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The Earth in Perspective (4)

As noted on the previous page, the Sun is in the largest 15% of stars in the galaxy, but of course it is the largest and brightest stars that shine most brightly in the night sky, and are thus best known.

Sirius is the brightest star in Earth’s sky, not because it is intrinsically an exceptionally bright star, but because it is relatively close, a ‘mere’ 8.6 light years away. It is a White Main sequence (A1 V) star about twice as massive as the Sun, and substantially hotter and brighter. It is also a binary star, its companion Sirius B being one of the most massive White Dwarf stars known. Sirius B was once the larger of the two - a blue-white supergiant star 4 or 5 times the mass of the Sun (like a smaller version of Rigel), but being larger it burnt itself out faster, and about 120 million years ago it first expanded into a red giant like Pollux and Arcturus, then its outer atmosphere drifted off to form a planetary nebula, and the core collapsed into a dense bright ember that is today barely larger than Earth (but still with a mass almost as much as that of the Sun). All that remains of the nebula today is a denser-than-normal halo of dust around the two stars and a higher-than-normal incidence of heavy elements in the atmosphere of Sirius A.

Pollux is an Orange-Red giant (K0 III) star 34 ly from the Sun, and is the 17th brightest star in the sky. It is about 1.7 times as massive as the Sun. Pollux has reached the stage in its existence where it has exhausted the supply of hydrogen that fuels nuclear fusion at the core of Main sequence stars (the normal, stable stars like Sirius and the Sun that make up 80% of stars). It has started fusing helium into heavier elements, causing its outer atmosphere to become unstable and begin to expand uncontrollably. As it expands, it cools down and its light emission shifts towards the red end of the spectrum, which gives these bloated stars their characteristic red colour. In 2006 a planet with 2.3 times the mass of Jupiter was detected in orbit around Pollux by radial velocity measurement, so it seems likely that Pollux has a solar system. If that were to include a planet with intelligent life, then they are having a really bad time right now.   

Arcturus is the third brightest star in the sky. It is another Orange-Red giant (K0 III) star, 37 ly away, and is bigger and more expanded but less massive than Pollux. In fact, despite being so much larger than the Sun, it is only slightly more massive (because it is so large and diffuse its exact mass is difficult to accurately determine). Arcturus is probably similar to what the Sun will be like in about 5000 million years once it has exhausted all its hydrogen fuel and leaves the Main sequence.