Our solar system has only one star, our Sun. This may seem to be stating the obvious, but in fact it’s far more common for stellar systems to exist around two or even more stars that are orbiting each other. For example, our nearest stellar neighbour, Alpha Centauri, is actually a system of three stars. Observations suggest that around 50% of sun-like stars are in binary systems, and the proportion is even higher for more massive stars.
Effect of gravity
In a binary system the effects of gravity are very different from what we see in our own system. A single star is by far the most massive object in its system so everything orbits around it, but in a binary system the stars orbit each other, and their gravitational effect on their surroundings will vary significantly in strength and direction depending on their alignment at any point in time. This can create some intriguing and beautiful effects when one of the stars starts to blow off layers of gas at the end of its life, creating a ‘planetary nebula’.
For example, there are two sun-like stars at the centre of the ‘Twin Jet Nebula’ (PN M2-9) shown above, one slightly larger and one slightly smaller than the Sun, that orbit each other approximately once every 100 years. The larger star is dying and shedding layers of gas. In a single-star system this gas would form a roughly spherical expanding nebula around it. In contrast, the twin lobes and the complex patterns within them that you see here are a result of the gravitational effects of the two stars’ combined motion.
Celestial Crab
A dying star in a binary system is also responsible for the Southern Crab Nebula (Hen 2-104) but the unlike the Twin Jet Nebula, the two stars here are very different – the dying star is a pulsating red giant, whereas its companion is a much smaller white dwarf that has already burned out. Gravitational effects have caused both stars to become embedded in a flat disc of gas ejected from the dying star, and this disc restricts the expansion of the gas in certain directions, resulting in the hourglass effect you can see here. In this image from the Hubble Space Telescope the different colours reflect the various gasses present in the nebula – sulphur appears red, hydrogen is green, oxygen is blue and nirogen is orange.
Sometimes, despite a nebula having clear twin-lobed appearance, it’s not easy to directly observe whether there is more than one star at its core, and other methods have to come into play. For example, an unusual type of infrared laser emission from the ‘Ant Nebula’ (Mz 3), observed by the Herschel Space Telescope, suggests that there is a hidden binary companion to the white dwarf at its heart.
- ESA/Hubble & NASA, Acknowledgement: Judy Schmidt https://www.spacetelescope.org/images/heic1518a/
- NASA, ESA and STScI http://hubblesite.org/image/4384/news_release/2019-15
- NASA, ESA and the Hubble Heritage Team (STScI/AURA) http://www.esa.int/spaceinimages/Images/2018/05/Ant_Nebula
Find out more:
https://phys.org/news/2014-06-binary-stars-common-thought.html
https://phys.org/news/2017-06-evidence-stars-born-pairs.html>
https://www.nasa.gov/feature/goddard/hubble-sees-the-wings-of-a-butterfly-the-twin-jet-nebula
http://hubblesite.org/image/4384/news_release/2019-15
https://www.universetoday.com/139263/the-ant-nebula-actually-has-intense-laser-emissions-coming-from-its-core/