Sirius A is currently a main-sequence star, but it is projected to evolve into a red giant in approximately 120 to 150 million years. When Sirius A transitions into a red giant, its size will increase significantly compared to its current state. However, it's important to note that predicting the exact appearance and behavior of a star millions of years into the future is challenging and subject to uncertainties.
As a red giant, Sirius A will expand in size, becoming much larger and more luminous. Its outer layers will extend outward, causing an increase in its radius. While I don't have access to the specific measurements for Sirius A's future size, I can provide some general information on red giants.
Red giants can reach sizes that are several times larger than their original main-sequence sizes. Some red giants have radii that are hundreds or even thousands of times larger than their original sizes. Consequently, when Sirius A becomes a red giant, it is expected to appear significantly larger in the sky compared to its current appearance.
Regarding the apparent magnitude, it is difficult to determine the exact change in magnitude for Sirius A in its red giant phase without specific measurements or modeling. The apparent magnitude of a star depends on both its intrinsic brightness (luminosity) and its distance from Earth. In the case of Sirius A becoming a red giant, while its luminosity will increase due to its expanded size, its distance from Earth is assumed to remain the same. As a result, any change in apparent magnitude would primarily be influenced by changes in its luminosity.
To determine the precise change in apparent magnitude, one would need detailed observations and calculations based on the specific properties of Sirius A as a red giant. Unfortunately, I don't have access to the exact measurements and predictions for Sirius A's future red giant phase.