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Why is it that some low mass stars have diameters greater than Sol?

...John...

Any particular stars in mind, John?

I-am-not-an-astronomer but...
perhaps these are low mass, old stars which have left or are about to leave the main sequence. If a star is burning helium it seems to bloat up into a subgiant stage which may well be larger and less dense than the Sun.

Yeah, what stars are you referring to? Granted, there's a bit of variation in the radii of stars as it is (another star with one solar mass may not necessarily have exactly the same radius as Sol), but I can't think of any low-mass K or M V stars that are bigger then Sol.

I'll look forspecifics tomorrow, but some of the low mass K-types listed in the Internet Stellar Database (possibly Sigma Draconis being one of them) often have stellar radii up to 1.5 that of Sol.

Okay, here are some:

http://www.stellar-database.com/Scripts ... a+Draconis

http://www.stellar-database.com/Scripts ... silon+Indi

Perhaps calculations are off? As far as I know, neither of these stars are particulaily old.

...John...

John,

Unfortunately, answering your question requires more knowledge of astrophysics than most people here have. You probably can do the appropriate research into the topic almost as easily as I can, for example. (I say "almost" only because I have access to Cornell's library services, not because I know much about the topic.)

One place to start is the catalog cited in the entries you list. It specifies the methods used to determine the radius of each of the stars listed in it. Some are measured directly, using an interferometer, for example, while some are determined indirectly from the stars' intrinsic brightness and color.

See http://vizier.u-strasbg.fr/viz-bin/Cat?II/224#sRM2.3

This is becuase there are swelling and their mass and thier mass is spreading out. Basiclly cause it is out of fuel. With the excess helium still trying to breach the surface, it swells up, like balloon. The swelling pushes their mass outwards. Eventully it will expand to wide and nova.

I'm might not be a astrophysist, but I know astrophysics.

If a star is burning helium it seems to bloat up into a subgiant stage which may well be larger and less dense than the Sun

Stars don't really burn. They fuse. But they fuse Hydrogen into helium.

Hi Rocket Man...

The only poblem with your explanation is that some of these stars are quite young. Sigma Draconis, for example, is estimated to be about 3.3 billion years old. With a mass of only 0.89 that of Sol, this is far too young to have begun to leave the Main Sequence and evolve towards subgiant status.

...John...

I think one of the explanations would be that certain stars start "burning" (read - "fusing") heavier elements, not just hydrogen. You need a certain composition and pressure to engage heavy fusion, and the energy balance is different. A star doesn't "choke" or "gasp for hydrogen", it just starts a superior fusion cycle (rather than the usual proton-proton). Which yields more energy, which makes the star expand.

That's why some of the stars, even though they're young, if the composition is just right, might engage these cycles and swell up or become superhot.

I'm am young, so I'm just trying put what I know is an understandable way. So I'm might leave out some parts but I'm grateful for your corrects.

Dollan said:

The only poblem with your explanation is that some of these stars are quite young. Sigma Draconis, for example, is estimated to be about 3.3 billion years old. With a mass of only 0.89 that of Sol, this is far too young to have begun to leave the Main Sequence and evolve towards subgiant status.

True, There are young stars like that.
There was a show on the Science Channel about these young stars which it has exhuasted its fuel and start to "die". This might a simular star.

You can go to http://www.nasa.gov , http://jpl.nasa.gov , or try looking through this site http://www.space.com to further your knowledge of this

Interestingly, SolStation quotes a value of 0.79 solar radii for Sigma Draconis, and its source is more modern (2001) than the one the ISD uses, which is dated 1988... Similar for Epsilon Indi.

You really need to use professional catalogs rather than "popular" summaries as your references if you're trying to find accurate values. The most recent measurement is not necessarily the best.

In particular, the catalog to look at is "Catalogue of Stellar Diameters (CADARS)" (Pasinetti-Fracassini+ 2001). It lists the measurements of radii for 9733 stars. See http://cdsweb.u-strasbg.fr/cgi-bin/Cat?II/224

It lists 7 measurements of the radius of SIG Dra. They vary from 0.0013 to 0.0020 mas, equivalent to 0.70 to 0.85 solar radii.