Which Star is larger?

Question

Say that a red and white star have the same apparent brightness and are the same distance from the Earth. Which is larger and why?

Answer 1

Well, we know that the white star must be hotter...for stars, the bluer it is, the hotter. Similarly, the redder it is, the colder it is.

In general, white stars have a surface temperature around 10,000 Kelvin, while red stars are around 3000 Kelvin. For simplicity's sake, let's say the white star is 3 times hotter.

By The "Stefan-Boltzmann" law, hot things emit more light...a whole lot more, in fact: the amount of light emitted goes as the temperature to the fourth power. A chunk of white star material that's 3 times hotter than a chunk of red star material will emit 3 raised to the fourth power, or 81 times as much light.

So, if they're the same brightness and distance, there must be a whole lot more "cool chunks" in the red star to appear just as bright as the "hot chunks" of the white star...it's the same way that two 50-watt light bulbs will be just as bright as a 100-watt light bulb.

Therefore, the red star is much larger.

Answer 2

It's not very likely that a white star and a red one would be the same distance from Earth and have the same apparent magnitude, but generally red stars are much larger than white ones. White stars are often young, hot stars. Red stars are very old, cooler stars that have swollen enormously as their nuclear fuel is depleted.

Answer 3

VY Canis Majoris
Solar diameter 1950 (1800-2100)

VV Cephei is an eclipsing binary star system located in the constellation Cepheus. It contains a red supergiant which fills its Roche lobe when closest to its companion blue star, which appears to be on the main sequence; matter flows from the supergiant onto the blue companion. The stars are located about 3000 light years away from Earth.

VV Cephei A
VV Cephei A, the supergiant, is the largest star known. It is of spectral type M2 and is about 1600-1900 times the Sun's diameter. If it replaced the Sun in our solar system, it would extend to the orbit of Saturn. It is 275,000-575,000 times as luminous as the Sun. The mass of the star is unknown. The mass estimated from its orbital motion is about 100 solar masses. One other hand, the mass estimated from its luminosity is about 25-40 solar masses.

VV Cephei B
VV Cephei B, the blue main sequence star, is separated from the larger star by a distance of 16 - 25 AU; it is a B0 star about 10 times the Sun's diameter and about 100,000 times its luminosity. The orbital period of the system is 7430 days (20.3 years). The eclipse lasts about 1300 days, and the midpoint of the last eclipse was in early January 1998.

You could get more information from the 2 links below...

Answer 4

if they are equal distance from us and appear to be the same brightness, they should be about the same size.

Answer 5

there needs to be more description on the stars.