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Q2.23:
The Sun is a hot plasma (ionized matter) with its inner core at a temperature exceeding (10^{7}) K, and its outer surface at a temperature of about 6000 K. At these high temperatures, no substance remains in a solid or liquid phase. In what range do you expect the mass density of the Sun to be, in the range of densities of solids and liquids or gases ? Check if your guess is correct from the following data : mass of the Sun = (2.0 \times 10^{30}) kg, radius of the Sun = (7.0 \times 10^{8}) m.

Solution :

Step 1: Theoretical Analysis of Solar State

The Sun consists of ionized matter (plasma). At temperatures ranging from $6 \times 10^3 \, \text{K}$ to $10^7 \, \text{K}$, thermal energy exceeds the binding energy of molecular and atomic structures. While plasma behavior often mimics an ideal gas [Per the Kinetic Theory of Gases], the gravitational pressure in the Sun's core is immense, forcing atoms into a state of high compression. Given the extreme mass and gravitational collapse, one might expect the density to be closer to that of solids and liquids ($10^3 \, \text{kg/m}^3$) rather than a low-pressure terrestrial gas ($1 \, \text{kg/m}^3$).

Step 2: Mathematical Formulation of Density

The mass density ($\rho$) of a spherical body is defined as the ratio of its total mass ($M$) to its volume ($V$):

$$\rho = \frac{M}{V}$$

For a sphere of radius $R$, the volume is given by the geometric formula:

$$V = \frac{4}{3} \pi R^3$$

Step 3: Data Input and Calculation

Given variables:

  • Mass ($M$) = $2.0 \times 10^{30} \, \text{kg}$
  • Radius ($R$) = $7.0 \times 10^8 \, \text{m}$

Substitute $V$ into the density equation:

$$\rho = \frac{M}{\frac{4}{3} \pi R^3} = \frac{3M}{4 \pi R^3}$$

Performing the numerical substitution:

$$\rho = \frac{3 \times (2.0 \times 10^{30})}{4 \times 3.14159 \times (7.0 \times 10^8)^3}$$

$$\rho = \frac{6.0 \times 10^{30}}{4 \times 3.14159 \times 343 \times 10^{24}}$$

$$\rho = \frac{6.0 \times 10^{30}}{4310.27 \times 10^{24}}$$

$$\rho \approx 1.39 \times 10^3 \, \text{kg/m}^3$$

Step 4: Comparative Analysis of Densities

State Typical Density ($\text{kg/m}^3$)
Gases (at STP) $\approx 10^0$
Sun (Calculated) $\approx 1.4 \times 10^3$
Solids/Liquids $10^3 - 10^4$
1 kg/m³ (Gas) Sun Density (1.4 x 10³ kg/m³) 10³-10⁴ kg/m³ (Solid/Liquid)

Step 5: Conclusion

The calculated average density of the Sun is approximately $1.4 \times 10^3 \, \text{kg/m}^3$. This value falls squarely within the range of the densities of solids and liquids. This is physically consistent with the fact that, despite the high temperature, the extreme gravitational pressure from the overlaying layers of the Sun compresses the ionized gas to a density comparable to that of water or common metallic solids.

Final Solution: The mass density of the Sun is approximately $1.4 \times 10^3 \, \text{kg/m}^3$, which falls in the range of densities of solids and liquids.


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