Why do you think we need to physically travel to the Sun to verify its properties?

Measuring the sun’s output across each wavelength of light lets us work out it’s average surface temperature, and this gives a figure that is approximately 5,000 C. Any matter at this temperature is a plasma. While fire can sometimes be classified as a plasma, fire is also a specific type of oxidation reaction, which doesn’t occur on the Sun due to its relatively low concentration of oxygen. The Sun’s spectral properties show that it’s composed primarily of hydrogen, thus its energy output is driven entirely by nuclear fusion.