Answer:
Because Betelgeuse is at a distance larger than 10 Parsecs.
Explanation:
The apparent magnitude is defined as the brightness of a star seeing from the Earth.
The brightness of a star seeing from the Earth can variate with the distance or with the radius of the star (since emits more light)
.
For example, a person is looking at two stars at the night sky. Star A is fainter than star B. That can suggest star A is farther away than star B, but this is not always the case. In the other hand, star B can be the one that is farther away, but since its radius is bigger it emits more light than star A.
Hence, to know the intrinsic brightness from a star, a quantity called absolute magnitude is established.
So, to measure the intrinsic brightness, all the stars are put at a distance of 10 Parsecs¹ (parallax of arc seconds, pc) from the Earth.
So, Betelgeuse has a larger distance than 10 pc since its apparent magnitude is less than the absolute magnitude.
Summary:
Absolute magnitude: Brightness of a star at a distance of 10 pc from the Earth.
Apparent magnitude: Brightness of a star at any distance from the Earth.
Brightness variates with the distance.
For Betelgeuse more light will be received at 10 Parsecs than at its real distance from the Earth (absolute magnitude [tex]>[/tex] apparent magnitude).
Key term:
¹Parsec: Unit of distance in Astrophysics.
