enough to reach Mercury.

What exactly causes the differences I wonder? That is intriguing.
 

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Re: Do stars appear larger than their mass suggests?
« Reply #7 on: 10/10/2013 12:16:16 »

 

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