About the same mass as the Sun, and with a radius around 25 times greater: Arcturus gives us a fairly good preview of how our star will be in about 5 billion years.
Have you ever asked yourself how the Sun will look once it has exhausted the hydrogen in its core (the main fuel that is powering its luminosity and stability as you are reading this)? Chances are that we can actually see this future event, or at least something very similar to it, right now. If you get the chance to enjoy a clear night sky during or around spring time, all you have to do is locate the “Big Dip” constellation, imagine prolonging the curve of its handle until you reach a bright red star, the most luminous one of the Bootes constellation. That is Arcturus.
Because of its high apparent luminosity, Arcturus is one of the most popular stars in the sky even among amateur astronomers. One of its main characteristics is its mass which is very close to that of the Sun, but at the same time its red color indicates a significantly lower surface temperature (around 4290 K, compared with 5772 K on the Sun’s surface). This is a direct consequence of a hugely extended envelope, whose radius exceeds the solar one by 25 times. These data indicate how Arcturus, whose age is estimated to be around 7 billion years, is in an advanced evolutionary stage (called “red giant” phase), having exhausted its central hydrogen and having started to burn its leftovers in a thin and dense shell around its helium-core, at a much higher temperature than during its past Main-Sequence phase (the current evolutionary phase of the Sun), aka its long lost “youth”.
Looking into the future of the Sun
Yet despite these many differences, Arcturus and the Sun also have much in common. In fact, the single most important parameter that determines the life of a star more than anything else is its mass. Since the mass of Arcturus is very similar to that of our star, we can argue that one day in the future, in about 4 to 5 billion years, our Sun will be very similar to Arcturus. Indeed, looking at Arcturus is like looking into the future of the Sun, like cosmic clairvoyants. But we also know that the current age of our Sun is around 4.6 billion years, which means that it will enter the red giant phase when it is almost 10 billion years old, 3 billion years older that Arcturus. How is that possible? This is because Arcturus is made of plasma about 3 times poorer in metals, such as carbon, nitrogen, oxygen, iron, and other elements heavier than helium, which heavily determine the opacity of the star layers. Less metals produce a less opaque plasma, which means that the light emitted by the star interacts less with the stellar matter in its interior and escapes more easily, producing a remarkably lower outward radiation pressure on its layers. As a consequence, the whole stellar structure is denser, hotter, and more compact, making the hydrogen burn at a faster pace and shortening the total lifetime of the star.
All hail the queen of the spring night sky!
References:
Ramirez et al. The Astrophysical Journal, 743:135 (14pp), 2011
Williams, D.R. (1 July 2013); “Sun fact sheet”
