# Size of VY Canismajoris and gravity

1. Apr 10, 2013

### SecretOfnumber

Smallest known star is OGLE-TR-122b and the biggest star in the universe is Canismajoris VY , now my question is :
what cause these two to be so different in shape? if the gravity is the planer for size, then how VY could become so huge in size before start collapsing?what volume of gas setup to create such a monster?!

Thanks

2. Apr 10, 2013

### Staff: Mentor

Stable stars are in an equilibrium of pressure (outwards) and gravity (inwards). Pressure is mainly radiation pressure and comes from the release of energy in the core (due to fusion). Heavier stars release more energy, so they tend to have a higher radiation pressure, pushing away the outer parts. The age and the composition of the star is relevant, too.

3. Apr 10, 2013

### Bandersnatch

Let's not get ahead of ourselves. It's one of the largest stars by volume known in our galaxy.
This is not to say there are no larger ones, even within Milky Way. E.g., https://en.wikipedia.org/wiki/NML_Cygni. These stars are barely ~4-6kly away, at the edge of our ability to measure properties of individual stars. Compare to the diameter of the Galaxy: 100kly.
And let's not even mention the whole universe.

As for the VY CMa, it's only ~18 times as massive as the Sun and 180 times as massive as OGLE-TR-122b.
Therefore, you need 180 times as much of relatively cold gas(by mass) to make the larger one than you need for the smaller one.

Let's compare the volumes of gas clouds:

$$ρ\frac{4}{3}πR_1^3=m_1$$
$$ρ\frac{4}{3}πR_2^3=m_2$$
ρ is the density of the gas cloud

substituting
m1=180m2

$$R_1^3=180R_2^3$$
$$R_1=180^\frac{1}{3}R_2$$
$$R_1~=5.6R_2$$

So you need just over five times larger(in radius) a gas cloud to produce CV CMa than OGLE-TR-122b.

4. Apr 10, 2013

### SecretOfnumber

Thanks Bandersnatch,

You R right 1 of the Biggest "Known" star!

The other thing I would love to know is What causes the different size of stars? how "gravity" decide to make different sizes ?(I suppose gravity should become to the action after she made the star "GR gravity" wasn't the gas homogenous after cooling down of the universe?

Cheers

Last edited: Apr 10, 2013
5. Apr 10, 2013

### Mordred

The size of stars is determined by the availability of gases

As far as gases being homogeneous in the early universe one needs to consider that matter such as gases will quickly start to condense. So that even distribution will quickly change.
I should also point out that the term homogeneous is a term that in many descriptives or models is a term that can only be described at different size scales.

for example we describe our current universe as homogeneous, on a scale below 100Mpc we have galaxies,stars, large empty spaces etc.
So in this case we can only apply the homogeneous term on scales larger than 100 Mpc

6. Apr 10, 2013

### Bandersnatch

Let me elaborate on Mordred's comment.

Perfect homogenity is unstable, in the same way as a needle standing on its sharp end is. It tends to be disrupted by the slightest of random motions and never reverts to the initial state.

Once a localised density spike appears in a uniform gas cloud, it gets exaggerated as it collapeses, and forms structures like groups of galaxies, galaxies themselves, stars and planets, depending on scale. Further collapse can be stopped by the conservation of angular momentum and (on small scales) internal kinetic energy of gas molecules(temperature) and radiation pressure.

Here's a nice simple program on Khan Academy that simulates a small, rotating cloud of particles collapsing under its own gravity:
It might help in visualising the "lumping" of material.
You can change the variables, including the rotation and "star" formation rules.

Anyway. Gravity works well if the time scale is huge, or spatial scale is small(stars, planets etc.). So, once you end up with lumps of gas floating around a galaxy, they tend to stay roughly undisturbed. They're very diffuse, and often hot, so significant gravitational collapse might not happen for a very long time.

Usually there's some event that triggers the collapse, like an earlier star exploding and sending shockwaves through the medium to compress it enough. How much material ends up in any given star ends up pretty much random. (the program linked above illustrates this unpredictability)

So now you've got a number of stars with a range of masses. In the early stages of their lives(http://en.wikipedia.org/wiki/Main_sequence), it is only mass that determines their size. The larger the bigger(and brighter and hotter).
More massive stars' cores are compressed more highly than low-mass ones, so they produce more heat that pushes the outer parts outward. So the mass-radius relation is different from what you might expect from e.g., the constant density calculations in my post #3.
It looks something like this:
(it changes a bit depending on mass range)

The most massive stars theoretically possible have(iirc) ~250 solar masses(more massive would produce enough energy to blow away the extra mass). This one is close:
http://en.wikipedia.org/wiki/R136a1
And it's only ~35 times the radius of the sun.

The really huge stars are all in their last phases of life, swelling to enormous proportions due the changing composition of their cores.
http://en.wikipedia.org/wiki/Stellar_evolution

Edit: the one factor I can think of that might influence the size of a star is the elemental composition of the collapsing molecular cloud.
Clouds with heavier elements will produce denser stars that will start fusion earlier. The resultant radiation flux will blow away the surrounding material that otherwise would accrete onto the star.

Last edited: Apr 10, 2013