How dense is a blue star?

Answer 1

A blue star has a spectral class of O. It is one of the largest types of star on the main sequence.

As the colour is based on the surface temperature, and not on size, there is no definitive size.

However, using Alntak (See related link) - a star in the constellation Orion - as an example of a blue star, we can expect a blue star to be around 20 times as big as our Sun. (See related question)

See related link for a size comparison with our Sun.

Answer 2

It's impossible to give an accurate answer, as all blue "Type O" stars are different.

However, as a general rule, the largest blue stars are greater than 100 solar radii (1 solar radii = our Sun).

On the Harvard Spectral Classification [See Link] blue stars are greater than 6 solar radii.

Answer 3

Generally speaking, the more massive a star is, the less dense it is.

One should be a little bit careful here though because the density of a star changes over it's lifetime. Our sun will eventually start burning hydrogen and become a red giant. It won't increase in Mass, but it will greatly increase in size and it's density will drop precipitously, so, at the end of a stars life, it's density can change a lot, either through collapse or through expansion.

And I believe, though don't know for sure, that young stars are somewhat less hot and more dense than older stars, but that's a guess. Stars lose mass over time through Coronal mass ejections and they tend to get hotter over their lifetimes too and more heat means more size.

But, rule of thumb, small stars are much more dense, big stars much less dense and the reason for this is simple. Big stars create more heat with pushes to expand the star. The hotter the star, the more the heat wins and gravity looses.

Our sun is about 333 thousand times as heavy as the earth and it's about 1.29 million times as voluminous, so the density of our sun is almost 4 times less than the earth, which makes our sun about 1.4 times as dense as water.

This chart below doesn't list density (Density isn't a very interesting statistic to most astronomers when discussing stars, though it's very interesting when discussing planets)

http://www.atlasoftheuniverse.com/startype.html

You can compare density by taking the mass and dividing it by the cube of the radius.

The B8 star, the smallest blue in that chart, which is 3.0 times the radius (27 times the volume) of our sun has only 3.8 times the mass, so it's 1/7th as dense as the sun or 1/5th the density of water.

The most dense stars (not counting neutron stars or white dwarfs) are the lightest/dimmest stars, where gravity is sufficiently stronger than the expansion from heat from the fusion process.

The least dense (not counting red super giants in their final stages) is the large blue giants.

Answer 4

Blue stars are very dense compared to our sun, with densities ranging from about 10 to 100 times greater. This high density is due to the intense gravitational forces at play in these massive and hot stars.