Special relativity and stars' masses

[omiros]

Hello everybody I am a first year student and I have a question about stars' masses.

I've been thinking about, what happens to a star's mass cause of the high speeds of the particles there. Do we consider the 'extra' mass, 'caused' by relativity (really high speed particles are heavier than those that are slow) or do we just think about its rest mass?

Thanks in advance :)

 

[jtbell]

You're thinking about the random motion of the particles associated with the star's temperature, right? In that case, yes, one way to look at it is that the star's mass includes the "relativistic mass" of those particles.

Another way, which is more in line with the way most physicists think of mass in relativity now (that is "mass" = "rest mass"), is to say that the star's mass is its total energy divided by c², in the reference frame in which the star as a whole is at rest (its total momentum is zero).

The star's total energy in turn consists of

• the energy associated with the "rest masses" of the individual particles
• the kinetic energies of those particles
• the (negative) potential energy that keeps all those particles together as a star

 

[omiros]

You're thinking about the random motion of the particles associated with the star's temperature, right? In that case, yes, one way to look at it is that the star's mass includes the "relativistic mass" of those particles.

Yes, I forgot to specify it. I've been thinking of a star with speed<<c.

So now we might say that this stars has a mass of 10kg let's say, however in reality is it more massive?

And if we had the case of fast moving star 1/3c would that affect its 'mass' too?

 

[Dale]

In special relativity the invariant mass (in units where c=1) of a system is given by m²=E²-p². In the star's rest frame p=0 for the star as a whole, so m=E (or in units where c≠1: E=mc²). In any frame where the star is moving both p and E increase so that m remains the same.

 

[sardar]

Hello, thank you for your question. The concept of mass in special relativity can be a bit confusing, but let me try to explain it. In special relativity, mass is not a constant value and can change depending on the observer's frame of reference. This is known as the "relativistic mass." However, this concept is not commonly used in modern physics and has been replaced by the concept of "rest mass," which is the mass of an object when it is at rest.

Now, to answer your question about stars' masses, we typically consider their rest mass, as it is the most accurate and consistent measure of their mass. The high speeds of particles within a star do not significantly affect its rest mass, so we do not need to consider the "extra" mass caused by relativity. However, special relativity does play a role in understanding the behavior of particles within a star, as it affects their energy and momentum. I hope this helps to clarify your question. Keep exploring and asking questions!